Microstructure and Shape Memory Characteristics of Powder-Metallurgical-Processed Ti-Ni-Cu Alloys
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TI-NI alloys are attractive functional materials not only as shape memory alloys with high strength and ductility but also as those exhibiting unique physical properties such as pretransformation behaviors, which are enriched by various martensitic transformations. Ti-Ni-Cu alloys have been attracting more attention by their high performance of shape memory effect and decrease in thermal and stress hysteresis compared with Ti-Ni binary alloys.[1,2] Their significant properties are attributed to a thermoelastic martensitic phase transformation from a high-temperature phase (B2 structure) to a low-temperature phase called martensite (B19 orthorhombic).[3] Ti-Ni-Cu alloys with a Cu content of more than 10 at. pct are known to show the B2 (cubic)B19 (orthorhombic) martensitic transformation.[4] The YEON-WOOK KIM, Professor, is with the Department of Advanced Materials Engineering, Keimyung University, Dalseo-gu, Daegu 704-701, Republic of Korea. Contact e-mail: [email protected] YOUNG-SOO CHUNG, Professor, is with the Department of Civil Engineering, Chungang University, Seoul 156-756, Republic of Korea. EUNSOO CHOI, Professor, is with the Department of Civil Engineering, Hongik University, Seoul 121-791, Republic of Korea. TAE-HYUN NAM, Professor, is with the Department of Materials Engineering, Gyeongsang National University, Jinju 600-701, Republic of Korea. Manuscript submitted March 30, 2011. Article published online November 9, 2011 2932—VOLUME 43A, AUGUST 2012
shape memory characteristics associated with the B2-B19 transformation lie in-between those of B2-R (triclinic) and B2-B19’ (monoclinic) transformation,[2] and therefore, the Ti-Ni-Cu alloys are considered to be attractive as actuator materials. It is well known that Ti-Ni-based alloys are ductile; 60 pct cold working is possible under certain conditions, although they are a kind of intermetallic compounds. However, Ti-Ni-Cu alloys with Cu-content more than 12. 5 at. pct are so brittle that they do not deform plastically. R} osner et al.[5] reported that the formation of plate-like precipitates could lead to embrittlement. Therefore, the poor workability restraints the practical applications of high Cu-content Ti-Ni-Cu shape memory alloys. Powder metallurgy is a promising method for the production of near-net-shape components of the brittle Ti-Ni-Cu alloys because expensive machining, thermomechanical treatments, and associated materials losses can be avoided or minimized.[6] Recently, pure elemental Ti and Ni powders have been often used as raw materials for the preparation of bulk TiNi alloys through self-propagating high-temperature synthesis.[7] Generally, the alloy is formed in the solid state via interdiffusion reactions between Ti and Ni powders. The TiNi alloys synthesized by hot isostatic pressing and the conventional sintering method exhibited martensitic transformations but often with reduced latent heat, suggesting the incomplete formation of the TiNi B2 METALLURGICAL AND MATERIALS TRANSACTIONS A
phase. In fact, all TiNi-based alloys synthesize
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