In response to the increased emphasis on reducing carbon emissions, the demand for lightweight, high-performance structural materials is quickly increasing, and Mg alloys, because of their having the lowest density among the common engineering metals, have demonstrated considerable advantages and prospective applications in contemporary industry. High-pressure die casting (HPDC), owning to its high efficiency and low production cost, is the most extensively utilized technique in commercial Mg alloy applications. The high room-temperature strength–ductility of HPDC Mg alloys plays an important role in their safe use, particularly in the automotive and aerospace industries. With respect to HPDC Mg alloys, their mechanical properties highly rely on their microstructural characteristics, particularly the intermetallic phases, which are further dependent on the alloys’ chemical compositions. Therefore, the further alloying of traditional HPDC Mg alloys, such as Mg-Al, Mg-RE, and Mg-Zn-Al systems, is the most adopted method to further improve their mechanical properties. Different alloying elements lead to different intermetallic phases, morphologies, and crystal structures, which can have helpful or harmful effects on an alloy’s strength or ductility. The methods aimed at regulating and controlling the strength–ductility synergy of HPDC Mg alloys have to arise from an in-depth understanding of the relationship between the strength–ductility and the components of the intermetallic phases of various HPDC Mg alloys. This paper focuses on the microstructural characteristics, mainly the intermetallic phases (i.e., components and morphologies), of various HPDC Mg alloys with good strength–ductility synergy, aimed at providing insight into the design of high-performance HPDC Mg alloys.

An ultrathin wall Mg-8Zn-8Al (wt%) alloy was manufactured by high-pressure die casting (HPDC), and exhibits standout strength of -248 MPa with an acceptable elongation of-2.3%. Interestingly, this alloy contains relatively coarse ix-Mg grains (-5.6 mu m) and ultra-fine beta-Mg grains (-0.67 mu m), and many nano -sized icosahedral quasicrystal phase (I-phase) particles in eutectic regions. Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations reveal possible amorphous regions around I-phase particles, C-containing phase and/or MgO aggregations, and Zn-rich clusters and pyramidal precipitates. Both ultra-fine grains along with the skeleton consisted of nano-sized I -phase particles and solutes including Zn-rich clusters in Mg matrix contribute the ultra-high strength. On the other hand, this work indicates that interaction of alloy melt with CO2 is avoidless during ultra-thin wall HPDC and pyramidal precipitation is possible at least in Zn-containing magnesium alloys.(c) 2022 Elsevier B.V. All rights reserved.

Effects of 3 wt% RE (rare earth) addition on microstructures and mechanical properties of a high-pressure die casting Mg-8Zn-6Al (ZA86) alloy before and after aging treatment was thoroughly studied in this work. Microstructural characterizations present that RE addition has indiscernible influence on the well-defined dendrite structure and grain size, but would additionally introduce Al-RE intermetallic phases, which were identified as Al3RE and Al11RE. Simultaneously, two groups of Mg-Al-Zn ternary phases were detected in alloys with/without RE addition, while their volume fraction was clearly reduced by RE addition. After aging at 90 degrees C for 320 h, a large density of precipitates were observed particularly near grain/cell boundaries, and were identified as MgZn2, Mg4Zn7, and I-phase. Although RE addition seems to have no influence on the components of precipitates, it obviously improves their volume fraction and slightly increases their size. Finally, the yield strength of the ZA86 alloy after aging was improved by -33 MPa by RE addition. Therefore, the die cat ZA86 alloy exhibits high-strength and certain precipitation strengthening ability, and RE addition can significantly enhance the precipitation strengthening, accomplishing the highest yield strength in the reported HPDC magnesium alloys, being -267 MPa. (c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Effects of various Sm additions on microstructures and mechanical properties of a hot-extruded Mg-14Gd-0.5Zr alloy were studied in this work. The results indicate that Sm addition firstly clearly improves the strength and then decrease as the Sm content increases at both room temperature and high temperatures. The highest yield strength was obtained to be similar to 495 MPa and similar to 512 MPa under tension and compression, respectively, at room temperature with the Sm concentration of 1 wt% while those were to be 384 MPa and 248 MPa at 250 degrees C and 300 degrees C, respectively, with the Sm content of 0.5 wt%. Electron backscatter diffraction mappings reveal that Sm addition significantly promotes dynamic recrystallization, resulted in much more fine recrystallized grains and lower texture intensity. Furthermore, transmission electron microscopy observations show that Sm addition increases dynamic precipitates particularly in non-recrystallized regions, leading to relatively less aging precipitates during artificial peak-aging. Finally, discussion based on grain boundary strengthening and dispersion strengthening suggests that Sm addition improving the yield strength of the extruded Mg-14Gd-0.5Zr alloy is mainly attributed to more grain boundaries and dynamic precipitates at room temperature and high temperatures, respectively.

The corrosion behavior and mechanism of as-cast, solid-solution treated, and as-extruded Mg-14.4Er-1.4Zn-0.3Zr (wt.%) alloys are investigated. The microstructure characteristic of as-cast alloy, i.e., the semi-continuous 18R-long period stacking ordered (LPSO) phase as cathode with the potential difference (PD) of 83 mV, is the main reason for the relatively strong tendency of micro-galvanic corrosion. The lower micro-galvanic tendency by the decreased size, number, and PD (30 mV) of LPSO-phase particles, and the existence of a few Er3+ in corrosion film, are mainly responsible for the improved corrosion resistance of solid-solution treated alloy. The as-extruded alloy exhibits a superior corrosion resistance (corrosion rate: 1.11 mm y−1 in 3.5 wt.% NaCl solution) as compared to many reported Mg alloys, which mainly attributed to the formation of nano-spaced basal plane solute-enriched stacking faults (SESFs) within fine dynamic recrystallized (DRXed) grains. The nano-scale SESFs as the weak anode with PD of 26 mV weaken the galvanic tendency to form a relatively homogeneously electrochemical microstructure. Moreover, the preferential corrosion of SESFs releases sufficient Er3+, promoting the quasi-passivation state of corrosion film. The construction of weak anodic nano-lamellar SESFs structure within fine grains is a feasible method for the synergetic improvement of strength and corrosion resistance of Mg alloys.