Surface impurity diffusion-induced recrystallization of ultrafine-grained molybdenum

Abstract

Methods of scanning and transmission electron microscopy are used to perform a comparative analysis of the structurе evolution of ultrafine-grained molybdenum subjected to nickel-free annealing and to diffusion annealing accompanied by nickel penetration from the surface into the bulk of Mo(Ni) specimens. The kinetics of nickel diffusion-induced recrystallization of ultrafine-grained molybdenum in the temperature interval 1123-1223 K is investigated and the recrystallization activation energy is determined. Plausible reasons for a decrease in the nickel diffusion-induced recrystallization temperature of ultrafine-grained molybdenum as compared to its fine-grained cousin are discussed. The nickel diffusivity in ultrafine-grained molybdenum along grain boundaries is estimated on the basis of the results obtained.

Effect of Grain Boundary State on Diffusion and Diffusion-Controlled Processes in Ultrafine-Grained Materials Processed by Severe Plastic Deformation

“…Thus, at room temperature tension of UFG nickel the localized deformation zones with boundary migration and recrystallization are revealed [27], whereas after severe rolling the structure of UFG nickel appears to be recrystallized even at liquid nitrogen temperature [28]. The effect of the state of grain boundaries on recrystallization and grain growth is well illustrated by the results of comparative studies on activated recrystallization in fine-grained (grain sizes of ~ 5 µm) and UFG (element sizes of the grain-subgrain structure of about 0.2-0.5 µm) molybdenum [29]. The activated recrystallization of metals occurs at reduced temperatures during diffusion of substitutional impurity atoms from the external medium (coating).…”

The applied stress effect on the diffusion-activated recrystallization of ultrafine-grained molybdenum

“…The latter, as a rule, have equilibrium grain boundaries. Submicrocrystalline molybdenum produced by high-temperature torsion was found to exhibit recrystallization activated by grain-boundary diffusion fluxes of substitutional impurity atoms from the molybdenum surface at temperatures 150-200 K lower than for coarse-grained polycrystals [2].…”