Radiation-Hardening and Recovery in Mild Steels and the Effects of Interstitial Nitrogen

Abstract

The effects of interstitial nitrogen on the hardening produced in mild steels by irradiation at 50° C (323 K) to doses up to 2·6 × 10¹⁸ n/cm² (fission) have been investigated. These studies were performed using annealed silicon-killed steel with a finite concentration of nitrogen in solution, annealed silicon-killed steel in which the interstitial nitrogen had been progressively reduced by ageing at 650° C (923 K), and aluminium-grain-size-controlled mild steel with zero nitrogen in solution. The recovery of the radiation-hardening in the annealed silicon-killed steel has also been studied. The irradiation-induced increases in the room-temperature tensile lower yield stresses were used as a measure of the radiation-hardening and the interstitial nitrogen contents were determined qualitatively from the strain-ageing behaviour and quantitatively by low-frequency internal friction. The results confirm that interstitial nitrogen enhances the radiation-hardening. The nitrogen in solution in the annealed silicon-killed steel is progressively removed with increasing neutron dose, the strain-ageing being virtually eliminated and the interstitial nitrogen content reduced to zero after exposure to ∼ 1·2 × 10¹⁸ n/cm² (fission). The nitrogen returns to solution and the strain-ageing and radiation-hardening are recovered in the same annealing range (300–400° C) (573–673 K). An analysis of the experimental data indicates that during irradiation at 50° C nitrogen reduces the “effective” self-interstitial jump rate by a factor of ∼ 4 × 10⁴; furthermore it is suggested that the nitrogen atoms are trapped at the peripheries of small vacancy and interstial clusters created during irradiation, tentatively assumed to be faulted (110) loops. The net effect is the nucleation and stabilization of small defect loops whose diameters have been calculated to be less than the resolution limit of the electron microscope (∼ 25 Å). It is concluded that nitrogen enhances the radiation-hardening in mild steels by increasing the defect-loop density and/or by stabilizing the small faulted (110) loops. The latter are inherently “stronger” with respect to dislocation interaction than the perfect loops on (111) planes observed in b.c.c. metals irradiated to high doses, which are reported to be formed by shear and rotation when (110) loops achieve a critical size.