Thermal diffusivity, microstructure and nanohardness of laser-welded proton-irradiated Eurofer97

Eurofer97 steel, a candidate structural material for future fusion reactors, was examined following 1.9 MeV proton-irradiation up to 0.91(5) dpa at 450 degrees C with and without a prior post-weld heat-treatment at 760 degrees C for 4 hours in a laser-welded state. A nanoindentation study found a pile-up-corrected nanohardness of 4.0(4) GPa in the as-welded fusion zone, decreasing to 2.1(3) GPa in the parent material. Irradiation temperature and post -weld heat treatment were both found to have a recovery effect on weld hardness, with the latter being entire. Proton-irradiation damage was not found to contribute to nanohardness at the temperature investigated. X-ray diffraction analysis found increased 1-dimensional dislocation density in the as-welded fusion zone, diminishing to 3(2) - 19.2(1.4)x1014 cm-2 in the parent material, dependent on irradiation and heat-treatment. Transient grating spectroscopy of Eurofer97 was attempted, finding a systematic underestimation of thermal diffusivity of average 15.6% from room-temperature to 600 degrees C when compared to laser flash analysis. Transient grating spectroscopy was, nevertheless, applied determining a room-temperature thermal diffusivity of 7.8(3) mm2 s-1 in the parent material and 6.7(4) mm2 s-1in the as-welded fusion zone. Irradiation at 450 degrees C alleviated this difference in thermal diffusivity; recovery of weld-induced changes was observed up to 20% in the fusion zone due to irradiation conditions, distinct from temperature effects alone. Such results bode well for Eurofer97'sapplication in fusion reactors, where welding will be essential. Thermal diffusivity has also been mapped at a fine scale across a heterogeneous structure, a technique applicable widely outside the realm of radiation materials science.

Automated summary

Eurofer97 steel was studied to investigate the effects of irradiation and heat treatment on weld hardness and thermal diffusivity. The results showed that both the irradiation temperature and post-weld heat treatment had a recovery effect on weld hardness. Proton-irradiation damage did not contribute to nanohardness. X-ray diffraction analysis revealed an increased dislocation density in the fusion zone, which decreased in the parent material. The thermal diffusivity was found to be systematically underestimated by transient grating spectroscopy compared to laser flash analysis.