A new precipitate phase in a TiNiHf high temperature shape memory alloy

A precipitate phase in a Ti36.5Ni48.5Hf15 high temperature shape memory alloy aged at 873K for 150 h has been studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The precipitate phase of the approximate composition of (Ti0.6Hf0.4)Ni is identified to be face-centered orthorhombic lattice with lattice parameters A = 1.287nm, B = 0.874nm and C = 2.622 nm and space group F2/d 2/d 2/d.. The lattice correspondence between the precipitate phase and B2 matrix is determined to be A = 4.17c, B = 2(a + b) and C = 6(-a + b), where A, B and C are the basis vectors of the precipitate phase and a, b and c are those of the B2 matrix. The precipitate has an oblate spindle like shape with its habit plane being the (100)(P) parallel to(001)(B) plane. Six orientation variants and translation domains have been observed. These variants are related by those symmetry operations which belong to the BZ matrix but are lost in the precipitate phase. (C) 1997 Acta Metallurgica Inc.