Thesis (Ph. D.)--Ohio State University, 2006. / Constitutive modeling of slip, twinning, and untwinning in AZ31B magnesium. Includes bibliographical references (p. 139-152).

Gamma titanium aluminide alloys are emerging as a lightweight replacement to nickel superalloys, with current application in turbine stages of aero-engines, as well as in high performance automobiles and potentially the nuclear industry. The lack of toughness of its two constitutive intermetallic phases, γ-TiAl and α2-Ti3Al, prevents a conventional damage tolerant approach to fatigue lifing. To gain confidence in the use of γ-TiAl alloys and extend the temperature-stress envelope of applicability, the present work aims to achieve an understanding of the development of plasticity and flaw formation during cyclic loading. The general plasticity of a γ-TiAl alloy, Ti-45Al-2Nb-2Mn(at.%)-0.8vol.%TiB2, in compression was investigated by mapping the development of localised strain at the specimen surface. Methods were developed to produce speckle patterns for high resolution digital image correlation that were stable at test temperatures of 700 °C in air, in order to study the extent of plasticity generated by differing deformation mechanisms at application-relevant temperatures, with nano-scale resolution. At the colony scale (i.e. single stacks of co-planar α2-Ti3Al and γ-TiAl lamellae, where each stack is formed from a single high temperature disordered α-TiAl grain), macroscopic deformation bands were observed to develop at only a few percent strain. Within such bands, which propagated across many colonies of differing lamellar orientations, considerable lattice curvature and localised slip and twin operation occurred. This correlated with colony boundary failure in such bands. Twinning of the γ-TiAl phase parallel to the lamellar interfaces, longitudinal twinning, has rarely been studied, despite generalised twinning in equiaxed γ-TiAl grains being known to cause boundary decohesion. Here, the occurrence of longitudinal twinning in both microcompression and polycrystalline testpieces was investigated up to 700 °C by electron backscatter diffraction. The strength of constraint by surrounding lamellar domains was found to be the determining factor in the increased prominence of twinning at 700 °C, and hence determined whether twinning shear-induced flaws formed at colony boundaries. Using the high temperature digital image correlation strain mapping and electron backscatter diffraction techniques developed, along with transmission electron microscopy, the onset of plasticity at temperatures up to 700 °C was studied in both micro-scale and macro-scale test specimens for different lamellar thicknesses. Testpieces were loaded below the macroscopic yield stress in both monotonic and high cycle fatigue regimes, to 107 cycles, at a tensile stress ratio of R = 0.1. Longitudinal plasticity occurred in most colonies with soft mode lamellar orientations, and was located just 30 - 50 nm from lamellar interfaces. Lamellar refinement caused an increased number of slip bands to develop. In most cases, plastic strains decreased to zero by the colony boundary and strain transfer across such boundaries in high cycle fatigue was rare. At room temperature, the maximum applied stress was found to influence the number of slip bands more than the number of loading cycles.

Zirconium is used inside nuclear reactors as fuel cladding. The in-reactor performance of zirconium alloys is strongly influenced by the properties that develop during thermo-mechanical processing, such as the microstructure and crystallographic texture. Optimising the combination of properties would enable improved reactor efficiency, longer component lifetimes and reductions in nuclear waste. Achieving the desired texture and microstructure requires a mechanistic understanding of the processes that govern them: deformation and recrystallisation. These mechanisms are influenced by numerous variables including temperature, strain-rate, and the initial state of the material. This work aims to clarify how texture develops as a result of the active deformation mechanisms of slip and twinning and how these mechanisms are influenced by temperature. The alloy chosen for this is Zircaloy-4.This work has shown that texture evolution varies with deformation temperature. The activation of {10-12}<10-11> tensile twinning dramatically alters the texture up to at least 300°C. In the absence of much twinning at 500°C prismatic slip appears to govern the texture evolution up to moderately high strain. Prismatic slip is generally considered the easiest slip system in zirconium. This work highlights its distinct effect upon both texture and microstructure evolution. In particular the extent of grain fragmentation by prismatic slip is shown to depend upon the initial grain orientation. As a result the break-up of the microstructure takes place heterogeneously. This then has implications for the microstructure and texture development during subsequent recrystallisation treatments. Experimental data indicates that the slip anisotropy between <c+a> and prismatic <a> slip increases with temperature. Crystal Plasticity simulations suggest that the variation of both the twin variant selection and the grain fragmentation with temperature are consistent with increasing slip anisotropy, in contrast to previous experimental and modelling studies on high purity zirconium alloys. The character of {10-12}<10-11> tensile twins and the texture change they induce is influenced by temperature, strain path and weakly influenced by the neighbouring orientations. Increasing temperature causes twin fraction variation, thicker twins and an increased frequency of less favourable twin variants. Plane strain compression also causes less favourable variants to activate more often. Looking at the twinned orientations highlights the importance of grain orientation. Poorly orientated grains do still twin. This work shows that in these instances neighbouring interactions can play a role. In summary, this work contributes to the current understanding of deformation in hexagonal close packed metals. It is hoped that this aids the development of improved physically based crystal plasticity models.

The motivation to use lightweight materials in the construction of the automotive structure is the resultant increased fuel efficiency. However, these materials possess certain drawbacks that make it challenging to adopt them into current automobile manufacturing processes. In this dissertation the microstructural response observed in a magnesium alloy, AZ31, and an advanced high strength steel alloy, QP1180, to uniaxial deformation is analyzed and the results are presented. In AZ31 the required slip modes are not activated at room temperature leading to its low ductility at room temperature. The resulting activity of these twins in response to uniaxial tension is analyzed and its correlations with the microstructure features is reported. Additionally, a neighborhood viscoplastic self-consistent model is developed that will allow more accurate simulation of twin response to outside deformation. Furthermore, activity of slip modes that are usually observed at high temperatures (>200°C) are also observed at lower temperatures (<125°C) and they are compared to the relative twin activity at these temperatures. It is observed that larger grains, with high schmid factors, longer grain boundaries and have misorientation with its neighboring grain greater than 27° are more favorable for twin formation and transmission in the AZ31 microstructure in response to uniaxial tension. The nature of retained austenite (RA) transformation into martensite that gives QP1180 its enhanced ductility, is not clearly understood primarily because of challenges present in characterization of these metastable RA. Further, a 2 dimensional characterization method does not provide the complete information of the RA grain. These challenges are overcome by characterization of a 3 dimensional volume element using serial sectioning and EBSD followed by reconstruction using DREAM3D. The influence of 3d morphology and orientation direction on RA transformation is studied using as-is and uniaxially deformed samples. A novel shear affinity factor is introduced as a metric to describe the ease of RA transformation under uniaxial tension. The 3d nature of the information collected allows a new classification of disk shape in addition to globular and lamellar shapes for RA. It is found that RA that are low volume laths and have low shear affinity factor transform later compared to disk shaped RA’s. Through these guidelines the preparation of a microstructure that is conducive to RA transformation under uniaxial tension is possible.

Methods designed for incorporation into multiscale modeling polycrystals are presented in this work in two tasks. This work contains mesoscale methods for capturing the effects of both the interactions of slip dislocations encountering twin grain boundaries and the simultaneous growth of multiple twin grain volume fractions on mechanical hardening and texture evolution. These are implemented in a crystal plasticity framework using the Los Alamos visco-plastic self consistent code, VPSC-7. Presented here, the effects of simultaneous growth in multiple twin variants on textural evolution is tracked using a Kalidindi-type twin volume transfer scheme. In Task 1, the implementation of this scheme in order to simulate the texture of Twinning Induced Plasticity steels (TWIP) subjected to Equal Channel Angular Pressing (ECAP) are summarized. In Task 2, the hardening effects of two types of interactions between slip dislocations and encountered twin grain boundaries, namely dislocation transmutation and dissociation, are captured by way of modifying the dislocation density based hardening model of [14]. Interactions of the first type are presented in a constitutive relation calculating the amount of dislocation density apportioned to a given slip system contained within the encountered twin volume fraction from each interacting slip system in the parent volume fraction. The amount transmuted from each interacting slip system described using the Correspondence Method, an onto mapping of slip systems in a parent grain to slip systems in considered twin grains. Interactions of the second type are then introduced into this constitutive relation as a disassociation parameter, the value of which is established by observations gleaned from the results of the molecular dynamics simulations of [11] and [36]. These methods are implanted to simulate the anisotropic hardening behavior of HCP magnesium under multiple load paths.

Deformation twinning in Magnesium alloys both facilitates slip and forms sites for failure. Currently, basic studies of twinning in Mg are facilitated by electron backscatter diffraction (EBSD) which is able to extract a myriad of information relating to crystalline microstructures. Although much information is available via EBSD, various problems relating to deformation twinning have not been solved. This dissertation provides new insights into deformation twinning in Mg alloys, with particular focus on AZ31. These insights were gained through the development of new EBSD and related machine learning tools that extract more information beyond what is currently accessed.The first tool relating to characterization of deformed and twinned materials focuses on surface topography crack detection. The intensity map across EBSD images contains vital information that can be used to detect evolution of surface roughness and crack formation, which typically occurs at twin boundaries. The method of topography recovery resulted in reconstruction errors as low as 2% over a 500 μm length. The method was then applied to a 3 μm x 3 μm area of twinned Tantalum which experienced topographic alterations. The topography of Ta correlated with other measured changes in the microstructure. Additionally, EBSD images were used to identify the presence of cracks in Nickel microstructures. Several cracks were identified on the Ni specimen, demonstrating that cracks as thin as 34 nm could be measured.A further EBSD based tool developed for this study was used to identify thin compression twins in Mg; these are often missed in a traditional EBSD scan due to their size relative to the electron probe. This tool takes advantage of crystallographic relationships that exist between parent and twinned grains; common planes that exist in both grains lead to bands of consistent intensity as a scan crosses a twin. Hence, twin boundaries in a microstructure can be recognized, even when they are associated with thin twins. Proof of concept was performed on known twins in Inconel 600, Tantalum, and Magnesium AZ31. This method was then used to search for undetected twins in a Mg AZ31 structure, revealing nearly double the number of twins compared with those initially measured by standard procedures.To uncover the driving forces behind deformation twinning in Mg, a machine learning framework was developed to leverage all of the data available from EBSD and use that to create a physics based models of twin nucleation and growth. The resultant models for nucleation and growth were measured to be up to 86.5% and 96.1% accurate respectively. Each model revealed a unique combination of crystallographic attributes that affected twinning in the AZ31.

The optimisation of compressor stage aerofoil and fan blade design remains an important area of titanium alloy research and development for aerospace gas turbines. Such research has important implications for critical and sensitive component integrity and efficiency. In particular, a better understanding of how deformation twinning interacts with microstructural features in titanium alloys is required, because such twinning facilitates plastic deformation at a higher strain rate than dislocations. To investigate this behaviour, commercial purity titanium and the titanium alloy Ti–6Al–4V were subjected to ballistic impact testing at room temperature with a high strain rate of 10³s⁻¹. In addition, a detailed analysis was conducted of three manufacturing processes of Ti–6Al–4V (wt. %) that are likely to cause deformation twinning: metallic shot peening, laser shock peening and deep cold rolling. The results presented in this thesis have furthered the understanding of the role of deformation twinning in the plastic deformation of α-phase titanium. Key findings of the research include the characterisation of deformation twinning types and the conditions that favour certain deformation twinning types. From the analysis of the ballistic testing of commercial purity titanium, the first definitive evidence for the existence of {112‾4} twinning as a rare deformation twinning mode at room temperature in coarse-grained commercial purity titanium is presented. In addition, the ballistic testing results of the Ti–6Al–4V alloy highlighted very different deformation twinning characteristics. Commercial purity titanium deformed plastically by a combination of {101‾2} and {112‾1} tensilve twinning and {112‾4} and {112‾2} compression twinning modes. By contrast, the deformation twinning of Ti–6Al–4V was limited to only the {101‾2} and {112‾1} tensile twinning modes. The two tensile deformation twinning types have very different morphologies in equiaxed fine grained Ti–6Al–4V. {112‾1} deformation twins span multiple grain boundaries and {101‾2} deformation twins reorient entire grains to a twinned orientation. This observation provides evidence for whole grain twinning of equiaxed fine grained Ti–6Al–4V by {101‾2} twinning. Grain boundary interactions between various deformation twinning types and alpha phase grain boundaries in commercial purity titanium and Ti–6Al–4V are reported and analysed. In commercial purity titanium {101‾2} as well as other deformation twinning types were observed interacting across alpha phase boundaries and higher angle alpha phase grain boundaries. The analyses of the manufacturing processes of Ti–6Al–4V highlight the very different dislocation and deformation twinning structures in surfaces processed by these techniques. A notable feature of material processed by laser shock peening is the almost complete absence of deformation twinning, contrasting with the frequent observation of extensive deformation twinning observed in the material processed by metallic shot peening and deep cold rolling. Therefore, the findings suggest that there is a strain rate limit above which deformation twinning is suppressed. The implications of this research are that a better understanding of the conditions that that favour certain deformation twinning types or propagation behaviours will enable more accurate plasticity modelling and better alloy design. This is important for the design and the manufacturing of titanium components and the high strain rate deformation to which titanium components in aerospace gas turbines can be subjected because of bird strike, foreign object debris ingestion or fan blade failures.

Thesis. 1978. Sc.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Sc.D.

Although the captive reproduction of non-avian reptiles has increased steadily since the 1970’s, a dearth of information exists on successful management practices for large captive populations of these species. The data reported here come from a captive population of ball pythons (Python regius) maintained by a commercial breeding company, The Snake Keeper, Inc. (Spanish Fork, UT). Reproductive data are available for 6,480 eggs from 937 ball python clutches. The data presented suggest that proper management practices should include the use of palpation and/or ultrasound to ensure breeding occurs during the proper time of the female reproductive cycle, and that maintenance of proper humidity during the incubation of eggs is vitally important. Ball python reproduction traits (clutch size, clutch mass, relative clutch mass, egg mass, hatch rate, egg length, egg width, hatchling mass, healthy offspring per clutch, week laid, and days of incubation) were recorded for the clutches laid during this study. For the 937 clutches, the identity of the dam and sire were known for 862 (92%) and 777 (83%) of the clutches, respectively. A multivariate model that included nine of the 11 traits listed above was compiled. Heritability and genetic and phenotypic correlations were calculated from the multivariate analysis. The trait that showed the most promise for use in artificial selection to increase reproduction rates was clutch size due to considerable genetic variation, high heritability, and favorable genetic correlations with other reproduction traits. Although large datasets have been published for twinning in avian species, relatively few are available for non-avian reptiles. Reported here are 14 sets of twins produced from 6,480 eggs from 937 ball python clutches. The survival rate for twins during the first 3 months of life in our study was 97%. Interestingly, 11 of the sets of twins were identical in sex and phenotype, and additional genetic data suggested the rate of monozygotic twinning within this captive population of ball pythons was higher than that of dizygotic twinning. Further, using microsatellite analysis we were able to generate data that shows three sets of python twins were genetically identical.

In this thesis, the relationship among friction stir processing (FSP) parameters, microstructure evolution, texture development, and mechanical hehavior of AZ31B Mg alloy was investigated. First of all, in order to reveal the correlation among the deformation conditions, dynamic recrystallization (DRX) mechanisms, and microstructure evolution in the Mg alloy, hot compression tests at a wide range of Zener-Hollomon parameter (Z) values were conducted. Through optical microscopic examination, it was found out that above a critical Z value, twinning influences the DRX process resulting in a more effective grain refinement, which is manifested in a significant change in the slope of the Z-drec relationship, where drec is the recrystallized grain size. Moreover, EBSD examination revealed that the twinning also contributed to a distinct change in the recrystallization texture. Compression tests were performed along both through-thickness and in-rolling-plane directions of the plate to study the orientation dependency of twinning activities and its influence on the DRX process. X-ray line profile analysis (XLPA) provides further insights by highlighting the differences in the dislocation density/types, subgrain sizes, and twin densities during the DRX processes operating with or without the twinning. Secondly, the constitutive behaviour study was applied to the investigation of microstructure evolution during FSP. By varying the key FSP parameters systematically, i.e. rotation and travel rates of the tool, a series of FSP specimens were prepared with a wide range of thermo-mechanical inputs in terms of Z. The resulting tensile behavior in the stir zone (SZ) showed a dramatic change as a function of Z, caused by a systematic change in the texture within SZ measured by neutron diffraction. A three-dimensional transient model was developed to investigate the detailed deformation history including the temperature and strain rate profiles and material flow pattern during FSP of the Mg alloy. Such deformation history can be combined with the constitutive study from the compression tests in order to analyze the developments of micro-texture and DRX grains during FSP, which will, in turn, dominate the mechanical properties. Based on the studies above, new fundamental understandings were gained on the governing mechanisms for the deformation and recrystallization processes during FSP and the influence of thermo-mechanical input during FSP on ductility enhancement in the Mg alloy.