Comparative analyses of the tensile and damage tolerance properties of tri-modal microstructure to widmanstätten and bimodal microstructures of TA15 titanium alloy

For a two-phase titanium alloy, the tensile and damage tolerance properties exhibited in tri-modal microstructure were investigated and compared to the same properties exhibited in widmanstätten and bimodal microstructures. These investigations and comparisons were conducted by analyzing the microstructure characteristics and their influences on deformation behavior and fracture features. The results show that the tri-modal microstructure exhibits superior strength and plasticity that are close to bimodal microstructure and remarkably higher than the widmanstätten microstructure. The underlying reason is that the multiple types of interfaces and refined grains of tri-modal and bimodal microstructures produce greater interface strengthening and better overall deformation homogeneity than the widmanstätten microstructure. Furthermore, the better deformation homogeneity also contributes to suppressing the low-cycle fatigue crack initiation of tri-modal microstructure. Besides, the unique lamellar α in tri-modal microstructure with disordered distributions on both geometric and crystallographic orientations effectively hinders the fatigue crack propagation. Thus, the low-cycle fatigue life (the number of reversals to failure) of tri-modal microstructure is the longest, which is 3.93 times as long as that of widmanstätten microstructure and 1.12 times as long as that of bimodal microstructure. For the fracture toughness, the fracture crack paths of bimodal and tri-modal microstructures with the small sized grains present far lower fluctuation and tortuosity compared to those of widmanstätten microstructure, which means lower energy consumption. However, the crack path of tri-modal microstructure is still more deflected than that of bimodal microstructure due to the contribution of the disorderly distributed lamellar α, which increases the energy consumption to some extent. Thus, the fracture toughness of tri-modal microstructure is lower than the widmanstätten microstructure but higher than the bimodal microstructure. These results indicate that tri-modal microstructure exhibits more excellently combined tensile and damage tolerance properties compared to the other two microstructures.