Investigation On Precipitation And Valence Electron Structure Of Phases In Mg3AllCelSb Alloy

AZ31 is one of the most widely used wrought magnesium alloys, however, its typical close-packed hexagonal structure, poor plastic deformation character, generally low productivity efficiency and high processing costs have restricted its wider application. Alloying with addition of RE is an effective means to improve the combined properties of magnesium alloy, but the acicular phases in the alloy lead to a deterioration to alloy's plastic property. So how to control the shape of RE phase and to increase the plastic deformation ability of magnesium alloy are the problems which need to be solved urgently.The influences of RE and Sb on Mg3Al magnesium alloy's microstructure and liquid quenching alloys'morphology at different temperatures were investigated by using optical microstructure(OM), X-ray diffraction(XRD) and scanning electric microscopy(SEM). The strengthening mechanisms of acicular rare earth phase and sphere-like phase in the alloys and its precipitation process in the melt were studied by empirical electron theory of solids and molecules (EET) with phase structure formation factor. The obtained results could provide experimental data and theory references for composition design, preparation and further application of high performance deformation magnesium alloy.The results showed that the amount of coarsen divorced eutectic P-Mg17Al12 phases decreased by adding 1%Ce into Mg3Al alloy, with a great deal of acicular Al4Ce phases distributing along the grain boundary. After adding 1% Sb, the number of acicular shaped rare earth phase disappeared, however, a small amount of granular CeSb phase and irregular bulkβ-Mg17Al12 phase generated in the grain boundary.Liquid quenching experiments showed that, in the process of adding Mg-Ce master alloy in the Mg3Al alloy and high temperature depositing, Al and Ce atoms first formed atomic clusters in the melt spontaneously, and then began to form small, dispersed Al-Ce particles. During cooling solidification, Al and Ce atoms enriched in the front of liquid/solid interface and transition of atomic clusters to the Al4Ce phase occurred, which gradually grew into an independent and long acicular-like structure and distributed along the grain boundaries. As the temperature further reduced, the eutectic reaction occurred and formedβ-Mg17Al12 phase. After adding Ce and Sb to Mg3Al alloy, the acicular-like Al-Ce atomic clusters formed firstly in the melt. Then Sb atoms gradually diffused towards Al-Ce atom group. The acicular-shaped Al-Ce atom group changed into granulated Ce-Sb atom group gradually. In the continuous cooling process, Ce-Sb clusters in the super-cooled liquid would form the nuclear and grow up gradually. When it reached the temperature of Eutectic reaction, Eutectic reaction occurred andα-Mg+β-Mg17Al12 eutectic was formed.The empirical electron theory of solids and molecules (EET) was used to calculate the valence electron structure of pure magnesium and a-Mg solid solution. After Zn, Mn elements dissolved into the a-Mg matrix, the number of the valence electron in the strongest bond increased and the bonding capacity of atoms in structural unit became stronger. The strength of the alloy increased, thus they played a solid solution strengthening role in matrix.Space valence electron structures of Al4Ce and CeSb phase are calculated. The number of valence electron on their strongest bonds is much larger than that on the strongest bonds of a-Mg, thus the bonding capacity is much stronger. The role in hindering the dislocation slip and grain boundary migration during the deformation process is much stronger than a-Mg matrix.The structure stability of Al4Ce and CeSb phase is analyzed. The number of valence electron on the strongest Al-Ce bond in Al4Ce phase is much smaller than that of the strongest bond in its structural unit, so this is the weak point of the Al4Ce structure, which would be inclined to bring about stress concentration in the process of deformation and destroy the phase structure, then lead to failure. CeSb phase was a uniform structural unit which was mainly interconnected by the nearest neighbor Ce and Sb atom, and the valence electron on the covalent bond distributed evenly, so there is no weak point. Therefore we can draw a conclusion that the strengthening effect of CeSb is better than Al4Ce in Mg-Al base alloys.The solidification sequence is analyzed by the phase structure factor S, the number of valence electron on the strongest bond nA, the number of valence electron between the atoms na. Al4Ce phase is impossible to precipitate, under the condition that Sb atoms exist, and Ce element can only exist in the form of Cesb phase in this alloy. Considering the phase structure factor S and the nA value, the eutectic reaction betweenβ-Mg17Al12 and a-Mg phase occurred and the Eutectic (α-Mg+β-Mg17Ali2) phase precipitated.