Tensile-stress-induced growth of ellipsoidal ω-precipitates in a Ti-20wt%Mo alloy

The effects of applied tensile stress on the growth of ellipsoidal omega phase precipitates have been investigated for a Ti-20wt%Mo alloy aged at 300 A degrees C. The application of tensile stress accelerates the growth of omega-precipitates when the misfit strain epsilon (Ie) of the precipitates along the loading direction is greater than 0; however, it does not significantly affect the growth of precipitates in cases where epsilon (Ie) < 0. Whereas the growth of precipitates under no stress or in the case where epsilon (Ie) < 0 under tensile stress is governed by the diffusion of Mo from the omega/beta interface toward the beta-Ti matrix, precipitate growth is instead interface-controlled in the case where epsilon (Ie) > 0 under a tensile stress of 400-550 MPa. The growth velocity of precipitates in the case where epsilon (Ie) > 0 is proportional to the tensile stress. This result, together with the misfit strain dependence of the growth of precipitates, is discussed on the basis of the interaction energy between the stress acting on an omega-precipitate and its misfit strain. The activation energies for the diffusion-controlled and interface-controlled growth are estimated to be approximately 190 and 130 kJ/mol, respectively. The value of 190 kJ/mol is consistent with the activation energy for volume diffusion of Mo in beta-Ti.