Grain size and hydrogen concentration effects on the ductility return in a refractory alloy

The effects of grain size (in the range of 30 to 220 μm) and hydrogen concentration on the ductility return of a hydrogen doped 25 at. pct V-75 at. pct Nb alloy was investigated at low strain rates. Since this alloy has a large terminal hydrogen solubility and since the hydrogen additions were kept low, hydride formation seems not to contribute to the alloy's embrittlement. It was found that the ductility return is present only for small grain sizes and low hydrogen concentrations. Cracks leading to failure were initiated intergranularly. It appears to be unlikely that the ductility return, if present, is caused by the low diffusivity of hydrogen in that temperature range. A qualitative model is proposed to explain the observations