Intermediate phases in alloys

Intermediate phases in. alloys may be grouped broadly into three main classes! (a) electrochemical compounds? (b) Size factor compounds? and (c) electron compounds.

The electrochemical compounds are formed when one element is strongly electropositive and the other is strongly electronegative. When the electrochemical factor is inappreciable the formation of the phases depends on the atomic size factor and valency electron concentration.

Size factor compounds are of two main types; those, where one atom is much smaller than the other, and those, where the difference in atomic diameters is about 20 to 30 per cent. An important group of the batter type is called the Laves phases2 with compositions according to the formula AB2 (e.g. .AgBe2 ? MgZn2 ? MgNi2 ? CaMg2 ). The chief reason for the existence of these phases is the fact that when the participating atoms differ in size by about 22*5% they can pack together in the crystal structure of higher coordination then the maximum (12) that can be achieved in structures of equally sized spheres. In a Laves Phase of formula AB2 each A atom has -12 B neighbours and4^4 neighbours giving-a co-ordination number of 16. The co-ordination number for each B atom is 12? so that the average coordination number in the structure is 13.33. These high coordination numbers are of course entirely consistent with.the metallic bond but not with ionic or homopolar bonds.

Hume Kothery and Westgren first observed that*over a •wide range of alloy systems phases of similar crystal structures are formed at the same ratios of valency electrons to atoms. These phases are termed electron compounds. The electron concentrations at which they are observed are 3/2, 21/13 and 7/4, the numerator being the number of electrons and the denominator,being the number of atoms.

Electrons compounds are also found in certain ternary alloys at the appropriate concentration and, in general, they can exist over a small range of composition about the exact ratio. Both ordered and disordered phases are found.

The existence of these phases is a direct consequence of the nature of the metallic bond. The stability of such a phase 1 p depends essentially upon the electron concentration and the pattern of atomic sites, while the actual distribution of atoms** amongst thase sites is of minor importance. This is an entire agreement with the zone theory of alloy phases. 


For more information click on the following link:

http://shodhganga.inflibnet.ac.in/bitstream/10603/157497/15/15_chapter%2010.pdf