Phase Transformation Kinetics and Mechanisms in Titanium Alloys Ti-6.2.4.6,ß-CEZ and Ti-10.2.3

Abstract
The precipitation mechanisms and kinetics in three titanium alloys of different ß-phase stabilities - the ß-rich a α+β titanium alloy Ti-6Al-2Sn-4Zr-6Mo and the near ß-titanium alloys Ti-5Al-2Sn-2Cr-4Mo-4Zr-1Fe(ß- CEZ) and Ti-10V-2Fe-3Al - were investigated by isothermal holding in the α+β field after a ß-solution treatment. For this purpose, the amount of transformation was determined by electrical resistivity measurements. The TTT diagrams of these alloys, plotted in the whole α+β field, show several independent C-curves. It is apparent from the cornparison of nose temperatures, that the transformation schemes should be quivalent for the thrre alloys in the two upper C-curves when temperatures are related to ß-transus temperatures, and for ß-CEZ and 10.2.3 in the two lower C-curves wheu absolute temperatiires are considered. This was confirmed by microstructiual observations (OM, SEM and TEM) of isothermal treated specimens. The nature and the morphologies of phases (ß-, α-, α"-, ω-phase) are specified. Furthermore, the ß-phase decomposition kinetics can be analysed by means of the Johnson- Mehl-Avrami relation from the electncal resistivity data at each studied temperahue. For the three alloys, the same n values are determined in the whole temperature range corresponding to similar C-curves. These coefficient values could be attributed to particular transformation mechanisms described by classical phase transformation theory. and coincide with the sequence of precipitation observed in these alloys.