Superelastic Response and Substructure Analysis of Ni-Ti Microcrystals

In an effort to develop a fundamental understanding of the relationship between the martensitic transformation and plasticity in solutionized 50.7at% Ni-Ti, FIB-machined micropillars of 5 micron and 20 micron diameters have been compressed along a <110> crystallographic axis using a modified nanoindentor . It is demonstrated for the first time that isolated martensitic transformations can be studied with direct measurement of mechanical response (stress, strain, work output) for individual variants. Post-mortem TEM characterization of the remnant substructure in the microcrystals is enabled using FIB-based extraction of TEM foils. The remnant dislocation configurations appear to have been generated as a result of the martensitic transformation. Analytic modeling coupled with post-mortem TEM images suggests that a single martensite plate was operative in the microcrystals. Subsequent finite element modeling of these pillars is used to study the coupling between the martensitic transformation and matrix plasticity, via a constitutive formulation that includes both rate-dependent crystal plasticity and rate-independent transformation. This coupled experimental-modeling approach permits detailed study of the interplay between martensite transformation versus matrix plasticity over a range of crystal orientations and microcystal size.