A Study on Phase Transformation Exhibited in Ti 49± x Ni 49∓ x Fe 2 ( x = 0 to 1) Shape Memory Alloys

The phase transformations and damping capacities in as solution-treated and 450 °C aged Ti 49± x Ni 49∓ x Fe 2 ( x = 0 to 1) SMAs were investigated. In Ti 48 Ni 50 Fe 2 , Ti 48.5 Ni 49.5 Fe 2 and Ti 49 Ni 49 Fe 2 SMAs, no transformation occurred in the as solution-treated state, and B2 ↔ R1 transformation induced by Ti 3 Ni 4 precipitates appeared when the SMAs were aged at 450 °C. DMA measurement showed that these as solution-treated SMAs exhibited strain glass behavior. The Ti 49.5 Ni 48.5 Fe 2 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 Ti 50 Ni 48 Fe 2 SMA showed a two-stage B2 ↔ R2 ↔ B19′ transformation in both solution-treated and 450 °C aged specimens. Microstructural observations showed Ti 3 Ni 4 precipitates in Ti 48 Ni 50 Fe 2 , Ti 48.5 Ni 49.5 Fe 2 , Ti 49 Ni 49 Fe 2 and Ti 49.5 Ni 48.5 Fe 2 SMAs aged at 450 °C. The morphology and distribution of Ti 3 Ni 4 precipitates are likely related to the amount of B2 ↔ R1 transformation.