Phase Stability of Single Crystalline Co-Ni-Ga Shape Memory Alloy

Single crystals of Co48Ni22Ga30 have been synthesized using a modified Bridgman method. The ability to solidify and retain single phase B2 austenite was found to depend not only on the starting composition and growth rate, but also the ability to maintain sufficiently high cooling rates to avoid the precipitation of a Co-rich FCC phase during post-solidification cooling. DSC measurements on the single crystal found the Ms, Mf, As, and Af to be 35.7°C, -1.8°C, 34.1°C and 72.2°C, respectively. On subsequent heating the B2 phase was found to partially decompose into the Co-rich phase at temperatures exceeding 380°C. Decomposition of the single phase B2 phase was tracked by microstructural observation, DSC, powder diffraction and low temperature heat capacity measurement. Restoration of the crystal to single phase B2 austenite required annealing of the crystal at temperatures above 1125°C followed by rapid cooling.