Fatigue crack growth behavior in niobium-hydrogen alloys

Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (ΔKth) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration (Ccr), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds Ccr, ΔKth increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, ΔKth decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.