A Study on Phase Transformation Exhibited in Ti49±xNi49∓xFe2 (x = 0 to 1) Shape Memory Alloys

The phase transformations and damping capacities in as solution-treated and 450 °C aged Ti49±xNi49∓xFe2 (x = 0 to 1) SMAs were investigated. In Ti48Ni50Fe2, Ti48.5Ni49.5Fe2 and Ti49Ni49Fe2 SMAs, no transformation occurred in the as solution-treated state, and B2 ↔ R1 transformation induced by Ti3Ni4 precipitates appeared when the SMAs were aged at 450 °C. DMA measurement showed that these as solution-treated SMAs exhibited strain glass behavior. The Ti49.5Ni48.5Fe2 specimen exhibited a two-stage B2 ↔ R2 ↔ B19′ transformation in the as solution-treated state, but after aging at 450 °C, the R2 ↔ B19′ was suppressed and B2 ↔ R1 appeared with B2 ↔ R2 remaining. The B2 ↔ R2 transformation is related to Fe, which causes point defects in the matrix. The Ti50Ni48Fe2 SMA showed a two-stage B2 ↔ R2 ↔ B19′ transformation in both solution-treated and 450 °C aged specimens. Microstructural observations showed Ti3Ni4 precipitates in Ti48Ni50Fe2, Ti48.5Ni49.5Fe2, Ti49Ni49Fe2 and Ti49.5Ni48.5Fe2 SMAs aged at 450 °C. The morphology and distribution of Ti3Ni4 precipitates are likely related to the amount of B2 ↔ R1 transformation.