Investigations on ductility improvement and reloading yielding during stress relaxation of dual phase Ti–6Al–4V titanium alloy

Abstract

Transient stress relaxation test can be used to estimate activation volume for plastic deformation. The analysis is complicated due to the non-constancy of mobile dislocation density during relaxation. The reloading yielding post stress relaxation has a direct correlation with the change in mobile dislocation density during relaxation. However, this feature is ignored in the past studies. In the present work, the reloading yielding phenomenon in dual phase (α+β) titanium alloy (Ti–6Al–4V) is investigated using phenomenological stress relaxation model. Based on indirect experimental evidences, it is shown that the reloading yielding is due to regeneration of new dislocations and not due to unpinning of sessile dislocations, as assumed previously. The improvement of ductility due to stress relaxation follows a similar trend reported in literature. The ductility improvement is analyzed using both microstructure changes and thermal gradient. Fractographs of failed samples were investigated to understand the fracture characteristics and quantitative analyses were performed using image processing to correlate the microstructure mechanisms responsible for enhanced ductility. Thermal history at select locations were recorded using thermocouple and were used to analyze the contribution of thermal gradient in ductility. The necessity of a comprehensive approach to model ductility improvement in non-monotonic stress relaxation test is explored.