Improving The Strength/Toughness Of The Magnesium Alloys By Alloying

The application of Magnesium alloys is dramatically limited by the low mechanical properties, few alloys systems and the high cost of Rare earth Magnesium alloys. Author wants to find new alloying element and supply a new strengthening design method of Mg alloys.Author summarized atomic information of 24 type of alloying elements including Magnesium element. Both methods of alloying of magnesium alloys were supposed according to the strengthening mechanism. The both methods were followed:(1) Indissolvable elements in Magnesium alloys were added to Magnesium alloys in order to improve the critical shearing stress of twin and oppose the grain boundary slipping in deformation process. However, the ductility of magnesium alloys was reduced. According to the phase diagrams and atomic information, Antimony and Germanium were selected and experimented to prove practical in practice.(2) The strengthening of Magnesium alloys was researched from the subjedt of material design and Gallium was chosen according to the electron concentration and the influencing factors of metallic bond. This is a new method for strengthening the Magnesium alloys.The dissertation included morphology ofα-Mg₃Sb₂ precipitates in Mg-Sb binary alloys, microstructure and mechanical properties of AZ31 with the addition of Sb; Microstructure evolution and mechanical properties of as-cast Mg-Ge binary magnesium alloys, Microstructure and mechanical property of as-cast AZ31 with the addition of Germanium, the effect of Al and Zn on the morphology of Mg₂Ge in Mg alloys, microstructure and mechanical properties of Mg-Ga binary alloys and the microstructure and mechanical properties of Mg-xGa-3Al-Zn alloys. All of the alloy ingots were prepared by die casting.The results of Sb alloying of Mg alloys indicated that the intermetallic compoundα-Mg₃Sb₂ soluted with many Mg atoms because of the difference between Pauling electro-negativity values of them. A double layer alloy ingot with Mg-9(wt.%)Sb and Mg-13(wt.%)Sb was gotten when the Mg-10(wt.%)Sb was prepared because a large number of theα-Mg₃Sb₂ grains with high density were synthesized and some of them deposited on the bottom of crucible. Then the amount of the addition of Sb to Magnesium alloys was limited. Theα-Mg₃Sb₂ precipitates were rectangle block in as-cast Mg-13(wt.%)Sb alloy while in as-cast Mg-9(wt.%)Sb alloy majority of them were rod shape and minority were rectangle block. The experiment results of AZ31 with the addition of Sb indicated the rod-shaped intermetallic compoundα-Mg₃Sb₂ formed and distributed homogenously in substrate. The elongations of alloys with Sb addition are good (δ>10%). Both of tensile strength and elongation of alloys appreciably reduce with the increase of contents of Sb in alloys. Twin deforming and boundary sliding are the main deformation of AZ31 with and without Sb addition.The results of Ge alloying of Mg alloys indicated that Ge could refine the Mg alloys, the morphologies of Mg₂Ge precipitates reduced the tensile properties of Mg alloys and were affected by other alloying elements in Mg alloys. The Mg₂Ge precipitates in Mg-Ge binary alloys were rod-shaped and formed the eutectic of (α-Mg + Mg₂Ge) which could effectively enhance the grain boundaries ofα-Mg. In the microstructure of as-cast AZ31 with the addition of Ge, the Mg₂Ge intermetallic compound presented Chinese script, which reduced the tensile properties of alloys. The microstructure of as-cast Mg-3Al-Ge and as-cast Mg-1Zn-Ge alloys indicated that both of Al and Zn elements could affect the morphology of Mg₂Ge.The experiment results of Mg-Ga binary alloys presented that Ga dissolved in Mg alloys dramatically reduced the lattice parameters and the average distance between of atoms ofα-Mg grains, the tensile properties of pure Mg were improved dramtically. The morphology and distribution of Mg₅Ga₂ in as-cast Mg-Ga binary alloys were associated with the content of Ga. If the content of Ga was below the 1.5 at.% (included) in as-cast alloys, the Mg₅Ga₂ precipitates were particles and homogeously distributed in matrix. If the content of Ga was above 2.0 at.% (included) in as-cast alloys, the Mg₅Ga₂ precipitates were block and distributed on grain boundaries. The solid solubility of Ga in Mg-xGa binary alloy could be improved by solution treatment. A lot of tiny rod-shaped Mg₅Ga₂ precipitates (φ<100 nm) were precipitated in aging treatment and the yield tensile strengthed of the alloys was improved.In as-cast Mg-xGa-3Al-1Zn alloys the morphology and distribution of Mg₅Ga₂ precipitates were similar with the rule of them in as-cast Mg-Ga binary alloys. The elongations of Mg-xGa-3Al-1Zn alloys were better after solution treatment. The effects of aging treatment on Mg-xGa-3Al-lZn alloys were similar with it on Mg-Ga binary alloys. The ultimate tensile strengthen of Mg-4.5Ga-3Al-1Zn alloy was higher, 207 MPa after aging treatment. The aging microstructure of Mg-xGa-3Al-1Zn alloys indicated both Al and Zn could accelerate the precipitation of Mg₅Ga₂ phase. The Mg-Ga system alloys should be further study to tap the potential and develop new Magnesium alloys with high performance.The Ga alloying results of Mg alloy proved that the atom cluster theory and the influencing factors of metallic bond were useful to find new alloying elements to strength Mg alloys and the Mg-Ga system alloys have large potentiality to be developed new high performance Mg alloys. At last, general characters of magnesium alloying elements except for Rare earth elements were summarized according to Ga, Al, Zn, Mn, Li, Sn, Si, Sb and Ge elements. The contents of the principle were followed:(1) The alloying elements could be dissolved in Mg alloys even at ambient temperature;(2) The radius of them should be much smaller than that of Mg atom;(3) The charge of basic ion of them should be more;(4) The Pauling electro-negativity values of them should be smaller than 1.9.