Microstructure tailoring and impact toughness of a newly developed high strength Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe alloy

To achieve preferable impact performance of a newly designed high strength Ti-5321 alloy, samples with three kinds of microstructures, i.e., bimodal microstructure (BM), tri-modal microstructure (TM) and lamellar microstructure (LM) were prepared for instrument Charpy tests at room temperature. Results of impact tests indicated that LM presented the higher impact toughness of 37.5 J/cm² than that of BM (11.25 J/cm²) and TM (25 J/cm²). Moreover, both crack initiation energy and crack propagation energy of LM were obviously raised. Further analysis containing fracture surfaces and the cross-sectional microstructures of the impact samples by the usage of SEM and EBSD provided insights into the influence of microstructure on crack initiation and propagation during impact process. The largely plastic deformation of primary α near the V-notch tip impeded crack initiation and enhanced the crack initiation energy. The interweaved α colonies with high angle boundaries deflected the crack propagation direction and thus increased the crack propagation energy. Besides, TEM analysis showed that the typical deformation characteristic under impact test with high strain rate. Except for dislocation lines, the pile-up of highly dense dislocations, twinning, dislocation cells, sub-grains, shear bands and fragmentation of α phase can be found. These approaches presented in this study could provide certain insights into fabricating typical microstructure in the high strength titanium alloys to ameliorate impact properties.