Effect of Mo on the Microstructure and Superelasticity of Ti-Ni-Cu Shape Memory Alloys
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Effect of Mo on the Microstructure and Superelasticity of Ti-Ni-Cu Shape Memory Alloys Guangwei Zhao, Jian Chen, Dong Fang, Yongsheng Ye, Caihua Huang, and Xicong Ye Submitted: 7 August 2020 / Revised: 27 October 2020 / Accepted: 8 November 2020 In this paper, the microstructure and superelasticity of Ti(502x)Ni44Cu6Mox (x = 0-2.5) alloys were studied. The main phase in Ti(502x)Ni44Cu6Mox alloys at room temperature is B2 austenite. The martensite transformation finish temperatures of the 0Mo and 0.2Mo samples are approximately 2 34.2 and 2 45.4 °C, respectively, while the transformation temperature cannot be detected above 2 50 °C for the samples with Mo contents greater than 0.2%. The content of Mo has little effect on the compressive strength of the Ti(502x)Ni44Cu6Mox alloy, but the fracture strain decreases with increasing Mo content. The 0.6Mo sample shows elastic deformation characteristics and has the lowest fracture strain due to the precipitation of Ni-rich compounds. Cyclic compression tests with an increased and constant prestrain were adopted to study the superelasticity properties of the alloys. The residual stress can increase approximately 5-6% after 5 cycles when the prestrain increases from 2 to 10%, while the residual strain drops approximately 2-3% after 20 cycles when the prestrain is constant at 6%. In both experiments, the results show that the recoverable strain can be generally improved by substituting Mo for Ti in Ti(502x)Ni44Cu6Mox alloys. Keywords
cyclic compression, Mo element, shape memory alloys, superelasticity
1. Introduction The Ti-Ni-Cu alloy system has attracted the attention of researchers because of its good memory and mechanical properties (Ref 1-4). Moreover, various fourth components are added to this alloy to further improve its properties. Among these fourth elements that have been added, it was found that adding elements Zr (Ref 5), Pd (Ref 6, 7), Hf (Ref 8), and Pt (Ref 9) into Ti-Ni-Cu alloys played a positive role in improving the transition temperature, thermal hysteresis and memory performance. Adding Nb can improve the thermal stability, ductility and recovery strain of Ti-Ni-Cu alloys (Ref 10-12). Y is also an element that is often added into Ti-Ni-Cu alloys, and an appropriate content of Y can improve the superelasticity, damping properties and transition temperature of this alloy (Ref 13, 14). In addition, there are investigations on the transformation behaviors after adding V (Ref 15), Cr (Ref 16), Mn (Ref 17), Fe (Ref 18) and Al (Ref 19) in Ti-Ni-Cu alloys. Mo is also an important alloying element for Ti-Ni (Ref 20, 21) and Ti-Ni-Cu shape memory alloys (Ref 22-24). Nam et al. found that adding from 0.3 to 1.0% Mo into Ti50Ni(45 X)Cu5.0 and Ti50Ni(47 X)Cu3.0 alloys can induce the transformation of B2-B19¢ and improve the superelastic recovery from 33 to 91% (Ref 23, 24). The transformation behaviors of Ti50Ni(47 X)Cu3MoX alloys depend largely on the Mo content and the transformation temperature decreases rapidly with increasGu
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