Dynamic SEM characterization of phase transformations in Ti-Ni alloys processed by various treatments

The phase transformations in Ti-Ni alloys processed by various treatments were examined by in- and ex-situ SEM observations. In a solution treated Ti-Ni alloy, the nucleation of the V-shaped 2 habit plane variants cluster (HPVsC) triggers the formation of the hexangular 6 HPVsC in the grain interior. The autocatalytic transformation spreads from the 6 HPVsC to the grain boundary and the reverse transformation proceeds like a rewind of the forward transformation. The relation between the dislocations induced by thermal cycling and the self-accommodation morphologies in Ti-Ni alloy was investigated by electron channeling contrast imaging. The dislocations are confined within the self-accommodated HPVsC and there are no dislocations in the untransformed parent phase area. Many of dislocation loops are observed along <011> type II twin trace near the habit plane. In a solution treated Ti-Ni-Fe alloy, the nucleation and growth of the trigonal R-phase are clearly visualized. The herringbone structure consisting of four R-phase variants is converted to single variant and/or two variants of alternated platelets with the formation of B19f martensite. In other words, the stress induced reorientation of R-phase takes place during subsequent martensitic transformation. The forward and reverse movements of stress induced martensite were characterized in superelastic wires annealed at various temperatures after severe cold drawing by in-situ tensile observations. Isothermal and multistage martensitic transformations in aged Ni-rich Ti-Ni with Ti3Ni4 precipitates were also visualized. We demonstrate that advanced SEM with in-situ stages has a great potential for dynamic visualization of phase transformations in shape memory alloys.