| The Mg-Al series magnesium alloys are the most extensively used magnesium alloy so far. However, because of their widen solidification range, theβ-Mg17Al12 phase frequently ll precipitate along theα-Mg grain boundary in the shape of network by the reaction of divorce eutectic, result in the poor mechanical properties of the Mg-Al series magnesium alloys component. This limited the Mg-Al series magnesium alloys application in the industry of automobile and aerospace. At present, the most frequently used method to increase the mechanical properties of magnesium alloy is alloying. During the used alloying elements, those who can increase the strength and ductility of magnesium alloys are Al,Zn,Ca,Ag,Ce,Ga,Ni,Cu,Th, ect. There are lots of literatures have studied the effects of elements of Al,Zn,Ca,Ag,Ce,Ni,Cu,Th on the microstructure and mechanical properties of Mg–Al series magnesium alloys, but about the effect of Ga on the microstructure and mechanical properties of magnesium alloys are very limited. The aim of the present thesis is to investigate the effect of Ga on the microstructure and mechanical properties of Mg–Al series magnesium alloys.The nominal chemical composition of magnesium alloys studied in this thesis are Mg-8%Al-(0-4)%Ga,Mg-(8-12)%Al-2%Ga and AZ91-(0-2)%Ga. The thesis analysis the effect of Ga on the microstructure and mechanical properties of casting magnesium alloys, the emphasis was laid on the effect of heat treatment and rolling technology on the second phase precipitation pattern of Mg-8%Al-2%Ga magnesium alloy, and the strengthening mechanism of the alloys was discussed at the same time. The experimental results demonstrated that the microstructure of casting Mg-Al magnesium alloy is consisted ofα-Mg, the eutectic ofβ-Mg17Al12 andα-Mg, and the secondary phase adjacent of eutectic phase. The secondary phase is in the shape of lamella alternate withα-Mg; the eutectic is transformed by the primary divorce eutecticβ-Mg17Al12 phase which is formed under the condition of high solidification rate.Most of the Ga addition is mainly concentrated inβphase and in the interface ofαandβphase. Ga addition not only can refine the casing microstructure but also can suppress the formation of eutectic and decrease the grain size of divorced eutectic phase ofβ-Mg17Al12. For the Mg-8%Al-(0-4)%Ga magnesium alloy, with increasing the Ga addition, it is mainly rich in the interface ofβphase andαmatrix. When the Ga addition is increased to 4wt.%βphase is broken and Mg-Al-Ga ternary eutectic phase is observed in the interface ofαandβphase, there is a new phase of Ga2Mg formed in the alloy when the Ga addition is increased to 4wt.%.For the magnesium alloy of Mg-8%Al the optimal addition of Ga is 2wt.%. When 2wt.%Ga is added to the Mg-(8-12)%Al alloy the supersaturation of Al in the matrix is decreased, as a result the ambient mechanical properties of the alloy is increased obviously. The ultimate tensile strength and elongation of the alloy in the as-cast state is 248.4MPa and 10.4%, respectively, while the ultimate tensile strength and elongation of the alloy in the T6 state is 288.0MPa and 4.7%, respectively.When the Mg-8%Al-2%Ga magnesium alloy is undergone solid solute treatment, the first dissolved phase is second lamellaβ-Mg17Al12 phase, then is the divorced eutecticβ-Mg17Al12 phase. With increasing the solid solution time the grain size of the matrix is increasing and during the subsequent aging the amount of discontinues precipitationβ-Mg17Al12 phase is decreasing. The different microstructure will be obtained after the Mg-8%Al-2%Ga magnesium alloy is solid solution-treated at 415℃then cooling at different method, the supersaturation microstructure with the straight and smooth grain boundary will be obtained after quench cooling, while microstructure featured of undulant grain boundary with discontinues precipitation ofβ-Mg17Al12 phase after air cooling, when the alloy is furnace cooling after solid solution, there is a great amount of lamellarβ-Mg17Al12 phase precipitated within the grain.After rolling deformation the microstructure of Mg-8%Al-2%Ga magnesium alloy is consist of great amount of twins, implying that the rolling deformation is mainly by twinning. Rolling deformation change precipitation site and morphology of the secondary phase. The precipitation patterns ofβphase are continuous and discontinuous before rolling, but theβphase is mainly precipitated in the site of defects(including grain boundary, twin grain boundary and with the twins).Rolling deformation can increase the hardness of Mg-8%Al-2%Ga magnesium effectively. The recrystallization of the alloy didn't not occure when it is annealing at 150℃to 300℃after rolling deformation, while annealing at 400℃the recrystallization take place. When the alloy is annealing at 150℃to 200℃the phase is only precipitated at grain and twin boundaries at small and dispersed morphology, when the annealing temperature is increasing to 300℃except above motionedβphase, there is, the long lamellarβphase precipitated along (0001) plane of theαmatrix is also observed, when the annealing time increased to 3 hours theβphase is in the shape of lamellar diamond and the angle between two lamellar is 120°(60°). When the alloy is annealing at 150℃,200℃,300℃, respectively, the hardness of the alloy is decreasing at first then increasing with increasing the annealing time, then lever off. The higher the annealing temperature, the higher the hardness value, that is because the higher the annealing temperature, the more theβprecipitation amount.The 2wt.%Ga addition to the AZ91 magnesium alloy can alternate the casting microstructure apparently, the eutectic is diminished and the divorced eutecticβphase tends to be block and spheroidization, and there is lamellar secondary phase formed around theβphase. The Ga is mainly concentrated aroundβphase, and Zn segregate aroundβphase because of the addition of Ga. The ambient mechanical properties of AZ91 magnesium alloy is increased apparently by 2wt.%Ga.
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