PNNL

Abstract

Specimens of 316 L stainless steel were irradiated to 0.5-10.3 dpa at 30-80 ºC with a mixture of 500-800 MeV protons and spallation neutrons at the Los Alamos Neutron Science Center (LANSCE). Tensile test results of irradiated 316 L that were reported earlier had showed hardening and embrittlement with increasing irradiation dose, with significant irradiation hardening occurring at a dose of as low as 0.5 dpa. Transmission electron microscope (TEM) examination of the irradiated microstructure of 316 L showed black spot damage (small loops) and somewhat larger faulted Frank loops to produce the hardening. There was an initial decrease in uniform elongation at low dose levels from 49% (unirradiated) to 30% at 1.1 dpa, followed by a second, rather abrupt loss of uniform elongation at higher doses (~2.5 dpa) from 21% at 2.5 dpa to 0.5% at 3 dpa. This second drop in ductility was not accompanied by any visible new or enhanced microstructural development of microstructure. The total elongation did not drop as abruptly as the uniform elongation and never approached near-zero values, but plateaued out at ~10%. In the current study additional transmission electron microscope investigation was conducted on both as-irradiated and irradiated plus subsequently deformed 316 L in the vicinity of the second abrupt ductility loss at ~2.5 dpa. The steel was observed to deform mainly by twinning and no brittle phases were found in the deformation microstructure. It is proposed that gas accumulation with increasing dpa, especially of hydrogen, may be a contributor to this second abrupt decrease in uniform elongation. Although the retained gas (helium and hydrogen) levels approached ~1 at.% total at the highest exposure level, no discernible cavities were observed.