2.2.1 Special remarks

This chapter discusses special remarks of bcc metals. The bcc structure receives attention in defect research for the transition metals, for the alkali metals, and for the bcc high-temperature phases of group IV B (Ti, Zr, Hf). Defect parameters of bcc metals are tabulated. The threshold energy T_d for the displacement of a lattice atom by irradiation with fast particles depends on the direction of the incident particle with respect to the orientation of the crystalline specimen, as studied by e^--irradiation experiments. For e^--irradiations defect creation can be described by a geometrical model for the threshold-energy surface. The resistivity recovery following e^--irradiations shows low-temperature recovery stages related to the annihilation of “close Frenkel pairs” (“close-pair recovery”). For bcc metals stage I could be observed so far only in Cr, Mo, W, and a-Fe. The bcc metals which allowed a detailed investigation of stage I, (Mo, W, a-Fe, while Cr suffers by impurity problems), show a modification of correlated and uncorrelated recovery in comparison to fcc metals. Low-temperature recovery spectrum of the bcc metals is illustrated. Correlated and uncorrelated recovery in the bcc metal i.e. Mo, is plotted. Several bcc metals show intrinsic first-order recovery stages above the onset of long-range migration. Arrhenius plot of self-diffusion against T_m/T for bcc metals is illustrated. The metals contain usually large amounts of H, 0, N, C as interstitial solute atoms. The action of H, and O, N, C on stage III, as observed in Nb and Ta, is schematically outlined.