Latest research advances on magnesium and magnesium alloys worldwide

Research advances in magnesium and magnesium alloys worldwide in 2020

Tribocorrosion is an irreversible transformation of a material resulting from simultaneous physico-chemical and mechanical surface interactions in a tribological contact. Electrochemical methods are well suited for the study of tribocorrosion phenomena because they allow one to simulate the corrosive effect of the environment by imposing a fixed potential. Furthermore, the measurement of the anodic current permits one to determine the amount of material removed by oxidation as opposed to mechanical wear. In the present paper, experimental and theoretical aspects of applying electrochemical methods in tribology are discussed and recent results obtained with passivating metals in the authors' laboratory are presented. The importance of controlling the mechanical parameters and the contact geometry is stressed, and it is shown that these parameters can significantly affect the electrochemical response of a tribocorrosion system.

Electrochemical methods in tribocorrosion: a critical appraisal

Alloying behavior, microstructure and mechanical properties in a FeNiCrCo0.3Al0.7 high entropy alloy

Corrosion of hot extrusion AZ91 magnesium alloy. Part II: Effect of rare earth element neodymium (Nd) on the corrosion behavior of extruded alloy

A multitemperature multi-axial forging (MAF) procedure has been developed to produce a fine-grained AZ61 alloy. The results show that the microstructure can be effectively refined with increasing equivalent strain during MAF. Once dynamic recrystallisation is completed, the alloy is in a steady state and further deformation of the alloy cannot change the grain size. The multitemperature MAF procedure can produce a fine-grained AZ61 alloy giving a grain size of 8 μm. This fine-grained alloy has an excellent strength accompanied by reasonable good tensile ductility. The success of the development of this multitemperature MAF procedure proves that MAF can offer a good opportunity for the development of magnesium alloys with good mechanical properties.

Grain refinement in an as-cast AZ61 magnesium alloy processed by multi-axial forging under the multitemperature processing procedure

Compression tests of Mg–Gd–Y–Nd–Zr alloy have been performed in the compression temperature range from 623 K to 803 K and the strain rate range from 0.005 s −1 to 5 s −1 . The high temperature deformation behavior was characterized based on an analysis of the stress–strain behavior, kinetics and processing map. The constitutive equation of the Mg–Gd–Y–Nd–Zr alloy is established, which describes the flow stress as a function of the strain rate and deformation temperature. The average apparent activation energy for hot deformation is calculated to be 250.0 kJ/mol. Based on the dynamic material model and the Prasad's instability criterion, the processing maps for the alloy are constructed at strains of 0.1 and 0.7. The processing map developed at strain of 0.7 exhibits two domains with higher value of power dissipation: one occurring in the temperature range of 720–760 K and in the strain rate range of 0.005–0.01 s −1 , which is associated with dynamic recrystallization and dynamic precipitation; the other occurring in the temperature range of 760–803 K and strain rate range of 0.01–0.5 s −1 . According to the processing map and microstructure observation, the optimum hot working condition for Mg–Gd–Y–Nd–Zr alloy is determined to be 773 K and 0.05 s −1 .

Hot deformation behavior and processing map of coarse-grained Mg–Gd–Y–Nd–Zr alloy

Microstructure development and thixoextrusion of magnesium alloy prepared by repetitive upsetting-extrusion

As-cast AZ91D +  x Y ( x  = 0, 0.5, 1, 2 mass%) magnesium alloys were prepared by a simple addition of Y-rich interalloy to AZ91D alloy. Influences of Y on the microstructures and tensile properties of AZ91D alloy were investigated. Influences of extrusion temperature on the microstructure and tensile properties of AZ91D + 2Y alloy were investigated. Moreover, microstructures and the resulting tensile of extruded AZ91D + 2Y alloy were also investigated under different heat-treatment conditions. The results show that the addition of Y to AZ91D alloy refines primary α-Mg matrix and β-Mg 17 Al 12 phase. The addition of 2 mass%Y forms rod-shaped Al 2 Y phase. After hot extrusion, the grain sizes of AZ91D and AZ91D + 2Y alloys are greatly refined through dynamic recrystallisation. At extrusion temperature below 300 °C, the microstructure of AZ91D + 2Y alloy is characterized by fine and equiaxed grains, which leads to a high tensile strength but relatively low elongation. With increasing extrusion temperature up to 400 °C, more equiaxed grains with larger size are obtained. A low strength and large elongation are achieved compared with those extruded at low temperature. The best combination of both high strength and large elongation takes place at 325 °C. As for extruded AZ91D + 2Y alloy, solution treatment at 413 °C (T4) causes parts of broken β-Mg 17 Al 12 phase to be dissolved into α-Mg matrix and the deterioration of tensile properties. During aging at 216 °C (T5), discontinuous precipitates appear at grain boundaries. Solid solution followed by aging (T6) treatment increases ultimate tensile strength but decreases yield strength and elongation.

Microstructures and mechanical properties of AZ91D alloys with Y addition

Based on conventional extrusion and equal channel angular extrusion (ECAE), a new severe plastic deformation (SPD) method called compound extrusion is developed to fabricate fine-grained AZ91D magnesium alloys. The fine grain size of 6 μm is obtained as the accumulated strain increased to 9.146. The AZ91D alloy treated by compound extrusion exhibits good mechanical properties, with a yield strength of 202.2 MPa, a tensile strength of 323.1 MPa and an elongation to fracture of 14.8%. The good mechanical properties of AZ91D alloy treated by compound extrusion are due to grain refinement and to the homogeneous distribution of intermetallic particles. The success in development of compound extrusion proves that compound extrusion can offer a good opportunity for the development of good mechanical properties of as-cast magnesium alloys.

Qiang Chen

Papers

Grain refinement in an as-cast AZ61 magnesium alloy processed by multi-axial forging under the multitemperature processing procedure

Hot deformation behavior and processing map of coarse-grained Mg–Gd–Y–Nd–Zr alloy

Microstructure development and thixoextrusion of magnesium alloy prepared by repetitive upsetting-extrusion

Microstructures and mechanical properties of AZ91D alloys with Y addition

Microstructure and mechanical properties of AZ91D magnesium alloy prepared by compound extrusion