Mitchell K. Meyer

Oxide scale formation and isothermal oxidation behavior of Mo–Si–B intermetallics at 600–1000°C

Initial scale formation in the range 600–1000°C and isothermal oxidation behavior at 1000°C was investigated for Mo–Si–B intermetallics containing 81–88 wt% molybdenum. All compositions exhibited an initial transient oxidation period consisting of a mass gain due to MoO3 and SiO2 formation, followed by a rapid mass loss starting at 750°C due to MoO3 volatilization. After the initial transient oxidation period, oxidation proceeded at a much slower rate. During isothermal oxidation at 1000°C the oxidation rate was found to vary inversely with the ratio of B/Si in the intermetallic, indicating that viscous flow of the scale was an important factor in determining the isothermal oxidation rate at 1000°C.

Compressive creep behavior of Mo5Si3 with the addition of boron

Abstract Mo5Si3 shows high creep resistance but poor high temperature oxidation resistance. Previous work has shown that the oxidation rate of Mo5Si3 can be decreased with the addition of boron. By adding 1.3 wt % boron to a silicon deficient composition, a three phase microstructure composed of Mo5Si3 (T1), Mo5Si3, and a ternary Mo5(Si,B)3 (T2) phase was synthesized. The compressive creep rate of this composition was evaluated at 1240–1320 °C and 120–180 MPa. The average creep stress exponent and activation energy for the three phase material were found to be n = 4.3 and E a = 396 kJ mol . 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 while polygonal sub-grain structures were observed in Mo3Si.

Oxidation and creep behavior of Mo*5*Si*3* based materials

Mo{sub 5}Si{sub 3} shows promise as a high temperature creep resistant material. The high temperature oxidation resistance of Mo{sub 5}Si{sub 3} has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo{sub 5}Si{sub 3} exhibits mass loss in the temperature range 800{degrees}-1200{degrees}C due to volatilization of molybdenum oxide, indicating that the silica scale does not provide a passivating layer. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1050{degrees}-1300{degrees}C. The oxidation rate of Mo{sub 5}Si{sub 3} was decreased by 5 orders of magnitude at 1200{degrees}C by doping with less than two weight percent boron. Boron doping eliminates catastrophic {open_quote}pest{close_quote} oxidation at 800{degrees}C. The mechanism for improved oxidation resistance of boron doped Mo{sub 5}Si{sub 3} is due to scale modification by boron.