Among the various shape-memory alloys (SMAs), NiTi alloys are the most preferred ones due to stability and feasibility of shape memory effect (SME). However, they are limited to being used below 100 °C because of their relatively low transformation temperatures (TTs) such as martensite finish temperature (Mf), austenite finish temperature (Af). In order to overcome this limitation, numerous studies were conducted on increasing the TTs by adding highly soluble alloying elements, such as Hf, Zr, Pd, Co, Cu and Pt [1, 2].Recently, high-entropy alloys (HEAs) have attracted extensive attention as a new class of metallic materials due to their unique microstructure and mechanical properties. 

In this study, we developed a new SMA using the HEA concept, i.e. to develop a equiatomic quaternary alloy with a single-phase b2 microstructure. As a result of this strategy, an equiatomic NiTiCoHf HE-SMA was designed. The NiTiCoHf HE-SMA showed formation of single B2 phase formation at both as-cast and annealed state. The B2 crystal structure of NiTiCoHf alloy has (NiCo)(TiHf) system because Co and Hf are highly soluble into NiTi B2 structure (Fig. 1). We further investigated on the shape memory behaviour and transformation temperatures of NiTiCoHf HE-SMA using X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. Structural difference between conventional NiTi SMA and the HE-SMA was also investigated by electron backscatter diffraction and transmission electron microscopy.