Ductility and fracture toughness of molybdenum with MgAl2O4 additions

New interest in controlling the ductility of molybdenum follows from the hope that multiphase Mo-Si-B alloys may represent the next generation of gas-turbine blade materials. While the addition of MgAl2O4 spinel particles up to 5 vol.% has been previously found to improve the tensile ductility of molybdenum, the present study indicates that there is no corresponding improvement in the fracture toughness. Based on these results, the ductilizing effect is attributed to the associated reduction in grain size, rather than the actual presence of the spinel particles. For spinel additions >2.5 vol.%, the additional crack initiation sites associated with the brittle particles embrittle the molybdenum and hinder the ductilizing grain size effect, resulting in a decrease in ductility beyond 2.5 vol.% spinel. Thus, it would appear that controlling the molybdenum grain size, rather than adding spinel particles, is a more effective means of controlling ductility of molybdenum when fracture occurs intergranularly.