Refinement Mechanism, Microstructure And Properties Of Mg-A1 Alloy With Addition Of Rare Earth Elements

Mg-Al series alloys including casting alloy and deformation alloy are widely used because of their excellent casting properties and corrosion resistance. However, the comprehensive mechanical properties of the alloys have yet to be further improved because the precipitation hardening effect of the b-Mg17Al12 phase in the as cast microstructure is weak. In the solidification process, the distribution of the b-Mg17Al12 phases can be more uniform with refinement of a-Mg grain, which is one of the effective methods to improve the comprehensive properties of magnesium alloy. And the method of adding rare earth elements has the advantages of low cost, simple processing, easy controlling and significant grain refinement effect. It is generally believed that the Y element in the intermediate alloy can react with the Al in the melt to form a large number of dispersion, refractory A12 Y particles, which is in a state of suspension and acts as a nucleation substrate during the solidification process. Mg-Al-Y alloy was studied in this paper, in order to reveal heterogeneous nucleation mechanism of a-Mg in Mg-Al alloy, and the interface characteristic between a-Mg and heterogeneous core was also carried out by the experimental research and theoretical calculation.In this paper, the morphological character of Al-Y intermetallic compound in Mg-9Al alloys and the mechanism of Y’s effect on heterogeneous nucleation and the effects of different Y content on hardness of magnesium alloy at room temperature were studied by using OM, SEM, EDS and XRD. Results show that A12 Y particles are produced and act as the potent heterogeneous nuclei for primary Mg. The a-Mg grain can be greatly refined when the content of Y is below 0.5wt%. The minimum average grain size 116μm can be obtained due to the formation of numerous small ball-like and short rod-like Al₂ Y intermetallics, which is helpful to grain refinement. In contrast, when the content of Y increased to 0.7 wt.%, a-Mg grains have a tendency to become coarse. The addition of rare earth elements can obviously change the mechanical properties of the alloy at room temperature. When the content of Y in the alloy is less than 0.5wt.%, the hardness of the alloy increases with the increase of Y content.The maximum hardness is 62.4HV, and increase 19% compared with that of the alloy without addition of rare earth. but when the content of Y increases to 0.7wt.%, the mechanical properties of the alloy have a tendency to decrease.The electronic structure and energy calculation of Mg/A12 Y interface are studied with first principles calculation. Results show that the surface relaxation of A12Y（044）mainly occurs in the first three layers of the surface structure, both to the A1 termination and A1-Y termination type surface.When the layer thickness is more than 9 layers,the layer changes gradually stable, namely to achieve convergence. Al termination type Al₂Y（044）surface’s atomic layer spacing change is smaller than Al-Y termination type Al₂Y（044）surface’s atomic layer spacing change. Therefore, Al terminated surface is more stable than Al, Y termination type surface structure. Calculation and analysis of the electronic structure and the bonding conditions of Al₂ Y show that there are metallic, ionic and covalent bonds in bulk Al₂ Y. The solid-to-liquid interfacial energy between A12 Y and magnesium melt is higher than that of the Mg/A12 Y solid-to-solid interfacial energy, which is responsible for the good heterogeneous nucleation of a-Mg on A12 Y surface.The binding energy, energy band structure, density of states, difference charge density and elastic constants of AlY, MgY, Al₂ Y and Mg17Al12, which have the same cubic structure in Mg-Al-Y intermetallic compounds, have been calculated and analyzed by the first principle method. By comparing and analyzing their binding energy, the stability from strong to weak is AlY>Al₂Y>MgY> Mg17Al12. The alloying ability in the order of strong to weak is Al₂ Y >AlY >Mg17Al12>MgY by forming energy. According to the formation energy analysis we can know that the Al atom in the alloy solidification process will give priority to the occurrence of Al-Y compound with rare earth Y element. Four phases all have metal, ionic and covalent bond. Moreover, four phases are brittle because the values of C12-C44 are negative numbers, which is consistent with the results of shear modulus and bulk modulus ratio（B/G）. Young’s modulus is a method for the hardness of solid materials. The greater the value is, the better of the hardness of solid materials. The hardness of the four phase from strong to weak is AlY >Mg17Al12>MgY>Al₂Y.The Poisson’s ratio can quantify the stability of the crystal shear resistance. Since the larger the value is, the better of the plastic property, the plastic property of four phases is in the order of MgY, Mg17Al12, AlY, Al₂ Y from strong to weak by comparing the Poisson’s ratio.