FIRST-PHASE FORMATION AT INTERFACES - COMPARISON BETWEEN WALSER-BENE AND EFFECTIVE HEAT OF FORMATION MODEL

The only two models which make definite and unambiguous predictions re garding first phase formation during solid-state interaction are the W alser-Bene and effective heat of formation (EHF) models. Although thes e models appear to have certain features in common, such as the choice of the liquidus minimum composition as a measure of atomic mixing at the interface, a careful analysis and comparison reveals fundamental d ifferences between them, mainly owing to the fact that the EHF model a lso makes use of heats of formation. The EHF model in effect gives a q uantitative reason for the relatively large margin of success of the W alser-Bene rules. A major feature of the EHF model is that it often pr edicts that there is not much to choose thermodynamically between the formation of several phases, whereas the Walser-Bene rules can only pr edict a single phase. In 53 out of 84 binary systems investigated it h as been found that there is complete agreement between the predictions of the two models, whereas for 14 systems the Walser-Bene and EHF mod el predict the same first phase, the EHF model also predicts other pha ses, which usually are confirmed by experimental measurements. In seve n binary systems, namely Al-Au, Al-Fe, Al-Gd, Au-Cd, Au-V, Au-Zn and P t-Ti, the EHF model correctly predicts the first phase while Walser-Be ne does not, There are also 10 systems where both models fail to predi ct the correct phase mainly because the liquidus minimum of the binary system is not well defined or the thermodynamically favoured phase ha s problems in nucleating.