Phase transformation kinetics and mechanisms in titanium alloys Ti-6.2.4.6,beta-CEZ and Ti-10.2.3

The precipitation mechanisms and kinetics in three titanium alloys of different beta-phase stabilities - the beta-rich alpha+beta titanium alloy Ti-6Al-2Sn-4Zr-6Mo and the near beta-titanium alloys Ti-5Al-2Sn-2Cr-4Mo-4Zr-1Fe (beta-CEZ) and Ti-10V-2Fe-3Al- were investigated by isothermal holding in the alpha+beta field after a beta-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 alpha+beta field, show several independent C-curves. It is apparent from the comparison of nose temperatures, that the transformation schemes should be equivalent for the three alloys in the two upper C-curves when temperatures are related to beta-transus temperatures, and for beta-CEZ and 10.2.3 in the Two lower C-curves when absolute temperatures are considered. This was confirmed by microstructural observations (OM, SEM and TEM) of isothermal treated specimens. The nature and the morphologies of phases (beta-, alpha-, alpha ''-, (omega-phase) are specified. Furthermore, the beta-phase decomposition kinetics can be analysed by means of the Johnson-Mehl-Avrami relation from the electrical resistivity data at each studied temperature. 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.