Texture Development in the Ni 47 Ti 44 Nb 9 Shape Memory Alloy During Successive Thermomechanical Processing and Its Eff
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ONE of the important thermomechanical responses of shape memory alloys (SMAs) is the shape memory effect (SME), which is strongly dependent on crystal orientation.[1,2] Often, in practical applications, SMAs were generally prepared by vacuum melting, followed by hot working with some cold working and finally suitable heat treatment. Thus, as-processed polycrystalline SMAs have specific crystallographic textures.[3–6] Among many SMAs developed so far, the nearequiatomic TiNi-based alloys have attracted much attention due to their good functional and mechanical characteristics.[7–9] Most characteristic texture types in the austenitic B2 phase of TiNi alloys were determined to be the a-fiber I,[10,11] a-fiber II,[11–16] and c-fiber,[11,12,16,17] and the relationship between the recoverable Y. YAN, Associate Professor, is with the Institute of Materials Physics and Chemistry, College of Sciences, Northeastern University, Shenyang 110004, People’s Republic of China. Contact e-mail: [email protected] W. JIN, Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. X.W. LI, Professor, is with the Institute of Materials Physics and Chemistry, College of Sciences, and also with the Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110004, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted December 31, 2011. Article published online October 17, 2012 978—VOLUME 44A, FEBRUARY 2013
performance and these textures was studied.[11–13,18–20] The a-fiber exhibits remarkable anisotropy of the recoverable strain in the rolled sheet,[11–13] while the c-fiber is associated with the least anisotropy.[11,12] Currently, studies on textures of SMAs are mainly focused on the final texture type in products obtained by a series of processing,[4–6,11,13–17] whereas texture evolutions and corresponding intensity during successive processing are rarely investigated.[3,12] Furthermore, on exploring the effect of textures on the recoverable strains and mechanical properties, most studies have focused on the variations of recoverable strains and mechanical properties with the angle of the sampling directions to the rolling direction (RD) of the sheet,[11–13] while the effects of other factors, such as grain size, texture weakening induced by grain growth after recrystallization, and b-Nb particle precipitation in the NiTiNb SMA, are seldom considered. The phase transformation temperature hysteresis (As-Ms) in ternary NiTiNb SMAs is widened due to the existence of soft b-Nb particles,[21] which allows the expanded joining, fastening, and sealing device to be stored at ambient temperature in the martensitic state. Therefore, such SMAs have great potential applications as mechanical components in the space technology field. At present, the Ni47Ti44Nb9 alloy was considered as the standard wide hysteresis SMAs for commercial applications. Investigations on textures in the Ni47Ti44Nb9 METALLURGICAL
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