The mechanism of hydrogen embrittlement of niobium has been studied using a high voltage electron microscope in the temperature range between 120 and 300 K. Crack propagation by repeated hydride formation and its failure has been demonstrated to be operative in niobium. In contrast to the embrittlement behavior of bulk niobium reported in the literature, the brittle temperature range in thin foils is much more restricted; in thin foils, ductile to brittle transition starts at a temperature only slightly higher than the solvus, while in bulk specimens, it sets in at a temperature more than 70 K above the solvus temperature. Possible sources of the different behavior are suggested. The two-step ductile-brittle transition reported for bulk specimens has been explained by considering a large thermal hysteresis of precipitation and dissolution of niobium hydride.
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