TEM investigation of microstructures in low-hysteresis Ti ₅₀Ni_50-xPd_x alloys with special lattice parameters

A clear relationship between the hysteresis of shape memory alloys and the middle eigenvalue of their transformation stretch tensor (λ₂) has recently been demonstrated experimentally and theoretically. As λ₂ gets closer to 1, the geometric: compatibility between austenite and martensite increases as predicted in the geometric nonlinear theory of martensite (GNLTM)[1] and leads to a drop in hysteresis. Changes in microstructure are investigated by conventional transmission electron microscopy and electron diffraction for the Ti₅₀Ni_50-xPd_x system over a range of compositions around the compatibility condition. Alloys sitting far enough above λ₂ = 1, i.e. x>20, show a common microstructure for SMAs, consisting of parallel bands of martensite twinned along the {111} type I lattice invariant shear (LIS). This microstructure changes gradually as we tune the composition to approach the compatibility condition. The twinning ratio of martensite plates decreases until the plates become a single variant. The banded morphology is replaced by an apparent random mosaic of twinless martensite plates where self-accommodation seems to play an important role. In addition the in-situ growth of martensite in the austenite near the compatibility condition was observed and showed exact austenite-martensite interfaces along the predicted habit planes.