Mechanism of low-temperature θ-CuGa2 phase formation in Cu-Ga alloys by mechanical alloying

The mechanism on the formation of the theta-CuGa2 phase in binary Cu-Ga alloys has been investigated through mechanical alloying (MA) of blended elemental powders by varying process variables such as milling time and milling temperature. The particle size distribution was very broad at the beginning of milling but became narrower as the milling time increased and steady-state equilibrium was reached. The average powder particle size reached a peak value of 270 mum at 30 min of milling and then continued to decrease gradually to 6 mum on milling for 20 h. Formation of the theta-CuGa2 phase started to occur even after milling for 2 min and was completed after 1 h of milling. Melting of Ga was noted in the early stages of milling, probably due to the rise in powder temperature. To discount the possibility that the melting of Ga was responsible for the theta-phase formation, milling was conducted at lower temperatures by dripping liquid nitrogen on to the container. The theta-phase still formed, suggesting that its formation was not related to the melting of Ga and that it was formed by a solid-state reaction even at low temperatures. The compositional homogeneity range of the theta-phase was also extended under the MA conditions. Details of the mechanism of phase formation, as observed by x-ray diffraction and scanning electron microscopy methods, are presented. (C) 2004 American Institute of Physics.