Analysis of the composition of α plates isothermally formed in titanium binary alloys

The solute distribution within and near the proeutectoid α plates which were isothermally formed from the β solid solution in Ti-V, Cr, and Fe alloys was measured in a scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray (EDX) analyzer. In the Ti-4.9 at. pct Cr and 5.1 at. pct Fe alloys in which all measurements were made above the calculated T_o temperature (the maximum temperature at which the diffusionless β to α transformation can occur), the solute concentration in the α plates was close to final equilibrium as long as the analyzed plates had grown to the sizes comparable to the STEM spatial resolution (∼50 nm). In the Ti-5.4 at. pct V and 2.6 at. pct Fe alloys, the solute concentration in the α plates was already close to equilibrium at very short reaction times at temperatures more than 100 °C below T_o. The ledgewise diffusion growth mechanism can roughly account for the thickening behavior of these plates, consistent with the results of the composition analysis. In the Cr alloy, a microstructure resembling the lower bainite in steels, i.e., nonlamellar distribution of TiCr₂ particles in thick α plates, was formed by coalescence of solute-depleted thin α plates which were formed in aggregates (in sheaves) at an earlier stage of growth. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.