Abnormal Two-way Shape Memory Effect Induced by Low-temperature Aging in a Rapidly Solidified Ni51Ti49 Alloy

Thus far, many studies focused on the two-way shape memory effect (TWSME) of NiTi alloys obtained through constraint-aging in 350~500 ºC, so as to achieve high recovery ratio. However, less attention is paid to TWSME in low-temperature (<350 ºC) aged NiTi alloy. This study reports an abnormal TWSME in a rapidly solidified and constraint-aged Ni₅₁Ti₄₉ alloy, which shows inverse deformation direction on cooling compared with normal TWSME. First, rapidly solidified Ni₅₁Ti₄₉ alloy strips were solution treated at 850 ºC for 3 h followed by constraint-aging at 250 ºC and 300 ºC for 1 h, 10 h and 100 h by placing them in an arch mold. Then, shape change, phase transformation behavior and microstructure were characterized by photography, DSC and TEM. Results show that the low-temperature aged NiTi alloy strip bends toward the constraint-position on cooling, showing an abnormal TWSME. The above as-prepared NiTi alloy consists of B2 and R-phase and only B2-R transformation occurs between -80~80 ºC. The occurrence of R-phase indicates the existence of Ni₄Ti₃, which cannot be observed under TEM. However, Ni₄Ti₃ precipitates are too small to form strong coherent stress after low-temperature aging, consequently the residual stress can directly influence R-phase and martensitic transformations, thus induce the abnormal TWSME. To further verify the mechanism of the abnormal TWSME, constitutive model analysis is applied to simulating the process of martensitic transformation.