Training two-way shape memory alloy by reheat treatment

Abstract

Unlike one-way shape memory alloy (OWSMA), which can only remember the high temperature (austenite) shape, two-way shape memory alloy (TWSMA) remembers both high temperature and low temperature shapes. It is believed that training process plays a key role in the introduction of two-way shape memory from an initially one-way shape memory alloy. Various training methods have been proposed [1–6]. In general, either thermal/mechanical cycling or severe (plastic) deformation is essential. From fabrication and stability points of view, they are either non-convenient or have a lack of reliability. In this paper, we take a look at the feasibility of a new training method: by reheat treatment. In most cases, shape memory alloys are supplied as raw materials, i.e. without shape memory. To memorize a certain shape, heat treatment is normally required. This is to say, fixing the shape memory alloy into a required shape and then heating it up for a certain period of time at a high temperature. After this, one-way shape memory alloy is formed. Next, same procedure is repeated again but with different clamped shape of SMA and different heat treatment time. This is training TWSMA by reheat treatment. A 0.5 mm diameter NiTi shape memory alloy wire with one-way shape memory at room temperature was used for experiments followed. First, it was immersed into boiling water to measure its initially remembered high temperature, circular shape. An average diameter of 47.7 mm was noted. The NiTi wire was then cut into several pieces of required length. Wire-holders were fabricated to constrain the NiTi wires at the required diameters of 8 mm and 12 mm. The specimens undergoing different treatment were indicated as: a1 (8 mm), b1 (12 mm), a2 (8 mm), b2 (12 mm), a3 (8 mm) and b3 (12 mm). Reheat treatment was then conducted on the secured specimens at a high temperature of 500 ◦C with varying reheat treatment time of 10, 30, 60 min for a1 and b1, a2 and b2, a3 and b3, respectively. The specimens were rapidly quenched in cold water upon retrieval. To demonstrate the two-way shape memory introduced into the reheat treated specimens, thermal cycling was performed. The cold condition for thermal cycling was set at ice water temperature (Tc:0 ◦C) while hot condition was set at boiling water temperature (Th:100 ◦C). A full thermal cycle will see the specimens subjected to the hot condition first before transferring into the cold condition. 100 thermal cycles were carried out on each specimen. Measurements were done at 1st, 10th, 25th, 50th, 75th and 100th cycles of both hot and cold shapes. The evolution of diameter was then plotted (see Fig. 1, where symbol “o” stands for hot shape and “∗” for cold shape). Figs 1(a1) and (b1) show that at the first thermal cycle, specimens (a1) and (b1) expanded significantly to a larger diameter (a1 : 13.5 mm, b1 : 24.0 mm) in the hot condition from their initially preset shape. Further expansion was observed when exposed to the cold condition. Subsequent thermal cycling showed the specimens remembered a hot shape of a slightly smaller diameter at hot condition while expanding to a cold shape of a larger diameter at the cold condition, i.e. the trace of two-way shape memory. As the number of thermal cycles increases, the change in diameter between both shapes decreases. In Figs 1(a2) and (b2), the first thermal cycle shows that both specimens (a2) and (b2) having their diameter remained relatively close to their reheat treatment shape when subjected to hot condition. This shows that the specimens remember the new trained shape as their hot shape. A significant increase in diameter took place when both specimens were immersed into the cold condition. Thereafter, throughout the 100 thermal cycles, both specimens alternate between the hot and cold shapes, showing the effectiveness of training TWSMA by reheat treatment. Figs 1(a3) and (b3) show minute shape change for specimen (a3) and (b3) which were reheat treated for 1 h at 500 ◦C. Figs 2 and 3 show the importance of reheat treatment time in obtaining the optimum two-way shape memory.