Microstructural evolution of ω assisted α precipitates in β-CEZ alloy during ageing process

The microstructural evolution in beta-CEZ (Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe wt%) alloy, after quenching and ageing, has been investigated by combining results from orientation imaging microscopy, transmission electron microscopy, high-resolution TEM (HRTEM). Ultrafine Athermal omega precipitates (about 1 nm) are formed in this alloy by quenching from above the beta transus temperature. In the ageing process, these omega precipitates coarsen, accompanied by the form of needle-like alpha phase. The scale of omega assisted alpha increased from 5 to 10 nm in width and about 50 nm in length to 10 nm in width and 100-300 nm in length while extending ageing time from 2 h to 5 h at 350 degrees C. However, when going on extending the ageing time from 5 h to 20 h at 350 degrees C, the scale of alpha didn't grow dramatically but the density of alpha increased clearly. When increasing the ageing temperature from 350 degrees C to 450 degrees C, the scale of omega assisted alpha grow dramatically. Furthermore, the structural relationship of omega and alpha is observed by HRTEM images. It can be found that omega precipitates were formed by {111}beta planes shift 0.5d222 along the < 111 > direction, and the superfine alpha particles are formed in close proximity of the omega/beta interface.