The effect of molybdenum on the microstructure and creep behavior of Ti-24Al-17Nb-xMo alloys and Ti-24Al-17Nb-xMo SiC-fiber composites

The effect of molybdenum (Mo) on the microstructure and creep behavior of nominally Ti - 24Al - 17Nb (at.%) alloys and their continuously reinforced SiC-fiber composites (fiber volume fraction = 0.35) was investigated. Constant-load, tensile-creep experiments were performed in the stress range of 10 - 275 MPa at 650 degrees C in air. A Ti - 24Al - 17Nb - 2.3Mo (at.%) alloy exhibited significantly greater creep resistance than a Ti - 24Al - 17Nb - 0.66Mo (at.%) alloy, and correspondingly a 90 degrees-oriented Ultra SCS-6/Ti - 24Al - 17Nb - 2.3Mo metal matrix composite (MMC) exhibited significantly greater creep resistance than an Ultra SCS-6/Ti - 24Al - 17Nb - 0.66Mo MMC. Thus, the addition of 2.3 at.% Mo significantly improved the creep resistance of both the alloy and the MMC. An Ultra SCS-6 Ti - 25Al - 17Nb - 1.1Mo (at.%) MMC exhibited creep resistance similar to that of the Ultra SCS-6/Ti - 25Al - 17Nb - 2.3Mo (at.%). Using a modified Crossman model, the MMC secondary creep rates were predicted from the monolithic matrix alloys' secondary creep rates. For identical creep temperatures and applied stresses, the 90 degrees-oriented MMCs exhibited greater creep rates than their monolithic matrix alloy counterparts. This was explained to be a result of the low interfacial bond strength between the matrix and the fiber, measured using a cruciform test methodology, and was in agreement with the modified Crossman model. Scanning electron microscopy observations indicated that debonding occurred within the carbon layers of the fiber-matrix interface.