Pii: S1359-6462(99)00347-4

The TiNiHf alloys are newly developed as high temperature shape memory alloys with the high transformation temperatures and with lower cost in comparison with TiNiX (X 5 Pd, Pt) alloys. Recently a lot of investigations were focused on the TiNiHf alloys, in which Hf contents are ranged from 1% to 30% [1–8]. Zhu et al. [9] proposed that the decrease of the transformation temperatures of the Ti51.5-xNi48.5Hfx (x 5 15, 30) alloys in the early period of high temperature duration at 723K was due to the precipitated second phase though no further experimental results were provided. In the Ti36.5Ni48.5Hf15 alloy aged at 873K for 150h, a new precipitate phase of the composition of (Ti0.4Hf0.6)Ni of a spindle-like shape with a habit plane of (100)P//(001)M and a long axis of [001]P//[1#10]M was observed by Han et al. [10]. It is well known that, in the Ni-rich binary TiNi SMAs, the precipitation sequences are as follows [11]: