ANALYSIS OF THE COMPOSITION OF ALPHA-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 To 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 To. 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. © 1990 The Metallurgical of Society of AIME.