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Cu-Zn-Al Shape Memory Alloy with Enhanced Hardness and Oxidation resistance
Chemical and Materials Engineering
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In this research, a novel polycrystalline Cu-21Zn-6Al shape memory alloy was fabricated and examined. This alloy has unique binary stable phases under recovering stage and two different types of martensite that can be observed during the deforming stage. During recovering stage, with proper condition control, the two phases crystallized and grow into lamellar structure. This structure can significantly enhance the microhardness and, therefore, improve the mechanical properties accordingly. Both microstructure and Vicker’s microhardness test verified the effect of this binary phased alloy. Under recovering stage, two different types of martensite were identified and examined. One of them was inherited from β phase which has a zigzag shaped martensite, and the other one was inherited from γ phase which has a plate martensite. Three different Cu alloys (Cu-21Zn-6Al, Cu-30Zn-6Al, and Cu-6Al) were fabricated to compare the change of microhardness and microstructure. Compared with Cu-30Zn-6Al, an increase of more than 30 % of microhardness was observed in Cu-21Zn-6Al. Compared with Cu-6Al, a similar microhardness was achieved with the presence of Zn. The impact of homogenization to microhardness was investigated. Different homogenization times were used to test the change of microhardness. Combining with microstructure, the change of microhardness was analyzed and discussed. Different types of martensite were identified in different compositions. The formation of different martensite was discussed and the modeling of the martensite evolution was presented. Shape recovery test was performed to characterize the shape memory effect. Under different conditions, different shape recovery results can be achieved. To improve oxidation resistance at high temperature (900 ℃) for betalization, aluminization was performed. Scanning electron microscopy and X-ray diffraction (XRD) was used to determine the aluminization condition. And an aluminization model was established from the diffusion equation.