Nitrogen vacancy complexes in nitrogen irradiated metals

Abstract

I -Gas desorption and positron annihilation techniques have been employed to study@ $ *F metals has a rather high affinity to vacancy type defects. The results obtained for low irradiation dose show that substitutional nitrogen (NV; with V-vacancy) is formed. The nitrogen vacancy complex dissociates at temperatures ranging from 350 K for Ni to 900K for Mo and 1 100 K for W. At high doses defects are fonned which can be characterized as nitrogen saturated vacancy clusters. These defects, as observed by helium probing, disappear during annealing for nickel at SOOK, and for Mo at 1100 K. The direct observation of the desorbing nitrogen for nickel and molybdenum reveals a very fast desorption transient at the dissociation temperature of the clusters. This is the characteristic desorption transient of a small nitride cluster,e,g., by shrinkage with constant rate. For iron the nitrogen desorption is more complicated because of a general background that continuously rises with temperature. With the positron beam technique depth information was obtained for defects in iron and the defect character could be established with the help of the information provided on annihilation with conduction and core electrons of the defect trapped positrons.nitrogen associated defects in nitrogen irradiated metals: Fe, Ni, Mo and W. Nitrogen O in T O f t ese

Theoretical Study of Nitrogen Absorption in Metals

“…Nitrogen binding in the bcc metals and their alloys has been widely reported both theoretically and experimentally. In the literature, multiple experiments to measure nitrogen binding and diffusion have been performed for various metals (e.g., V, ,− Nb, , Ta, ,,, Cr, − Mo, ,− and Fe). , Also, theoretical studies focusing on nitrogen in the bulk metals have supported the O-site binding of nitrogen in V, W, and Fe. , Lattice expansion often occurs due to the large atomic size of nitrogen compared to the size of interstitial sites. ,, When other defect sites, such as vacancies, grain boundaries, and dislocations, exist in metal structures, those sites tend to exhibit stronger binding of nitrogen. ,, However, only limited number of nitrogen binds to those sites since the defect concentrations are limited, and the mobility of defects decreases with nitrogen binding. Introducing nitrogen to metals is often referred to as “nitriding.” In steel manufacture, nitriding is a metal strengthening process to enhance mechanical properties of steels.…”