Influence of constrained aging treatment on martensitic transformation and functional properties of Ni-48.85Ti-0.3Cr shape memory alloy

This study systematically investigates the effects of the constrained aging process on the microstructure, martensitic transformation, and functional properties of Ni-48.85Ti-0.3Cr shape memory alloys using DSC, TEM and DMA. The experimental findings reveal that following constrained aging, directionally distributed Ni₄Ti₃ phase is discretely precipitated within the alloy matrix. Under the condition of 400 ℃/100 MPa, extending the constrained aging time leads to a significant increase in the length of Ni₄Ti₃ phase. When subjected to the process of 1 h/100 MPa, raising the temperature not only increases the length and width of the precipitated phases simultaneously but also transforms their morphology from fine granular to larger lenticular, resulting in a corresponding improvement in the aspect ratio. At 400 ℃/100 MPa, when the constrained aging time is prolonged from 0.5 h to 2 h, both the R transformation and B19′ martensitic transformation temperatures rise. This can be attributed to the precipitation of Ni₄Ti₃ phase, which reduces the Ni element content in the matrix. Under the condition of 1 h/100 MPa, aging at 400 ℃ enlarges the size of the precipitated phases and weakens their strengthening effect on the matrix, thereby facilitating the increase in the transformation temperature. At 500 ℃, the transformation process shifts from a two-step to a three-step, with A→R→M₁ transformation occurring in the regions near the precipitated phases and A→M₂ transformation in the regions far from them. Isothermal entropy change tests indicate that at 400 ℃/1 h, the isothermal entropy change decreases as the constrained stress increases. At 400 ℃/100 MPa, the isothermal entropy change initially increases and then decreases with the extension of the aging time. Similarly, at 1 h/100 MPa, the isothermal entropy change also exhibits a trend of first increasing and then decreasing as the aging temperature rises.