No / Martensitic transformations are of considerable technological importance, a particularly promising application being the possibility of using martensitic materials, possibly proteins, as tiny machines. For organic crystals, however, a molecular level understanding of such transformations is lacking. We have studied a martensitic-type transformation in crystals of the amino acid DL-norleucine using molecular dynamics simulation. The crystal structures of DL-norleucine comprise stacks of bilayers (formed as a result of strong hydrogen bonding) that translate relative to each other on transformation. The simulations reveal that the transformation occurs by concerted molecular displacements involving entire bilayers rather than on a molecule-by-molecule basis. These observations can be rationalized on the basis that at sufficiently high excess temperatures, the free energy barriers to concerted molecular displacements can be overcome by the available thermal energy. Furthermore, in displacive transformations, the molecular displacements can occur by the propagation of a displacement wave (akin to a kink in a carpet), which requires the molecules to overcome only a local barrier. Concerted molecular displacements are therefore considered to be a significant feature of all displacive transformations. This finding is expected to be of value toward developing strategies for controlling or modulating martensitic-type transformations.

The work in this thesis is inspired by the fabrication of Zn₄₅Au₃₀Cu₂₅. This is the first alloy undergoing ultra-reversible martensitic transformations and closely satisfying the cofactor conditions, particular conditions of geometric compatibility between phases, which were conjectured to influence reversibility. With the aim of better understanding reversibility, in this thesis we study the martensitic microstructures arising during thermal cycling in Zn₄₅Au₃₀Cu₂₅, which are complex and different in every phase transformation cycle. Our study is developed in the context of continuum mechanics and nonlinear elasticity, and we use tools from nonlinear analysis. The first aim of this thesis is to advance our understanding of conditions of geometric compatibility between phases. To this end, first, we further investigate cofactor conditions and introduce a physically-based metric to measure how closely these are satisfied in real materials. Secondly, we introduce further conditions of compatibility and show that these are nearly satisfied by some twins in Zn₄₅Au₃₀Cu₂₅. These might influence reversibility as they improve compatibility between high and low temperature phases. Martensitic phase transitions in Zn₄₅Au₃₀Cu₂₅ are a complex phenomenon, especially because the crystalline structure of the material changes from a cubic to a monoclinic symmetry, and hence the energy of the system has twelve wells. There exist infinitely many energy-minimising microstructures, limiting our understanding of the phenomenon as well as our ability to predict it. Therefore, the second aim of this thesis is to find criteria to select physically-relevant energy minimisers. We introduce two criteria or selection mechanisms. The first involves a moving mask approximation, which allows one to describe some experimental observations on the dynamics, while the second is based on using vanishing interface energy. The moving mask approximation reflects the idea of a moving curtain covering and uncovering microstructures during the phase transition, as appears to be the case for Zn₄₅Au₃₀Cu₂₅, and many other materials during thermally induced transformations. We show that the moving mask approximation can be framed in the context of a model for the dynamics of nonlinear elastic bodies. We prove that every macroscopic deformation gradient satisfying the moving mask approximation must be of the form 1 + a(x) ⊗ n(x), for a.e. x. With regards to vanishing interface energy, we consider a one-dimensional energy functional with three wells, which simplifies the physically relevant model for martensitic transformations, but at the same time highlights some key issues. Our energy functional admits infinitely many minimising gradient Young measures, representing energy-minimising microstructures. In order to select the physically relevant ones, we show that minimisers of a regularised energy, where the second derivatives are penalised, generate a unique minimising gradient Young measure as the perturbation vanishes. The results developed in this thesis are motivated by the study of Zn₄₅Au₃₀Cu₂₅, but their relevance is not limited to this material. The results on the cofactor conditions developed here can help for the understanding of new alloys undergoing ultra-reversible transformations, and as a guideline for the fabrication of future materials. Furthermore, the selection mechanisms studied in this work can be useful in selecting physically relevant microstructures not only in Zn₄₅Au₃₀Cu₂₅, but also in other materials undergoing martensitic transformations, and other phenomena where pattern formation is observed.

We study the plasticity mechanisms of diffusionless martensite phase transformation in Nickel-Titanium, one of the most widely used shape memory alloys. The research here involves four thrusts focusing on different length and time scales: (I) Molecular statics and dynamics simulations are applied to study the nanotwin structures and temperature-driven B2 → B19′ phase transitions. (II) Molecular dynamics simulations are performed to explore the stress-driven martensitic phase transformation governing the pseudoelasticity and shape memory effects in NiTi nanopillars. (III) Monte Carlo simulations are conducted to characterize the temperature- driven B2 → B19 phase transition and the patterning of martensitic nanotwins in NiTi thin films. (IV) Phase field simulations are performed to predict the formation and evolution of complex martensitic microstructures, including the detailed analysis of twin compatibility under complex loading conditions. We also study the nucleation-controlled plasticity mechanisms in different metals of Cu, Al and Ni. Our work focuses on understanding how dislocations nucleate in single crystals. Interatomic potential finite element method is applied to determine when, where and how dislocations nucleate during nanoindentation in metals such as Cu, Al and Ni.

Transformation Induced Plasticity (TRIP) steels undergo a phase transformation when subjected to high levels of mechanical strain. This transformation from a paramagnetic austenitic parent phase to a ferromagnetic martensitic phase is irreversible and the resultant magnetic properties may therefore be used as a measure of strain history. The transformation behaviour of TRIP steels has been recognised as a potential smart characteristic and various proposals have appeared aimed at producing a structure that performs its primary structural function as well a strain sensing function simultaneously. However the strain induced nature of the transformation implies that transformation will occur in areas of high stress concentration and therefore engineered stress concentration features will be required to provide a consistent measure of the changes in the magnetic properties of the material as a function of applied load. In order to predict the performance of smart TRIP steel sensors, an analysis method capable of quantifying the effectiveness of a component in its dual role as structure and sensor is needed. The thesis addresses the development of a methodology for correlating the changing magnetic permeability of TRIP steel sensors and structures with martensitic transformation behaviour. The prediction of the deformation behaviour including transformation is implemented by considering a mechanical analysis based on the finite element method and a constitutive model incorporating strain-induced martensitic transformation kinetics. .Extensions to the model which allow for a wide range of deformation rates and temperatures are also discussed. In order to demonstrate the application of the methodology, an analysis of a simple tensile element used in strain measurement applications is presented. The analysis also includes the effect of temperature on the performance of the sensor. An analysis of a design proposal for a smart aircraft bolt is also included to investigate the effects of geometry, particularly engineered stress concentrations, and sensor placement. / Thesis (Ph.D.)-University of Natal, Durban, 2003.

The work contained herein is in pursuance of the Development of the SMART aircraft bolt. Failure of the bolt in the aircraft wing is taken for granted in the project, and the consequent repairs are presently very costly. The SMART material investigated in this work is the TRIP steel, and any reference to SMART material, in this work, shall at all time mean TRIP steel. Investigation of the stresses pertaining to bolts in general is carried out, based on the bolt theories and using the finite element analysis. An optimal bolt based on impact resistance only has also been suggested. Metallurgical behaviour of materials similar to TRIP materials is also investigated, including a section dedicated only to TRIP steels. Therefore, the work contained herein acts as a good base for further research. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2002.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (p. [139]-145).

<p>  </p> <p>Stress corrosion cracking (SCC) is a longstanding critical materials challenge in austenitic stainless steels (AuSS). Recently, there has been mounting concern regarding the potential for Chloride-induced stress corrosion cracking (CISCC) along arc weld seams on austenitic stainless-steel canisters used as spent nuclear fuel (SNF) dry storage containers, due to the residual stress from the welding process and exposure to chloride-rich coastal air at storage sites. To ensure the safety of the SNF storage, fundamental understanding and mitigation methods of CISCC are critical in both engineering design and maintenance of the storage canisters before and after their deployment. With the recent development of high-resolution characterization and analysis techniques, a more robust and comprehensive understanding of the fundamental TGCISCC mechanism starts to be more accessible. In this thesis, comprehensive state-of-the-art techniques, including SEM, EBSD, HREBSD, FIB, ATEM, TKD, potential dynamic measurement, XRD, and nanoindentation will be used to further understand the mechanistic mechanism of TGCISCC in AuSS from macroscopic scale down to atomistic scale. </p>

The need to develop light-weight and high strength materials for car frames which improve fuel efficiency and provide increased passenger safety during dynamic events such as automobile crashes has been the focus of the steel and automobile industries for the past 30 years. In recent years, the development of high strength steels such as multi-phase TRIP (Transformation-Induced Plasticity)-aided steels have shown great promise due to their excellent combination of high strength and ductility. The savings in automobile weight is provided by the inherent strength of TRIP steels which allows for the use of thinner sections. The TRIP effect is characterized by the phenomenon known as strain-induced martensitic transformation (SIMT) which enhances the work hardenability of such steels as the austenite phase transforms to the much harder martensite phase during plastic straining. This results in a resistance to local necking which subsequently enhances the strength, ductility, and formability of such steels. However, various factors exist which affect the mechanical behaviour of TRIP steels. This study will aim, through the use of finite element models, to investigate the role and influence of each of these factors on the TRIP effect in type 304 austenitic and multi-phase TRIP steels. These factors include the rate at which the martensitic transformation proceeds, the state of stress to which the material is subjected to, the interaction between the surrounding matrix and embedded retained austenite islands in multi-phase TRIP steels, and the volume fraction and morphology of the retained austenite islands. Investigation of these factors will provide further insight on each of their contributions to the TRIP effect in order to exploit the potential benefits offered by these steels.

Fretting, which can occur when two bodies in contact undergo a low amplitude relative slip, can drastically reduce the fatigue performance of a material. The extent of fretting damage is dependent on the material combination and is affected by many parameters, making it difficult to design against fretting. Some of these parameters include contact force, displacement amplitude, and contacting materials. This work develops a method for quantifying the extent of damage from fretting as a function of these parameters for a thin sheet of AISI 301 stainless steel in the full hard condition in contact with both ANSI A356 aluminum and AISI 52100 steel contacting bodies. Fretting experiments were conducted on a Phoenix Tribology DN55 Fretting Machine using a fixture which was developed for holding thin specimens. The displacement amplitude and normal force were systematically varied in order to cover a range that could typically be experienced during service. The tribological behavior was studied by analyzing friction force during cycling and inspecting the resulting surface characteristics. Fretting damaged specimens were cycled in tension in a servohydraulic test system to failure. The decrease in fatigue life caused by fretting damage was determined by comparing the stress-life (S-N) response of the fretted specimens to the S-N response of the virgin material, thus characterizing the severity of the fretting damage. The conditions that lead to the greatest reduction in life were identified in this way. Using the fracture mechanics based NASGRO model, an Equivalent Initial Flaw Size (EIFS) was used to quantify the level of fretting damage, thus separating the life of the component into crack nucleation and subsequent propagation. This method and data will allow engineers to design more robust components that resist fretting damage, thus increasing the safety and reliability of the system.

No description available.