Mg-Al-Y合金中Al-Y金属间化合物的相稳定性、弹性和热力学性质的第一原理计算

通过基于密度泛函理论的第一原理计算方法,对Mg-Al-Y合金中的主要强化相,即Al_2Y和Al_3Y的相稳定性、电子结构、弹性性质以及热力学性质进行计算。相生成热的计算结果表明:Al_2Y和Al_3Y均可稳定存在,Al_2Y的结构稳定性更强,因此,在合金的凝固过程中,Al_2Y优先析出。Al_2Y和Al_3Y的电子态密度(DOS)和差分电荷密度计算的结果表明:Al_2Y和Al_3Y两相可以稳定存在的内在本质在于Al原子与Y原子的价电子轨道发生强烈的相互作用,形成了spd杂化。两相内的原子成键均为共价键、离子键和金属键。体模量B、剪切模量G、弹性模量E、泊松比v和各向异性因子A等力学性质参数的计算结果表明:这两种相为强硬的脆性相并都为各向同性,因此,具有相似的强化效果。两相熔点较高表明其具有很好的热稳定性,能够提高合金的高温性能。声子谱和声子态密度计算以及德拜温度的计算结果进一步验证了两相具有结构稳定性较高。两相的热力学性质符合一般热力学规律,其中自由能的计算结果表明:两相的稳定性顺序没有发生变化。随着温度的升高,Al_2Y的结构稳定性仍强于Al_3Y的。

Structure stability and mechanical properties of high-pressure die-cast Mg-Al-Ce-Y-based alloy

With the aim to further improve the mechanical properties of Mg-A1-RE-based alloy, Mg-3.0Al-1.8Ce-0.3Y-0.2Mn alloy was prepared by high-pressure die-casting technique. The microstructure, thermal stability of intermetallic phases and mechanical properties were investigated. The results show that the alloy is composed of fine primary a-Mg dendrites and eutectic in the interdendritic regions. The intermetallic phases in eutectic are Aln（Ce,Y）3 and A12（Ce,Y） with the former being the dominant one. The thermal stability of Al11（ce,Y）3 is conditioned. It is basically stable at temperature up to 200℃ within 800 h, while most of the Al11（Ce,Y）3 intermetallics transform to A12（Ce,Y） at higher temperature of 450 ℃ for 800 h. The alloy exhibits remarkably improved strength both at room temperature and 200℃, which is mainly attributed to the reinforcement of dendrite boundaries with Alll（Ce,Y）3 intermetallics, small dendritic arm spacing effect as well as the solid solution strengthening with Y element.

Modeling of deformation energy at elevated temperatures and its application in Mg-Li-Al-Y alloy

To control the superplastic flow and fracture and examine the variation in deformation energy,the stress and grain size of Mg-7.28Li-2.19Al-0.091Y alloy were obtained using tensile testing and microstructure quantification,and new high temperature deformation energy models were established.Results show that the grain interior deformation energy increases with increasing the strain rate and decreases with increasing the temperature.The variation in the grain boundary deformation energy is opposite to that in the grain interior deformation energy.At a given temperature,critical cavity nucleation energy decreases with increasing strain rate and cavity nucleation becomes easy,whereas at a given strain rate,critical cavity nucleation energy increases with increasing temperature and cavity nucleation becomes difficult.The newly established models of the critical cavity nucleation radius and energy provide a way for predicting the initiation of microcrack and improving the service life of the forming parts.

Effect of Y on microstructure evolution and mechanical properties of Mg−4Li−3Al alloys

To obtain magnesium alloys with a low density and improved mechanical properties,Y element was added into Mg−4Li−3Al(wt.%)alloys,and the effect of Y content on microstructure evolution and mechanical properties was investigated by using optical microscopy,scanning electron microscopy and tensile tests.The results show that mechanical properties of as-cast Mg−4Li−3Al alloys with Y addition are significantly improved as a result of hot extrusion.The best comprehensive mechanical properties are obtained in hot-extruded Mg−4Li−3Al−1.5Y alloy,which possesses high ultimate tensile strength(UTS=248 MPa)and elongation(δ=27%).The improvement of mechanical properties of hot-extruded Mg−4Li−3Al−1.5Y alloy was mainly attributed to combined effects of grain refinement,solid solution strengthening and precipitation strengthening.