Fabrication of a (Ti,Hf)-Rich NiTiHf Alloy Using Graphitic Mold and Crucible

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.

کلیدواژه ها:

Carbide ، High temperature shape memory alloy ، Martensite ، Microstructure ، NiTi ، Transformation temperatures