Improving the strength and maintaining good ductility of as-forged Ti6Al4V alloy by regulating the microstructural defects

Abstract

Solution-aging treatment is a conventional method for enhancing the strength of titanium alloys contributed by the decomposition of metastable phases, like martensite. The traditional viewpoint suggests that the martensite transformation occurring after water quenching does not significantly contribute to the strengthening of titanium alloy and this perspective actually overlooks the high strength resulting from the high-density defects formed in the martensite microstructure due to rapid cooling. So, previous studies have not fully taken advantages of the excellent strengthening capability of martensite. This study adopted a technique which combined solid solution and incomplete aging treatment to control the decomposition behavior of martensite and as a result, the strength of the as-forged Ti-6Al-4V (TC4) alloy had been obviously improved and good ductility had been maintained. Furthermore, the changes of phase structure and microstructural defects in TC4 titanium alloy during solution and aging process have been detailly studied by XRD refinement, EBSD, and TEM. Specifically, the highest tensile strength increased by 28.8 % which could reach 1224 MPa, and the elongation at fracture could still remain at 13 %. The reason is as follows. The formation of martensite after water quenching led to a substantial increase in internal microstructural defects, which were partly retained during the subsequent aging process because of lower temperature and short aging time, and the remaining martensite and microstructural defects can help maintain high strength. What's more, a small part of defects disappeared during this incomplete aging process due to martensite decomposition and recovery, which was beneficial to ductility.