부산대학교 도서관

Different cooling rates during duplex-annealing of mill-annealed Ti-6Al-4V 1-in. plate produce essentially indistinguishable microstructures and similar tensile properties, but significantly varying fracture properties. Duplex-annealing was performed by an initial heating at 1775°F (968.3°C) for 1/2 h and air-cooled; then reheated at 1450°F (787.4°C) for 1 h and cooled (1) by water quench, (2) in the air, or (3) in the furnace. Yield strength values, relatively unaffected by the annealing and cooling rate treatments, were grouped at 140±2 ksi (965 ± 14 MPa). Fracture toughness, however, varied from a low of 38 ksi √in. (42 MPa · √m) in the material with the fastest cooling rate (1) to nearly similar values of 50 and 51 ksi √in. (55 and 56 MPa · √m) for cooling rates (2) and (3), respectively. The stress-corrosion crack-growth, threshold stress intensities for the three cooling rates were (1) 34 ksi √in. (37.5 MPa · √m), (2) 30 ksi √in. (33 MPa · √m), and (3) 22 ksi √in (24.2 MPa · √m). Differences in fatigue crack propagation resistance brought about by the three cooling rates are illustrated with safe operating lifetimes obtained by computer simulation for a typical engineering application. The results reveal a fivefold advantage in safe life for air-cooled material. Although the fracture properties exhibited significant variation with cooling rate from the final anneal, light optical microscopy to a magnification of 3000 diameters did not reveal significant microstructural differences. Scanning electron fractographic evidence, however, is consistent with an inference that local ordering, that is, a precipitation process involving Ti3 Al, is the most probable reason for the furnace-cooled material's more rapid fatigue-crack propagation rates and enhanced susceptibility to stress-corrosion cracking.