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

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-Al2O3 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 quasi-constrained 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 alloy machine chips using high-pressure torsion

The high-pressure torsion processing technique was used to consolidate and process magnesium-based chips. Chips were prepared by machining commercially pure magnesium and a magnesium alloy AZ91 separately. Optical microscopy and microhardness measurements showed good consolidation of pure magnesium. The magnesium alloy continued to exhibit the boundaries between the chips even after 5 turns of HPT suggesting poor bonding. The results show that soft chips are easier to consolidate through HPT than harder alloys

Interface structures in Al-Nb₂O₅ nanocomposites processed by high-pressure torsion at room temperature

Extremely thin Nb2O5 nanowires and Al powder were successfully consolidated at room temperature by using high-pressure torsion (HPT), producing a novel metal matrix nanocomposite with exceptional mechanical properties. It is shown that minor additions of Nb2O5 increase sharply the hardness of commercially pure Al. For instance, hardness of over 180 Hv was developed at the edge of samples with 10% nanowires and processed through 10 turns of HPT. This is markedly higher than any other value reported for pure aluminum matrix composites having this level of reinforcement phase. A detailed characterization of the interface structure using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed a pronounced grain refinement of the Al matrix at the nanoscale and the occurrence of the aluminothermic reduction of the Nb2O5. The latter led to: (i) the formation of Al2O3 nanolayer at the Al/Nb2O5 interface and (ii) the nanosegregation of metallic Nb (with few atomic layers) along grain boundaries and dislocations. The pronounced increase in hardness is attributed to the formation of this interface nanostructure

Magnesium-based bioactive composites processed at room temperature

Hydroxyapatite and bioactive glass particles were added to pure magnesium and an AZ91 magnesium alloy and then consolidated into disc-shaped samples at room temperature using high-pressure torsion (HPT). The bioactive particles appeared well-dispersed in the metal matrix after multiple turns of HPT. Full consolidation was attained using pure magnesium, but the center of the AZ91 disc failed to fully consolidate even after 50 turns. The magnesium-hydroxyapatite composite displayed an ultimate tensile strength above 150 MPa, high cell viability, and a decreasing rate of corrosion during immersion in Hank’s solution. The composites produced with bioactive glass particles exhibited the formation of calcium phosphate after 2 h of immersion in Hank’s solution and there was rapid corrosion in these materials