Abstract
The deformation kinetics of Ti-O and Ti-C alloys (0.04∼0.45 at% O and 0.07∼0.36 at%C) with 2 to 15 μm grain size were investigated over the temperature range of 77 to 700°K employing strain rate cycling during an otherwise constant strain rate test. The value of the thermal component of the flow stress σ* was obtained directly from the strain rate cycling tests through the relation σ*=m*(∂σ⁄∂ln\dotε)T, where m* is the value of (∂ln\dotε⁄∂lnσ)T extrapolated to zero strain and infinite grain size. σ* was found to be proportional to the square root of the oxygen or carbon content, the proportionality constant at 0°K being 0.23 C66 or 0.17 C66 for Ti-O and Ti-C alloys respectively. The Gibbs free energy of activation (σ*=0, 0°K) for plastic flow of the Ti-O and Ti-C alloys are 1.1 and 0.8 eV respectively. The maximum iorces exerted by the obstacles on the dislocations are 63×10−6 and 52×10−6 dynes in Ti-O and Ti-C alloys respectiveiy and the distance at which the force first arises rapidly ∼2b, all independent of the interstitial content and grain size. These results are in accord with the overcoming of interstitial solute obstacles as the rate controlling mechanism of σ*.
The athermal component of the flow stress σμ increased with decrease in grain size according to the Hall-Petch relation. σμi(d−1⁄2=0) is equal to 0.05 C66 times the square root of the oxygen or carbon content, whereas the Hall-Petch constant K is relatively independent of the interstitial content. Based on previous work, it is concluded that the effects of grain size and interstitial content on σμ is through the dislocation density produced during straining.
