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

Abstract

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 °C followed by air cooling (900-AC alloy) and the other comprising of annealing at 910 °C followed by water quenching and aging (910-Q&A alloy), respectively. Both bi-lamellar microstructures encompass lamellae of primary α phase and β transformed structure which consists of ultrathin secondary α lamellae and β thin layers. Thanks to the higher Al concentration in the primary α lamellae associated with the higher annealing temperature and finer scale of βt structure lamellae, the α and βt 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 α and βt 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 α and βt lamellae accounted for approximately 35% and 28% of the total strength increment, respectively, while the heterostructure induced hardening associated with the inhomogeneity between the α and βt 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.