EFFECT OF MICROSTRUCTURE ON SUSTAINED LOAD CRACKING BEHAVIOR OF TI-6AL-4V ALLOY

Sustained load cracking behavior of Ti-6Al-4V was studied by constant load testings and constant cross head speed tensile testings using compact tension specimens for four different microstructures. Annealed specimens and duplex annealed specimens showed stable crack growth without incubation time during constant load testings for initial K values which were considerably low, compared with K(IC), while beta annealed specimens and solution treated and aged specimens showed no such behaviors. In the stable crack growth, the rate of COD change decreased with decreasing the load. Fractography and micrography revealed that the stable crack propagation occurred with quasi-cleavage fracture of primary alpha phase. In constant cross head speed tensile testings, the fracture toughness of annealed specimen decreased with decreasing the cross head speed, while beta annealed specimens showed no such behavior. The stable crack growth of annealed specimen initiated at a stress intensity factor about half of K(IC) at the lowest cross head speed.