CORROSION BEHAVIOR AND ITS MECHANISM OF Y 2O3 DISPERSED W COMPOSITE IN MOLTEN-METAL

Yttrium oxide dispersed tungsten (Y2O3/W) composite has been developed as a crucible material for melting of reactive metals. In this study, the corrosion behavior and its mechanism of the Y2O3/W composite in m olten metal were investigated in comparison with that of pure W. The 2 0 vol%-Y2O3/W and pure W crucibles whose relative densities became ove r 99% by HIPing were used for melting experiments descrived below. Ytt rium (Y, melting point: 1775 K), which was one of the reactive metals, was melted with 20 vol%-Y2O3/W and pure W crucibles at 1923 K for 3.6 ks in Ar atmosphere. The contamination of W in the molten Y in the ca se of using the 20 vol%-Y2O3/W crucible was found to decrease less tha n that for the pure W crucible. This observation shows that the Y2O3 d ispersed crucible has excellent corrosion resistance in molten Y. On t he contrary, a remarkable penetration of molten Y into grain boundarie s was observed in the case of the pure W crucible. It is considered th at the difference of W contamination in molten Y for both cases depend s on the difference in the corrosion resistances of the grain boundari es. From an auger electron spectroscopy analysis, a large amount of ph osphorus (P) was detected at the grain boundaries of the pure mi, whic h shows that the attack of grain boundaries observed in the pure W cru cible was caused by the reaction between the segregated P and the molt en Y. For the Y2O3/W composite, the P contents at the W grain boundari es were lower than that in the pure W, because P or P oxide (PO4) was partly taken into the Y2O3 particles during sintering. The growth of Y 2O3 particles in the Y2O3/W composite, which contained a small amount of Y-2(WO4)(3) complex oxide, occurred in the surface region after hea ting above 1573 K, and the W grain boundaries were covered with Y2O3, which was considered to be a main reason for the excellent corrosion r esistance of this composite.