GOALI: Micromechanical Experiments and Modeling of Shape Memory Response in Ni-Ti Based Alloys

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TECHNICAL SUMMARYShape memory alloys (SMAs) are materials with remarkable properties that stem from a martensitic transformation. The crystallographic aspects of the martensitic transformation have been calculated and verified in a number of systems; however, there are fundamental aspects of shape memory and pseudoelastic behavior that are not understood. Principal among these is how the matrix accommodates the large strain associated with the transformation. Theoretically, accommodation may be achieved either by matrix plasticity or by inducing additional transformation variants. With plentiful evidence for plasticity in the literature, understanding the mechanism of defect generation has become a critical component for mitigating functional fatigue in thermomechanical cycling applications.By combining micromechanical testing, in situ and post mortem scanning transmission electron microscopy (STEM), and multi-scale computational modeling efforts, the current study aims to develop a more detailed picture of the microstructural evolution as a function of cycling. The fundamental study of defect generation and multiplication is being facilitated by the observation of plastic deformation associated with individual martensite transformation modes in small volumes of material (micropillars). By characterizing the defects generated during these isolated transformation events, the nature of the coupling between plasticity and the transformation is being illuminated. The results from micromechanical testing are also compared to the substructure development observed in pure thermal and combined thermomechanical cycling via in situ STEM experiments. This experimental work is being supplemented by microstructure-sensitive modeling at various length scales, including an Eshelby-type, variant-level prediction of the local stresses developed by specific martensite modes. In previously published work, this model's output has shown remarkable agreement with the active slip systems observed experimentally. This combination of a variety of novel experimental and computation techniques allows for an unprecedented insight into the fundamental mechanisms driving functional fatigue in NiTi-based SMAs and will eventually lead to more fatigue-resistant alloy design for future applications. NON-TECHNICAL SUMMARYShape memory alloys (SMAs), such as NiTi, are unique materials that are able to retain a "memory" of their original shape after cyclic heating or loading. This makes these materials extremely attractive for applications in the medical industry such as stents and surgical devices, in the automotive industry as solid-state actuators, and in the technological world for use in micro-electro-mechanical systems (MEMS). Unfortunately, these remarkable properties rapidly degrade with repeated cycling, making them unsuitable for many potential applications. The current work is focused on studying the mechanisms of functional fatigue in NiTi-based shape memory alloys through the use of a variety of novel experimental techniques. By employing micromechanical testing, in situ and post mortem scanning transmission electron microscopy (STEM), and computational modeling, the study aims to develop a more detailed understanding of how the microstructure evolves with both mechanical and thermal cycling. Specifically, this includes observation and analysis of the material?s behavior and resultant defect accumulation for pure mechanical cycling, pure thermal cycling, and combined thermomechanical conditions at different length scales. In addition, this program facilitates industrial and international collaborations with General Motors Research and Development, the Ruhr University in Bochum, Germany, and others. The results of this work will eventually aid in the development of fatigue-resistant alloys that can out-perform current state-of-the-art materials in demanding applications.