beta Phase decomposition in Ti-5Al-5Mo-5V-3Cr

The decomposition of the beta phase during rapid cooling of the near beta titanium alloy Ti-5Al-5Mo-5V-3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971:19) that embryonic omega structures form by the correlation of linear (111)beta defects at high temperatures. Further Cooling causes increased correlation of these defects and the formation of athermal omega structures within the beta matrix at temperatures similar to 500 degrees C. Post-quench aging at 570 degrees C resulted in the nucleation of alpha laths after similar to 90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of alpha precipitates with a lath morphology similar to 1.5 mu m x 0.2 mu m in size composed of both the expected Burgers variants. Mechanical property data suggests that the omega structures alone have no real effect: however, hardness increases were observed as the alpha phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the alpha phase in near beta titanium alloys and hence control mechanical properties. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved