<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>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/17144384 |
| Date | 20 December 2021 |
| Creators | Shivam Tripathi (11516983) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/EXPLORING_THE_TUNABILITY_OF_MARTENSITIC_TRANSFORMATION_IN_SHAPE_MEMORY_ALLOYS_VIA_COHERENT_SECOND_PHASE/17144384 |
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