Chadwick Sinclair

Microstructure evolution of materials is derived by kinetic processes that are atomistic in nature. Phenomena like grain boundary migration, the formation and growth of crystalline phases in bulk metallic glasses and structural relaxation in amorphous materials are examples of microstructural phenomena that are derived from atomic scale dynamics. Probing such processes in disordered atomic environments is challenging experimentally since they operate at small length scales (nanometers) and time scales (nanoseconds). In this work, we employ molecular dynamics simulations and a variety of dynamical coarse-graining methods to bridge the gap between microscopic processes and macroscopic observables. First, the diffusion kinetics of carbon in Fe-C glasses is studied. By detecting individual atomic hops, we quantify the parameters that control the diffusivity, namely jump length, residence time and correlation factor. Our results help explain the experimentally observed increase in stability of metal-metalloid glasses against crystallization with increasing carbon concentration. Next, the dynamical processes and structural relaxations in a model glassy system are explored using a machine learning algorithm involving neural networks combined with Markov State Models with the aim of identifying previously unexplored dynamical processes that may be crucial for understanding the complex behaviour of metallic glasses. Finally, the kinetic processes governing grain boundary (GB) motion are studied using the same approach as was used for the glasses. The GB mobility is extracted from three GBs in iron using both conventional techniques as well as Markov State Models. The Markov State Model is shown to also provide insights into the intra-GB processes that govern the temperature dependence of GB motion.

View record

The mechanisms leading to the alpha fibre texture development in hot rolled ferritic stainless steels are investigated. This texture, and particularly its rotated cube {001} component, needs to be avoided to improve the mechanical properties of commercial alloys. The industrial hot rolling process is decomposed into carefully designed experiments that aim to study the contributions of plastic deformation and static recrystallization on texture development. On the one hand, it is shown that rolling of pancaked grain microstructures needs to be avoided as it strengthens the deleterious rotated cube {001} component. The experimental observations are supported by crystal plasticity simulations. On the other hand, it is shown that the alpha plus cube fibre textures obtained after static recrystallization of hot rolled products (i.e. rolled >=900°C) differ fundamentally from the more classic gamma fibre texture in recrystallized warm and cold rolled products. The origin of this difference is attributed to the activation of different mechanisms of nucleation of recrystallized grains (bulging vs. intragranular) as a function of the specific deformation microstructures developed for each rolling condition. Under the assumption that nucleation occurs by abnormal subgrain growth, a model was developed to predict, using the characteristics obtained from a deformed microstructure, the static recrystallization texture. The experiments and model show that while static recrystallization of hot rolled products develop deleterious orientations, it also helps to maintain a weak texture. It is thus concluded that the alpha fibre texture cannot be avoided during the hot rolling process but that frequent recrystallization of the material during the interpasses of rolling can help to reduce its strength.

View record

Aluminum-magnesium alloys are commonly used as wrought products in the automotive industry. Cold forming of such alloys leads to strengthening by work hardening but some of this strength can be lost by exposure to elevated temperatures leading to recovery. Such softening by recovery occurs when car body panels are subjected to the industrial paint bake cycle (160-200 C for 30 min).It has been previously shown that small (
View record

The production and crystallization of amorphous iron-carbon alloys has been investigated experimentally. A physical vapour deposition (PVD) technique has been developed at UBC to create films with a variety of carbon contents. A system using controlled flow of reactive gas has been developed to allow the variation of carbon content, and films have been shown to have a reproducible amorphous structure and consistent chemistry. Characterization of the chemistry and structure of the as-sputtered alloys has been performed. Amorphous films were annealed to assess the kinetics of crystallization. For films containing less than 25 at.% carbon, a two-stage crystallization involving the formation of ferrite followed by cementite was observed at low temperatures. The structure and chemistry of these crystallization products were characterized by x-ray diffraction, electron microscopy and atom probe tomography. In-situ annealing was also per- formed in transmission electron microscopy, allowing for direct observation of the nucleation and growth of the product phases. This annealing study showed a significant decrease in the nucleation and growth rate of ferrite within the amorphous matrix. Simple models of diffusion-controlled and interface-controlled growth were not able to capture this slowing of the transformation. In addition to thermodynamic factors, it is proposed that the ferrite growth rate is strongly affected by a decrease in diffusivity arising from aging of the amorphous matrix and its enrichment in carbon during crystallization. Alloy films containing more than 25 at.% carbon were also found to crystallize in a two-stage process during annealing. This crystallization involved the initial formation of ferrite and cementite, with a secondary formation of cementite to fully consume the original structure. This two-step process has not been previously reported in the related literature. The secondary crystallization led to large grains of cementite that exhibited a systematic lattice compression in the [010] direction. The final cementite structure was found to have a super-stoichiometric carbon concentration that can only be possible via a process of ‘chemical twinning’. While faulting on the correct lattice plans was observed, no diffraction evidence for such ‘chemical twinning’ could be identified. This is proposed as an area for future research.

View record

Microstructure evolution during material processing is determined by a number of factors, such as the kinetics of grain boundary migration in the presence of impurities, which can take form of solid solution, second-phase precipitates or clusters. The dynamic interaction between grain boundaries and clusters has not been explored. In this work, a variety of simulation tools are utilized to approach this problem from an atomistic perspective. Atomistic simulations are first implemented to explore the parameter space of the solute drag problem, i.e. grain boundary migration in a binary ideal solid solution system, via a kinetic Monte Carlo framework. Depending on their diffusivity, solute atoms are capable of modifying the structure of a migrating boundary, leading to a diffusion-dependent drag pressure. A phenomenological model adapted from the Cahn model is proposed to explain the simulation results. The interaction between clusters and a migrating grain boundary is studied next using molecular dynamics simulations. The iron helium (Fe-He) system is chosen as the object of the study. A preliminary step towards such a study is to investigate the grain boundary migration in pure bcc Fe. An emphasis is placed upon demonstrating the correlation between the migration of curved and planar boundaries. Evidence that verifies such a correlation is established, based on the analyses on the shapes, the kinetics and the migration mechanism of both types of boundaries. Next, the formation of He clusters in the bulk and grain boundaries of Fe is examined. The cluster formation at the boundary occurs at a lower rate relative to that in the bulk. This is attributed to the boundary being a slow diffusion channel for interstitial He atoms. The overall effect of clusters on the boundary migration is twofold. Clusters reduce the boundary mobility via segregation; the magnitude of their effect can be rationalized using the Cahn model in the zero velocity limit. Clusters also act as pinning sources, delaying or even completely halting the boundary migration. A phenomenological model adapted from the Zener pinning model is used to discuss the role of clusters on grain boundary migration.

View record

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.

View record

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.

View record

The influence of precipitate state and annealing temperature on the recrystallization of a Mg-2.8wt.%Nd alloy has been investigated. Precipitation kinetics at 190°C, 350°C and 400°C were studied in order to understand precipitate evolution during recrystallization. Precipitation was studied primarily through electrical resistivity measurements, and modelled using a mean radius model. It was found that predicting the kinetics required the spatial distribution of solute to be considered. Pre-aging conditions were selected in order to study the influence of either pre-existing or concurrently formed precipitates during recrystallization. After aging, the samples were cold rolled to a strain of 20%. The microstructures were characterized primarily through EBSD, and also through hardness measurements. Pre-aging the samples at 400°C for three hours resulted in a dispersion of stable β precipitates during annealing. This led to a recrystallized microstructure with recrystallization nucleation sites similar to those previously reported in the literature. Pre-aging the sample at 190°C for 24 hours lead to the formation of metastable β'' precipitates which strengthened the sample, but dissolved rapidly upon annealing at higher recrystallization temperatures. When samples previously solutionized at 545°C or aged at 190°C were subsequently annealed at 350°C, recrystallization stagnated. This was attributed to concurrent precipitation pinning grain boundaries. In all samples, irrespective of aging condition, recrystallization was observed primarily in twins and shear bands. The twins which recrystallized were found to be {10-11} contraction twins and {10-11}{10-12} contraction-extension twins. As the nuclei forming within these regions were not randomly oriented, recrystallization in these alloys did not randomize the texture. The work presented in this thesis increases understanding of recrystallization in Mg-Nd alloys. In particular, the means by which Nd interacts with and affects the recrystallizing microstructure have never been studied in detail. Furthermore, this work points to possible ways in which new magnesium alloys and thermomechanical processes could be designed to improve final material properties.

View record

This thesis develops a physically based model for the work hardening of model (pure or solute strengthened) FCC alloys having grain sizes of between 2 μm and 100 μm. This model builds on a previous study of Sinclair et al. (2006) on fine-grained pure Cu and extends it to Cu16at%Ni, Cu50at%Ni and Cu1.5at%Al alloys. Through careful and systematic mechanical testing coupled with microstructural observations several basic hypotheses were tested. The yielding behaviour of fine-grained materials showed an extension of the elasto-plastic transition (over the generally accepted 0.002 offset strain) with decreasing grain size and increasing solute content. This resulted in Hall-Petch plots which showed that the grain size effect was more pronounced with increasing solute content. In all of the materials tested with a sufficiently fine grain size, the stress-strain plots showed an inflection (i.e. region of a low work hardening or “plateau”). While the work hardening rate dropped significantly in this portion of the test, image correlation was used to show that the drop in work hardening was not sufficient to cause strain localization. The stress-strain plots were differentiated and work hardening behaviour was analyzed using a Kocks-Mecking model. An important observation was that with increasing solute content from pure Cu to Cu50at%Ni, a grain size dependent separation between the work hardening plots appeared for tests performed at room temperature. This observation was initially hypothesized to be due to backstresses as proposed in the original model of Sinclair (Sinclair et al. 2006). This idea was tested using strain-rate sensitivity experiments. Strain-rate sensitivity tests showed that a single mechanism (forest hardening) controls the work hardening behaviour beyond the initial few percentage of straining. To unify all these experimental observations in a self-consistent work hardening model, the original Sinclair model was modified through the addition of a new variable, n*f , which accounts for additional dislocation storage by the forest dislocations blocked at grain boundaries. It was hypothesized that the effects of dislocation/grain boundary interactions on screening/annihilation of dislocations could be used to capture the initial high rate sensitivity at the “plateau” in the stress-strain curve of fine grained alloys.

View record

The influence of cold rolling and final annealing on the development of ridging during tensile deformation of an industrial AISI 445 ferritic stainless steel has been investigated. The relationship between microstructure, (micro)-texture and ridging was evaluated by comparing full-field crystal plasticity calculations (VPFFT) to experimental measurements of surface roughness and microstructure. Results showed that the major parameter responsible for ridging is the through-thickness fraction of orientations with high out-of-plane shear strain rate. This was found to dictate the amplitude of the surface displacement and the spacing between corrugations observed on the sheet surface. To examine the origins of these regions, the process of annealing from the cold rolled state to the final product was next characterized by means of electron back-scattered diffraction (EBSD), with a focus on the formation of regions with similar shearing behaviour as defined by the crystal plasticity calculations. The combined effect of preferential nucleation and growth advantage of {111} orientations from deformed {111} grains is able to explain the bulk texture change from the deformed to the recrystallized state. On a microscopic scale, these orientations (within +/- 15° of the ideal orientation) have low out-of-plane shearing intensity of both negative and positive sign. Their nucleation and growth during annealing leads to the replacement of large grains with high out-of-plane shearing intensity with finer grains having lower out-of-plane shearing tendency. As a consequence, both the intensity and the spatial distribution of orientations with various out-of-plane shearing tendency are modified, leading to a reduction of ridging. As the final processing stage of the steel sheet (cold rolling and final annealing) reduces the non-random distribution of grains having a particular out-of-plane shearing tendency (“clustering”) and because ridging is still present in the final product, it was concluded that ridging originates in the upstream process. A generalized description of the origins of ridging and the heredity of microstructure and texture from the slab to the final product is proposed based on simplified crystal plasticity calculations and microstructure observations of the casting and the transfer bar.

View record

The influence of carbon distribution and carbide precipitation on the mechanical properties of the as-quenched and quenched and tempered 300M martensitic steel has been investigated. The microstructure, investigated by transmission electron microscopy (TEM) and three dimensional atom probe tomography (APT) was found to be relatively homogeneous in the as-water-quenchedstate, but signifi cantly evolved upon tempering and variation of quench rate. This evolution included carbon segregation to dislocations andgrain boundaries and carbide precipitation. A simple mean- field precipitationmodel assuming heterogeneous nucleation onto the dislocations provedto satisfactorily capture the evolution of precipitation upon tempering at120C and 150C. The material was found to behave, mechanically, as a composite and in accordance, the Bauschinger stress-strain behaviour wassuccessfully modeled using a continuous Masing model. This model, when related to the microstructure, showed that the composite behaviour arose from the mechanical contrast between the laths, this being controlled by thelocal dislocation density and carbon segregation and/or precipitation onto them. Carbon segregation and carbide precipitation were observed to have adirect impact on alpha in the Taylor-like equation that was shown to control the local yield stress within the laths. When applied to martensites containing various amount of carbon, the model allowed for an empirical assessment of the e ffect of the nominal carbon content on alpha , which was found to be linearly dependent on the nominal carbon content.

View record

In this work, the deformation mechanisms of an austenitic stainless steel (grade 301LN) have been investigated with particular attention on the strain-induced phase transformations from austenite to ε and α’ martensites. The average grain size of this alloy was varied in the range 0.5-28 µm, and two strain paths, namely uniaxial tension and simple shear, were analyzed. At the macroscopic level, the work-hardening response was examined in relation to the formation of ε and α’ martensites, followed by X-ray phase quantification and Feritscope measurements. At a microscopic level, the microstructures after deformation were investigated using electron back-scatter diffraction, energy-dispersive X-ray spectroscopy and transmission electron microscopy. It was found that the grain size refinement was responsible of a change in nucleation mechanisms of α’-martensite, thereby affecting the macroscopic volume fraction of α’-martensite. The switch from tension to shear was not found to affect the mechanisms of formation of ε and α’ martensites, but significantly reduced the work-hardening, an effect too large to be attributed to the slight reduction of the kinetics of α’ volume fraction. The stresses borne in the α’-martensite were quantified using a novel method based on the magnetomechanical effect. These stresses, together with the determination of the intrinsic constitutive laws of austenite and α’-martensite, were used to design a one-dimensional physically-based model of the work-hardening in this alloy. This model, based on the “dynamic composite" effect of the formation of fresh α’-martensite in austenite, successfully predicted the measured stress-strain behaviour in tension, as well as the tensile instabilities encountered in this class of materials.

View record

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.

View record

In this study the effect of recovery on the yield strength and work hardening of a modelAl-Mg-Sc alloy in the presence of A1₃Sc precipitates was investigated. Recoveredmicrostructures containing A1₃Sc precipitates were obtained through a series of thermo-mechanical treatments including pre-aging, cold rolling and annealing.Recovered microstructures were characterized in terms of precipitate size distribution,subgrain sizes and dislocation structures. Yield strength and work hardening ofprocessed microstructures were examined by tensile testing at 77K. The results show thatthe effect of precipitates arises directly from precipitation strengthening as well asindirectly from their effectiveness at controlling the recovered microstructure. Physical based models were developed to describe the tensile response of recoveredmicrostructures consistent with previous models on single phase recovered Al-Mg alloys. For the first time, the impact of precipitates on recovery kinetics wascaptured by coupling recovery models of single phase Al-Mg alloys and creep models.In this new modelling approach a transition stage from dislocation annihilation to a climb controlled mechanism was defined. In addition, the effects of both recovery andprecipitates on work hardening have been incorporated in the previous models. Finally,model limitations as well as their potential applications to improve mechanical propertiesincluding yield strength, ultimate tensile strength and uniform elongation of Al-Mg-Scalloys by controlling thermo-mechanical processing and chemical composition wererevisited.

View record

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.

View record