Initial microstructure effect on the mechanical properties of Ti-6Al-4V ELI alloy processed by high-pressure torsion

Abstract

effect of initial microstructure on the mechanical properties and fracture mode of the Ti-6Al-4V ELI alloy subjected to the high-pressure torsion (HPT) processing at room temperature and 500 °C was investigated. In this purpose, the four different microstructures (fully lamellar, martensitic, equiaxed and globular microstructures) were developed by the proper heat treatments. The results showed that the application of the HPT processing provided substantial microstructural refinement, independent on the alloy initial microstructure. As a consequence, the alloy hardness, tensile strength and ductility significantly increased. At the same time, the elastic modulus of the HPT-processed alloy is almost 2 times lower as compared to that of the alloy in the initial heat treated state. The obtained improvements strongly depend on the alloy initial microstructure and HPT processing temperature. It was found that the initial martensitic microstructure is beneficial to improve the hardness, tensile and fracture properties. The HPT processing at 500 °C enhanced them more effectively. After the warm HPT processing, alloy with the initial martensitic microstructure exhibited exceptional hardness (455 MPa), high ultimate tensile strength (1546 MPa), high elongation to failure (18.8%) and low elastic modulus (78.6 GPa). Although the alloy with initial fully lamellar microstructure displayed the lowest elastic modulus (68 GPa), the ultrafine and homogeneous (α + β) two-phase microstructure produced by the HPT processing of the alloy with initial martensitic microstructure offered the best combination of the strength-ductility balance and strength-to-elastic modulus ratio (19.8 × 10−3). The complex fracture process, involving transgranular quasi-cleavage and ductile dimple fracture mode, corresponded to the changes in ductility.