Warren Poole

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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