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|Title:||Origin of Microstructural Irreversibility in Ni-Ti Based Shape Memory Alloys during Thermal Cycling|
|Citation:||METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, 43A(4)1277-1287|
|Abstract:||Different microstructures of Ni-Ti- and Ni-Ti-Fe-based shape memory alloys were subjected to thermal cycling: dipping in liquid nitrogen, for approximately 5 minutes, and then bringing it back to room temperature or austenite (cubic: B2) <-> martensite (monoclinic: B19') reversible solid-state phase transformation. Direct electron backscattered diffraction (EBSD) observations could bring out aspects of microstructural irreversibilities: namely, changes in grain size, mis-orientation buildup, and presence of retained martensite. The average changes in grain size (Delta d) differed by almost 2 to 4 times between different microstructures. The highest Delta d was typically observed in structures having maximum clustering of fine (d < 5 mu m) grains. The sample with highest Delta d was also subjected to multiple thermal cycling. Although Delta d scaled linearly with d after the first thermal cycle, the scatter increased during subsequent thermal cycles. Grain or orientations deviating from the linear behavior were clearly anisotropic crystallographically. With repeated thermal cycling, the patterns of changes in Delta d, austenite misorientation, and retained martensite content were similar. A phenomenological model or hypothesis, based on 40 deg < 001 > orientation relationship between austenite and martensite phases, was proposed to address the observed patterns of microstructural irreversibility.|
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