Stress Corrosion Cracking of Alpha Titanium Alloys at Room Temperature

Abstract

Abstract Transgranular stress corrosion cracks are formed in Ti-5Al-2.5Sn alloy immersed in a 3 percent NaCl aqueous solution when tensile specimens are dynamically strained over a narrow range of rates. Metallographic evidence suggests that the critical process during crack propagation is entry of hydrogen into the alloy at the crack tip immediately following creation of fresh metal surface. Fractographic examination reveals that cracks propagate by a discontinuous cleavage mechanism. As each incremental growth is arrested, the embrittlement process resumes. Ductile fracture is observed in specimens strained (a) at high tensile rates because there is insufficient time for embrittlement to occur, and (b) at low tensile strain rates because repassivation occurs more readily and hydrogen entry is substantially reduced. In methanolic solutions containing HCl, an identical cleavage crack propagation process is observed. In addition, a slow intergranular dissolution mechanism is found in alloys susceptible and nonsusceptible to cleavage-type failure. This is initiated in specimens that have regions of high residual stress, e.g., sheared edges and continues until the mechanical strength of the alloy is reduced to a very low value. During this process hydrogen is picked up by the metal. Clevage has been observed in specimens broken in air after exposure. Vacuum annealing substantially reduces but does not eliminate this slower form of attack by removing initiation sites. Anodic polarization at low current densities produces extremely severe intergranular attack. The significance of dislocation arrangements, mechanical properties, and electrochemical reactions at the crack tip are discussed in detail. In particular, it is suggested that cathodic polarization can prevent cracking by forming films which reduce the rate of hydrogen ingress. In 10N HCl solutions, cathodic polarization does not prevent cracking.