Due to the rapid development of rail transportation, aerospace and new energy vehicles, the strong demand for lightweight new material technology and the strategic considerations from pressures in energy, economy, resources, etc., developed countries around the world have given magnesium alloys strong Attention and huge investment have pushed the development of magnesium alloy technology to a critical moment. Magnesium alloy is the lightest metal structural material with a density of only 2/3 of aluminum and 1/4 of steel. It has broad application prospects in high-speed rail, subway, automobile, 3C electronics, aerospace, defense and military industries. Based on the age hardening effect of adding rare earth elements to magnesium, a variety of high-strength magnesium alloys with Gd, Y and other rare earth as the main alloying elements have been developed at home and abroad.
However, the addition of high content rare earth elements brings the following bottlenecks:
1) The addition of a large amount of rare earth elements causes the density of the magnesium alloy to increase;
2) The magnesium alloy with high rare earth content has poor forming performance, low yield rate and high processing cost;
3) The addition of a large number of heavy rare earth elements will inevitably increase the cost of magnesium alloys, thus limiting its wider application.
How to greatly increase the absolute strength of low-cost (low / no rare earth) magnesium alloys is the key issue to continue to broaden the practical application of magnesium products. In recent years, based on the phase equilibrium and thermodynamic calculation of magnesium alloys, the team of professor Qin Gaowu of Northeastern University has discovered a new mechanism for grain refinement of defect-induced dynamic segregation of Ca solutes, and successfully designed and prepared accordingly A series of Mg-Ca based alloys with excellent mechanical properties.
In 2015, the tensile strength of the Mg-Ca binary alloy can reach 330 MPa, which is ~ 100 MPa higher than the strength value of the same composition Mg-Ca deformed alloy reported by the Korean Kim group, and the elongation can also reach ~ 10% (J . Alloys Compd., 2015, 630: 272-276).
In 2017, the Mg-Ca binary alloy was prepared based on traditional one-step extrusion. The grain size of the matrix can continue to be refined to ~ 0.7 mm, and the tensile strength at room temperature therefore reached ~ 400 MPa (Mater. Lett., 2019, 237: 65-68).
In 2018, the R & D team found that the addition of a small amount of Ca element in the conventionally extruded Mg-2Ca-2Sn non-rare earth alloy can induce the segregation of Ca at the grain boundary / sub-grain boundary and the dynamic precipitation of nano-Mg2Ca. Plays the role of alloying elements and fine crystals in the extrusion process, and finally obtains a-Mg matrix refinement to submicron size (~ 0.32 um) that is difficult to achieve by conventional extrusion, so it shows excellent mechanical properties (yield strength ~ 443) MPa).
Microstructure characteristics of ultra-high-strength Ca-containing deformed magnesium alloy
In particular, the alloy can achieve ultra-high strength at a solute content of ~ 4 wt.%, That is, to achieve a "high-strength low-alloyed magnesium alloy" (Acta Materialia, 2018, 149: 350-363).
The inverted relationship between the strength and plasticity of metal structural materials has been an eternal research topic in the field of structural materials. Magnesium alloys are no exception. Conventional alloy design strategies that use large-angle grain boundaries and non-coherent precipitation phase interfaces to inhibit magnesium alloy dislocation slippage are often accompanied by greater brittleness while achieving the strengthening effect. By developing low-energy interfaces with low mismatches (such as coherent precipitation phase boundaries, small-angle grain boundaries, twin boundaries, etc.), it is expected to break through the strong plastic inversion relationship of magnesium alloys. Based on this, Professor Qin Gaowu's team recently proposed a new design concept based on the dynamic segregation of multi-component solute elements to construct a high-density and low-energy interface magnesium alloy to achieve excellent strong plastic matching characteristics. Extruded Mg-Ca-Al-Mn-Zn alloy (total solute content ~ 2.4 wt.%) Achieves yield strength ~ 425 MPa, tensile strength ~ 442 MPa, and elongation ~ 11% (Acta Materialia, 2020, 186 : 278-290).
Microstructure and property characterization of a new low-cost high-strength plastic Mg-Ca-Al-Zn-Mn alloy
This strategy successfully solved the bottleneck problem of mutual exclusion of strength and plasticity of magnesium alloys, and provided a new development path for the design of new high-performance deformed magnesium alloy materials.
Based on the above idea of dynamic segregation of solute atoms in crystal defects, the team will further combine first-principles, molecular dynamics and phase diagram calculations to design and prepare low-cost, high-strength plastic deformed magnesium alloys with more comprehensive properties Materials to meet the actual needs of engineering in different fields.