Damage Structures and Mechanical Properties of High-Purity Fe–9Cr Alloys Irradiated by Neutrons

Abstract

The tensile and impact behavior and microstructures of neutron-irradiated Fe–9Cr alloys with purity levels of 99.99 and 99.8 mass% have been examined. The neutron irradiation was mainly performed at about 255°C or 290°C to 0.3 dpa in the JRR-3M (Modified Japan Research Reactor-3). The increment of yield strengths in the high-purity (HP) and low-purity (LP) alloys induced by the irradiation was 225 and 170 MPa, respectively. The elongation in the HP alloy was remarkably reduced by the irradiation, and it was smaller than that in the LP alloy. The shift of the ductile-brittle transition temperature of HP alloy was about 175°C, and the shift was larger than that of the LP alloy. In TEM observations, dislocation loops were observed in HP and LP alloys. α′-like precipitates, about 6 nm in diameter, were observed on loops in the HP alloy, but in the LP alloy only dislocation loops were observed. Based on the theory of dispersed barrier hardening, the barrier strength factors of dislocation loops in HP and LP alloys are estimated to be about 0.4 and 0.3, respectively. The increase of the strength factor of loops in the HP alloy is thought to relate to the formation of the α′-like precipitates on the loops. From the relations between these mechanical properties and microstructures, the decrease in ductility of the Fe–9Cr alloy may be caused by the formation of α′-like precipitates on the dislocation loops.