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

Abstract

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 ∘C with and without a prior post-weld heat-treatment at 760 ∘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)×1014 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 ∘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−1 in the as-welded fusion zone. Irradiation at 450 ∘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's application 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.