| Al-Si-Cu-Mg cast alloy has higher strength, lower density and excellent corrosion resistingproperty as an alloy can be reinforced by heat treatment. Most of the proportion of Aluminumalloy market is Al-Si-Cu-Mg cast alloy because of its excellent performance. Different materialshave different performance and the performance of the material decided to materialorganization. In order to get excellent macro-performance must be knowledge of the material’smicrostructure. For casting Al-Si-Cu-Mg alloy, its microstructure is closely related to the size,structure and distribution of nanoscale precipitated phase in the process of aging.This article investigates the precipitation phase and precipitation sequence in the process ofaging of casting Al-10Si-1.5Cu-0.5Mg by using the transmission electron microscope (TEM)and three dimensional atom probe (3DAP). At the same time, research the structural stability,thermal stability and elastic properties of precipitation by first principles calculation. Given areasonable explanation of the Al-Si-Cu-Mg alloy "twin peaks" phenomenon from bothexperiment and theory directions.Dispersed Q’ phase can be observed in alloy substrate after aging at175℃for1h. Fewneedle-like θ"phase can be observed after aging4h. After aging6h, the size of θ" phase anddensity is small, but Q phase has reached the peak of the first peak of the total hardness curve;with the extension of aging time (6h-12h), Q phase and θ metastable phase continue to grow up,but density of Q phase decreases gradually, while the advantage of θ metastable phase reflected.After aging12h, number and size of θ metastable phase reached the peak peak, embodied in thesecond peak hardness of the alloy hardness curve. Aging in the late (after12h) the precipitationhas apparent coarsening phenomenon.The experimental results of3DAP show that after aging175℃for15min, Mg and Siatoms exist different degrees of partial, at the same time a large number of Cu atoms are still ina state of uniform distribution, has not yet formed Mg/Si/Cu composite clusters in the alloysubstrate; after aging for2h, the segregation degree of Si and Mg in the alloy substratedeepening, and the segregation degree of Cu atom also have certain segregation, Mg/Si/Cucomposite clusters appear in alloy substrate; after aging for4h, the segregation degree of Siã€Mgand Cu in the alloy substrate further deepening, the size and amount of Mg/Si/Cu composite clusters have been increased. The above test results are consistent with the conclusion of TEMexperiment.The calculated cohesive energies and formation heats show that Q’ phase is morestable than θ’ and θ", Q’ phase precipitates first in the process of aging. The addition of Cuimproves the structure stability in Al-Si-Mg alloy through the formation of Al2Cu.Stiffness of Al2Cu is larger than Mg2Si, but shearing resistance of Al2Cu is smaller thanMg2Si. Plasticity of Al2Cu is better than Mg2Si. Al2Cu is toughness and has better ductility,Mg2Si is brittle phase. The anisotropic degree of two phase were small. |
