Effects of Neutron Irradiation on Mechanical Properties of Iron-Nitrogen Alloys

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Abstract
Fe-N alloys containing 150 ppm N were neutron irradiated in the JMTR to 5.5×1019 n/cm2, then deformed in tension at temperatures ranging from that of liquid nitrogen to 250°C. Irradiation was found to increase markedly the yield, tensile and fracture stresses, and to decrease the elongation. Irradiation further had the effect of shifting upward the range of testing temperature in which serrations appeared in the stress-strain diagram, and to lower the work hardening coefficient. The alloys were examined by transmission electron micrography both as irradiated and after being deformed to observe the changes in internal structure. Irradiation was found not to induce deffects in the iron matrix; the precipitates, on the other hand, revealed in some parts the presence of radiation defect clusters. Heat treatment at 300°C for 100 hr removed the precipitates in the irradiated specimens, leaving many dislocation loops in the matrix. These dislocation loops were of the vacancy type with Burgers vectors corresponding to a/2 , and lay on the {111} plane. Radiation anneal hardening was observed in the irradiated specimens in the occurred range of heat treatment temperature between 100 and 600°C. It would appear that the stable phase Fe4N and the remaining entangled dislocations tend to enhance the hardening due to the post-irradiation annealing, with an effect similar to that produced by nitrogen-vacancy complexes, interstitial cluster, metastable precipitates and dislocation loops.

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