Investigation of a Cu-Zn-Al alloy with two-way shape memory effect by the cycled constrained heating/cooling technique

Abstract

The two-way shape memory effect (TWSME) is a phenomenon whereby a material can retain both high temperature and low temperature shapes, while one-way shape memory alloy can only retain the high temperature (austenite) shape. Therefore, many studies have focused on obtaining TWSME by using a variety of alloys, such as Ni-Ti [1–4], Cu-Al-Ni [5], Ni-Al-Fe [6], Ni-Cu-Ti-Hf [7], and Cu-Zn-Al [8–10]. Cu-based shape memory alloys have received more attention in the past few years owing to their low price, easy fabrication, and excellent conductivity of heat and electricity, and in our previous work [11], we have investigated the induction of TWSME by bending CuZn-Al alloy samples around cylindrical structures and by employing a constrained heating/cooling technique. In the present work, in order to enhance the strength of TWSME, we have used a thermomechanical cycling method, consisting of the constrained heating/cooling technique. The alloy investigated had a composition of Cu-24.1Zn-5.6Al (wt%). The samples were prepared in an induction furnace and subsequently forged and hot-rolled. Finally, rectangular specimens with dimensions of 170 mm × 12 mm × 2.2 mm were machined from the ingot. They were heated to 800 ◦C, held there for 30 min, then quenched in water to room temperature. The characteristic transformation temperatures are: Ms = 20 ◦C, Mf = 0 ◦C, As = 30 ◦C, and Af = 61 ◦C (Ms = start temperature of martensitic transformation, Mf = finish temperature of martensitic transformation, As = start temperature of austenitic transformation, and Af = finish temperature of austenitic transformation). The strip-shaped specimens were then bent around a cylindrical mold of 50 mm diameter placed in liquid nitrogen, and subsequently thermally heated in the temperature range of 100–240 ◦C, in the constrained state. The specimens were then rapidly cooled in liquid nitrogen in the constrained state, and following this, the constraint was removed. This routine was repeated up to 4 times. The strength of TWSME was evaluated by cycling the samples in the unconstrained state between temperatures below Mf and above Af and a schematic diagram of the manner of measurements is shown in Fig. 1. The TWSME was evaluated using the following equation: strength of TWSME = 2(CD−AB)/EF. The microstructures were observed using a transmission electron microscope (TEM) (Jeol 200 CX), whose specimens were jet-polished with a nitric acid and methanol solution. In order to observe the induction of TWSME in Cu24.1Zn-5.6Al alloy, we have varied the number of thermomechanical cycles and the constrained heating temperature respectively, in the range of 1–4 cycles and 100–240 ◦C. Fig. 2a indicates that the TWSME increases significantly on increasing the constrained heating temperature from 130 to 160 ◦C regardless of the number of cycles, revealing that the constrained heating temperature plays a significant role in improving the TWSME. It is noteworthy that the TWSME of the 240 ◦C-constrained heated sample is below 0.2, regardless of the number of cycles. Therefore, we find that there is an optimal range of constrained heating temperature for the induction of TWSME. Fig. 2a also indicates that the TWSME increases with the increasing number of cycles in the range of 1–4, when the constrained heating temperature ranges from 100–180 ◦C. Fig. 2b shows the variation of the strength of TWSME in Cu-24.1Zn-5.6Al alloy with variation of the constrained heating time, at a constrained heating temperature of 160 ◦C. The strengths of TWSME with 1-cycle treatment are about 0.18 and 0.55, respectively, with constrained heating times of 60 and 120 s. The TWSME of the 120 s-treated samples is higher than that of the 60 s-treated samples, regardless of the number of cycles. We have thus revealed that the constrained heating time also affects the induction of TWSME. We have compared the microstructures of Cu24.1Zn-5.6Al alloys with and without thermomechanical cycling treatment, in which the sample with the treatment has a TWSME strength higher than 0.7.