Effects of microstructures on the notch tensile fracture feature of heat-treated Ti–6Al–6V–2Sn alloy

The notch tensile strength (NTS) of Ti–6Al–6V–2Sn (Ti–6–6–2) specimens heat-treated using distinct processes was investigated at various aging temperatures and correlated to microstructures. Supratransus-processed and subtransus-processed specimens were, respectively, solution-treated at 965 °C (above the β transus) and 900 °C (below the β transus), before being water-quenched and aged at specific temperatures (482 °C, 593 °C, and 704 °C). Following the water quenching, titanium martensite (αʹ or α″) formed and decomposed into α and β phases with various sizes while aging. Additionally, the specimens that were aged at 482 °C achieved the peak hardness, but they were susceptible to notch brittleness and exhibited inferior NTS because a predominantly transgranular quasi-cleavage fracture separated along the α/β interface, regardless of the type of solution treatment. As the aging temperature increased, the NTS improved remarkably. This was attributed to the coarse α+β colony structure that formed in the aged specimen. The structure can facilitate crack-branching or micro-cracking under tensile straining, thereby improving fracture toughness and notch blunting while testing the NTS. Furthermore, under the same aging treatment, the NTS of the subtransus-processed specimens was greater than that of the supratransus-processed specimens, particularly for the specimens aged at 482 °C and 593 °C, which resulted from the retarding effect of the primary α on the crack front, higher volume fraction of the β phase, and absence of the intergranular fracture in the subtransus-processed specimens while executing the notch tensile test.