X-ray diffraction analysis of concentration and residual stress gradients in nitrogen-implanted niobium and molybdenum

Large biaxial residual strains are developed after a 5-at.% implantation of N into Nb and Mo. The results indicate that the dominant source of internal strain arises from N located in interstitial sites. For Nb implanted at liquid-nitrogen temperature, the N atoms are located in octahedral sites. However, the data allow for some clustering as di- or tri-interstitials at the highest concentration (∼5 at. % N). Radiation damage is present as small vacancy and interstitial loops. Since vacancies and self-interstitials are present in nearly equal concentrations, the overall bulk dilatation cancels. However, because of their small size, a lesser core expansion has been included as a correction to the overall residual strain. Although one can obtain an estimate of the N distribution from TRIM, a more accurate description must include the distribution of knock-on energy. The latter has an important influence on the redistribution of N relative to that predicted by TRIM. Both host lattices (Nb and Mo) behave like ''rigid containers'' in directions parallel to the free surface and give a magnified elastic response normal to the free surface