Comparison of the microstructure, tensile, and creep behavior for Ti-24Al-17Nb-0.66Mo (atomic percent) and Ti-24Al-17Nb-2.3Mo (atomic percent) alloys

The effect of small molybdenum additions, 0.66 and 2.3 at. pct, on the microstructure, tensile, and creep behavior of a nominally Ti-24Al-17Nb (at. pct) alloy was investigated. The alloy containing 2.3 at. pct Mo contained higher body-centered-cubic (bee) phase volume fractions, which was expected as Mo stabilizes the bee phase. Constant load, tensile-creep experiments were performed in the stress range of 29 to 275 MPa and the temperature range of 650 degrees C to 710 degrees C, in both air and vacuum environments. In-situ creep experiments were performed inside a scanning electron microscope chamber in order to identify the deformation evolution from surface observations. From these experiments, it was evident that alpha(2) intergranular cracking was prevalent and initiated the fracture process where the crack path followed the alpha(2) grain boundaries. The Ti-24Al-17Nb-23Mo (at. pct) alloy exhibited significantly lower creep rates than the Ti-24Al-17Nb-0.66Mo (at. pct) alloy, and this was associated with less contiguity of the alpha(2) phase.