Microstructural and Mechanistic Study of Primary Creep in Titanium Alloys at Lower Temperatures

摘要

Primary creep is a dominant mode of deformation for commercial, two- phase alpha-beta titanium alloys such as Ti-6242 which are used extensively in the fan and compressor sections of jet aircraft engines. This program has improved our understanding of the mechanisms and phenomenology of primary creep in these alloys. Using a phenomenological approach based on the Holomon equation, we have shown that the extensive low temperature creep behavior is linked to the exceptionally small values of strain hardening exponent exhibited by these alloys, and that the long term creep response can be predicted from the results of short-term, constant-strain rate tests. This insight has suggested possible processing paths for improving the creep response of these materials, including the minimization of short-range ordering. A second focus of the program has shown that the Widmanstatten colony structure exhibits a dramatic plastic anisotropy. Single colony crystals of Ti-5Al-2.5Sn-0.5Fe, were obtained in collaboration with J. M. Scott of UES and Wright Laboratory. Extensive TEM investigation has revealed that this anisotropy can be directly correlated with the transmission of dislocation across the beta laths, and the accumulation of residual dislocation content near the interfaces. This information is essential order to better model creep deformation in these materials.