Hydrogen-induced slow crack growth in Ti-6Al-6V-2Sn

The effect of hydrogen and temperature on threshold stress intensity and crack growth kinetics was studied in Ti-6Al-6V-2Sn containing 38 ppm hydrogen. A slight decrease in threshold values occurred as temperature decreased from 300 K while they increased significantly above 300 K. For a given test temperature, crack growth rates exhibited an exponential dependence on stress intensity over a major portion of growth. At 300 K the rates reached a maximum. Slow crack growth occurred predominately by cleavage of α grains which has been associated with hydride formation. The stress intensity required for hydride formation at a crack tip can be determined from hydrogen concentration and solubility considerations under stress. As these values differed from observed thresholds, a strong influence of microstructure was suggested and subsequently revealed by crack front examination. Quantification of this effect with a modified Dugdale-Barenblatt model relates the effective stress intensity at the crack tip to the applied stress intensity. Microstructure was also found to exert a strong influence on slow crack growth behavior when examined in terms of the effective stress intensity, K _eff. From Arrhenius plots of crack growth rates for various K _eff, activation energies of 27.0 to 32.8 kJ/mol were obtained and related to the diffusion of hydrogen through the β phase. The increase in crack growth rates with increasing temperatures up to 300 K is attributed to the temperature dependence of hydrogen diffusion. The decrease in crack growth rates above 300 K is related to a hydride nucleation problem.