Investigation On Evolution Of Microstructure And Phase In Al-Mg Alloy Solidifying Under High Pressure

and transmission electron microscope etc testing method, change rules of microstructure and phase in Al-Mg alloy and Al-Mg-Zn alloy solidifying under high pressure as well as stability of the phases in Al-Mg alloy were investigated in the paper.Comparing with solidification under normal pressure, the second dendrite arm spacing decreases when Al-Mg binary alloy and Al-Mg-Zn ternary alloy solidify under high pressure. The morphology of the dendrite is perfect and the primary arm gets long under high pressure. In addition, the amounts of the second phases decrease, its sizes also reduce, and the morphology of the second phase become from plate to graininess.Solid solubility of Mg in the matrix phase(α′-Al) increases as Al-9.6%Mg alloy solidifies under high pressure. For Al-11%Mg-4.5%Zn alloy, solid solubility of Mg and Zn inα-Al phase all has an increase with the pressure increasing, and the solute in the second phase(Mg32(Al,Zn)49) presents the following rules: Zn content increases, Mg content changes little, and that of Al decreases. It is found that the lattice constant ofα′-Al phase in Al-9.6%Mg alloy increases from 4.08A (normal pressure) to 4.10A (4GPa solidification) and 4.11A (6GPa solidification) as the pressure increases. The lattice constant ofα-Al phase in Al-Mg-Zn alloy increases slightly under high pressure. While, the lattice constant of Mg32(Al, Zn) 49 phase decreases from 14.55A (normal pressure) to 14.39A (4GPa solidification) and 14.37A (6GPa solidification) with the pressure increasing.There are all the metastable phases produded when Al-9.6%Mg alloy solidified at 4GPa high pressure with the melting temperature of 860℃and 6GPa high pressure with the melting temperature of 1000℃besides Al₃Mg₂ (FCC) phase. At 4GPa high pressure, the metastable phase has a size of 70nm or so, the crystal structure is FCC structure with the lattice constant of 6.438A, and phase constituent is near Al₉Mg₂, phase transformation temperature range is 451.17⁴57.59℃. For Al-9.6%Mg alloy solidifying at 6GPa, the metastable phase has a hexagonal structure having the lattice constant of a=4.42A, and its phase constituent is near Al4Mg with a size of 30nm or so. Phase transformation temperature is in the range of 450.65⁴57.53℃. As 4.5%Zn added to Al-11%Mg alloy which solidified under high pressure, only Mg32 (Al, Zn) 49 phase formed without any other metastable phase.For Al-9.6%Mg alloy solidifying at 6GPa high pressure with the lower cooling rate, the dendrite is coarsening apparently, and Mg content in the matrix changed little comparing with large cooling rate of the same pressure. There is still the metastable phase produced besides Al₃Mg₂ phase. For Al-9.6%Mg alloy solidifying at 4GPa high pressure with the large superheat of the melt, the dendrite structure is coarsening, and there are Al₃Mg₂ phases with two different crystal structure formed, one is face-centered (FCC) structure having the lattice constant of 28.3A, the other is close-packed hexagonal (HCP) structure having the lattice constants of a=11.26A, c=16.5A. Besides, a new metastable phase produced whose crystal structure is calibrated to be body-centered cubic (BCC) structure, its lattice constant is 8.495A, and morphology is like nodular. The phase constituent is near Al16Mg9, and the temperature range of the metastable are 458⁴65.38℃.The theory analysis is carried out for the solidification process of Al-Mg alloy solidifying at 4GPa,6GPa high pressure, the diffusion coefficient of the solute decreases three orders, which is the main reasons of the solute trapping and the formation of the metastable phase as well as the decrease of the second dendrite arm spacing. In addition, thermal undercooling, constituent undercooling and kinetic undercooling of the melt increase under high pressure, which has the main effects on the morphology of the dendrite.Research on stability of the high pressure phases is performed by aging treatment. The results show that the phases formed in Al-Mg alloy at 6GPa high pressure become instability,α′-Al phase decomposed asα-Al phase and Al₃Mg₂ phase at both low temperature aging and high temperature aging. Al4Mg phase transformed to another metastable phase-Al2Mg at the lower aging temperature, with the aging temperature increasing, Al2Mg phase transformed to Al₃Mg₂ phase. Transformation rules of high pressure metastable phase after aging treatment is that transforms from the metastable phase to stable phase, and from low Mg content to high Mg content.