Abstract
One of the major challenges of near-equiatomic NiTi shape memory alloys is their limitation for high-temperature applications. To overcome this barrier, researchers have tried to enhance the transformation temperatures by addition of alloying elements or even by introducing the concept of high-entropy alloys (HEAs). In this study, the CuNiHfTiZr HEAs were developed for high-temperature shape memory effect. Based on their solubility and electron configurations, the alloying elements are divided into two groups, (CuNi)50 and (HfTiZr)50. The content of Cu in (CuNi)50 is modulated to investigate the influences of Cu on martensitic transformation of the HEAs by studying structural evolution and transformation behavior. The results of x-ray diffraction and thermal expansion tests revealed that Cu15Ni35Hf16.67Ti16.67Zr16.67 possesses high transformation temperature, narrow hysteresis temperature loops, and good dimensional stability within this HEA system.
Highlights
One of the major challenges of near-equiatomic NiTi shape memory alloys is their limitation for high-temperature applications
The Cu0, Cu5, Cu15, and Cu25 alloys were developed from the CuNiHfTiZr high-entropy alloy system
The as-cast CuNiHfTiZr high-entropy alloys consist of dendrites and interdendrites
Summary
One of the major challenges of near-equiatomic NiTi shape memory alloys is their limitation for high-temperature applications. Based on the theoretical modeling and experimental results mentioned above, it can be concluded that the martensitic transformation of either binary intermetallics or TiZrHfCoNiCu high-entropy SMA is mainly dominated by the competing A-B and B-B interactions, which decide the total energy and the stability of the martensite or austenite during changing of temperature[13,14] All these findings provide important insights into the design and development of high-temperature SMAs. Apart from the (CoCuNi)(HfTiZr) system, in a very recent work, Ni-Ti based high-entropy SMAs (Ni,Pd)50(Ti,Hf,Zr)[50] with austenite finish temperature beyond 700 °C were designed with the shape memory behavior tested by Canadinc et al.[15] They speculated that the enhanced configurational entropy of the alloys by multi-element alloy design is responsible for the significantly increased TTs and recovered strain at elevated temperatures. Despite the promising shape memory properties of the (Ni,Pd)50(Ti,Hf,Zr)[50] alloys, the high cost arising from large proportion of Pd addition is another critical issue to deal with
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