M(sub 5)Si(sub 3) (M equals Ti, Nb, Mo) Based Transition-Metal Silicides for High Temperature Applications; Doctoral thesis

摘要

Transition metal silicides are being considered for future engine turbine components at temperatures up to 1600 degrees C. Although significant improvement in high temperature strength, room temperature fracture toughness has been realized in the past decade, further improvement in oxidation resistance is needed. Oxidation mechanism of Ti(sub 5)Si(sub 3)-based alloys was investigated. Oxidation behavior of Ti(sub 5)Si(sub 3)-based alloy strongly depends on the atmosphere. Presence of Nitrogen alters the oxidation behavior of Ti(sub 5)Si(sub 3) by nucleation and growth of nitride subscale. Ti(sub 5)Si(sub 3.2) and Ti(sub 5)Si(sub 3)C(sub 0.5) alloys exhibited an excellent oxidation resistance in nitrogen bearing atmosphere due to limited dissolution of nitrogen and increased Si/Ti activity ratio. MoSi(sub 2) coating developed by pack cementation to protect Mo-based Mo-Si-B composites was found to be effective up to 1500 degrees C. Shifting coating composition to T1+T2+Mo(sub 3)Si region showed the possibility to extend the coating lifetime above 1500 degrees C by more than ten times via formation of slow growing Mo(sub 3)Si or T2 interlayer without sacrificing the oxidation resistance of the coating. The phase equilibria in the Nb-rich portion of Nb-B system has been evaluated experimentally using metallographic analysis and differential thermal analyzer (DTA).