Aging-induced two-stage reverse martensitic transformation behavior in Co₄₆Ni₂₇Ga₂₇ high-temperature shape memory alloy

The effect of austenite aging at 823 K on the microstructures and martensitic transformation behavior of Co₄₆Ni₂₇Ga ₂₇ alloy has been investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and differential scanning calorimeter (DSC). The microstructure observation results show that the unaged Co₄₆Ni₂₇Ga₂₇ alloy is composed of the tetragonal nonmodulated martensite phase and face-centered cubic γ phase. It is found that a new nanosized fcc phase precipitates in the process of austenite aging, leading to the formation of metastable age-affected martensite around the precipitates with composition inhomogeneity. Two-stage reverse martensitic transformation occurs in the samples aged for 2 and 24 h due to the composition difference between the age-affected martensite and the original martensite. For the Co₄₆Ni₂₇Ga₂₇ alloy aged for 120 h, no reverse transformation can be detected due to the disappearance of the metastable age-affected martensite and the small latent heat of the original martensite. The martensitic transformation temperatures of the Co₄₆Ni₂₇Ga₂₇ alloy decrease with an increase in aging time.