Reduction and removal of martensite stabilization in Cu-Zn-Al-Mn-Zr shape memory alloys
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Reduction and Removal of Martensite Stabilization in Cu-Zn-Al-Mn-Zr Shape Memory Alloys C.W.H. LAM, C.Y. CHUNG, W.H. ZOU, and J.K.L. LAI Cu-Zn-Al alloys are currently the most popular Cu-based shape memory alloys (SMAs) available in the market. However, their thermal stability problem and poor mechanical strength are a hindrance to their application. Severe martensite stabilization usually occurs when the alloy is aged in the martensite state,[1] which may take place even at room temperature. Aging in the parent phase may also cause changes in the degree of order and the formation of precipitates.[2] Because of the large anisotropy and large grain size of the alloy, intergranular cracking occurs during processing and servicing.[3] Therefore, the properties of CuZn-Al need to be improved. It has been reported that minor additions of alloying elements such as B, V, Zr, and Ti can refine the grain size.[4] Zirconium can effectively suppress grain growth during annealing at 800 7C for the Cu-Zn-Al C.W.H. LAM, Lecturer, C.Y. CHUNG, Assistant Professor, and J.K.L. LAI, Chair Professor in Materials Science, are with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Hong Kong. W.H. ZOU, formerly Research Fellow with the Department of Physics and Materials Science, City University of Hong Kong, is Research Fellow, Department of Physics, Hong Kong University of Science and Technology, Clearwater Bay, Kowloon, Hong Kong. Manuscript submitted May 14, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloy. Moreover, the addition of Mn in Cu-Al-Ni SMAs can enhance the thermoelastic and pseudoelastic behaviors.[5] Manganese and Zr (1 and 5 pct, respectively) were added to the Cu-Zn-Al alloy in the present study. In our previous work, successful grain refinement was achieved.[6] Zirconium-rich precipitates, which are responsible for the grain refinement, were observed.[7] The mechanical strength was also improved. Furthermore, when the alloy was subjected to parent phase aging, the reordering phenomenon was not serious when the aging temperature was below 200 7C. Similar aging at 200 7C will significantly decrease the monoclinicity and lower the shape memory capacity of the CuZn-Al alloy.[8] The thermal stability of the parent phase of the Cu-Zn-Al-Mn-Zr alloy was found to be better than that of the Cu-Zn-Al alloy. In the present investigation, we studied the thermal stability of the Cu-Zn-Al-Mn-Zr alloy in greater detail. This is important because martensite phase aging may cause martensite stabilization and destroy the thermoelasticity of martensitic transformation. Remedial treatment has been developed to revitalize the stabilize
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