Transient liquid phase bonding, interfaces, and M chromium aluminum-yttrium coating of superalloy/sapphire composites

Using light, ion, and electron microscopy, structural and chemical analyses of Ni-base superalloy/sapphire fiber composites were conducted to understand (1) the mechanism of transient liquid phase (TLP) bonding, (2) the nature of the matrix/sapphire interfaces, and (3) the MCrAl-Y coating effects on the oxidation of composite. It was found that the TLP bonding process does not take place under local thermodynamic equilibrium conditions of solid/liquid interfaces, contrary to the primary assumption of traditional TLP bonding models. To achieve better TLP bonding, four modifications are recommended: (i) use less boron (present as a melting point depressant) for TLP bonding, (ii) use finer sapphire fibers, (iii) increase the homogenization time and temperature, and (iv) create smaller starting grain sizes in the superalloy matrix. Extensive interfacial reactions occurred between the fiber and matrix, resulting in the formation of MgAl{dollar}sb2{lcub}rm O{rcub}sb4{dollar}, MgAl{dollar}sb{lcub}11{rcub}{dollar}LaO{dollar}sb{lcub}19{rcub}{dollar} and various types of oxide clusters, as part of a sequence of reduction-oxidation-dissolution reactions. It was found that close-packed fibers and Cu seem to induce the formation of deleterious clusters. Evaluation of the MCrAl-Y coating performance shows that the coating has several positive effects on the oxidation resistance of the composite: (a) suppression of chromium oxide formation, (b) induction of the stable "external" aluminum oxide layer formation, and (c) modification of the aluminum oxide layer.