Effect of hydrogen on the tensile properties of vanadium at moderate temperatures

Tensile tests in the temperature range of 293 and 573 K and microscopic observations were carried out on vanadium and V-high hydrogen concentration (1240-3410 wt ppm H) alloys. Examining the true stress (σ)-true plastic strain (epsilon sub(p)) curves and the temperature and strain-rate dependence of the tensile properties, the following results were obtained: (1) The relation between σ and epsilon sub(p) was well represented by the equation σ=K sub(epsilon sub(p)) sup(m), where K is a constant and m is the strain hardening exponent. The value of m was measured to be 0.34 for vanadium and 0.11 -- 0.15 for V-H alloys at 293 K, which slightly increased with increase in test temperature up to 473 K for vanadium, but decreased for V-H alloys. This decrease of m for V-H alloys might involve the resolution of hydride: under the tensile test at a temperature where hydride is completely resolved into the matrix, the alloys show the same behavior as vanadium subjected to prestraining to a stress level equal to the yield stress of the alloys at the specified temperature; consequently the value of m for the alloys is smaller than that of vanadium. This fact was attributed to the dislocation structure which formed when the hydride was produced and still remained after the hydride was resolved; the existence of the structure was confirmed by electron microscopy in the dehydrogenated alloy. Hydride, if it exists in the alloys, contributes to the strengthening of the alloys but induces embrittlement. (2) The temperature and hydrogen concentration dependence of the tensile properties were also explained by consideration (1) mentioned above. (3) As the ductile-brittle transition temperature of the alloys was lower than the solvus temperature at which hydride precipitates, it was considered that the hydride itself contributed mainly to the embrittlement of the alloys. (4) Anomalous embrittlement of V-H alloys containing about 3000 wt ppm or more H was observed at 423 K. (auth.)