Oxidation and creep behavior of Mo5Si3 based materials

Abstract

Mo[subscript]5 Si[subscript]3 shows promise as a high temperature creep resistant material. The high temperature oxidation resistance of Mo[subscript]5 Si[subscript]3 has been found to be poor, however, limiting its use in oxidizing atmospheres. Mo [subscript]5Si[subscript]3 exhibits mass loss during oxidation at 800°-1200° C due to volatilization of molybdenum oxide, indicating that the silica scale that forms does not provide a passivating layer. Catastrophic \u27pest\u27 oxidation occurs at 800° C. The oxidation rate at 1200° C is on the order of 10[superscript]3 mg cm[superscript]-2 hr[superscript]-1. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1050°-1300° C. The oxidation rate of Mo[subscript]5 Si[subscript]3 was decreased by 5 orders of magnitude at 1200° C by doping with less than two weight percent boron. Boron doping eliminates catastrophic \u27pest\u27 oxidation at 800° C. Oxidation and scale formation on several Mo[subscript]5 Si[subscript]3 based Mo-Si-B multiphase intermetallics was studied at 600° C-1300° C using SEM, XRD, ESCA, and thermogravimetric analysis. The mechanism for the improvement in oxidation resistance was found to be scale modification by boron. Boron additions of as low as 0.14 wt% promote formation of a passivating scale by allowing viscous flow to occur;The compressive creep rate of an MoSi[subscript]3 based Mo-Si-B composition was evaluated at 1240°-1320° C and 140-180 MPa. The composition tested had a three phase microstructure composed of Mo[subscript]5 Si[subscript]3 (T1), Mo[subscript]3Si, and a ternary Mo[subscript]5( Si,B)[subscript]3 (T2) phase. The average creep stress exponent and activation energy were found to be n = 4.3 and E[subscript] a = 396 kJ/mol. The boron modified Mo[subscript]5 Si[subscript]3 was found to have approximately the same creep rate as Mo[subscript]5 Si[subscript]3 at 140 MPa. TEM analysis of the crept microstructure of the boron modified material reveals no evidence for dislocation activity in T1. Only basal slip was observed in the T2 phase. \001 dislocations and polygonal subgrain structures were observed in Mo[subscript]3 Si. Mo[subscript]3 Si and T2 were found to stop cracks that nucleate in the T1 phase during creep