The system iron-gallium

Abstract

The equilibrium diagram of the system Fe-Ga has been determined by a combination of thermal analysis, microscopical, and X-ray methods. The system is of the γ loop type, and gallium dissolves in αδ iron up to a maximum of 48.4 at. %. In contrast to the system Fe-Al, there are no superlattices in the b.c.c. solid solution, but in the region of 30 at. % Ga, f.c.c. phases of slightly variable composition are formed from the b.c.c. phase, and these have the Cu 3 Au type of ordered structure at low temperatures, and a random f.c.c. structure at high temperatures. It is significant that metastable f.c.c. structures have been reported in the system Fe-Al, and it is probable that the more favourable size factor in the system Fe-Ga is responsible for the greater stability of the close-packed f.c.c. phase. At higher proportions of gallium, intermetallic compounds Fe 3 Ga 4 and FeGa 3 have been established, and the crystal structure of the latter has been determined, and is similar to that of CoGa 3 (tetragonal space group D 8 2d - P 4 n 2 ) but with slightly different positions of the atoms in the unit cell. The structures of alloys of Fe, Co, and Ni with Zn, Ga, and Al are discussed. The differences are ascribed to (1) the greater tendency of Ni and Co to fill the 3 d shell, as compared with Fe, (2) the more electropositive nature of Al as compared with Ga. The ordered b.c.c. phases are stabilized both by electron concentration effects, and by superlattice factors which favour a structure in which an atom has neighbours of the opposite kind. The great stability of the NiAl and CoAl b.c.c. phases (CsCl type) results from the fact that if a transition metal has a zero valency, it is only with a 3-valent solute that the equiatomic ratio gives the electron: atom ratio 3/2 which favours the b.c.c. structure, and so permits the formation of the CsCl arrangement in which each atom is surrounded by eight of the opposite kind.