Copper Content Dependence of the Lattice Parameters of Ti(NiCu)<SUB>2</SUB>

The ternary Ti–Ni–Cu system forms a tetragonal Ti(NiCu)2 phase with C11b-type structure, in a wide range of copper content. The copper content dependence of the lattice parameters of the Ti(Ni1−YCuY)2 (0.22≤Y≤0.75) alloys and those of the Ti(NiCu)2 precipitate formed in the Ti49(Ni1−XCuX)51 (0.06≤X≤0.4) alloys, were determined by powder X-ray diffraction. The parameters a and c both exhibit a minimum, but the ratio c⁄a decreases almost linearly as the copper content increases. The coherency between the matrix and the precipitate becomes worse as the copper content increases. The relation between the copper contents X and Y will be approximated as Y=1.24X+0.09. While the X-ray diffraction patterns can be indexed by the C11b-type structure, diffuse 1/2{110} reflections appear in the electron diffraction pattern. The Vickers microhardness of Ti(NiCu)2 decreases as the copper content increases.