Abstract:
Mo-Si-B alloys and Ultra-high temperature ceramics are potential materials for high temperatures beyond Ni based superalloys. In Mo-Si-B based systems and with SiC addition to ZrB2, protective SiO2-rich glassy scale is formed at the surface. Diffusion through the scale and scale viscosity play an important role in understanding the oxidation behaviour of the system. In the present work, diffusion coefficient and activation energies of the diffusing species have been calculated using Molecular Dynamics (MD) as these govern the scale growth and surface coverage. Diffusion was modelled in SiO2 glass in the temperature range of 1200-1700 ℃ with elemental additions. To predict oxidation kinetics and to simulate oxide growth in Mo-Si-B alloys, a mathematical model based on Cellular Automata (CA) was developed. This model simulates the transient oxidation phenomenon both visually (microstructure evolution at each time step) as well as numerically (mass change at each time step). The microscopic nature of the model allows us to explore the roles of material chemistry and the microstructure on the oxidation behaviour quantitatively. Using the model as a guide, oxide evolution of Mo-Si-B alloys with addition of W, Ta and Al was studied at 1100 and 1350 ℃ experimentally and the results were comparable with those predicted by the developed model. For example, the model predicted higher mass loss on adding tungsten and lower mass loss on adding Ta to the alloy 711 at 1100 ℃. The oxidation behaviour of ZrB2-SiC based ceramics was studied at 1600 with AlN, TaC and CeO2 additions. Poor oxidation resistance was observed in 70ZrB2-20SiC-10AlN composite which was attributed to the lower oxide scale viscosity resulting in faster diffusion through the scale and increased oxidation. This agreed with the diffusion modelling results obtained using MD where increased self-diffusion of species was observed with Al addition to SiO2 glass. This work involves the use of a combination of computational and experimental approaches for better design of ultra-high temperature materials that form silica and borosilica scales.
