Martensite Transformation of (HfxTi50-x)Ni50Shape Memory Alloys
Department of Mechanical Engineering, Florida International University, Miami, FL 33199, U.S.A.
Despite the growing body of research emerging from the exciting field of High Temperature Shape Memory Alloys, little is as yet known about the characteristics of martensite transformation behavior in NiTi-Hf alloys. In this paper, transformation behavior including temperature, enthalpy and entropy changes, hysteresis width, Young's modulus, internal friction, and even martensite structure, are investigated. Five primary conclusions are drawn from the results of the study: 1) The martensite of the NiTi-Hf alloys is thermoelastic martensite. 2) The transformation temperatures and thermodynamic parameters depend on the Hf content. 3) The crystal structure of the high temperature parent phase is of the CsCl-type regardless of the alloy composition. The primary product of the martensite transformation at room temperature for alloys with x <6 is monoclinic B19' martensite, while for 20< x <30at%, the crystal structure is orthorhombic B19. 4) The stability of martensite structure of NiTi-X systems can be successfully predicted using Pettifer's phenomenological method. The martensite structure changes from monoclinic to orthorhombic when the MNi> 10.206 + 1.123 MTi.
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