Enhancement of superelasticity in Cu-Al-Mn-Ni shape-memory alloys by texture control
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Cu-BASED shape-memory alloys (SMAs), such as CuZn-Al and Cu-Al-Ni, are commercially attractive systems for the practical exploitation of the shape-memory effect (SME) and superelasticity (SE) and stand next in line to NiTi as suitable alloys for SM applications, because of their low cost and their advantages with regard to electrical and thermal conductivities.[1,2] However, Cu-based SMAs with a polycrystalline structure are too brittle to be sufficiently cold worked and possess a low degree of shape recovery in the SME and SE.[1,2] The brittleness of the  -polycrystalline Cu-based alloys arises from the high degree of order in the parent  phase with a B2, DO3, or L21 structure and the unusually high elastic anisotropy ratios of A ⫽ 2c44/(c11 ⫺ c12) ⫽ 13 and A ⫽ 15 for Cu-Al-Ni and Cu-Zn-Al SM alloys in the parent  phase, respectively,[1] where c44, c11, and c12 are the elastic stiffness. The coarse grain structure associated with the  phase in these alloys exacerbates this proneness to brittleness even further. Many attempts to improve the ductility by grain refining have been made, but only limited success has been reported.[1,3] Figure 1 shows the vertical section of the phase diagram of the Cu-Al-10 at. pct Mn system,[4] where the single-phase  region is seen to be broadened by the addition of Mn. It can also be seen that the transition temperatures associated with two types of order-disorder transitions,  (A2) → 2 (B2) and 2 → 1 (L21), decrease with decreasing Al content. Recently, Kainuma et al. have found that the Cu-Al-Mn– based alloys with an Al content below 18 at. pct (low degree of order) show excellent cold workability, good ductility, and also exhibit the SME and SE based on cubic 1 (L21) to monoclinic 1⬘ (18R) martensitic transformation.[5,6] However, the SE strain is still limited to the region below 2 pct in these alloys and is not yet sufficient for practical applications in many fields. Very recently, the present authors Y. SUTOU, JSPS Researcher, T. OMORI, Graduate Student, R. KAINUMA, Associate Professor, and K. ISHIDA, Professor, are with the Department of Materials and Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Contact e-mail: kainuma@ material.tohoku.ac.jp N. ONO, Professor, is with the Department of Mechanical Systems on Information Technology, Hachinohe Institute of Technology, Hachinohe 031-8501, Japan. Manuscript submitted October 5, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
have investigated the effect of alloying elements such as Ni, Co, etc. on the SME and SE of these alloys[7] and found that the Ni addition considerably enhances the ductility of the specimens after annealing at temperatures between 773 and 1073 K,[8] although the SME and SE properties are not significantly improved. Commercial Ni-Ti–based SMAs are prepared by conventional casting techniques followed by thermomechanical processing (TMP), which is a combination of cold working or hot rolling and low-temperature annealing at 673 to 773 K that yields a favo
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