EFFECT OF HYDROGEN AS A TEMPORARY BETA-STABILIZER ON MICROSTRUCTURE AND BRITTLE-FRACTURE BEHAVIOR IN A TITANIUM ALUMINIDE ALLOY

Microstructures of Ti-24Al-11Nb were solution treated in either the alpha-2 + beta or beta phase field under vacuum or hydrogen gas, followed by air (AC), furnace (FC), or controlled cooling (CC), and then aged or outgassed at 800-degrees-C. All mechanical tests were on hydrogen-free specimens. If 1147-degrees-C beta solution treatment (ST) was followed by AC, hydrogen as a temporary beta stabilizer clearly decreased the size of the alpha-2 phase and increased greatly the retained or transformed beta phase content. As a result, room-temperature tensile strength, fracture strength (sigma-F), ductility (epsilon-f), and fracture toughness (K-ic) resulting from 1147-degrees-C solution treating in hydrogen increased by 120, 100, 60, and 42 pct, respectively, as compared with that under vacuum. If beta ST was followed by FC, hydrogen as a beta stabilizer increased only the beta phase content but did not decrease the size of alpha-2 phase or colony; and made ultimate tensile strength (UTS), sigma-F, epsilon-f, and K(Ic) increase only moderately. Cooling from the alpha-2 + beta phase field (1000-degrees-C or 1075-degrees-C), hydrogen as a beta stabilizer changed the size of transformed alpha-2 phase and UTS, sigma-F, epsilon-f, and K(Ic) increased slightly. Properties were also a function of crosshead speed. For all microstructures, alpha-2/beta boundries appeared to be effective barriers to slip and crack initiation.