Characterization and Modeling Of Transformation-Induced Defects In Pseudoelastically-Deformed NiTi Microcrystals

The present study investigates the effects of orientation and specimen size on the pseudoelastic response of NiTi shape memory alloy. The primary goal in this investigation is to determine the means by which the matrix accommodates the large strain associated with the martensitic transformation.  This information is critical for extending the working life of components under cyclic loading/heating.  We demonstrate that micron-scale pillar testing can isolate individual stress-induced martensite plates, allowing for the investigation of the microstructural evolution related to particular plate types in the absence of interactions between competing transformation variants. FIB-machined, single crystal micropillars of various crystal orientations have been tested in compression and analyzed via mechanical response measurements and post-mortem scanning transmission electron microscopy (STEM) observations.

In addition, microstructural FEM simulations that incorporate anisotropic elasticity, the crystallographic theory of martensite, and crystal plasticity as competing deformation mechanisms are conducted to determine the dominant martensite plate type based on the loading orientation. The effects of platen-specimen contact and slight loading misorientation are considered. Finally, local stress fields and plastic slip activity surrounding the internally-twinned martensite plate are analyzed through micromechanics computations for an ellipsoidal plate morphology with discrete internal twin interfaces. The results are compared with STEM observations, and issues surrounding the martensite-induced micro-plasticity are discussed. 

Financial support from the National Science Foundation award #DMR- 0907561 is gratefully acknowledged.