Abstract:

In this research, fabrication of a (Ti,Hf)-rich NiTiHf alloy by using vacuum induction melting (VIM) process and a graphitic crucible was investigated. For this purpose, casts with the nominal composition of Ni₄₉Ti₃₆Hf₁₅ were prepared in graphitic crucible and mold. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and differential scanning calorimetry (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 the DSC curve of this sample, indicating a drastic decline in the transformation temperatures. In the succeeding cast, 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 decreased remarkably. This cast exhibited transformation temperatures above 100 °C, 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 the fabrication of (Ti,Hf)-rich NiTiHf high temperature shape memory alloys.

Key words: High temperature shape memory alloys, NiTi, Martensite, Casting, Transformation temperatures, Microstructure