The influence of ageing treatment on the microstructural evolution and the mechanical properties of the TI-6AL-4V alloy

Abstract

The Ti-6Al-4V alloy is used in aerospace parts and its microstructure and properties can be controlled by solution treatment or solution treatment followed by ageing. However, there are different views on whether solution treatment above the beta transus temperature followed by ageing can improve the microstructure and mechanical properties of the Ti-6Al-4V alloy. This study investigated the influence of ageing treatment conditions (ageing time, temperature and cooling rate) on the microstructural evolution and the response of the mechanical properties. Specimens of the Ti-6Al-4V alloy were solution-treated at 1050 C for 30 minutes in a furnace in an oxygen atmosphere and water-quenched (WQ), or furnace-cooled (FC) to room temperature. The specimens were subsequently aged at temperatures of 500 C to 900 C for 0.5 hours to 48 hours, followed by either FC or WQ. Microstructural analysis, hardness measurements, tensile properties, fracture toughness, and fatigue crack growth rate were determined. The ageing temperatures of 500 C to 650 C produced greater hardness, yield strength, and ultimate tensile strength with optimum ageing times of 0.5 hours for samples that were solution-treated and WQ. As ageing temperature and time increased, hardness, yield strength and ultimate tensile strength decreased due to the increase in the volume fraction of the α phase and the dissolution of the martensitic needles. The best strength-ductility balance was obtained from specimens that were solution-treated and WQ, followed by ageing at 900 C for 24 hours and FC. Ageing of samples that were solution-treated and FC (colony lamellar) showed hardness, yield strength, and ultimate tensile strength to increase as ageing time and temperature increased. The increase was attributed to the increase in the beta content with ageing and subsequent martensitic phase transformation after quenching. Solution treatment and WQ followed by ageing was found to result in improved mechanical properties of the Ti-6Al-4V alloy for use in the aerospace industry. The colony lamellar specimens gave greater fracture toughness values than the fully martensitic specimens. In addition, the fatigue crack growth resistance was superior for the colony lamellar specimens than for the fully martensitic specimens. The random orientation of the α/β-colonies in the lamellar microstructure led to crack branching and formation of secondary cracks, resulting in higher fracture toughness and fatigue crack growth resistance. The fully martensitic morphology had brittle fractures leading to smoother fracture surfaces. The aged, fully martensitic specimens gave rise to greater fracture toughness than the fully martensitic specimens due to the larger α-plates, while the fatigue crack growth resistance was found to be similar.