M. J. Mills, The Ohio State University, Columbus, OH; P. Sarosi, General Motors, Warren, MI
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.
Summary: Micropillars of 5 micron and 20 micron diameters are machined from solutionized 50.7at% Ni-Ti and 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. 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.