Warren Poole


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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Microstructure evolution in the coarse grain heat affected zone of line pipe steels (2023)

Line pipe steels are manufactured by thermomechanically controlled processing (TMCP), which combines selected alloying additions and processing conditions, to produce engineered microstructures that confer steels their properties. During pipeline construction, segments of pipe are sourced from numerous suppliers, each of whom may have different alloying strategies, and welded by varied technologies and procedures, leading to numerous different resulting microstructures in the weld heat affected zone (HAZ). Consequently, it is critical to develop a chemistry sensitive model that describes austenite grain growth and decomposition in the coarse grain heat affected zone (CGHAZ).In this work, austenite grain growth in the CGHAZ is investigated by using Laser Ultrasonics for Metallurgy (LUMet) in 37 steels of chemistries in a range relevant to X80 line pipe steels. A model which considers the effect of solutes on the mobility of the austenite grain boundary and the pinning pressure exerted by Ti-rich precipitates was developed. The amount of C and Nb in solution were found to reduce the mobility of the austenite grain boundary. The size distribution of Ti-rich precipitates was analyzed and verified by scanning transmission electron microscopy (STEM). The model was validated for laboratory thermal cycles and industrial welding trials.Austenite decomposition kinetics were measured for 16 steels with systematic variations of C, Mo, Nb, and Cr, at continuous cooling rates of 3, 5 and 10 °C/s, and their final microstructures were characterized using electron backscatter diffraction (EBSD), optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Vickers hardness. Microstructures at 30 and 50°C/s were also produced and characterized with the same techniques. The effect of solutes in decomposition kinetics was analyzed and C, Nb, and Mo were found to decrease the transformation start temperature in continuous cooling. A novel finding was the role of Ti-rich particles. It was observed that a decrease in the interparticle spacing of these precipitates leads to an increase in the transformation start temperature, which was rationalized by their ability to pin moving bainite grain boundaries. A microstructure-properties model is proposed where the hardness in the CGHAZ is a function of the high angle grain boundary density.

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Prediction of grain structure and texture evolution after high temperature extrusion of aluminum alloys (2022)

To engineer the mechanical response of the material, it is crucial to predict the microstructure after thermomechanical processing and relate properties to the microstructure. In this study, the relationship between the features of the deformed state after axisymmetric extrusion of aluminum at high temperatures (e.g., the subgrain size distribution and the disorientation distribution) and the final recrystallized texture was investigated. It aims to systematically change the features of the deformed state by generating different initial synthetic microstructures and using them as inputs for a phase field model to predict the recrystallized texture. The characteristics of the deformed state and the recrystallized state were analyzed for extruded 3XXX aluminum samples with two different homogenization heat treatments. The subgrain size distribution and disorientation distribution in different texture fibres of the deformed state, i.e., ||ED and ||ED, were extracted to construct a baseline condition. After verifying that 2D simulations could be used to capture the essential phenomena in 3D, the role of the microstructural features was investigated using 2D simulations. First, the subgrain size distribution and disorientation distribution were changed from baseline condition by ±50% for individual grains with ||ED or ||ED. Second, for a mixture of grains with ||ED and ||ED orientations, ||ED baseline condition was used, then subgrain size distribution and disorientation distribution were changed by ±50% for ||ED grains. Third, an input microstructure from experimental EBSD measurements was simulated. For the first scenario, a narrower distribution and a wider disorientation distribution resulted in a larger final average subgrain size which was rationalized based on the evolution of these microstructural features. For the second scenario, the tail of the subgrain size distribution and the width of disorientation distribution within ||ED was found to have a large impact on the final texture. This was explained by the probability of large grains with high angle disorientation in ||ED fibres growing preferentially to pinch-off grains with ||ED orientations. For the third scenario, only ||ED volume fraction could be predicted accurately compared to the experiment. Possible sources of error were identified and discussed.

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The effect of microstructure on strain localization in the precipitate free zone of Al-Mg-Si alloys (2022)

An important parameter to consider for selecting Al-Mg-Si alloys for the automotive industry is their ductility as extruded parts may experience severe plastic deformation, e.g. in a crash. Several studies have investigated the effect of microstructural features on mechanical properties and, in particular, ductility as characterized by the true fracture strain in a tensile test. However, most of the data in the literature is based on qualitative experimental observations, or more recently, polycrystal plasticity simulations. The lack of quantitative measurements limits the predictive capability of such models. Therefore, the current study aims to make a quantitative approach to the measurement of microstructural parameters and the plastic strain distribution within grains and the PFZ. Having this perspective, the effect of cooling rate after solution treatment and the effect of aging on the size and number density of grain boundary precipitates and PFZ width was quantified. In T6 condition it was found that slow cooling (~ 6 C/s) results in the formation of ~ 300 nm wide PFZ near the grain boundaries whereas the PFZ width was ~ 55 nm in fast quenched (~ 1500 C/s) sample. It was observed that the magnitude of strain localization is affected by the magnitude of yield stress mismatch between the PFZ and the grain interior. The effect of significant strain localization within the PFZ in slow cooled samples is reflected in the observed fraction of inter-granular fracture and fracture strains. For example, in the T6 condition, the air cooled sample exhibited a fully intergranular fracture with strain to fracture of ~ 0.35 whereas the water quenched sample showed a mixture of intergranular and transgranular fracture with strain fracture of ~0.75. Finally, it is found that non-shearable precipitates and dispersoids make the distribution of slip within the grains more uniform. It should be noted that although the addition of dispersoids increased transgranular fracture mode, but it did not affect the fracture strain in this sample. This may be due to the competition between void nucleation on grain boundary precipitates and dispersoids inside grains.

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Microstructure and mechanical properties of simulated weld heat affected zones in X80 linepipe steel (2020)

Low carbon micro-alloyed steels are used for linepipe applications as they provide a good combination of strength, toughness and weldability. An important part of the construction for long distance pipelines is the in-field joining of pipes. Owing to the complex thermal cycles during welding, the microstructures and mechanical properties of the base material are altered adjacent to the weld in the region known as the heat affected zone (HAZ), in particular, the regions, i) next to the fusion line (the coarse grain heat affected zone, CGHAZ) and ii) where the thermal fields from multi-pass welds overlap (for example: the intercritically reheated coarse grain heat affected zone, ICCGHAZ).Using a Gleeble thermomechanical simulator, bulk microstructures, that were representative of the thermal conditions found in the CGHAZ and ICCGHAZ regions, were produced in two high strength low alloy steels. It was found that the cooling rate after the first weld pass had a large effect on the microstructure produced relevant to the CGHAZ. The second thermal excursion involves intercritical annealing of the initial microstructures (relevant to the ICCGHAZ). This produced a nearly continuous necklace of martensite along the prior austenite grain boundaries. The effects of i) the different morphologies of bainite and ii) the intercritical austenite fraction (which transforms to martensite-austenite (M/A) constituents during cooling) on the tensile and Charpy impact properties were systematically studied. The fine bainite microstructures formed at a cooling rate of 50℃/s were found to have the highest density of high angle grain boundaries resulting in the best combination of strength and ductile-brittle transition temperature. Upon intercritical annealing, the ductile-brittle transition temperature was significantly increased when a nearly continuous necklace of M/A formed on the prior austenite grain boundaries (for M/A ≥ 10%). This work presents a systematic study on the effect of the fraction of M/A constituents on the tensile stress-strain response and the ductile-brittle transition behaviour. The conclusions from this work have the potential to provide guidelines to linepipe steel producers (in terms of chemistry) and pipeline constructors on the input of different welding parameters on the properties in the heat affected zone of the base pipe.

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A study on the texture and microstructure development in extruded AA3003 alloys and the relevant mechanical behaviour (2018)

In this study, a model alloy (i.e. AA3003) was used to examine microstructure and texture development in the extrusions and the relevant mechanical response. In particular, two homogenization heat treatments were studied as the initial condition, i.e. at 375 °C for 24 h, which produces a high density of dispersoids, and at 600 °C for 24 h, which produces a condition with almost no dispersoids. Three nearly ideal deformation modes were studied, i.e. (i) axisymmetric extension (the central region of a round bar extrusion), (ii) plane strain deformation (the central region of a strip extrusion), and (iii) simple shear deformation (using torsion tests). Electron backscatter diffraction (EBSD) was the main technique to characterize the texture and microstructure of the materials.For the axisymmetric extension and plane strain deformation, it is proposed that the high density of dispersoids in the material causes a large Zener drag, which inhibits grain boundary migration and thereby maintaining the deformation texture and microstructure in the extrusions. For the materials with almost no dispersoids, it is proposed that continuous dynamic recrystallization (CDRX, which is characterized as the subgrain coarsening or the grain boundary migration) occurs during and after the extrusion, and therefore, the texture and microstructure transform from the as-deformed to the recrystallized state. In contrast, for simple shear deformation (i.e. torsion), there is little difference observed between these two materials. However, with an increase in the level of equivalent strain, the texture changes from a deformation to a recrystallized texture. Geometric dynamic recrystallization (GDRX, which is characterized as the phenomenon of the grains continuously pinched off during the deformation) is proposed as the mechanism for the case of the simple shear deformation. For the mechanical behaviour of the extruded materials, it was found that the surface layer has little effect on the measured stress-strain curves. However, the surface layer has a large effect on the R-values, i.e. a characteristic value of the material formability. It is suggested that the large R-value difference between the surface layer and the central region of the sample is attributed to the different textures.

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Microstructure evolution during homogenization and its effect on the high temperature deformation behaviour in AA6082 based alloys (2018)

There is a current trend in increased use of aluminum extrusion alloys in automotive applications. This trend is driven by the need to reduce the vehicle weight, which in turn, is to decrease energy use and/or emissions of the vehicles. In this work, Al-Mg-Si alloys, and especially variants of AA6082 with different Mn and/or Cr additions have been studied. The objectives of the study are to i) experimentally characterize the evolution of the constituent particles (phase, volume fraction and size) and dispersoids (chemistry, crystal structure, size and volume fraction), ii) rationalize the mechanisms of dispersoid evolution and iii) develop a physically based constitutive law for the high temperature flow stress.The characterization of microstructure evolution during homogenization was done using a combination of i) transmission electron microscopy, ii) field emission gun scanning electron microscopy, iii) electron microprobe microanalysis and iv) electrical resistivity measurements. The high temperature flow stress was characterized by uniaxial compression tests.The main results on microstructure evolution during the process of homogenization show that i) there is a transformation of the constituent particles from the β to α phase during homogenization and a concurrent speriodization of the particles, ii) dispersoids with the size range of 20-200 nm and a volume fraction of 0.25 – 1.3 % are initially formed during homogenization but they eventually dissolve as Mn is transported to the constituent particles and iii) a steady-state flow stress of between 20 and 45 MPa was measured for the test temperature between 550 °C and 580 °C with strain rates of 0.1 – 10 s-¹.The evolution of dispersoids during homogenization was rationalized by considering their nucleation, growth and coarsening. It is proposed that dispersoid coarsening initially involves long-range diffusion of Fe from the constituent particles to dispersoids and later Mn and Fe diffuse to the constituent particles. The Kocks-Chen constitutive model was extended to include the role of dispersoids on high temperature flow stress using an Orowan type model for precipitation hardening. This was found to predict the flow stress ± 5 % in ≈ 95 % of the cases that were studied.

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Examination of Deformation in Magnesium Using Instrumented Spherical Indentation (2016)

This investigation examines the use of instrumented indentation to extract information on the deformation behaviour of commercial purity magnesium, AZ31B (Mg-2.5Al-0.7Zn), and AZ80 (Mg-8Al-0.5Zn). In particular, indentation was conducted with spherical indenter using a range of spherical indenter tip radii of R = 1 µm to 250.0 µm. A detailed examination has been conducted for the load-displacement data combined with three-dimensional electron backscatter diffraction (3D EBSD) characterization of the deformation zone under the indenter after the load has been removed. It was proposed that the initial deviation of the load-depth data from the elastic solution of Hertz is associated with the point when the critical resolved shear stress (CRSS) for basal slip is reached. Also, it was observed that reproducible large discontinuities could be found in the loading and the unloading curves. It is proposed that these discontinuities are related to the nucleation and growth of {101̅2} extension twins during loading and their subsequent retreat during unloading. For the case of c-axis indentation, 3D EBSD studies showed that the presence of residual deformation twins depended on the depth of the indent. Further, a detailed analysis of the residual geometrically necessary dislocation populations in the deformation zone was conducted based on the EBSD data. It was found that residual basal dislocations were dominant in the deformation zone. This was consistent with crystal plasticity finite element method calculations where only basal slip was allowed albeit with some differences that can be rationalized by the presence of {101̅2} extension twins in the experiments. Using different spherical diamond tips, it was concluded that the quantitative values for the RSS0.1% offset for basal slip of magnesium obtained from the indentation test is indentation size dependent and it increases linearly with the inverse square root of the misorientation gradient under the indent. Finally, effects of chemistry on the CRSS for basal slip was also successfully measured by conducting the indentation tests on AZ31B and AZ80 alloys. It was shown that the CRSS of basal slip increases linearly with c¹′², where c is the concentration of Al.

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On the Origin of Cluster Strengthening in Aluminum Alloys (2016)

Understanding the influence of atomic clusters formed during natural aging in aluminum alloys is a key problem to control more effectively the strength of alloys both during their processing and life. The yield strength of these alloys is controlled by the interaction between dislocations and solute atoms, and clusters. An empirical scaling law relating the dislocation-obstacle interaction force with the size of clusters has been developed and successfully used to predict the yield stress of a cluster strengthened AA6111 industrial aluminum alloy.It is proposed that the strengthening effect captured by the scaling law could come from the geometrical rearrangement of solute atoms from a random distribution to a clustered distribution, and/or from the change in strength of individual obstacles.A modified areal glide model was employed to investigate the statistical problem of a dislocation moving through a set of clustered point obstacles in the glide plane. The results of these simulations suggest that the degree of clustering of solute atoms does not influence the critical resolved shear stress.Then, molecular statics simulations were used to investigate the origin of the change in strength of individual clusters, in the simple case of Al-Mg alloys. A model based on elastic interaction between the solute atoms/clusters and an edge dislocation was developed and demonstrated to give good predictions for the maximum pinning force of single solutes, dimers and trimers.Using a detailed analysis of the model and the molecular statics simulations, it was shown that the strength of clusters principally comes from the elastic interaction between dislocations and solute atoms forming the clusters. Further, the change of topology of clusters was found to not significantly affect their strength at least in the case of Mg clusters in aluminum. Finally, this model was employed to determine the strengthening contribution of distributions of single solutes, dimers and trimers in binary Al-Mg alloys. The strength was found to roughly depend linearly on the size of clusters, however, its slope is lower than in the case of the AA6111 alloy which predominately contains a combination of Mg-Mg and Mg-Si clusters. The possible reasons for this discrepancy are discussed.

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Annealing Behaviour of Cold Deformed AA3003 Aluminum Alloys (2015)

AA3xxx series aluminum alloys which are used in automotive heat exchangers usually experience a complicated thermomechanical history with a number of processing steps including homogenization, extrusion, cold deformation and then annealing. The cold deformation can involve a wide range of strains, for example, a few percent strain for micro-multiport tubing and large strains for tube drawing. Often the parts are annealed either as a separate processing step or in conjunction with the brazing operation. Results from industry suggest that a wide range of microstructures can be observed after brazing, ranging from coarse multi-crystals to fine grained polycrystalline microstructures. As such, annealing behaviour of cold deformed AA3003 based alloys after extrusion was investigated in this work. Experimental work was conducted on two alloys homogenized prior to extrusion: 3003 (0.54 wt% Fe) and low Fe 3003 (0.09 wt% Fe). A variety of homogenization heat treatments were examined in order to produce the starting materials. Microstructure, yield stress, work hardening and recrystallization behavior of these alloys with different initial microstructures were investigated. A wide range of pre-strain (1-80%) was applied at room temperature using tensile test and rolling. Although most of the annealing treatments were done at 600 °C, samples with pre-strains larger than 0.1 were also annealed at 350-600 °C to study the effect of temperature on microstructure evolution. The minimum strain required to initiate recrystallization was experimentally measured for each condition using tapered samples. As expected, it was found that dispersoids significantly inhibit recrystallization process and can change the critical strain for recrystallization as high as 18%. In addition, contribution from different strengthening mechanisms on the yield stress and work hardening behaviour was calculated and then a model was developed to describe the stress-strain response. The model represents experimental data within ±10% for both yield stress and UTS over a wide range of conditions. Consequently, a physically based model was developed for the critical strain required to initiate recrystallization. This is the first attempt, to the author’s best knowledge, to model critical strain in a system with distribution of fine and relatively large precipitates.

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The tensile properties and toughness of microstructures relevant to the HAZ of X80 linepipe steel girth welds (2015)

In the transportation of oil and gas products from and over remote locations, such as Canada's Arctic environment, pipelines are used. Girth welding to join sections of steel pipelines creates a substantial heat affected zone (HAZ) within the base pipeline steel. While there is significant concern as to the fracture and mechanical properties of the HAZ as whole, detailed knowledge about the mechanical properties of the microstructural constituents is lacking. For this research, measurements of the temperature time profile in the HAZ of single and dual torch welds were made. This was then used to guide heat treatments of X80 steel in a Gleeble simulator to create samples of 8 different bulk microstructures with differing amounts and morphologies of bainite, ferrite and martensite-retained austenite (MA). From the heat treated samples tensile and Kahn tear test specimens were made for testing at ambient, -20⁰C, and -60⁰C. The highest strength microstructure proved to be the finest, lower bainitic microstructure, while the lowest strength microstructure was the coarsest, upper bainitic sample containing a significant amount of MA. This was observed to be true at all testing temperatures. As part of the tensile behaviour investigation, the Bouaziz dislocation based model for work hardening was applied and shown to fit well across all temperatures and conditions. The Kahn tear test, a machine notched, thin-sheet, slow strain rate test, showed all tests failed in a ductile manner. Relative toughness measurement from this test showed that the fine, lower bainitic microstructure was the toughest and the coarse, ferritic microstructure was the least tough. This work presents a novel measurement of dual torch temperature time profiles in a real HAZ, an extensive mechanical testing program of isolated microstructures relevant to the X80 HAZ at potential pipeline operating temperatures, and an applied a robust model to fit the work hardening behaviour for all conditions. This work has the potential for future application in microstructure evolution-property models, and in a combined mechanical model of the different microstructures to further improve understanding of HAZ mechanical responses.

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Microstructure evolution during intercritical annealing of a Mn-Cr dual-phase steel (2013)

A model was developed to describe the microstructure evolution during intercritical annealing of a low-carbon steel suitable for industrial production of dual-phase steels (DP600 grade) on a hot-dip galvanizing line. The microstructure evolution model consists of individual submodels for recrystallization, austenite formation in a fully recrystallized material and austenite decomposition after partial austenization. These submodels were developed using the Johnson-Mehl-Avrami-Kolmogorov approach and the additivity principle. The model parameters were obtained based on the results of systematic experiments addressing the effects of initial microstructures and processing conditions on the microstructure evolution in the course of intercritical annealing. The initial microstructures included 50 pct cold-rolled ferrite-pearlite, ferrite-bainite-pearlite and martensite. If heating to an intercritical temperature was sufficiently slow, recrystallization was completed before austenite formation, otherwise austenite formed in a partially recrystallized microstructure. The recrystallization-austenite formation interaction accelerated austenization in all three starting microstructures by providing additional nucleation sites and enhancing growth rates; this complex process could not be accounted for with the current modelling approach. A variety of austenite morphologies was produced by using different initial microstructures and/or by means of the interaction of recrystallization and austenite formation. Following the complete intercritical annealing cycle, the final microstructure was composed of ferrite, bainite and martensite; the latter two components inherited the distribution and morphology of those for intercritical austenite. The microstructure evolution model was validated using simulated industrial thermal paths for intercritical annealing. Laser ultrasonics was employed for in-situ monitoring of phase transformations to facilitate the validation of the microstructure evolution model.

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Multiaxial Deformation of AZBO Magnesium Alloy (2013)

The multiaxial deformation of magnesium alloys is important for developing reliable, robust models for both the forming of components and also analysis of in service performance of structures, for example, in the case of crash worthiness. This work presents a combination of unique biaxial experimental tests and biaxial crystal plasticity simulations using a visco-plastic self-consistent (VPSC) formulation conducted on AZ80 magnesium alloy in two different conditions - extruded and a more weakly textured as cast condition. The experiments were conducted on tubular samples which are loaded in axial tension or compression along the tube and with internal pressure to generate hoop stresses orthogonal to the axial direction. The results were analyzed in stress and strain space and also in terms of the evolution of crystallographic texture. In general, it was found that the VPSC simulations matched well with the experiments, particularly for the more weakly textured cast material. However, some differences were observed for cases where basal slip and {10¯12} extension twinning were in close competition such as in the biaxial tension quadrant of the plastic potential. The evolution of texture measured experimentally and predicted from the VPSC simulations was qualitatively in good agreement. Finally, experiments and VPSC simulations were conducted in which samples of the extruded AZ80 material were subjected to a small uniaxial strain prior to biaxial loading in order to further explore the competition between basal slip and extension twinning.

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Microstructure-property models for heat treatment of A356 aluminum alloy (2011)

The evolution of microstructure and mechanical properties during heat treatment of an industrially-cast A356 aluminum alloy was studied in an extensive experimental investigation. The temperature ranges of interest were; solution treatment at 500-560°C, natural ageing at room temperature, and artificial ageing at 150-200°C. The changes in dendritic composition and eutectic morphology due to solution treatment were quantified by microprobe and image analysis for a wide range of processing conditions. Subsequently, a microstructure model for solution treatment was constructed using sub-models for; i) the dissolution of Mg₂Si particles, ii) the fragmentation of eutectic fibres, and iii) the coarsening of the fragmented eutectic. For the ageing investigations, characterisation of mechanical properties was done by hardness and tensile testing, and the kinetics of precipitation was determined by an isothermal calorimetry technique. A model to predict the evolution of yield strength during artificial ageing was developed based on established physical theories. A yield strength model for natural ageing was also proposed using data from isothermal calorimetry tests performed close to room temperature. Two model Al-Si-Mg alloys were investigated in order to extend both ageing models to include the effects of; i) alloy chemistry, ii) incomplete solution treatment and iii) natural ageing prior to artificial ageing. The validity of the models was verified using independent experimental measurements and literature data, and they were subsequently used as a tool to identify potential optimisation strategies for industrial heat treatment processes. The linkages between the models revealed details of processing challenges arising from the interdependence of the heat treatment stages, such as reduced strengthening during ageing due to incomplete solution treatment, and delayed strengthening during artificial ageing as a result of prior natural ageing.

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The deformation behaviour of a Mg-8Al-0.5Zn alloy (2011)

In this work, the deformation behaviour of an Mg-8Al-0.5Zn (AZ80) alloy having a fixed initial grain size of ~ 32 µm was studied by varying the initial texture, temperature, stress state and microstructure. The work focused on investigating the influence of these variables on the mechanical properties, work hardening characteristics, texture evolution and deformation mechanisms of the alloy. The initial materials with different starting textures (i.e. strongly and weakly textured) and microstructures (i.e. solution-treated and aged) were obtained through a series of thermo-mechanical treatments including cold rolling, annealing and ageing. The uniaxial compression and tension deformation experiments were carried out on strongly and weakly textured solution-treated and aged samples at 77K and 293K. Neutron diffraction, slip trace analysis, high and low resolution EBSD were used to characterize the texture evolution and deformation mechanisms of the alloy. In addition, a visco-plastic self consistent (VPSC) model was used to predict the influence of initial texture and temperature on the deformation behaviour. The results show that temperature and loading direction with respect to initial texture has a pronounced effect on yield strength and work hardening. It is found that there is a substantial difference between the nature of twinning, slip system activity and texture development as a function of deformation temperature. It is shown that the VPSC model is effective in predicting the deformation response of alloy when it is dominated by slip. The same model however proved to be inadequate for twinning dominated deformation. The results illustrate that precipitates are capable of changing the balance of deformation mechanisms and texture development of the alloy. They were found to be extremely effective in reducing the well known tension compression yield asymmetry exhibited by magnesium and its alloys.

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Modelling the effect of grain size distribution on the mechangical response of metals (2009)

Recent experimental studies have pointed to the potential for producing metals withimproved mechanical properties based on manipulation of the grain size distribution. It is,however, unclear how these improvements are brought about and whether grain sizedistribution manipulation can effectively be used to tailor the mechanical response of metals.In this work, these issues are examined using two novel grain size dependent self-consistentmodels, an elastoplastic and a viscoplastic, where plasticity is assumed to occur bydislocation slip. For this purpose, monotonic deformation of a number of model f.c.c.polycrystals with moderate stacking fault energy (such as copper) is examined. Polycrystalswith lognormal distributions, having average grain sizes ranging from 100 nm to 50 p.m, andbimodal distributions are considered.It is found that increasing the width of the lognormal grain size distribution, whilekeeping the average grain size constant, decreases the yield strength of the polycrystal andincreases its work hardening rate. This behaviour is attributed to the increasing volumefraction of grains larger than the average, as the distribution is widened, which have lowerthreshold stresses and higher work hardening rates than the average. The simulation resultsare summarized in the form of new property maps where the range of ultimate tensilestrength-uniform elongation combinations which can be achieved through grain sizedistribution manipulation are shown. These maps also demonstrate that bimodal polycrystalsdemonstrate better overall properties as compared to lognormal polycrystals. The observedgrain size distribution effects are, however, found to be dependent on the nature of theconstitutive relationship assumed for the grains. The developed maps provide a first guide for materials engineers interested in the modification of the mechanical properties ofpolycrystals through grain size distribution manipulation.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Anisotropy of strength and fracture behavior in Al-Mg-Si-(Mn) extrusion alloys (2023)

Aluminum 6xxx extrusion alloys are attractive candidates for use in automotive applications to decrease vehicle weight. In this study, Al-Mg-Si alloys, and especially variants of AA6082 with two different Mn contents were extruded into 3 × 90mm strips on a pilot scale extrusion press with processing conditions designed to produce either recrystallized or unrecrystallized microstructures. The objectives of the study are to i) characterize the important microstructural features in the two alloys and ii) examine the relationship between orientation dependence of mechanical properties measured from uniaxial tensile and VDA bend tests with the microstructure.The characterization of microstructure and crystallographic texture was done using scanning electron microscopy and electron back scatter diffraction. The mechanical anisotropy was measured by tensile and VDA bend testing conducted at 0, 45 and 90o to the extrusion direction. The characterization analysis shows presence of crystallographic anisotropy, morphological and topological anisotropy of second phase particles in the studied alloys. It was found that the strength and fracture anisotropy in tension and bending were significantly affected by the microstructure. A linear correlation between the tensile true strain to fracture and the maximum bend angle prior to substantial damage was observed for the samples tested at 0 and 90o to the extrusion directions for the two alloys. However, the results for samples taken at 45o to the extrusion direction did not follow this trend.

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Estimating the yield stress in aluminum alloys using instrumented spherical indentation (2023)

Instrumented indentation tests using a spherical indenter are a potential source to obtain mechanical properties from small samples and also to obtain localized mechanical properties within a sample. Thus, the data obtained from indentation tests would provide valuable insights into the mechanical properties of materials which are not feasible with conventional tensile testing. Despite the potential advantages of indentation tests, there are still questions which need to be considered when conducting indentation test and analyzing the data. In particular, a robust procedure is required to determine the true contact point between the indenter tip and the sample.In the present work, instrumented indentation tests were used to extract stress-strain curves from the load, displacement and the continuous stiffness measurements. Indentation tests were conducted on an Al-Mg-Si precipitation hardened alloy with two different heat treatments (underaged and peak aged). The various parameters that must be considered when conducting indentation tests and analyzing the measured load, displacement, and continuous stiffness data have been examined. A new approach is proposed to determine the true contact point of the indenter to the sample using a combination of Kalidindi’s method and the deviation from Hertzian elastic contact mechanics. The critically resolved shear stress (CRSS) for the onset of plasticity was determined for indentation tests using indenter tip radii of 5, 10, 50, and 250?m and then compared to the CRSS estimated from polycrystal plasticity simulations fit to experimental tensile stress-strain results. A significant indenter size effect was observed and as such a conversion factor is proposed to relate the CRSS measured for different indenter tip radii to the bulk response.

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The effect of PCG layer thickness on bendability of aluminum extrusion alloys (2023)

Recently, there is an increase in demand for the usage of aluminum extrusion alloys in automotive applications to reduce the vehicle weight. Bendability (i.e. how much bending the material can sustain without cracking) is an important material property, both for the forming of components and in crash behavior.During the production of Al-Mg-Si extrusion alloys (AA6082 non- recrystallized) which was studied in this research, the properties of the surface can be substantially different from those of the interior as these alloys are prone to forming a surface layer known as the Peripheral Coarse Grain (PCG) zone. The objective of this study is to investigate the effect of the PCG layer thickness on bendability of aluminum alloys. Different PCG layers thicknesses were achieved in the range of 50 μm to 800 μm at different extrusion ram speeds by our industry partner.In this project, different mechanical tests were done to characterize the effect of PCG layer thickness, and heat treatment on the bendability of aluminum extrusion alloys. The microstructures were examined using Barker’s etch to observe the initiation of cracks in bend region. To see the effect of PCG layer on the bendability, the PCG layer was removed from one side of the sample and then the test was conducted with the PCG layer in tension and compression. The results show that the sample with a very thin PCG layer (50μm) showed similar load- displacement curves whether the PCG layer was in compression or tension. However, for the thicker PCG layers, the bend response was poorer when the PCG layer was in tension. Tests done on the samples in ED and TD direction (Note: ED- bend axis ⊥ to extrusion direction, TD- bend axis ∥ to extrusion direction) and results show that for all thicknesses of the PCG layer, tests done in TD direction showed a lower bend angle failure compared to the tests done in ED direction. Finally, to see the effect of cooling rate on bendability, cooling rate of 13-20 °C/s was chosen and after bend test the results showed that the lower cooling rate had 10-20% poorer bendability.

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Influence of Cu content on the artificial ageing response of Al-Mg-Si-Cu alloys (2021)

There is a current trend in increased use of aluminum extrusion alloys in automotive applications. This trend is due to the increasing demand for the lightweight materials in the automotive industry. The major reason for this is government regulation of internal combustion engine powered vehicles for vehicle fuel efficiency and the corresponding reduction of emissions. The main objective of this work is to quantify the effect of Cu content on the precipitation response of Al-Mg-Si-Cu alloy during artificial ageing. In this study, four alloys with different Cu contents (0-0.9wt%) and two different initial microstructures (unrecrystallized and recrystallized) were examined. The alloys were all solution treated at 550 °C for 5 minutes and then were artificially aged at four ageing temperatures (160°C, 180°C, 200°C and 220°C). Their mechanical properties were measured by hardness and tensile tests. It was found that the addition of Cu to Al-Mg-Si alloys increases the peak yield stress. The peak yield stress increased by 60MPa and 45MPa for the recrystallized and unrecrystallized alloys, respectively, when 0.9wt% Cu was added. Based on these data, a process model for Al-Mg-Si-Cu alloys based on the Shercliff-Ashby approach was developed to model the evolution of yield strength during artificial ageing. The model contains contributions from precipitation hardening, solid solution strengthening, dislocation hardening and the intrinsic strength of the alloy. The model was verified by the good agreement between the predictions and experimental results.

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Quench sensitivity of an Al-Mg-Si-Cu aluminum alloy (2021)

The use of aluminum extrusion alloys in automotive applications is increasing due to the requirement that vehicle weight be reduced. One of the steps involved in the processing of aluminum extrusion alloys is rapid cooling (or quenching) after the solution heat treatment. The term quench sensitivity refers to the relationship between mechanical properties and the cooling rate after solution treatment. In this study, two initial microstructures were produced from the as-received alloy using different thermomechanical processing routes to study quench sensitivity: in the first case, the as-received microstructure underwent a solution treatment of 5 minutes at 550 C to produce a predominantly unrecrystallized grain structure, and in the second case, the material was rolled at room temperature prior to a solution treatment of 5 minutes at 550 C to produce a recrystallized grain structure.The main objective of this study was to measure the quench sensitivity of the two initial microstructures. The variation of hardness with cooling rates was examined using a modified Jominy test, and the variation of yield strength and ultimate tensile strength with quench rates was characterized using tensile tests. Hardness, yield strength, ultimate tensile strength and ductility were found to decrease with decreasing cooling rates. The reduction in the mechanical properties was attributed to the loss of solute atoms which precipitates as coarse precipitates during slow cooling. Slow cooling typically results in a higher number of grain boundary precipitates and a wider precipitate free zone (PFZ) adjacent to the grain boundary, which increases the propensity of intergranular fracture. Finally, using a polycrystal plasticity model, the degree of strain localization within the PFZs was studied for different idealized geometries.

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The effect of quench rate and initial grain structure on the mechanical behaviour of an Al-Mg-Si-Mn aluminum alloy (2018)

The mechanical behaviour of AA6082 is a function of the extrusion conditions and in particular, the quench rate after extrusion. Controlling the quench rate after extrusion can affect the microstructure evolution to produce desirable mechanical properties for application in structural components for the automotive industry, and is therefore a key parameter of interest. In this study, a near industry alloy similar to automotive grade AA6082 containing 0.5 wt.% Mn and 0.15 wt.% Cr was direct chill (DC) cast, homogenized for 2 hours at 550 °C, and extruded at a temperature of 500 °C with a ram speed of 8 mm/s to form 3 mm x 42 mm strips. The microstructure in the as-extruded strip was unrecrystallized due to the Smith-Zener drag from the Mn/Cr dispersoids. Furthermore, when the as-extruded strip was cold rolled prior to heating, recrystallization occurred concurrently with the solution treatment. This allowed for 3 initial microstructures to be produced, i.e. unrecrystallized, and recrystallized with a grain size of 9 and 40 µm.The aim of the study was to measure the quench sensitivity for the 3 different initial grain structures after solution treatment at 560 °C for a sufficient time to dissolve the Mg-Si precipitates, followed by cooling at rates between 4 and 2000 °C/s. Controlled cooling experiments within this temperature range were conducted using the Gleeble 3500 thermomechanical simulator. The relationship between quench rate and precipitation of Mg-Si phases on heterogeneous nucleation sites were analyzed qualitatively by FEGSEM, as well as their effect on mechanical properties such as yield stress, ultimate tensile stress and fracture properties which were characterized by tensile tests. It was found that the yield stress decreased as the quench rate decreased, and that the unrecrystallized material had a much larger quench sensitivity with respect to the recrystallized initial microstructures, speculated to arise from its high density of heterogeneous nucleation sites.

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Grain Growth and Austenite Decomposition in Two Niobium Containing Line Pipe Steels (2016)

Niobium is a common microalloying element for line pipe steels for the promotion of fine ferrite microstructures following hot rolling. During construction via welding, niobium carbonitrides formed during coiling may dissolve depending on peak temperatures reached. This has a strong influence on the microstructure and mechanical properties of the heat affected zone. The primary objective of the current work is to study the effect of niobium microalloying on heat affected zone microstructures. Two steel compositions were selected for this study, possessing a high (0.091 wt%) niobium content and differing (0.028 and 0.058 wt%) carbon contents. To study the effect of niobium on grain growth, thermal histories were designed in the context of continuous heating and isothermal holding for a range of heating rates and holding temperatures. Laser ultrasonics were used to observe in-situ grain growth during these histories and experimental results were confirmed by ex-situ metallography using appropriate etchant. Niobium is shown to be effective in restricting grain growth behavior at temperatures when precipitates are stable. An increase in heating rate was found to reduce overall grain growth (from 48 to 25 μm between 10 and 1000°C/s), eventually reaching a limiting behavior at the maximum heating rate (i.e. 1000°C/s). To study the effect of niobium on austenite decomposition, thermal histories were designed to produce two grain sizes (5 and 35 μm) and two states of niobium (in and out of solution). The latter was investigated through precipitation experiments. Cooling rates were varied between 3 and 30°C/s and mechanical dilatometry was used to measure the phase transformation. As expected, an increase in cooling rate, carbon content, and prior austenite grain size results in lower transformation temperatures. The effect of niobium in solution was strongly dependent on prior austenite grain size. Microstructures formed by austenite decomposition were characterized via metallography and microhardness. The microstructural constituents include ferrite, bainite, and M/A. Samples representative of each microstructure were selected to quantify M/A fraction through etching and point counting. Niobium was shown to have a strong effect on hardness when the prior austenite grain size was large (up to 70 HV).

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Static Strain Aging in Low Carbon Ferrite-Pearlite Steel: Forward and Reverse Loading (2016)

The combination of static strain aging and plastic strain reversal is important to understand for both the forming of components and also analysis of in service performance, for example, in the case of fabrication of pipeline, motor shafts or structural components in buildings and ships.Static strain aging phenomenon has been experimentally studied for the cases of forward and reverse re-straining after aging on a low carbon steel (0.16 wt% C) with a ferrite-pearlite microstructure. Torsion tests on hollow tubular samples were used for the mechanical tests. The shear strain on the surface of the sample was measured with the digital image correlation. The influence of the amount of pre-strain, aging time and temperature, and the strain path reversals on the stress-strain response after aging has been measured experimentally. A maximum increase of 46 MPa was obtained in the yield stress of the samples re-strained after full aging in the same direction as the initial straining. This maximum increase in yield stress as well as the rate of increment in yield strength during aging was almost independent of the amount of pre-strain and the increase in the flow stress occurred without a significant variation in the work hardening behavior. Further, it was shown that a yield point phenomenon was absent if the direction of re-straining after aging was reversed and the increase in the flow stress level after aging was proportional to the amount of pre-strain and increased with extended aging time. In this case, the absence of a sharp yield point after prolonged aging time led to the speculation that the activation of dislocations sources, rather than unpinning of locked dislocations in re-straining after aging was the controlling mechanisms although proof of this requires further investigation. Although it is difficult to unambiguously identify all of the underlying physical mechanisms, nevertheless, a comprehensive set of experimental results has been measured which can be used by the design engineer when considering cases where static strain ageing and strain path reversals are relevant for a ferrite-pearlite steel with 0.16 wt% carbon.

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Effect of high temperature extrusion conditions on the microstructure of AA3003 aluminum alloy (2013)

The effect of high temperature extrusion conditions on the microstructure of AA3003 aluminum alloys has been investigated. The extrusion trials were conducted using a laboratory scale fully instrumented extrusion press at Rio Tinto Alcan’s Arvida Research and Development Centre (ARDC). Direct Chill (DC) cast billets were homogenized using one of three different treatments (e.g. 8 h at 500°C, 8h at 550°C, and 24 h at 600°C) prior to extrusion. Extrusions were conducted using a range of extrusion temperatures (e.g. 350°C to 500°C), ram speeds (e.g. 2 mm/s to 32 mm/s), and extrusion ratios (e.g. 17:1 ER to 280:1 ER). This provided a full range of as-extruded microstructures from recrystallized to unrecrystallized, with a wide range of final grain sizes.The microstructure of the extrudates was examined using optical microscopy and Electron Backscatter Microscopy (EBSD). The extrudate microstructures have been rationalized in comparison with the processing conditions from the extrusion trails. It was found that the extent of recrystallization is related to the homogenization treatment (i.e. dispersoid number density), and the ram speed (i.e. the temperature profile). It was also found that the ‘unrecrystallized’ deformed grain thickness could be approximated using a simple mass balance approach. The stability of ‘unrecrystallized’ as-extruded samples was investigated in post-extrusion annealing experiments, using a range of temperatures (e.g. 500°C to 550°C) and times (e.g. 10 min. at 550°C). It was found that most of the structures had enough stored energy to recrystallize.In addition, high temperature compression tests were conducted using a Gleeble® 3500 Thermo-mechanical Simulator. These tests were conducted to further investigate the constituent behavior of the aluminum alloy with regards to transient strain rates. Tests were conducted at a temperature of 500°C using a constant strain of 1 s-¹ and 10 s-¹, and transient strain rates. The yield stress, flow stress and work hardening were fit to a physically based flow stress model developed by Kocks and Chen. It was found that the strain rate history had an effect on the flow stress of the material, and that the model could not fully capture the behavior.

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Aging behaviour of Flexcast Al-Mg alloys with SC and Zr additions (2012)

AA5xxx series Al alloys have presented great potential in recent decades for packaging and automotive applications. A recent further development of these alloys has been the addition of Sc and Zr. The presence of these elements is known to develop strengthening phases that also influence other metallurgical phenomena such as recovery and recrystallization. However, the low solubility of both Sc and Zr in Al requires high cooling rates during casting in order to attain a supersaturated solid solution which conventional DC casting is unable to achieve.In this study, two Al- 3%Mg alloys, one Sc-free and the other containing 0.4% Sc were cast using the Flexcaster® available at the Novelis Global Technology Center (Kingston, Ontario). The Flexcaster® is a strip casting technology that transforms liquid aluminum into a directly rollable cast ingot that is not subject to scalping or homogenization processing. This technology is characterized by relatively high solidification and cooling rates. Aging experiments were performed on as-cast and cold-rolled samples of the two alloys to study the evolution of strengthening phases and their impact on recovery and recrystallization. Both alloys were artificially aged following casting at 200, 300 and 400°C for times ranging from 30s to 72 hours. The alloys were characterized by hardness and tensile tests and electrical resistivity measurements as well as optical and electron microscopy in order to monitor the aging behavior of the alloys. Results show enhanced strengthening in the Sc-containing alloy and superior high temperature microstructural stability in comparison to the Sc-free alloy as well as conventionally cast AlMgSc alloys.

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Austenite decomposition in an X80 linepipe steel (2012)

The final microstructure and resulting mechanical properties in the heat-affected zone (HAZ) of welded linepipes are predominantly determined by austenite decomposition during cooling after welding processes. Thus, a full understanding of continuous cooling transformation of austenite is a key step toward improving the overall performance of linepipes. The main objective of the current study is to investigate the influence of cooling rate, prior austenite grain size and niobium content of austenite on austenite decomposition kinetics and the resulting microstructures for an X80 linepipe steel. To consider the significant effect of the niobium solid solution level on the transformation of austenite, two thermal histories were developed. For the first case, Nb was dissolved in solid solution prior to austenite decomposition. In contrast, the second scenario involved the formation of Nb(C,N) precipitates prior to austenite decomposition, i.e. leaving a low level of Nb in solid solution. Austenite grain growth studies were conducted to obtain grain sizes similar to those observed in the HAZ of the girth-welded steel, i.e. 5-80μm. Furthermore, employing appropriate thermal cycles, continuous cooling transformation (CCT) tests were conducted to examine the effect of niobium condition, austenite grain size and cooling rate on austenite decomposition behavior of the steel. Cooling rates varied in the range of 3−100ºC/s and dilation measurements were utilized to capture the transformation kinetics of austenite upon cooling. The resulting microstructures, which usually consist of ferrite, bainite and martensite-austenite (MA) constituents, were examined using optical microscopy. They were revealed using appropriate etchants and the corresponding phase volume fractions were subsequently measured in accordance with ASTM standards. Hardness measurements were also conducted on CCT samples.

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Fabrication and Characterization of Steel-Magnesium Laminated Metal Composites (2012)

In this study, interstitial-free steel and commercial purity magnesium sheets were used to fabricate steel-magnesium laminated metal composites by roll bonding at 300℃. It was found that the steel and magnesium can achieve reasonable bonding after a 47% rolling reduction when the volume fraction of the laminate is 10-15% magnesium. The microstructure of the laminated composites was observed with scanning electron microscope. It was found that a continuous interface between the IF steel and the magnesium was produced during the roll bonding process. There was no evidence of intermetallic formation at the interface. A seven layer steel-magnesium laminate was fabricated by accumulative roll bonding at 300℃ with an overall reduction of 77 percent. Through-width cracks were found in the surface steel layers after the one cycle accumulative roll bonding process. The longitudinal cross-sectional microstructure of the laminate revealed that multi-localizations and even fracture occurred in steel layers inside the laminate. The mechanical properties, including tensile behavior, micro-hardness and bending behavior, of the laminated composites were assessed. The tensile property of the laminated composites was compared with those of monolithic steel and magnesium with equivalent deformation amount deformed under the same conditions. It was found that the UTS of the laminated composites obeyed the simple rule of mixtures. The fracture surfaces of the laminated composites were examined with SEM and compared with those of the monolithic IF steel and magnesium rolled under the same conditions. It was found that the fracture modes of each component were different in the laminated composites compared to the monolithic materials. Three-point bending test was conducted and it was observed that no debonding at the interface occurred for moderate strains. To investigate the fracture behavior of the laminats in bending, a series of U-shape bending tests were conducted and the bend tips were observed. Localization of the outer steel layer was observed, followed by the formation of a major crack at 45 degree to the maximum tensile stress direction. Shear cracks in the magnesium core were also found in some places adjacent to the major crack, and delamination between the steel and magnesium layers occurred.

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