Transformation processes and microstructural instability in a beta titanium alloy

The transformation processes and microstructural instability have been examined in a {dollar}beta{dollar} Titanium alloy Ti-15V-3Cr-3Al-3Sn over the temperature range 250 to 500{dollar}spcirc{dollar}C, for times up to 1000 hours.; The role of the {dollar}omega{dollar} phase as a precursor to the {dollar}alpha{dollar} phase is examined in Ti-15-3 alloy aged at 250 or 275{dollar}spcirc{dollar}C where the {dollar}omega{dollar} and {dollar}alpha{dollar} phases coexist. The {dollar}omega{dollar} particles provide preferential nucleation sites for {dollar}alpha{dollar} phase, thereby promoting a uniform dispersion of fine {dollar}alpha{dollar} precipitates. The effect of cold work prior to aging on the aging kinetics was examined in Ti-15-3 alloy cold rolled up to 60% and aged at temperatures in the range 200 to 350{dollar}spcirc{dollar}C. Cold deformation prior to aging accelerates the formation of {dollar}omega{dollar} phase when it is the sole aging product. However, when {dollar}omega{dollar} and {dollar}alpha{dollar} coexist, cold deformation suppresses the formation of the {dollar}omega{dollar} phase and favors the direct precipitation of the {dollar}alpha{dollar} phase for the aging condition employed.; The progressive increase in the values of the temperature coefficient of resistivity, dp/dT, (TCR) are directly associated with the precipitation of the {dollar}omega{dollar} and {dollar}alpha{dollar} phase depending on the aging temperature. TCR values are very sensitive to {dollar}omega{dollar} precipitation and can be used to monitor the onset and progress of the {dollar}omega{dollar} reaction during aging or deformation.; The resistometric data was used to follow the kinetics of the {dollar}omega{dollar} precipitation reaction in terms of phenomenological equations. The kinetics of {dollar}omega{dollar} precipitation in the Ti-15-3 alloy can be described by the Austin-Rickett log autocatalytic equation or the Cohen equation. The value of n, the time exponent in the integrate rate equations suggests that an interfacial process (the unpinning of {dollar}omega{dollar} embryos from interstitial oxygen) for the {dollar}omega{dollar} precipitation is relatively dominant during the early stages of aging, but changes to diffusion control during the later stages of aging.; The kinetics of {dollar}alpha{dollar} precipitation in the Ti-15-3 alloy aged below 400{dollar}spcirc{dollar}C were also examined. The activation energy for the formation of {dollar}alpha{dollar} phase decreases with the fraction transformed. This result is consistent with the enhancement of heterogeneous nucleation of the {dollar}alpha{dollar} phase by an increasing volume fraction of {dollar}omega{dollar} particles as the fraction transformed increases. (Abstract shortened with permission of author.)...