On 2-stage martensitic transformation behavior in aged Ti_50.5Ni_33.5Cu_11.5Pd_4.5 alloys with near-zero thermal hysteresis

The microstructures and martensitic transformation of a Ti-rich Ti_50.5Ni_33.5Cu_11.5Pd_4.5 alloy aged at 500°C for durations ranging from 15 min to 25 h were systematically investigated. With increasing aging time, the precipitates exhibit continuous growth; however, their quantity initially increases and subsequently decreases. The morphology of the precipitates evolves from granular to lamellar forms. Post-aging treatment, the transformation sequence transitions from a one-stage (B2-B19) to a two-stage process (B2₁-B19₁ and B2₂-B19₂) due to the precipitation of Ti₂Cu-type particles, maintaining near-zero hysteresis. The two-stage martensitic transformation originates from the combined effects of stress distribution heterogeneities and variations in Ni content induced by aging precipitation.