Martensitic Transformation and Mechanical Properties of Fe-added Au-Cu-Al Shape Memory Alloy with Various Heat Treatment
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Martensitic Transformation and Mechanical Properties of Fe-added Au-Cu-Al Shape Memory Alloy with Various Heat Treatment Conditions Akira Umise1*, Masaki Tahara1, Kenji Goto2, Tomonari Inamura1 and Hideki Hosoda1 1
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Japan Tanaka Kikinzoku Kogyo K.K., Hiratsuka, Japan. *Graduate Student, Tokyo Institute of Technology 2
ABSTRACT In order to improve shape memory properties of Au-Cu-Al based shape memory alloys, the possibility to utilize thermo-mechanical treatment was investigated in this study, and effects of heat-treatment temperature on microstructure, martensitic transformation and mechanical properties of cold-rolled Au-30Cu-18Al-2Fe (AuCuAlFe) alloy were clarified by X-ray diffraction analysis (XRD, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and tensile tests at room temperature (RT). Here, Fe addition to AuCuAl improves ductility. Cold rolling with the thickness reduction of 30% was successfully carried out in AuCuAlFe at RT. An exothermic heat was observed in DSC at temperature from 402K, suggesting that recovery started at 402K. Besides, the transformation temperature hysteresis increased by the cold-rolling. The alloy was completely recrystallized after the heat treatment at 573K for 3.6ks. Tensile tests revealed that the yield stress was raised by cold rolling and largely by the subsequent heat treatment at 433K, which corresponded to the recovery start temperature by DSC. The yield stress decreased with increasing heat treatment temperature over 453K, probably due to recrystallization. AuCuAlFe cold-rolled and subsequent heat-treated at 573K exhibited the lowest yield stress as well as stress-plateau region, indicating that the thermomechanical treatment is effective to improve shape memory properties of Au-Cu-Al based alloys. INTRODUCTION Au-Cu-Al shape memory alloys, in which a typical chemical composition is Au7Cu5Al4, are known as “Spangold” which has been strongly developed by, for example, Cortie and coworkers from early 1990s; Spangold is refers to a family of -type gold alloys formulated to undergo a phase transformation [1]. This class of gold-based alloys has been developed in use of jewelry. In the Au-Cu-Al alloys, several researches have been done for phase transformation, phase equilibria and alloy color, especially. The martensitic transformation behavior as well as the phase transformation behavior of the Au-Cu-Al alloys at elevated temperatures has been reported [2-7]. The martensitic transformation is from L21 parent phase (Heusler) at high temperature into the martensite M phase (doubled B19) at low temperature. The Au-Cu-Al phase diagrams of 76wt.%Au section [8] and isotherms at 500°C [9] and 750°C [10] have been also reported. The alloy color is described in Refs [1, 11, 12]. On the other hand, reports related to shape memory properties, mechanical properties, aging, effects of additional elements and applications are limited at present, at least the authors’ best knowled
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