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۱Fabrication of a (Ti,Hf)–Rich NiTiHf Alloy Using Graphitic Mold and Crucible
اطلاعات انتشار: دوازدهمین کنفرانس ملی مهندسی ساخت و تولید ایران، سال
تعداد صفحات: ۶
In this research, fabrication of a (Ti,Hf)–rich NiTiHf by using VIM process and a graphitic crucible was investigated. For this purpose, several casts with nominal composition of Ni49Ti36Hf15 were prepared in graphitic crucible and mold. OM, SEM, EDS, XRD, and DSC tests were employed to characterize the samples. Results demonstrated that microstructure of the first cast was composted of a B2 austenite phase as well as a great amount of two differently formed (Ti,Hf)C carbides. Moreover, no austenite ↔ martensite transformation peak was detected in DSC curve of this sample, indicating the drastic decline of the transformation temperatures. In the succeeding casts, however, owing to the formation of carbide layers on the inner surfaces of the graphitic crucible and mold during the initial casting process, the amounts of carbides remarkably decreased. These casts exhibited transformation temperatures above 100 ºC in DSC curves, while XRD pattern denoted the presence of B19´ monoclinic martensite phase at room temperature. All in all, results confirmed that VIM process using graphitic mold and crucible can be considered as an appropriate method for fabrication of (Ti,Hf)–rich NiTiHf high temperature shape memory alloys.<\div>

۲Heat Treatment of a (Ti,Hf)–Rich NiTiHf High Temperature Shape Memory Alloy
اطلاعات انتشار: دوازدهمین کنفرانس ملی مهندسی ساخت و تولید ایران، سال
تعداد صفحات: ۵
NiTiHf alloys are known as one of the most promising groups of shape memory alloys for operating at temperatures above 100 ºC. In this paper, effect of intermediate aging on the microstructure, transformation temperatures, and one–way shape memory behavior of a Ni49Ti36Hf15 alloy was investigated. For this purpose, an ingot was fabricated using vacuum induction melting of high purity Ni, Ti, and Hf precursors. The alloy was then homogenized at 1000 ºC, hot–rolled at 850 ºC, and solution treated at 1000 ºC, respectively, to be prepared for aging treatment. Aging treatment was carried out at 500 ºC and 600 ºC for 2h. OM, SEM, XRD, DSC, and the so–called bending test were used to characterize the alloy. Results demonstrated that microstructure of the aged Ni49Ti36Hf15 is composed of B19’ martensitic matrix, (Ti,Hf)2Ni second phase mostly at grain boundary regions, and a new phase, while an approximately 25 ºC decrease in transformation temperatures was observed after aging. It was also found that this heat treatment cannot provide better shape memory properties for the alloy. That aging provided no improvement was correlated to the microstructure of the alloy.<\div>
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