Effect of hydride precipitation behavior on fracture morphology for Ti-6Al-4V alloy

An investigation has been made into the relation between hydride precipitation behavior and the fracture morphology for Ti-6Al-4V alloy by means of fractography and internal friction measurement. As the specimens thermally hydrogenated at 1273K for 7.2ks were furnace-cooled (β annealed specimen), the hydrides precipitated finely from the β phase during cooling. The fracture morphology showed the fracture at the interface between β phase and the hydrides or inside the hydrides. The strength of the hydrides was 0.5 GPa and smaller than that of the matrix and the elongation was hardly present. The hydrides act as the fracture initiation site. As the specimens thermally hydrogenated at 1273K for 7.2ks were water-quenched (β quenched specimen), the hydrides hardly precipitated and hydrogen was solutionized by force in the β phase. As the β quenched specimens were plastic-deformed, the hydrides precipitated from the β phase. As the tensile strain rate was fast, the fracture morphology showed the morphology similar to the β annealed specimen thermally hydrogenated. It means that the hydrides precipitated finely from the β phase by plastic deformation, the fracture occurred at the interface between the hydrides and β phase or inside the β phase. As the tensile strain rate was low, the fracture morphology showed the terrace fracture inside the hydrides. It means that the hydrides precipitated from the β phase, grew to the large blocks independent of the microstructure, and then the fracture occurred inside the hydrides. (author)