Probing Microstructure-Martensitic Transformation Couplings in Metamagnetic Shape Memory Alloys

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Non-Technical Summary: Shape Memory Alloys (SMAs) undergo reversible shape changes, mediated by re-arrangements in their atomic structure, as a result of the application of forces and/or by imposing changes in temperature. Some types of SMAs also respond to magnetic fields and in some cases the reversible shape change is accompanied by dramatic changes in the magnetic properties of the material. These SMAs are known as meta-magnetic SMAs- 'meta' in this case indicates 'beyond' conventional magnetic behavior- and the very strong coupling between mechanical and magnetic fields make them ideal for sensors, actuators, and even solid-state refrigeration. Beyond their practical applications, meta-magnetic SMAs are fascinating materials because their shape-change behavior is extremely sensitive to minute changes in their configuration, which can in turn be changed by different processing techniques. While many groups have proposed different mechanisms by which heat treatments induce changes in the behavior of meta-magnetic SMAs, most explanations rely on changes to global properties of the alloys, such as the overall degree of order/disorder in their atomic configuration. Recent work by the PIs and others suggest that more local effects (i.e., microstructure) may play an unexpected role. This award supports a combined experimental/theoretical effort that seeks to elucidate the role of microstructure evolution on the response of these systems. The knowledge gained can be used to increase the degree of control over the behavior of meta-magnetic SMA-based devices. This award also supports the training of undergraduate (UG) and graduate students in state-of-the-art experimental and computational materials techniques. The PI and Co-PI currently lead an REU program on Multi-functional Materials and students recruited into this program will be mentored by graduate students involved in the project or undergraduate research projects related to meta-magnetic SMAs. Moreover, the PhD students involved in the project will receive training in materials science, informatics and design through enrollment in an interdisciplinary graduate training program (D3EM) directed by the PI. Elements of the research produced in this project will also be used as case studies at the Texas A&M Computational Materials Science Summer School, co-organized by the PI for over eight years.Technical Summary: The PIs propose an experimental/theoretical program to elucidate the underlying microstructural mechanisms and the corresponding length scale effects on the martensitic transformation (MT), in NiMnIn-based meta-magnetic Shape Memory Alloys. Meta-magnetic SMAs exhibit a wide range of complex phenomena resulting from the interplay between microstructure, configurational disorder and several phase transitions (ordering, ferromagnetic, ferroelastic/martensitic). Recent work by the PIs has demonstrated that NiCoMnIn alloys show complex, non-linear, non-monotonic dependence of the MTs, as well as their suppression and re-appearance, on small compositional changes and heat treatments. This behavior, however, cannot be explained by changes in global (thermodynamic) degrees of freedom, such as, vacancy concentration or overall degree of order/disorder. The underlying hypothesis of this project is the existence of a very strong connection between mesoscale microstructural features and the phase transformation behavior of meta-magnetic SMAs. The main goal of this project is to establish a fundamental understanding of the underlying microstructural mechanisms responsible for the observed complex chemistry and heat treatment dependence of multiple phase transitions in NiMnIn-based meta-magnetic SMAs through the close coupling of experiments and simulations. Advanced synthesis and characterization will be combined with state-of-the-art multi-scale computational materials science frameworks to understand the connection between microstructural features and the martensitic transformation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.