Simulating martensitic transformation in NiTi-Hf - effects of alloy composition and aging treatment

In the NiTi-Hf alloy system, coherent H-phase (orthorhombic) nanoprecipitates have been shown to have strong impact on the B2 (cubic) to B19' (monoclinic) martensitic transformation (MT) and the corresponding stress- strain behavior. Two typical martensite-precipitate microstructures have been observed: intra-martensite precipitates (IMP) and inter-precipitate martensites (IPM). In the former, high density small H-phase nano- precipitates are embedded in individual domains of different variants of the B19' martensite that form a herringbone structure, while in the latter, finer twinning B19' martensites are confined in between larger Hphase nanoprecipitates. In this study we investigate the mechanisms underlying the formation of these two distinct microstructures through computer simulations using the phase field method. In particular, we study temperature- and stress-induced MTs in the presence of H-phase nanoprecipitates, concentration variations and stress field in the B2 matrix created by the precipitation reaction, and the corresponding thermal and stress hystereses and superelasticity for different alloy compositions and aging treatments. The simulation results indicate that densely populated nanoprecipitates 7.20 nm in length and 3.8 nm in width lack the strength to impede the growth, coalescence and coarsening of martensitic domains, resulting in the formation of IMP. In contrast, plate-like nanoprecipitates with diameters of .100 nm and thickness of .20 nm effectively hinder the growth, coalescence and coarsening of martensitic domains, giving rise to IPM. The IPM microstructure is accompanied by an almost linear volume fraction vs. temperature curve with less than 1 degrees C hysteresis during thermal cycling and a slim stress-strain hysteresis as compared to that of the IMP microstructure. Therefore, thoughtful choices in alloy composition and aging treatment enable the attainment of distinctly varied precipitate and martensitic microstructures, each characterized by unique pseudoelastic or superelastic properties.