Simulating Thermal Cycling and Isothermal Deformation Response of Polycrystalline NiTi
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Simulating Thermal Cycling and Isothermal Deformation Response of Polycrystalline NiTi Sivom Manchiraju1, Darrell J. Gaydosh2, Ronald D. Noebe2 and Peter M. Anderson1 Materials Science and Engineering, The Ohio State University, Columbus, OH, 43210 2 N. A. S. A. Glenn Research Center, 21000 Brookpark Rd., Cleveland, OH, 44135 1
ABSTRACT A microstructure-based FEM model that couples crystal plasticity, crystallographic descriptions of the B2-B19′ martensitic phase transformation, and anisotropic elasticity is used to simulate thermal cycling and isothermal deformation in polycrystalline NiTi (49.9at% Ni). The model inputs include anisotropic elastic properties, polycrystalline texture, DSC data, and a subset of isothermal deformation and load-biased thermal cycling data. A key experimental trend is captured—namely, the transformation strain during thermal cycling is predicted to reach a peak with increasing bias stress, due to the onset of plasticity at larger bias stress. Plasticity induces internal stress that affects both thermal cycling and isothermal deformation responses. Affected thermal cycling features include hysteretic width, two-way shape memory effect, and evolution of texture with increasing bias stress. Affected isothermal deformation features include increased hardening during loading and retained martensite after unloading. These trends are not captured by microstructural models that lack plasticity, nor are they all captured in a robust manner by phenomenological approaches. Despite this advance in microstructural modeling, quantitative differences exist, such as underprediction of open loop strain during thermal cycling. INTRODUCTION Shape memory Alloys (SMAs) are often subjected to different thermo-mechanical loading conditions based on the intended application. Two examples are isothermal deformation above the austenite finish temperature and thermal cycling under a bias stress. Each presents a set of modeling challenges. An added complication is that two inelastic deformation mechanisms— martensitic transformation and plasticity—may be intricately coupled. Models for polycrystalline SMA can be characterized by their microstructural detail and phenomenological nature. Those that do not use a crystallographic description of the deformation processes [1] are computationally efficient but lack robustness. More advanced models adopt a crystallographic description and incorporate grain orientation and texture [2], making them more robust. Although some models in the literature employ a crystallographic description of the martensitic transformation, they either omit plastic deformation all together [2] or adopt a phenomenological description of plasticity [3]. A recent advance by Manchiraju and Anderson [4] couples crystallographic descriptions of both martensitic transformation and austenite-based plasticity. The coupling is achieved through the grain-to-grain redistribution of stress caused by each mechanism, so that one mechanism affects the driving force of the other. This model qualitatively capt
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