M Moshref-Javadi - School of Materials & Metallurgical Engineering, Iran University of Science & Technology, Tehran, Iran
M Belbasi - School of Materials & Metallurgical Engineering, Iran University of Science & Technology, Tehran, Iran
S. H. Seyedein - School of Materials & Metallurgical Engineering, Iran University of Science & Technology, Tehran, Iran
M. T. Salehi - School of Materials & Metallurgical Engineering, Iran University of Science & Technology, Tehran, Iran

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.

Aging; Heat treatment; High temperature shape memory alloy; Martensite; Microstructure; NiTi; Shape memory effect; Transformation temperatures