EXPLORING THE TUNABILITY OF MARTENSITIC TRANSFORMATION IN SHAPE MEMORY ALLOYS VIA COHERENT SECOND PHASE
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.
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.
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
to discover coherent precipitates within a base multi-component system by coupling highthroughput equilibrium thermodynamics calculations with strain-based lattice matching.