Multistage martensitic transformation in high temperature aged Ti48Ni52 shape memory alloy

• Transformation behavior of Ti48Ni52 SMA aged at high temperatures is investigated.
• 2-stage and 3-stage transf. occur in 600 °C and 550 °C aged specimens, respectively.
• Ni-content in the matrix nearby Ti3Ni4 and Ti2Ni3 ppts affects the transf. temp.
• A high temp. B2↔B19′ transf. is induced by introducing Ti2Ni3 ppts in the matrix.
• The occurrence of B2→R transf. is related to the coherent interface of Ti3Ni4 ppts.

Aging effects on the transformation behavior of Ti48Ni52 SMA aged at 600 °C and 550 °C were investigated. The transformation evolution of specimens aged at 600 °C and at 550 °C can be classified into three and two periods, respectively. During aging at 600 °C, the specimens undergo B2↔M1 in period I, B2↔M1 + B2↔M2 in period II, and B2↔M2 in period III (M represents the B19′ martensite). During aging at 550 °C, the specimens undergo B2→R→M1/M1→B2 in periods I and II, and an additional B2↔M2 in period II. In period I, only Ti3Ni4 precipitates (ppts) form in the B2 matrix. In period II, Ti3Ni4 and Ti2Ni3 ppts coexist, and Ti3Ni4 depletion zones occur around Ti2Ni3 plates, inducing B2↔M2. In period III, only Ti2Ni3 plates exist. The occurrence of B2→R→M1 or B2→M1 in period I is related to whether the Ti3Ni4 ppts have a coherent interface or not. Schematic diagrams for periods I ∼ III are proposed to interpret the observed transformation behavior. The difference in Ni-content in the matrix near the Ti3Ni4 and Ti2Ni3 ppts affects the transformation temperatures of B2↔B19′.