| Tensile tests were carried out at 20℃~-80℃. The scanning electron microscope was used to observe the fracture morphology and the distribution of dislocation slip bands in the tensile test specimen. The microstructure of fracture surface of specimen was observed by optical microscope. The stress distribution in the loading process was simulated by ANSYS finite element analysis software.When temperature drops from 20°C to-80°C, tensile strength and yield strength of Al-7.0Si-0.3Mg alloy increase and elongation slightly reduces. In Al-1.0Si-0.3Mg alloy, all of tensile strength, yield strength and elongation increase. The thermal activation of aluminum matrix was studied at different temperatures. In the process of room temperature tension, and the resistance of dislocation slipping and proliferation is less. At low temperature, the resistance of dislocation-slip and proliferation is larger, which leads to the increase of tensile strength and yield strength of Al-Si-Mg alloys.In Al-1.0Si-0.3Mg alloy, the micro holes are mainly formed by the falling off of Si phase. In the tensile process, dislocations pile up on grain boundary. And the stress concentration starts the dislocation sources in the neighboring grain. It leads to alloys with higher plastic deformation. The crack growth process is a combination process of both growth and connection of micro pores in the microstructure. At low temperature, deformation of the alloy is uniformly occured, so that the plasticity is improved and the elongation is increased. In Al-7.0Si-0.3Mg alloy, the micro-cracks are mainly formed by the rupture of Si phase. During the stretching process, the dislocations pile up on the Si phase of edge, which reduces the stress concentration. At low temperature, slip resistance increases and Si phase hinders more dislocation movement, which leads to further increase of alloy strength. In the process of crack propagation, Si phase fracture leads to the local deformation of adjacent aluminum matrix, then leads to the decrease of total plastic properties of the alloy.Effect of solid solution treatment on low temperature properties of Al-7.0Si-0.3Mg alloy was studied. The size of eutectic Si phase is large in the alloy without solid solution treatment. During the process of plastic deformation, a large number of dislocations lead to cleavage fracture. So strength and elongation of the alloy is low. At low temperature, the increase of dislocation resistance is increased and stress concentration in the Si phase is increased, which leads to elongation decrease. In the medium stage of solid solution, the sizes of Si particles are significantly decreased and the morphology is mainly granular. In the process of plastic deformation, the cleavage plane of the fractured Si phase is smaller. It leads to the increase of the strength and elongation. In the later stage of solution treatment, the average diameter of the Si phase becomes larger. Both the strength and the elongation decrease.The effect of Mg2 Si phase on low-temperature properties of Al-7.0Si-0.3Mg alloy was studied. In the initial stage of aging, the GP zone in aluminum matrix occurs. As time goes on, the GP zone transforms into β″ phase, whose morphology is acicular. At aging time of 10 h, β″ phase transforms to β′ phase. And the strength reaches to the maximum value. During the over-aging process, β′ phase transforms to β phase. And the morphology is lath-shaped. Due to the different contraction of the Al matrix and Mg2 Si particles with the decrease of temperature, the matrix is pressed by the Mg2 Si particle at low temperature and motion resistance of dislocations through the stress field increases. The edge dislocation whose excess half atom facing toward Mg2 Si experiences a repulsive force and the edge dislocation whose excess half atom backward Mg2 Si experiences a gravitational force.The effect of stress concentration on the tensile fracture behavior of Al-7.0Si-0.3Mg alloy at different temperatures was studied. With decrease of test temperature, the sensitivity of the alloy to the stress concentration is increased, the yield strength is not affected by the notch, and the tensile strength is sensitive to the stress concentration. The elongation of notched specimen is affected by the stress concentration. Local plastic deformation occurrs in the stress concentration region of the alloy during the tensile process, and than a large number of dislocations are generated causing the Si phase to crack. Initial crack of notched specimen is produced in the eutectic structure of notch root. The initial crack is generated in the eutectic structure near the internal defect of the smooth specimen. In the process of crack propagation in the alloy, stress concentration leads to a large number of dislocation generations. These dislocations pile up in the vicinity of Si and lead to Si phase fracture. With the stress concentration in the aluminum matrix between the Si phase and the crack further increasing, the distribution of cracks along the Si phase leads to the cracking of aluminum matrix. |
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