Microstructure Thermal Stability And Mechanical Properties Of Al-Mg Binary Alloys Solidified Under High Pressure

Solidification behavior of Al-Mg binary alloys under different pressures was studied in this thesis. The Mg contents are 20wt.%, 30wt.% and 40wt.%, respectively. The solidification pressures are normal pressure, 1GPa, 2GPa and 3GPa, respectively. Microstructure evolution and phase constitution in Al-Mg alloys solidified under different pressures were obtained and analyzed. The phase stability of the alloys solidified under high pressure was studied. Effect of high pressure on mechanical properties of Al-Mg alloys was investigated.Effect of high pressure on solidification microstructure and phase constitution of Al-Mg alloy shows that, the amount of fragileβphase of Al-Mg alloys decreases with increasing solidification pressure. The supersaturated Al(Mg) solid solution was obtained when the Al-20Mg alloy solidified under 2GPa and 3GPa. However, the distribution of Mg solutes in the solid solution is inhomogeneous, i.e. the Mg concentration in the dendrite is lower than that in the interdendriti region. The Al-30Mg and Al-40Mg alloys both contain the Al(Mg) solid solution andγphase solidified under high pressures, and theβphase can not be formed. Furthermore, when solidified under 3GPa, the matrix is the primary Al(Mg) solid solution and the eutectic colonies composed of the Al(Mg) solid solution andγphase distribute in the interdendritic region.Morphology of eutectic structure of the Al-30Mg and Al-40Mg alloys changes greatly with increasing pressure. Eutectic structure is composed of the dotted Al(Mg) solid solution andγphase (anomalous eutectic) under 1GPa, but the eutectic structure transforms to lamellar eutectic structure under 3GPa. The transition is determined by the kinetic and thermodynamic aspects during solidification process.Effect of high pressure on Mg solubility in Al(Mg) solid solution shows that, the solubility of Mg element in Al(Mg) solid solution can be greatly extended by solidified under high pressures. A bulk Al(20Mg) supersaturated solid solution can be obtained solidified under 2GPa. Thus, solidification under high pressure is a new method to prepare bulk Al(Mg) solid solution. The Mg solubility in Al(Mg) solid solution can be extended up to 41.6at.% in the Al-40wt.%Mg(Al-42.2at.%Mg) alloy solidified under 3GPa, but the eqlibrium solubility of Mg in Al is less than 1 at.% at room temperature and normal pressure.Al(Mg) supersaturated solid solution obtained by solidified under high pressures is metastable, and will transform to the stable direction during heating process. It was found that theγphase enriched of Mg atoms is formed during heating process of Al(Mg) solid solution. Prolonging ageing time or increasing temperature, the Mg atoms in theγ phase will diffuse to the Al(Mg) solid solution around it, resulting in the decreasing of Mg content. Finaly, theγphase will directly transform to the stableβphase.The precipitation of Al(20Mg) solid solution prepared by solidification under high pressures takes place in a non-uniform manner during heating process, i.e. the precipitates are tend to first formed in the solid solution with higher Mg solubility, and then formed in the solid solution with lower Mg solubility. In addition, the morphology evolution of theβphase will go through a few steps during ageing process: granular→stripe→agglomerate→bulk shape.Effect of high pressure solidification on mechanical properties of Al-Mg alloys shows that, the amount of intermetallics decreases and the amount of Al(Mg) solid solution increases in the Al-Mg alloy solidified under high pressure, resulting in the great enhancement of the mechanical properties. The Al-20Mg alloy is fragile and its fracture characteristic is cleavage fracture under normal pressure. However, it can transform to be a ductile material with elongation of 11% when solidified under 2GPa and 3GPa, meanwhile, its strength can be also greatly improved. The ultimate tensile strength (UTS) of Al-20Mg solidified under 3GPa is 8.0 times of that solidified under normal pressure. The strength of Al-30Mg and Al-40Mg alloys are remarkably improved with increasing pressure. The ultimate tensile strength (UTS) of Al-30Mg and Al-40Mgs alloy solidified under 3GPa are 14.3 and 11.2 times of that solidified under normal pressure, respectively. Furthermore, the elongation and yield strength of Al-20Mg and Al-30Mg alloys are also greatly improved. The fracture characteristic can be essentially altered under the condition of high pressure solidification. The Al-30Mg alloy is cleavage fracture under normal pressure, however, it transforms to the quasi-cleavage fracture when solidified under 3GPa. In addition, the supersaturated Al(Mg) solid solution of Al-40Mg alloy solidified under 3GPa exhibits typical brittle fracture characteristic, due to the increased amount of Mg segregation zones.