Development of a magnesium-alumina composite through cold consolidation of machining chips by high-pressure torsion

Abstract

High pressure torsion offers unique conditions for the consolidation of metallic particles at room temperature owing to the high hydrostatic compressive stresses combined with the high shear strain. A Mg-Al 2 O 3 composite was produced by consolidation of machining chips of pure magnesium with 10% in volume of alumina particles. The consolidation process was investigated by optical and scanning electron microscopy and X-ray microtomography. It is shown that shear deformation concentrates along thick alumina particle layers in the initial stage of deformation. A significant fraction of the hard phase particles are pushed into the outflow in quasiconstrained HPT and a homogeneous composite is achieved after significant straining. The composite exhibits a refined microstructure, a higher hardness and improved resistance against room temperature creep compared to pure magnesium.

Consolidation of magnesium and magnesium-quasicrystal composites through high‑pressure torsion

“…Composites with similar amount of hard quasicrystal particles could be consolidated by 10 turns of HPT using pure magnesium as matrix material but not with AZ91. It is worth noting that previous studies have shown that pure magnesium chips, pure or with the addition of alumina particles, could be consolidated after 5 turns of HPT [11,16]. On the other hand, machining chips of AZ91 were not consolidated in the same conditions [16].…”

“…HPT has been used to produce magnesium-based composites with different materials to meet the performance targets. For example, it was shown that the incorporation of alumina particles into pure magnesium matrix promotes a slight increase in hardness and in room temperature creep resistance [11]. Magnesium can also be mixed with pure aluminum to produce a ductile metastable composite which can be hardened by thermal treatment [12].…”

Developing magnesium-based composites through high-pressure torsion

“…A continuous magnesium matrix is developed and the ceramic phase is apparently well-dispersed. Similarly, a continuous magnesium matrix incorporating ceramic particles is observed in a Mg-10 % Al 2 O 3 composite [21]. However, as the size of the alumina particles is smaller, the average distance between the particles is also reduced.…”

“…As the size of the alumina particles is significantly smaller than the chips, a high heterogeneity of distribution of phases takes place in the early stages of processing and a thick agglomeration of alumina was observed after 1 turn (shown at centre). A high number of turns is required in order to produce a solid disc with a well-dispersed reinforcement phase [21]. Accordingly, homogeneous distributions of particles of bioactive glass and hydroxyapatite in magnesium matrices were also obtained after 10 and 50 turns of HPT, respectively [22].…”

Finite Element Modelling of High-Pressure Torsion: An Overview

“…was fabricated through cold-consolidation of Mg machining chips and alumina particles using a quasi-constrained HPT facility. 7) In this study, a preferential outflow of alumina particles towards the periphery of the discs was experimentally observed and this led to the formation of spiral-like structures composed by the hard-phase after one HPT turn. However, the influence of material properties and processing parameters on the flow process and solid-state reactions during powder consolidation by high-pressure torsion are not well understood and thereby FEM simulations could provide helpful insights on tailoring composites using this processing route.…”