Effect of microstructural parameters and Al alloying on creep behavior, threshold stress and activation volumes of molybdenum disilicides

Creep behavior of several molybdenum disilicides, with varying grain sizes, silica content, SiC reinforcement, or Al additions has been studied at 1200 °C, and compared with the deformation behavior at constant strain rates. The effect of grain size on creep rate supercedes all other factors. A high silica content is less deleterious at coarser grain sizes in the dislocation creep range. In addition, 20 vol% SiC reinforcement of MoSi2 can enhance creep strength, if grain size remains coarse. Furthermore, Al addition has been found to be beneficial up to a critical concentration, which helps in the removal of silica at grain boundaries, while retaining the tetragonal structure of MoSi2. A threshold stress for creep can arise due to the build up of internal stresses at second phase particles or dislocation networks. These stresses inhibit further movement of dislocations. The activation volume was found to vary between 27b3 and 2b3 with applied stress, where b is the Burgers vector. This implies that the rate controlling creep mechanism can be overcoming glide type of barrier (say, Peierls stress) at low stresses, and dislocation climb at higher stresses.