Strength enhancement of a bi-lamellar PM Ti-6Al-4V alloy without sacrificing ductility by Al partitioning and twinning-induced plasticity effect

Ti-6Al-4V alloy samples with two bi-lamellar microstructures were prepared by cost-effective thermomechanical powder consolidation and two inter-critical annealing treatments; one comprising of annealing at 900 degrees C followed by air cooling (900-AC alloy) and the other comprising of annealing at 910 degrees C followed by water quenching and aging (910-Q&A alloy), respectively. Both bi-lamellar microstructures encompass lamellae of primary alpha phase and beta transformed structure which consists of ultrathin secondary alpha lamellae and beta thin layers. Thanks to the higher Al concentration in the primary alpha lamellae associated with the higher annealing temperature and finer scale of beta t structure lamellae, the alpha and beta t lamellae of the 910-Q&A alloy exhibit a higher nano hardness than that of the 900-AC alloy (5.4 and 4.6 GPa vs. 4.5 and 4.3 GPa) as well as a larger difference in nano-hardness between the alpha and beta(t) lamellae (0.8 vs. 0.2 GPa). These microstructural and nano-hardness differences lead to a significant enhancement of the yield strength of the 910-Q&A alloy by 200 MPa compared to the 900-AC alloy with almost no decrease of tensile ductility. Quantitative analysis indicates that the hardening of alpha and beta(t) lamellae accounted for approximately 35% and 28% of the total strength increment, respectively, while the heterostructure induced hardening associated with the inhomogeneity between the alpha and beta(t) lamellae accounts for approximately 37% of the strength increment. Interestingly, the significantly higher flow stress of the 910-Q&A alloy during tensile deformation induces a significant higher degree of trans-lamellar twinning which is believed to be responsible for mitigating the strain localization and maintaining the good tensile ductility despite of a higher flow stress.