Strengthened flow instability in hot deformation of titanium alloy with colony structure: On the effect of microstructure heterogeneity

The effect of microstructure heterogeneity on flow instability is investigated during isothermal compression. The mechanism of plastic flow instability is studied from the perspective of the strain mode and geometrical orientation dependence of microstructure. It is found that the instability criteria underestimate the range of unsafe processing region of lamellar microstructure due to its stronger trend to flow instability. The key factors are the geometrical orientation and colony size of lamellar microstructure. In the colonies with hard geometrical orientation, deformation resistance is big since only the pyramidal slip system can be operated. The difference of the deformation resistance among colonies can reach 49% in a primary β grain. The incompatibility of flow among various colonies promotes the generation of micro shear bands in colonies with hard geometrical orientation, which facilitates the slip transmission across α/β interfaces and weakens the Hall-Petch strengthening effect. The large colony size facilitates the rapid expansion of micro shear bands leading to the continuous softening of colonies, which strengthens the strain localization and accelerates the occurrence of flow instability. The critical value of Semiatin's model is modified to 4 for lamellar microstructure. Based on this research, a method to confirm more precise unsafe processing regions of titanium alloys of different microstructures is proposed.