| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 95 === The shape memory effect and phase transformations of Cu-11.38Al-0.43Be (wt%) alloy were studied in this work. The alloy shows an extraordinary two-stage shape memory effect (TSSME) after aging at 505℃ for 20 min, which was first observed in Cu-based shape memory alloys. This aging treatment causes the precipitation of the α phase and results in the redistribution of Al atoms around the α precipitate, which creates two areas with different Al contents in the matrix. The first stage of transformation occurs in the area of higher Al content at low temperature, while the areas with lower Al content control the second stage at high temperature, resulting in the formation of TSSME. During this aging treatment, a large number of fine martensites are also formed in the matrix. The formation of these fine martensites allow a specimen with TSSME to retain an excellent SME, in spite of the α precipitates exist in the matrix. The refine treatment of martensite was also investigated in this work. The results show that the aging treatment at a temperature near the DO3=B2 transition temperature caused numbers of renucleated phase with oval-shaped in the matrix. Following the EDX analysis, a concentration gradient is formed because the Al content of renucleation phase is higher than in the surrounding matrix. Therefore, the occurrence immediately provides many nucleation sites for martensite. When a large number of martensites grow simultaneously, mutual collision and obstruction occur, creating clumps of fine martensitic structures and improving the recovery ability of the alloy.
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