Copper–cobalt f.c.c. metastable phase prepared by mechanical alloying

Abstract

The solid state reaction of the normally immiscible copper and cobalt powders has been studied at the equiatomic composition with the mechanical alloying (MA) technique, supplemented by the milling of cobalt and copper elemental powders. The diffraction of unmilled pure cobalt powder shows the coexistence of the two face-centred-cubic (f.c.c.) and hexagonal-close-packed (h.c.p.) allotropes. After 1 h of milling the neutron diffraction pattern reveals only the highly distorted h.c.p. phase. Further, the degree of distortion in the h.c.p. phase is highly dependent on the crystallographic directions. Mechanical alloying the Cu-Co equiatomic mixture creates an almost entirely f.c.c. single phase after zh of treatment. The lattice parameter of the Cu(Co) extended solid solution decreases on increasing the milling time. Moreover, a thermal treatment at 700°C of the powders M A 16 h demixes the pure constituents. A comparison with previous data on the Cu-Co system prepared by rapid quenching, evaporation and magnetron sputtering methods confirms the MA method to be an efficient synthesis tool of new metastable non-equilibrium phases.