Two-way shape memory effect of Cu-Zn-Al alloys induced by a constrained heating method

Abstract

Two-way shape memory effect (TWSME) is a phenomenon that a material can alternate between two distinct shapes as it undergoes a thermal cycle. The TWSME is not an intrinsic property of shape memory alloy but can be obtained by several training methods, most of which are based on the thermomechanical cycling [1, 2]. Although there have been some reports on the realization and improvements of TWSME in Cu-Zn-Al alloys using the thermomechanical cycling [3, 4], there are few reports of the induction of the TWSME without employing the repetitive cycling method. In this study, we have used a simple constrained heating technique to realize the TWSME and have investigated the effect of heating time and temperature. An alloy of 65.6Cu-33.1Zn-1.3Al (wt%) was prepared by an induction furnace and subsequently forged and hotrolled. Finally, rectangular specimens with dimensions of 170 mm × 12 mm × 2.2 mm were machined from an ingot. The specimens were then betatized at 800 ◦C for 30 min, and quenched into water at 25 ◦C. The transformation temperatures were calculated by measuring the electrical resistance of the sample, indicating that Ms = −70 ◦C, Mf = −100 ◦C, As = −48 ◦C, and Af = −20 ◦C (Ms = start of the martensitic transformation, Mf = end of the martensitic transformation, As = start of the austenitic transformation, and Af = end of the austenitic transformation). The specimens were subjected to the constrained heating to obtain the TWSME. The specimens were bent around a cylinder mold of 50 mm diameter in liquid nitrogen, and subsequently thermally heated in the temperature range of 40–220 ◦C, in the constrained state. The amount of TWSME was assessed by cycling the samples in the unconstrained state between the