The present work aims to reveal the microstructural evolution and post-processing mechanical behavior of difficult-to-work alloys upon severe plastic deformation. Severe plastic deformation is applied using equal channel angular extrusion (ECAE) where billets are pressed through a 90o corner die achieving simple shear deformation. Three different materials are studied in this research, namely Ti-6Al-4V, Ti-6Al-4V reinforced with 10% TiC and AISI 316L stainless steel. Microstructure and mechanical properties of successfully extruded billets were reported using light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), tension and compression experiments and microhardness measurements. The effects of extrusion conditions (temperature and processing route) on the microstructure and mechanical properties are investigated. The underlying mechanisms responsible for observed mechanical behaviors are explored. It is seen that ECAE shear deformation leads to refinement in α plates and elimination of prior β boundaries in Ti-6Al-4V. Decreasing extrusion temperature and increasing number of passes decreases α plate size and grain size. Refined α grain size leads to a significant increase in tensile and compressive flow stresses at room temperature. Texture produced by ECAE has a pronounced effect on mechanical properties. Specifically it leads to tension/compression asymmetry in flow strengths and strain hardening coefficients may be described by the activation of differing slip systems under tension and compression loading. ECAE of Ti-6Al-4V+10%TiC samples also improved mechanical properties due to α plate size refinement. Nevertheless, further extrusion passes should be carried out for tailoring reinforcement size and distribution providing optimum strength and ductility. ECAE deformation of AISI 316L stainless steel at high homologous temperatures (0.55 to 0.60 Tm) results in deformation twinning as an effective deformation mechanism which is attributed to the effect of the high stress levels on the partial dislocation separation. Deformation twinning gives rise to high stress levels during post-processing room temperature tension and compression experiments by providing additional barriers to dislocation motion and decreasing the mean free path of dislocations. The highest tensile flow stress observed in the sample processed at 700 oC following one pass route A was on the order of 1200 MPa which is very high for 316L stainless steel. The ultimate goal of this study is to produce stabilized end microstructures with improved mechanical properties and demonstrate the applicability of ECAE on difficult-to-work alloys.

Ultra-fine grained (UFG) materials have attracted considerable interest due to the possibility of achieving simultaneous increase in strength and ductility. Effective use of these materials in engineering applications requires investigating the processing-microstructure-property inter-relations leading to a comprehensive understanding of the material behavior. Research efforts on producing UFG hexagonal close packed (hcp) materials have been limited in spite of their envisaged utilization in various technologies. The present study explores multiple UFG hcp materials to identify the general trends in their deformation behaviors, microstructural features, crystallographic texture evolutions and mechanical responses under strain path changes. UFG hcp materials, including commercial purity Ti, Ti-6Al-4V alloy and high purity Zr, were fabricated using equal channel angular extrusion (ECAE) as a severe plastic deformation (SPD) technique following various processing schedules. Several characterization methods and a polycrystal plasticity model were utilized in synergy to impart the relationships between the UFG microstructure, the texture and the post-ECAE flow behavior. Pure UFG hcp materials exhibited enhanced strength properties, making them potential substitutes for coarse-grained high strength expensive alloys. Incorporation of post-ECAE thermo-mechanical treatments was effective in further improvement of the strength and ductility levels. Strong anisotropy of the post-ECAE flow response was evident in all the materials studied. The underlying mechanisms for anisotropy were identified as texture and processing-induced microstructure. Depending on the ECAE route, the applied strain level and the specific material, the relative importance of these two mechanisms on plastic flow anisotropy varied. A viscoplastic self-consistent approach is presented as a reliable model for predicting the texture evolutions and flow behaviors of UFG hcp materials in cases where texture governs the plastic anisotropy. Regardless of the material, the initial billet texture and the extrusion conditions, ECAE of all hcp materials revealed similar texture evolutions. Accurate texture and flow behavior predictions showed that basal slip is the responsible mechanism for such texture evolution in all hcp materials independent of their axial ratio. High strength of the UFG microstructure was presented as a triggering mechanism for the activation of unexpected deformation systems, such as high temperature deformation twinning in Ti-6Al-4V and room temperature basal slip in pure Zr.

Present times are sometimes referred to as "the golden era of regulation", as more and more areas of social life are regulated. But regulation is not only increasing; it is also changing. New regulators are emerging, and they are issuing new kinds of rules. These new kinds of regulation are frequently not legally binding, and are therefore labelled soft regulation as opposed to hard law. It is not compulsory to follow soft rules but many actors - including sovereign states - still do, and the thesis asks the question why this is so. Why do even states, which are powerful regulators themselves, abide by soft regulation, and wherein lies the regulative power of soft rules? Through an in-depth study of the European Union's pre-accession instrument Twinning an answer to the question of the power of soft regulation has been arrived at. Treating Twinning as a critical case of soft regulation, and using theories of imitation to grasp the meaning and evolution of Twinning projects, makes it possible to define three regulative elements involved in soft regulation. These are the combinative, co-productive and constitutive elements of soft regulation, from which the thesis suggests that it derives its power. First of all, soft regulation combines different kinds of rules, the regulation of identity and the regulation of activity, and a variety of sources of legitimacy. Second, it depends on regulators and regulatees interacting to co-produce regulation. And third, as its main result, it constitutes the rule-followers as formal, rational, and modern organisations. Accordingly, soft regulation has rather impressive regulative capabilities, builds on complex, dynamic, and social interactions, and embodies as well as promotes some of Western society's most strongly institutionalised ideas. The thesis argues that it is through these characteristics that actors, including states, are compelled to follow soft rules.

Magnesium, with a Hexagonal Close-Packed (HCP) structure, is the eighth most abundant element in the earth’s crust and the third most plentiful element dissolved in the seawater. Magnesium alloys exhibit the attractive characteristics of low densities and high strength-to-weight ratios along with good castability, recyclability, and machinability. Replacing the steel and/or aluminum sheet parts with magnesium sheet parts in vehicles is a great way of reducing the vehicles weight, which results in great savings on fuel consumption. The lack of magnesium sheet components in vehicle assemblies is due to magnesium’s poor room-temperature formability. In order to successfully form the sheets of magnesium at room temperature, it is necessary to understand the formability of magnesium at room temperature controlled by various plastic deformation mechanisms. The plastic deformation mechanisms in pure magnesium and some of its alloys at room temperature are crystallographic slip and deformation twinning. The slip systems in magnesium at room temperature are classified into primary (first generation), secondary (second generation), and tertiary (third generation) slip systems. The twinning systems in magnesium at room temperature are classified into primary (first generation) and secondary (second generation, or double) twinning systems. A new comprehensive rate-dependent elastic-viscoplastic Crystal Plasticity Constitutive Model (CPCM) that accounts for all these plastic deformation mechanisms in magnesium was proposed. The proposed model individually simulates slip-induced shear in the parent as well as in the primary and secondary twinned regions, and twinning-induced shear in the primary and secondary twinned regions. The model also tracks the texture evolution in the parent, primary and secondary twinned regions. Separate resistance evolution functions for the primary, secondary, and tertiary slip systems, as well as primary and secondary twinning systems were considered in the formulation. In the resistance evolution functions, the interactions between various slip and twinning systems were accounted for. The CPCM was calibrated using the experimental data reported in the literature for pure magnesium single crystals at room temperature, but needs further experimental data for full calibration. The partially calibrated model was used to assess the contributions of various plastic deformation mechanisms in the material stress-strain response. The results showed that neglecting secondary slip and secondary twinning while simulating plastic deformation of magnesium alloys by crystal plasticity approach can lead to erroneous results. This indicates that all the plastic deformation mechanisms have to be accounted for when modelling the plastic deformation in magnesium alloys. Also, the CPCM in conjunction with the Marciniak–Kuczynski (M–K) framework were used to assess the formability of a magnesium single crystal sheet at room temperature by predicting the Forming Limit Diagrams (FLDs). Sheet necking was initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field was assumed inside and outside the band, and conditions of compatibility and equilibrium were enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs were simulated under two conditions: a) the plastic deformation mechanisms are primary slip systems alone, and b) the plastic deformation mechanisms are primary slip and primary twinning systems. The FLDs were computed for two grain orientations. In the first orientation, primary extension twinning systems had favourable orientation for activation. In the second orientation, primary contraction twinning systems had favourable orientation for activation. The effects of shear strain outside the necking band, rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations were individually explored.

Ultra-fine grained (UFG) materials have attracted considerable interest due to the possibility of achieving simultaneous increase in strength and ductility. Effective use of these materials in engineering applications requires investigating the processing-microstructure-property inter-relations leading to a comprehensive understanding of the material behavior. Research efforts on producing UFG hexagonal close packed (hcp) materials have been limited in spite of their envisaged utilization in various technologies. The present study explores multiple UFG hcp materials to identify the general trends in their deformation behaviors, microstructural features, crystallographic texture evolutions and mechanical responses under strain path changes. UFG hcp materials, including commercial purity Ti, Ti-6Al-4V alloy and high purity Zr, were fabricated using equal channel angular extrusion (ECAE) as a severe plastic deformation (SPD) technique following various processing schedules. Several characterization methods and a polycrystal plasticity model were utilized in synergy to impart the relationships between the UFG microstructure, the texture and the post-ECAE flow behavior. Pure UFG hcp materials exhibited enhanced strength properties, making them potential substitutes for coarse-grained high strength expensive alloys. Incorporation of post-ECAE thermo-mechanical treatments was effective in further improvement of the strength and ductility levels. Strong anisotropy of the post-ECAE flow response was evident in all the materials studied. The underlying mechanisms for anisotropy were identified as texture and processing-induced microstructure. Depending on the ECAE route, the applied strain level and the specific material, the relative importance of these two mechanisms on plastic flow anisotropy varied. A viscoplastic self-consistent approach is presented as a reliable model for predicting the texture evolutions and flow behaviors of UFG hcp materials in cases where texture governs the plastic anisotropy. Regardless of the material, the initial billet texture and the extrusion conditions, ECAE of all hcp materials revealed similar texture evolutions. Accurate texture and flow behavior predictions showed that basal slip is the responsible mechanism for such texture evolution in all hcp materials independent of their axial ratio. High strength of the UFG microstructure was presented as a triggering mechanism for the activation of unexpected deformation systems, such as high temperature deformation twinning in Ti-6Al-4V and room temperature basal slip in pure Zr.

The present work aims to reveal the microstructural evolution and post-processing mechanical behavior of difficult-to-work alloys upon severe plastic deformation. Severe plastic deformation is applied using equal channel angular extrusion (ECAE) where billets are pressed through a 90o corner die achieving simple shear deformation. Three different materials are studied in this research, namely Ti-6Al-4V, Ti-6Al-4V reinforced with 10% TiC and AISI 316L stainless steel. Microstructure and mechanical properties of successfully extruded billets were reported using light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), tension and compression experiments and microhardness measurements. The effects of extrusion conditions (temperature and processing route) on the microstructure and mechanical properties are investigated. The underlying mechanisms responsible for observed mechanical behaviors are explored. It is seen that ECAE shear deformation leads to refinement in α plates and elimination of prior β boundaries in Ti-6Al-4V. Decreasing extrusion temperature and increasing number of passes decreases α plate size and grain size. Refined α grain size leads to a significant increase in tensile and compressive flow stresses at room temperature. Texture produced by ECAE has a pronounced effect on mechanical properties. Specifically it leads to tension/compression asymmetry in flow strengths and strain hardening coefficients may be described by the activation of differing slip systems under tension and compression loading. ECAE of Ti-6Al-4V+10%TiC samples also improved mechanical properties due to α plate size refinement. Nevertheless, further extrusion passes should be carried out for tailoring reinforcement size and distribution providing optimum strength and ductility. ECAE deformation of AISI 316L stainless steel at high homologous temperatures (0.55 to 0.60 Tm) results in deformation twinning as an effective deformation mechanism which is attributed to the effect of the high stress levels on the partial dislocation separation. Deformation twinning gives rise to high stress levels during post-processing room temperature tension and compression experiments by providing additional barriers to dislocation motion and decreasing the mean free path of dislocations. The highest tensile flow stress observed in the sample processed at 700 oC following one pass route A was on the order of 1200 MPa which is very high for 316L stainless steel. The ultimate goal of this study is to produce stabilized end microstructures with improved mechanical properties and demonstrate the applicability of ECAE on difficult-to-work alloys.

Magnesium, with a Hexagonal Close-Packed (HCP) structure, is the eighth most abundant element in the earth’s crust and the third most plentiful element dissolved in the seawater. Magnesium alloys exhibit the attractive characteristics of low densities and high strength-to-weight ratios along with good castability, recyclability, and machinability. Replacing the steel and/or aluminum sheet parts with magnesium sheet parts in vehicles is a great way of reducing the vehicles weight, which results in great savings on fuel consumption. The lack of magnesium sheet components in vehicle assemblies is due to magnesium’s poor room-temperature formability. In order to successfully form the sheets of magnesium at room temperature, it is necessary to understand the formability of magnesium at room temperature controlled by various plastic deformation mechanisms. The plastic deformation mechanisms in pure magnesium and some of its alloys at room temperature are crystallographic slip and deformation twinning. The slip systems in magnesium at room temperature are classified into primary (first generation), secondary (second generation), and tertiary (third generation) slip systems. The twinning systems in magnesium at room temperature are classified into primary (first generation) and secondary (second generation, or double) twinning systems. A new comprehensive rate-dependent elastic-viscoplastic Crystal Plasticity Constitutive Model (CPCM) that accounts for all these plastic deformation mechanisms in magnesium was proposed. The proposed model individually simulates slip-induced shear in the parent as well as in the primary and secondary twinned regions, and twinning-induced shear in the primary and secondary twinned regions. The model also tracks the texture evolution in the parent, primary and secondary twinned regions. Separate resistance evolution functions for the primary, secondary, and tertiary slip systems, as well as primary and secondary twinning systems were considered in the formulation. In the resistance evolution functions, the interactions between various slip and twinning systems were accounted for. The CPCM was calibrated using the experimental data reported in the literature for pure magnesium single crystals at room temperature, but needs further experimental data for full calibration. The partially calibrated model was used to assess the contributions of various plastic deformation mechanisms in the material stress-strain response. The results showed that neglecting secondary slip and secondary twinning while simulating plastic deformation of magnesium alloys by crystal plasticity approach can lead to erroneous results. This indicates that all the plastic deformation mechanisms have to be accounted for when modelling the plastic deformation in magnesium alloys. Also, the CPCM in conjunction with the Marciniak–Kuczynski (M–K) framework were used to assess the formability of a magnesium single crystal sheet at room temperature by predicting the Forming Limit Diagrams (FLDs). Sheet necking was initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field was assumed inside and outside the band, and conditions of compatibility and equilibrium were enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs were simulated under two conditions: a) the plastic deformation mechanisms are primary slip systems alone, and b) the plastic deformation mechanisms are primary slip and primary twinning systems. The FLDs were computed for two grain orientations. In the first orientation, primary extension twinning systems had favourable orientation for activation. In the second orientation, primary contraction twinning systems had favourable orientation for activation. The effects of shear strain outside the necking band, rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations were individually explored.

Zirconium and its alloys have been extensively used in both heavy and light water nuclear reactors. Like other Hexagonal Close-Packed (HCP) materials, e.g. magnesium, zirconium alloys develop different textures during manufacturing process which result in highly anisotropic materials with different responses under different loading conditions. Slip and twinning are two major deformation mechanisms during plastic deformation of zirconium. This dissertation uses various experimental techniques and a crystal plasticity scheme in the finite element framework to study deformation mechanisms in HCP materials with an emphasis on twinning in Zircaloy-2. The current study is presented as a manuscript format dissertation comprised of four manuscript chapters. After a literature review in Chapter 2, Chapter 3 reports steps in developing a crystal plasticity finite element user material subroutine for modeling deformation in Zircaloy-2 at room temperature. It is shown in Chapter 3 that the developed rate dependent equations are capable of capturing evolution of key features, e.g., texture, lattice strains, and twin volume fractions, during deformation by twinning and slip. Chapter 4 reports various assumptions and approaches in modeling twinning where results are compared against neutron diffraction measurements from the literature. It is shown in Chapter 4 that the predominant twin reorientation scheme can explain texture development more precisely than the other schemes discussed. Chapter 5 and 6 are two connected chapters where in the first one the formation of twins is studied statistically and in the second one, local inception and propagation of twins is studied. Numerical results of these two chapters are compared with 2D electron backscattered diffraction measurements, both carried out by the author and from the literature. Results from these two connected chapters emphasize the important role of grain boundary geometry and stress concentration sites on twin nucleation and growth. The four manuscript chapters are followed by summarizing conclusions and suggestions for future work in Chapter 7. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2012-04-23 11:50:33.751

Ni-Mn-Ga alloys close to the stoichiometric composition Ni2MnGa belong to the quite new family of ferromagnetic shape memory alloys. These alloys are characterized by the magnetic field induced strain (MFIS) based on the comparably easy motion of twin boundaries under a magnetic field. They are mostly chosen as a potential candidate for practical application especially promising for actuators and sensors because they are showing the largest MFIS so far. Depending on the chemical composition and heat treatment, at least three martensitic structures can be distinguished in the Ni-Mn-Ga system. However, the effect mentioned above only exists in two modulated structures. Since for the intended application of MFIS in technology polycrystalline materials seem to be more appropriate in contrast to single crystals, the specific polycrystalline aspects are considered. Factors important for decreasing the twinning stress and increasing the twinning strain of polycrystalline Ni-Mn-Ga alloys are texturing, adjusting the structure by annealing and training by thermomechanical treatments. To achieve pronounced MFIS in polycrystals, fabrication processes are needed to produce specific strong textures. The material texturing has been obtained by directional solidification and plastic deformation by hot rolling and hot extrusion as well as high pressure torsion (HPT). To examine the texture of coarse-grained Ni-Mn-Ga alloys (due to a solidification process or dynamic recrystallization), diffraction of synchrotron radiation and neutrons was applied. The texture results show that the texture of Ni-Mn-Ga subjected to directional solidification, hot rolling and hot extrusion is a fibre or weak biaxial texture. However, local synchrotron measurements reveal that the global fibre texture of the hot extruded sample is a ”cyclic” fibre texture, i.e. it is composed of components related to the radial direction rotating around the extrusion axis. This allows finding regions with a strong texture component. The texture after HPT is characterized by a strong cube with the cube favourably oriented. The initial microstructure of the Ni-Mn-Ga alloys is a typical self-accommodated microstructure of martensite. High resolution EBSD mappings show macro, micro twins and two types of microstructure. The twin plane is determined to be {110). In a typical martensitic transformation the high-temperature phase has a higher crystallographic symmetry than the low-temperature phase. Consequently, austenite may transform to several martensitic variants, the number of which depends on the change of symmetry during transformation. Generally, in a cubic-to-tetragonal transformation (5M case) three variants can form with the c-axis oriented close to the three main cubic axes of austenite. However, close examination of the high resolution EBSD mapping reveals that more than just three orientations, as expected from the Bain model, exist in Ni50Mn29Ga21. Each of three Bain variants may be split in some twin relations in different regions of the sample which differ from each other by about few degrees creating a much higher number of variants. The training process, as the last step in the preparation procedure of Ni-Mn-Ga alloys, consists of multi-axis compression finally leading to a single-variant state. Compression of polycrystalline samples leads to motion of those twin boundaries changing the volume fraction of particular martensitic variants in such a way that the shortest axis (c-axis) becomes preferentially aligned parallel to the compression axis. It allows reducing the twinning stress and maximizing the twinning strain. To understand the training process in more detail, the interaction of the twin variants with the neighbourhood of parent austenite grains was investigated.

Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: J. S. Preston. Includes bibliographical references (leaves 111-117).