Effect of Solution Treatment and Aging Temperature on α′ and Ti3Al(α2) Phase Formation and Mechanical Properties of Water-Quenched Ti–6Al–4V

Ti–6Al–4V is a well-known alloy used in the aerospace industry. In the present study, the effect of solution treatment (annealing) and aging temperature on microstructural and, consequently, mechanical properties were studied. Initially, heat treatment was performed at temperatures of 850, 900, 950 and 1000 °C for 1 h, followed by water-quenching, and then aged at 500 and 600 °C. The mechanical properties of the samples were examined by hardness and tensile strength tests. The microstructure of the samples was characterized by X-ray diffraction analysis and by optical and scanning electron microscopy, which showed that the structure of the obtained martensite has a great role in increasing the strength of the samples, especially after aging. The microstructure solution treated below the β transition temperature (unaged) consists of the martensitic needle phase (α′) with the spherical alpha (α p ) phase with light color and the beta phase (β) with dark color. The aging microstructure consists of the α p and β phases and the secondary alpha (α S ) and beta (β S ) phases resulting from the decomposition of the martensite phase. The mechanical strength indicated that the highest strength was obtained in the sample with a bimodal microstructure obtained by solution-treatment at 950 °C (temperature below the conversion of the α to the β phase), water-quenching, and aging at 500 °C. In fact, the bimodal microstructure containing Ti 3 Al (α 2 ) precipitation along with the α/β lamellar structure leads to strength increases and elongation decreases compared to the as-received sample. Increasing the aging temperature over 500 °C resulted in a strength decrease due to the growth of Ti 3 Al precipitation (over-aging) and the consequently increased precipitation distance.