Visualization of hydrogen distribution around blisters by tritium radio-luminography

Hydrogen distribution around blisters on aluminum (Al) and molybdenum (Mo) was examined by tritium radio-luminography, i.e. tritium autoradiography (TARG) and an imaging plate technique. Tritium accumulated in the blisters on Al surface was successfully visualized at the first time. The tritium density in the blisters was found to increase with their radius to the power of 2.3. This supports the blister mechanism of bubble coalescence but the blister shape was flattened along the surface with increasing their size. For Mo, tritium distribution was not well correlated with blisters, and the bubbles coalescence was not clearly observed, too. But the erosion or exfoliation of thick layers with wider area than blisters were observed and hydrogen was released by the exfoliation of the thick surface layers, remaining not tritium on the exfoliated surface. Such exfoliation is very likely caused by mechanical stress given by accumulated hydrogen at trapping site such as grain boundaries, intrinsic defect, or self trapping.