[SY-C9] Microstructural effects on strain rate sensitivity in dual-phase titanium alloys
Cold dwell fatigue failure in titanium alloys is a major concern in aero-engine applications and is inherently linked to the strain rate sensitivity (SRS) of the material. Recent crystal plasticity finite element (CPFE) work has shown the material SRS to be influenced by a number of microstructural factors [1]. However, these higher-level modelling studies do not include discrete aspects of slip and considerably overestimate the material rate sensitivity as compared to experimental studies. Thus, the significance and mechanism of microstructural effects on experimentally observed strain rate sensitivity in titanium alloys remain unclear.
In this study, a planar discrete dislocation plasticity (DDP) model is set up explicitly incorporating dislocation penetration across phase boundaries and material heterogeneity arising from alpha and beta titanium phases to simulate realistic microstructures. Stress relaxation tests are performed to investigate microstructural effects, such as the presence of colony or basketweave microstructures, textured or non-textured crystallography and grain size effects, on the material strain rate sensitivity. The SRS coefficients obtained from the simulations match closely with experimental values. It is found that changing the microstructure from pure alpha, colony to basketweave, and reducing alpha grain sizes leads to a significant reduction in material rate sensitivity, whereas dislocation penetration is found to not be as significant as previously considered for small hold strains. The mechanistic basis for these effects is argued to be changes in dislocation mean free-path and the total amount of plasticity in the specimen. Finally, DDP results are compared with corresponding CPFE simulations, to show that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. These results provide increased understanding of the mechanism of cold dwell fatigue failure in titanium alloys and are relevant for the design of dwell-insensitive microstructures.
In this study, a planar discrete dislocation plasticity (DDP) model is set up explicitly incorporating dislocation penetration across phase boundaries and material heterogeneity arising from alpha and beta titanium phases to simulate realistic microstructures. Stress relaxation tests are performed to investigate microstructural effects, such as the presence of colony or basketweave microstructures, textured or non-textured crystallography and grain size effects, on the material strain rate sensitivity. The SRS coefficients obtained from the simulations match closely with experimental values. It is found that changing the microstructure from pure alpha, colony to basketweave, and reducing alpha grain sizes leads to a significant reduction in material rate sensitivity, whereas dislocation penetration is found to not be as significant as previously considered for small hold strains. The mechanistic basis for these effects is argued to be changes in dislocation mean free-path and the total amount of plasticity in the specimen. Finally, DDP results are compared with corresponding CPFE simulations, to show that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. These results provide increased understanding of the mechanism of cold dwell fatigue failure in titanium alloys and are relevant for the design of dwell-insensitive microstructures.