Magnesium alloy is the lightest metal structural material and has broad application prospects in structural lightweighting in fields such as automobiles and aerospace. However, the low crystal symmetry of magnesium results in poor room temperature processability. This drawback increases the preparation cost of magnesium alloy parts and seriously weakens the competitive advantage of this material. The machinability of materials is influenced by two factors: tensile elongation and work hardening rate. Traditional methods such as adding rare earth elements, grain refinement, and precipitation strengthening can generally only improve one of the tensile elongation or work hardening rates of magnesium alloys, and it is difficult to improve both at the same time.
Researchers from the School of Materials Science and Engineering at Shanghai Jiao Tong University have designed a novel magnesium alloy AX61 (Mg-6Al-1Ca, wt.%) using two common alloying elements, Al and Ca. Under the extrusion condition of 250 ℃, a sub micron sized Al2Ca phase was precipitated in AX61 alloy, and its crystal orientation was randomly distributed. These Al2Ca precipitates hinder dislocation slip in the magnesium matrix and improve the work hardening rate of the material. On the other hand, the Al2Ca precipitate phase itself has a face centered cubic crystal structure, which can form dislocations and stacking faults on its {111} crystal plane to initiate plastic deformation. The internal plasticity of Al2Ca phase effectively eliminates local stress concentration, allowing the material to maintain high elongation. The precipitation of Al2Ca phase indirectly inhibits the formation of brittle Mg2Ca and Mg17Al12 phases. In addition, solutes Al and Ca inhibit twin nucleation and promote dislocations, which are beneficial for improving the elongation of the material.
Due to the above mechanism, the room temperature elongation of AX61 alloy is as high as 27%, and the work hardening amount (expressed as the difference between tensile strength and yield strength) can reach 135MPa. The above performance combination is higher than all known magnesium alloys. This new type of AX61 alloy with ultra-high processability does not require expensive rare earth elements and can be formed in one step through ordinary extrusion technology. Therefore, its production cost is very low and it has good practical application prospects.