TENSILE AND FATIGUE-STRENGTH OF HYDROGEN-TREATED TI-6AL-4V ALLOY

Tensile, fatigue and fractographic data on Ti-6Al-4V microstructures attained through a series of post-beta-annealing treatments which used hydrogen as a temporary alloying element are presented. Hydrogen-alloying treatments break up the continuous grain boundary alpha and colony structure, and produce a homogeneous microstructure consisting of refined alpha-grains in a matrix of discontinuous beta. These changes in microstructural morphology result in significant increases of the yield strength (974 to 1119 MPa), ultimate strength (1025 to 1152 MPa) and high cycle fatigue strength (643 to 669 MPa) compared to respective values for lamellar microstructures (902, 994, 497 MPa). The strengths are also significantly greater than the strengths of equiaxed microstructures (914, 1000, 590 MPa). The strengths of hydrogen-alloy treated samples are therefore superior to strengths attainable via other thermal cycling techniques. The fatigue fracture surfaces of the hydrogen-alloy treated samples were topographically similar to equiaxed samples. Fatigue crack initiation was characterized by faceted regions. As crack length and DELTA-K increased, the crack surface changed to a rounded, ductile topology, with microcracks and locally striated regions. Fracture primarily followed the alpha-beta interfaces. This is rationalized by the fact that hydrogen-alloyed microstructures are very fine Widmanstatten microstructures having reduced aspect ratios, and these microstructures fail along alpha-beta interfaces.