Performances And First-principle Calculation Of Al-Cu-Mg-Y Alloys

Aluminum alloys are widely used in aerospace, transportation, construction, container packaging, power electronics, machinery manufacturing, petrochemical and other areas of national economy and daily life because of their light weight, high specific strength, good corrosion resistance, and easy processing character. However, the development of new aluminum alloys is mostly confined to the traditional experimental model. In recent years,with the rapid development of computer technology, the simulation calculation on the performances of materials has become a new hotspot in the research of materials. In this dissertation, the simulation calculation is performed for the structure stability, elastic properties and thermodynamic performances of main precipitates in the Al-5Cu-1Mg-(Y)alloys with Materials Studio software and based on density functional theory and first-principle caculation method. And through performing the the microstructural observations amd mechanical property tests for the Al-5Cu-1Mg(-0.3Y) alloys, the theoretical calculation and experimental results are compared and analyzed in order to provide a theoretical reference for the development and composition design of new aluminum alloys.Through calculating the generating heat, binding energy and electron density of states for the Al-Cu-Mg(-Y) alloys, it is revealed that in the solidification process of Al-Cu-Mg alloy,Al2 CuMg phase will take precedence over other phases, but with adding a relatively small amount of Mg, Al2 Cu phase is more easy to precipitate. Therefore in the Al-Cu-Mg alloy, both Al2 CuMg and Al2 Cu precipitates may exist stably, which illustrates the alloying ability of Cu and Al elements is significantly higher than that of Mg and Al elements. When adding the trace amount of Y element into the Al-Cu-Mg alloy, the Al2 Y phase forms easily in the Al-Cu-Mg-Y alloy because the alloying ability of Y and Al elements is significantly higher than that of Cu, Mg and Al elements. When the Cu content reaches a certain value in the Al-Cu-Mg-Y series alloy, the Al8Cu4 Y phase also forms more easily. Therefore, both Al2 Y and Al8Cu4 Y phases are the main strengthening phase with better structural stability in the Al-Cu-Mg-Y alloy.With the simulation calculation on the elastic constants of several main strengthening phases, it can be seen that for main strengthening phases such as Al2 Cu, Al2 Y and Al8Cu4 Y inthe Al-Cu-Mg(-Y) alloys, their bulk modulus is higher than that of pure aluminum. It illustrates that the strength of these strengthening phases is higher than that of pure aluminum.The shear modulus G of three main strengthening phases is higher than that of pure aluminum and pure yttrium. It means that the brittleness increases while the strength and hardness get enhanced. The Young’s modulus of Al2 Cu phase is the smallest, and its plasticity is the best.However, the Yong’s modulus of Al2 Y phase is the largest, and its plasticity is the worst. The G/B value of Al2 Cu phase is less than 0.5, approaching to that of pure aluminum, and its toughness is the best. Both Al8Cu4 Y and Al2 Y phases generated with adding the rare earth element Y in the Al-Cu-Mg alloy are the brittle phases, and the brittleness of Al2 Y phase is larger. The Al2 Cu phase has a higher Poisson’s ratio, and shows the better plasticity. However,the Poisson’s ratio of Al2 Y phase is the smallest, and its plasticity is relatively the wrost.Al single crystal has three phonon spectrum curves which belong to the acoustic waves,and there is a phonon state density peak at 3.37 THz. There are eighteen dispersion curves in the phonon spectrum curves of both Al2 Y and Al2 Cu phases, including three acoustical lattice waves and fifteen optical lattice waves. The degeneracy of phonon spectrum curves of Al2 Y and Al2 Cu phases exists at some special points with high symmetry. The phonon state density peak appears in the range of 3THz to 5THz for the Al2 Y phase. And three significant phonon state density peaks appear in the ranges of 4THz to 4.5THz, 7THz to 8THz and 9THz to9.5THz for Al2 Cu. It implies that the lattice vibration near these frequency ranges is the most intense. With increasing the temperature, both entropy and enthalpy of Al2 Y and Al2 Cu phases increase, while their free energy decreases. When the temperature is ranging from 300 K to900K, the thermal stability of Al2 Y phase at different temperatures is higher than that of Al2 Cu phase, description Al2 Y of phase within the temperature rang than Al2 Cu phase. The relationship between the heat capacity at constant volume and temperature is basically the same for both Al2 Y and Al2 Cu phases. When the temperature is lower than 200 K, the heat capacity at constant volume increase quickly with increasing the temperature, then changes slowly, and is even close to a constant 3R.The results of mechanical property tests shows that with adding 0.3% rare earth element Y into the Al-5Cu-1Mg alloy, the tensile strengths of the alloy significantly increase, but the elongation to failure slightly decreases. With prolonging the aging time, the tensile strengths ofthe Al-5Cu-1Mg-0.3Y alloy firstly increase and then decrease. For both Al-5Cu-1Mg and Al-5Cu-1Mg-0.3Y alloys, the fracture mode under the tensile loding condition is ductile fracture. Under the low-cycle fatigue loading condition, the extruded Al-5Cu-1Mg and Al-5Cu-1Mg-0.3Y alloys with T6 state show the cyclic strain hardening and cyclic stability.The addition of rare earth element Y can improve the cyclic deformation resistance of the Al-5Cu-1Mg alloy at the lower total strain amplitudes, and enhances the low-cycle fatigue lives of the alloy at the higher total strain amplitudes, which is relevant to the formation of Al2 Y and Al8Cu4 Y phases. For the extruded Al-5Cu-1Mg and Al-5Cu-1Mg-0.3Y alloys with T6 state, the relationships between the plastic strain amplitude, elastic strain and reversals to fatigue fracture are linear. Under the low-cycle fatigue loading condition, the fatigue cracks initiate at the free surface of fatigue specimens in a transgranular manner, and propagate in a transgranular mode.