, Volume 15, Issue 7, pp 1709-1718

Melting behaviour and metastability of yttrium aluminium garnet (YAG) and YAlO3 determined by optical differential thermal analysis

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The melting point of yttrium aluminium garnet (YAG), reinvestigated by optical differential thermal analysis (ODTA), was found to be 1940±7° C. Above this temperature YAG liquids are opaque, suggesting the presence of two immiscible liquids. In the composition range 10.0 to 47.5 mol% Y2O3, crystallization of the equilibrium phases can only occur in the presence of YAG nuclei; otherwise solidification of YAlO3 and Al2O3 will take place. A metastable phase diagram has been defined with a metastable eutectic at 23 mol% Y2O3-77 mol% Al2O3 and 1702±7° C. YAlO3 (perovskite) was found to melt incongruently with a peritectic temperature of 1916±7° C and a liquidus temperature of 1934±7° C. YAlO3 formed during metastable solidification transforms to YAG in the presence of Al2O3 at 1418±7° C. It is suggested that the metastability arises from the difficulty of the aluminium to attain four-fold co-ordination in the YAG structure.