Inverse melting in binary systems: Morphology and microscopy of catatectic alloys
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INTRODUCTION
FIGURE 1 shows the general features, in the isobaric section of a two-component system, of a so-called ‘‘first class invariant equilibrium’’ (according to Rhines’ classification[1]) or ‘‘eutectic type (e-type) equilibrium’’ (according to other authors such as those in References 2 and 3). This equilibrium corresponds, at the invariant temperature Ti, to the following general reaction: →w 1 w w1 ← 2 3
with the transformation occurring from left to right on decreasing the temperature. Several equilibria, often characterized by distinctive names, belong to this type, for instance, eutectic, eutectoid, and monotectic. The following may be considered of special interest: → s 1 g or s ← →s 1 l l← 1 2
where g 5 gas, l 5 liquid, and s, s1, and s2 solid phases, which result, on cooling, in the partial transformation of the initial amount of substance into a more disordered phase (gas from liquid, or liquid from solid). Names such as inverse boiling (inverse melting)[4] or retrograde boiling (retrograde melting) have therefore been used. This, apparently anomalous behavior is also suggested by names such as ‘‘second boiling (second melting) point.’’[5] The inverse boiling invariant equilibria are very important in systems such as those formed by water with several high-temperature melting salts. Zernike,[5] for instance, underlined a practical side to this point. He observed that in order to drive out all the water from a salt, the heating temperature has to be carefully planned, taking into account the formation of the inverse boiling point and the corresponding value of the invariant temperature. As an instance of inverse boiling points encountered in nature, he noted that the seething masses of lava observed in volcanic eruptions are essentially composed of fused, water containing silicates, which crystallize out as they cool down while steam is being evolved.
R. FERRO, FASM, Full Professor, A. SACCONE, Associate Professor, `, S. DELFINO, Full Professor, and A.M. CARDINALE and D. MACCIO Researchers, are with the Division of Inorganic Chemistry and Metallurgy, Chemistry Department, Genova University, I-16146 Genova, Italy. Manuscript submitted January 23, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
The inverse melting is found in a number of binary systems. It has received alternative distinctive names such as ‘‘catatectic’’ by Wagner and Rigney[6] and ‘‘metatectic,’’ for instance, by Prince[7] and also sometimes designated ‘‘inverted peritectic’’ (see, for instance, References 8 and 9). Indicatively, we may refer to the Massalski compilation[10] of phase diagrams relevant to binary, mainly, twometal, systems. Among the different binary systems reported, the number of those showing tables of invariant equilibria (entitled ‘‘special points of the system’’) is 508. In these tables, the overall number of true binary invariant equilibria is '3750, after the deduction of the unary ones. Of these, the equilibria corresponding to the inverse melting are 53 (of which 33 are called catatectic and 20 metatecti
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