Thesis (M.Sc. (Physics)) --University of Limpopo, 2005 / Self-consistent electronic structure calculations have been performed on ordered cubic-based magnesium-lithium (Mgx-Li1−x) alloys spanning the concentration range 0 ≤ x ≤ 1, using an ab initio plane wave pseudopotential (PWP) method. The first principle pseudopotential planewave approach is used within the local density approximation (LDA) and generalized-gradient approximation (GGA)of the density functional theory (DFT) framework. We have calculated the binding energy curves and the systematic trends in various cohesive and elastic properties at zero temperature, as a function of Li concentration. The calculated equilibrium lattice parameters show a large deviation from Vegard’s rule in the Li-rich region whilst the bulk moduli decrease monotonically with increase in Li concentration. The heats of formation for different ground state superstructures predict that the DO3, B2 and DO22 structures would be the most stable at absolute zero amongst various phases having the Mg3Li, MgLi and MgLi3 compositions, respectively. This stability is reflected in the electronic density of states (DOS). Because of the special significance of the isotropic bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio for technological and engineering applications, we have also calculated these quantities from the elastic constants. The elastic constants indicate the softness of the material as more Li is added with the bcc-based phases becoming mechanically less stable for Li concentration less than 50%. Our results show good agreement within the estimated uncertainty with both experimental and previous theoretical results. / The National Research Foundation (NRF), South Africa-Royal Society (RS), Great Britain collaboration and Council for the Scientific and Industrial Research (CSIR)

Mg alloys have spurred a renewed academic and industrial interest because of their ultra-light-weight and high specific strength properties. Hexagonal close packed Mg has low deformability and a high plastic anisotropy between basal and non-basal slip systems at room temperature. Alloying with Li and other elements is believed to counter this deficiency by activating non-basal slip by reducing their nucleation stress. In this work I study how Li addition affects deformation mechanisms in Mg using atomistic simulations. In the first part, I create a reliable and transferable concentration dependent embedded atom method (CD-EAM) potential for my molecular dynamics study of deformation. This potential describes the Mg-Li phase diagram, which accurately describes the phase stability as a function of Li concentration and temperature. Also, it reproduces the heat of mixing, lattice parameters, and bulk moduli of the alloy as a function of Li concentration. Most importantly, our CD-EAM potential reproduces the variation of stacking fault energy for basal, prismatic, and pyramidal slip systems that influences the deformation mechanisms as a function of Li concentration. This success of CD-EAM Mg-Li potential in reproducing different properties, as compared to literature data, shows its reliability and transferability. Next, I use this newly created potential to study the effect of Li addition on deformation mechanisms in Mg-Li nanocrystalline (NC) alloys. Mg-Li NC alloys show basal slip, pyramidal type-I slip, tension twinning, and two-compression twinning deformation modes. Li addition reduces the plastic anisotropy between basal and non-basal slip systems by modifying the energetics of Mg-Li alloys. This causes the solid solution softening. The inverse relationship between strength and ductility therefore suggests a concomitant increase in alloy ductility. A comparison of the NC results with single crystal deformation results helps to understand the qualitative and quantitative effect of Li addition in Mg on nucleation stress and fault energies of each deformation mode. The nucleation stress and fault energies of basal dislocations and compression twins in single crystal Mg-Li alloy increase while those for pyramidal dislocations and tension twinning decrease. This variation in respective values explains the reduction in plastic anisotropy and increase in ductility for Mg-Li alloys.

<p>Shape memory alloys (SMAs) belong to an important class of active materials. Beyond shape memory, these alloys exhibit super-elasticity and pseudo-plasticity, all originating from a reversible phase transformation from a high-temperature austenitic phase to a low temperature martensitic phase. Their unique thermo-mechanical properties make these SMAs desirable for a wide range of applications in automobiles, robotics, aerospace, construction, and medicine. Only a fraction of the known metallic alloys exhibits martensitic transformations, and a relatively small subset exhibits shape memory. Given this limited pool of SMAs, tunability of this martensitic transformation and, hence, thermo-mechanical properties is a way to move forward for effectively designing the next-generation SMAs for specific applications. The modification in composition has always been at the heart of designing new SMAs for future applications. However, a relatively recent discovery of incorporating a second non-transforming phase in base martensitic materials to tune martensitic transformation to achieve unprecedented thermo-mechanical properties has shown great promise.</p><p><br></p><p>The objective of this work is to utilize the second phase to provide design guidelines for next-generation SMAs and to understand the detailed physics behind the experimentally observed unprecedented thermo-mechanical properties in SMAs as a result of the incorporation of coherent second phases. We first investigate Mg-Sc shape memory alloys that are attractive for a wide range of applications due to their low density. Unfortunately, the use of these alloys is hindered by a low martensitic transformation temperature (173 K). We observe from first-principles calculations that epitaxial strains arising from appropriate substrate or coherent second phase selection increase the martensitic transformation and operational temperature to room temperature. Next, we develop a novel approach to induce martensitic transformation in composite systems of two non-transforming materials. While we demonstrate this approach for the technologically relevant ultra-lightweight Mg/MgLi superlattices, however, our approach is general and will open a wide material space for the discovery and design of next-generation SMAs.</p><p><br></p><p>Finally, to bridge the gap between computationally studied single-crystalline materials and experimentally studied polycrystalline systems, we characterize the role of nanoscale precipitates on temperature- and stress-induced martensitic phase transformation in nanocrystalline Ni63Al37 SMAs using multi-million-atoms molecular dynamics simulations. Simulations provide the understanding of underlying atomistic mechanisms of experimentally observed unprecedented thermo-mechanical properties and the guidelines to design low-fatigue ultra-fine grain shape memory alloys. As a result of the exploration of novel thermomechanical properties in SMAs via coherent second phases, we also published a software package</p><p>to discover coherent precipitates within a base multi-component system by coupling highthroughput equilibrium thermodynamics calculations with strain-based lattice matching.</p>