A GEOMETRIC ANALYSIS OF SOLUBILITY RANGES IN LAVES PHASES

Laves phases nominally occur at the AB(2) stoichiometry but can exhibi t a range of solubility involving non-stoichiometric compositions in b inary alloys. The solubility trends in the reported binary C14, C15 an d C36 structures have been analyzed in terms of the atom size requirem ents that are known to stabilize the Laves phases. For example, Laves phases exist at metallic diameter ratios (D-A/D-B) between similar to 1.05 and 1.68 with the ideal diameter ratio existing at similar to 1.2 25. Although less than 25% of the Laves phases within the D-A/D-B rati os of 1.05-1.68 have defined ranges of homogeneity, the frequency of t he number of intermetallic phases exhibiting any solubility range is i ncreased by a factor of approximately two to three within specific D-A /D-B ratios of 1.12-1.26 (C14 and C36 phases) and 1.1-1.35 (C15 phases ). The upper and lower bounds for the C15 structures can be physically defined as the limits at which the A-B atom distance contractions are greater than the A-A atom distance and B-B atom distance contractions , respectively. For all three main polytypes the occurrence of solubil ity corresponds to a lattice-adjusted contraction between 0-15%. The c ontraction size rule is a geometric argument based upon the contractio n of the atoms forming the intermetallic structure and appears to be a n important relationship in describing ranges of homogeneity in Laves phases. The relationships developed are applied to interpret potential defect mechanisms and alloying behavior in binary and ternary Laves p hases. In addition, extended ternary solubility ranges normal to a pse udobinary direction can be predicted with suitable solute additions ha ving a metallic diameter between that of the A and B atoms.