Residual stresses in high-temperature ceramic eutectics

This paper explores residual thermal stresses in directionally solidified ceramic eutectics, a class of materials that has much promise for high-temperature structural applications. Residual strain tensors of both phases in a eutectic composite are measured by single crystal X-ray diffraction techniques. In the analysis the material is treated as fully anisotropic and the strain tensors, subsequently converted to stress tensors, are measured. Results are presented for two oxide eutectics, NiO-cubic ZrO₂ and YAG-Al₂O₃, the former having a large thermal expansion mismatch between the two phases and the latter having similar thermal expansion properties. It is discovered that large residual stresses (of the order of 1 GPa) can be present at room temperature in as-processed eutectic materials unless the thermal expansion behaviors of the constituent materials are very similar. Ultimately these measurements not only elucidate the stress state but, when compared to theory, give information about the degree of interfacial constraint between the two phases.