Fabrication of NiTiHf High Temperature Shape Memory Alloy by Alloying and Casting in Vacuum Induction Furnace

By adding ternary or quaternary elements to NiTi alloys, transformation temperatures can be increased or decreased in order to qualify the alloy for a broader range of applications. Due to their high transformation temperatures, good shape memory behavior and lower price, NiTiHf high temperature alloys proved to be promising for high temperature technical applications. This research deals with fabrication and characterization of the Ni49Ti36Hf15 alloy. The ingots were fabricated by alloying and casting of pure materials in a vacuum induction furnace under two different vacuum conditions of 10-1 and 10-4 mbar. After homogenization at 1000°C, ingots were hot rolled at 850°C in order to ameliorate the microstructure. Microstructural investigations were carried out using optical and scanning electron microscopy, and compositional and phase examinations were performed by using XRD and EDS analysis. Results demonstrate that the microstructures of the alloys are composed of B19´ martensite, but different second phases. In the sample prepared under the vacuum of 10-1 mbar, second phase was Titanium-Hafnium Oxide Second phases, especially oxide kinds, not only deteriorate ductility but also lessen transformationtemperatures. Rolling process at elevated temperature results in the annihilation of voids and breaking as well as better distribution of the second phase in the alloy prepared at higher vacuum condition.