Microstructure, Grain Growth and Hardness of Nanostructured Ferritic ODS Steel Powder during Annealing

Abstract

Nanocrystalline oxide-dispersion strengthened ferritic alloy formation and its annealing behavior were examined through modern X-ray diffraction pattern analysis and supplemented by microhardness and microscopic measurements. The basic microstructure features, with particular emphasis on evolution of domain size distribution and defect content during mechanical and thermal treatment, were quantified via the whole powder pattern modeling approach. The microstructure of the powdered alloy, formed during mechanical alloying, evolved toward nanocrystalline state consisting of narrow dispersion of very fine crystallites with substantial dislocation density, which exhibited relatively high stability against elevated temperature. It was shown that crystallite size is seriously sustained by the grain-boundary strain, therefore coarsening of grains begins only after the density of dislocations drops below certain level. Obtaining correct results for the annealing-related data at specific temperature range required the incorporation of the “double-phase” model, indicating possible bimodal domain size distribution. The dislocation density and grain size were found not to be remarkably affected after consolidation by hot isostatic pressing.

Structural evolution of mechanically alloyed ODS steel powders during ball milling and subsequent annealing treatment

“…Concomitant influence of grain growth and formation of intermetallic, which were unfavorable and favorable impact on hardness resulted in such an indiscernible difference on hardness performance. One reason for this case could be originated from combined influences grain coarsening and disappearing of structural defects inside of the powders due to thermal softening triggered by elevating annealing temperatures as reported by Nowik and Oksiuta [42].…”