Dislocation hardening and phase transformation-induced high ductility in Ti-6Al-4V with a heterogeneous martensitic microstructure under tensile load

Abstract

In this work, a kind of heterogeneous martensitic microstructure was successfully fabricated within the fine β grains of Ti-6Al-4V through a rapid heating and cooling processing (RHCP) method, resulting in excellent ductility during tensile deformation. A total elongation was reached at 25.1%, while a considerable tensile strength of 1118 MPa was maintained. The main reasons for achieving this good ductility were the occurrence of dislocation hardening and deformation-induced phase transformation during tensile deformation. We used transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) to characterize and analyze the dislocation configurations and new face-centered cubic (FCC) phase during tensile deformation. We found that, to accommodate plastic deformation between the microcrystalline soft lamellae and ultrafine hard lamellae, many geometrically necessary dislocations (GNDs) were generated, which inevitably led to dislocation hardening, thereby greatly improving the ductility of the alloy. As a result of dislocation hardening, the improved ductility induced two types of FCC phase transformations, and the FCC phase quantity increased significantly during further tensile deformation. Plastic deformation can be effectively accommodated by the formation of a new FCC phase so that the ductility of the material can be further improved. These accommodating plastic deformation mechanisms reveal the nature of the good ductility of the material under a tensile load and provide a new strategy for fabricating strong and ductile titanium alloys.