恒应力约束时效Ni-48.85Ti-0.3Cr形状记忆合金的马氏体相变与功能特性的影响OA

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.