COPPER COBALT FCC METASTABLE PHASE PREPARED BY MECHANICAL ALLOYING

The solid state reaction of the normally immiscible copper and cobalt powders has been studied at the equiatomic composition with the mechan ical alloying (MA) technique, supplemented by the milling of cobalt an d copper elemental powders. The diffraction of unmilled pure cobalt po wder 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 n eutron diffraction pattern reveals only the highly distorted h.c.p. ph ase. Further, the degree of distortion in the h.c.p. phase is highly d ependent on the crystallographic directions. Mechanical alloying of th e Cu-Co equiatomic mixture creates an almost entirely fc.c. single pha se after 2 h of treatment. The lattice parameter of the Cu(Co)extended solid solution decreases on increasing the milling time. Moreover, a thermal treatment at 700-degrees-C of the powders MA 16 h demixes the pure constituents. A comparison with previous data on the Cu-Co system prepared by rapid quenching, evaporation and magnetron sputtering met hods confirms the MA method to be an efficient synthesis tool of new m etastable non-equilibrium phases.