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

Abstract

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 MoSi 2 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 MoSi 2. 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 27 b 3 and 2 b 3 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.