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

摘要

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 ℃ (above the p transus) and 900 ℃ (below the p transus), before being water-quenched and aged at specific temperatures (482 ℃, 593 ℃, and 704 ℃). Following" the water quenching, titanium martensite (α' or α") formed and decomposed into α and β phases with various sizes while aging. Additionally, the specimens that were aged at 482 ℃ 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℃ and 593 ℃, 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.