Effect of hydrogen as a temporary β stabilizer on microstructure and brittle fracture behavior in a titanium aluminide alloy

Abstract

Microstructures of Ti-24A1-11Nb were solution treated in either the α2 + β or β phase field under vacuum or hydrogen gas, followed by air (AC), furnace (FC), or controlled cooling (CC), and then aged or outgassed at 800°C. All mechanical tests were on hydrogen-free specimens. If 1147 °C β solution treatment (ST) was followed by AC, hydrogen as a temporary β stabilizer clearly decreased the size of the α2 phase and increased greatly the retained or transformed β phase content. As a result, room-temperature tensile strength, fracture strength (σF), ductility (ef), and fracture toughness(KIc) resulting from 1147 °C solution treating in hydrogen increased by 120, 100, 60, and 42 pct, respectively, as compared with that under vacuum. If β ST was followed by FC, hydrogen as a β stabilizer increased only the β phase content but did not decrease the size of α2 phase or colony; and made ultimate tensile strength (UTS), σf, ef, andKIc increase only moderately. Cooling from the α2 + β phase field (1000 °C or 1075 °C), hydrogen as a β stabilizer changed the size of transformed α2 phase and σf, ef, andKIc increased slightly. Properties were also a function of crosshead speed. For all microstructures, α2/β boundaries appeared to be effective barriers to slip and crack initiation.