Formation and Microstructural Evolution of Ferritic ODS Steel Powders during Mechanical Alloying

Abstract

Ferritic ODS steel elemental powder compositions with various Zr content (0.3–1.0 wt.%), ground in a Pulverisette 6 planetary ball mill, were extensively studied by X-ray diffraction line profile analysis, microscopic observations, microhardness testing and particle size measurements. A characteristic three-stage process of flattening the soft powders, formation of convoluted lamellae and, finally, formation of nanocrystalline grains was observed. In order to quantify the microstructural properties, expressed mainly in terms of crystallite size and dislocation density, a methodology for detailed and accurate microstructure analysis of nanosized and severely deformed materials was proposed by the Whole Powder Pattern Modelling (WPPM) approach. In the case of the proposed ODS alloy composition, the overlapping of Fe and Cr Bragg reflections makes the microstructure analysis certainly more complicated. The results showed that the microstructure of powders evolved towards the nanocrystalline state consisting of fine (diameter of ~15 nm) and narrowly dispersed domains, with extensive dislocation density exceeding 1016 m−2.

Effect of Nitrogen Atmosphere Annealing of Alloyed Powders on the Microstructure and Properties of ODS Ferritic Steels

“…After an initial slight increase between the first and fifth hour of the process, there was a significant decrease in the parameter d-from 38 to 14 nm after 20 h. The refinement of the crystal structure results from an extreme increase in defect densities (especially dislocations) [28], which occur when the powder particles collide with the grinding balls and the walls of the container. The estimated crystallite sizes are similar to those reported by Nowik et al [27]. Additional XRD analysis of the Y 2 O 3 powder added during the ODS preparation proved that it was a single-phase crystalline material (Figure 6e).…”

“…Similar behavior was observed for the 0.6% Y 2 O 3 content. This effect is well known in MA processes and is attributed to the dissolution of alloying elements in the crystal lattice of the matrix (here Fe) during milling and to phenomena related to severe plastic deformation [27]. Based on the analysis of the crystallite size, longer milling times resulted in significant refinement of the crystalline structure of the material.…”

Fabrication and Arc Erosion Behavior of Ag-SnO2-ZnO Electrical Contact Materials

“…After 2 h of milling, the morphology of certain agglomerates changes from lamellar to equiaxed. This occurs due to an increase in the density of dislocations and the powders’ embrittlement, causing the fracture to prevail over plastic deformation [ 41 ].…”