Study On The Microstructure And Tensile Deformation Mechanism Of Cast Al-Li-Cu-Mg-(Ni, Sc, Er, Si) Alloys

The lightweight of alloy is an important means to reduce the weight of components,and it is also a key way to reduce environmental pollution,achieve energy conservation,reduce carbon emissions,and achieve carbon peak.Al-Li alloy is widely used in aerospace,transportation,biomedicine,key equipment and other fields because of its advantages of light weight and high strength.However,its poor heat resistance restricts the promotion and application of Al-Li alloy under more severe service conditions.At present,alloying is a common method to improve the mechanical properties of alloys.Based on above,this paper study the influence of the transition group elements Ni,rare earth elements Sc and Er,and the non-metallic element Si on the as-cast and as-aged microstructure,and the mechanical behavior at ambient/high-temperature of Al-2.5Li-1Cu-1.5Mg alloy,and reveals the correlation between alloying elements,microstructure and mechanical behavior of cast Al-2.5Li-1Cu-1.5Mg alloy.The observation of the as-cast microstructure shows that the addition of（0.2,0.4）wt.%Ni,0.2 wt.%Sc or Er element will slightly refine the as-cast microstructure of the alloy.Moreover,Sc alloying can significantly refine the grains.The addition of alloying element Ni will introduce Al₃Ni phase within the alloy and reduce the amount of intergranular phase.In addition,the addition of 0.2 wt.%Ni will additionally induce the formation of T₂-Al₆CuLi₃icosahedral quasicrystal phase.The results of the solidification path shows that the addition of 0.2 wt.%Ni deviates the solidification path of the alloy and induces the solid-state phase transition of R-Al_4.8CuLi₃→T₂-Al₆CuLi₃.The Sc or Er alloying will not only introduce Al₃（Sc/Er,Zr）phase into the alloy,but also reduce the number of intergranular phases.Adding Si element to the alloy will promote the formation of columnar-like AlLiSi phase along the grain boundary,and the growth of AlLiSi intergranular phase will be inhibited with the increase of Mg content.With the increase of Si addition,the intergranular phase of AlLiSi changes from columnar-like to dendritic-like.When the addition amount of Si is 3 wt.%,dendrimer-like AlLiSi intergranular phase and bulk-like AlLiSi hypereutectic phase are both formed in the alloy at the same time.The addition of 0.2 wt.%Ni will inhibit the precipitation ofδ’-Al₃Li precipitates,and the addition of 0.4 wt.%Ni will not affect the precipitation amount ofδ’-Al₃Li precipitates.The addition of 0.2 wt.%Sc or Er elements can promote the precipitation of a large number of core/shell Al₃（Sc/Er,Zr,Li）composite structure particles in the alloy.Si alloying will induce the precipitation of short rod-shapeβ’-Mg₂Si phase and globular-like AlLiSi phase in the alloy.Ni,Sc and Er alloying will inhibit the develops ofδ’-PFZs.In addition,the intergranular phase of AlLiSi in Si containing alloys will undergo spheroidization/passivation after heat treatment.Al₃Li phase is the main strengthening phase in cast Al-2.5Li-1Cu-1.5Mg alloy,and its strengthening mechanism is dislocation cutting mechanism.The addition of Ni can inhibit the growth ofδ’-PFZs and promote the formation of Al₃Ni phase.The exist of a transition layer（～5 nm）makes the lattice strain distribution more uniform on both sides of interface between Al₃Ni/matrix.The above two factors comprehensively improve the elongation of the alloy at ambient/high-temperatures.In particular,the addition of 0.2 wt.%Ni will also introduce T₂-Al₆CuLi₃icosahedral quasicrystal phase into the alloy.The T₂quasicrystal will produce compressive strain on the side of the matrix,which can inhibit the initiation of microcracks at the interface of T₂/matrix and harden the matrix.ALCM-0.2Ni alloy showed the best ambient temperature properties（UTS=396.3 MPa,EL.=8.15%）.The addition of rare earth element Sc/Er can promote the precipitation of core/shell composite particles in the alloy.The core/shell composite structure particles strengthen the alloy followed by the dislocation cut through+Orowan bypass mixing mechanism at ambient temperature.While the core/shell composite particles still strengthen the alloy by only Orowan bypass mechanism at high temperature,due to the dissolution of Al₃Li shell.ALCM-0.2Sc alloy still maintains good strength plasticity synergy（UTS=264.3 MPa,EL.=16.8%）at 200℃.The precipitation strengthening of core/shell composite particles and the fine grain strengthening effect of Sc alloying on the alloy are the main strengthening mechanisms.The main strengthening phases in Si-containing alloys areβ’-Mg₂Si and AlLiSi precipitates.Geometric phase analysis（GPA）results show that strengthening mechanisms ofβ’-Mg₂Si and AlLiSi are follwed by Orowan bypassing mechanism and dislocation cutting mechanism,respectively.The calculation results of the density distribution of geometrically necessary dislocation（GND）near the fracture surface of the tensile specimen show that there is a discontinuous Cu rich phase at the grain boundary within the cast Al-2.5Li-1Cu-1.5Mg alloy.In the process of tensile deformation,the local stress concentration easily occurs at the grain boundary,which leads to premature failure of the alloy.Adding 0.4 wt.%Ni to the alloy can reduce the amount of intergranular phase,which makes the GND distribution between adjacent grains more uniform,and the deformation is easy to transfer between grains,thus significantly improving the elongation of the alloy.Meanwhile,the Al₃Ni phase can hinder the slip of GND and also play a toughening role on the alloy.The addition of Sc or Er promotes the precipitation of a large number of alloy core/shell Al₃（Sc/Er,Zr,Li）composite structure particles.These composite structural particles can retain a large amount of GND within the grains,which alleviates the local stress concentration and improves the deformation resistance of the alloy at ambient/high-temperatures.The addition of 1.6 wt.%Si can promote the formation of double-scale microstructure of the alloy.The nanoscaleβ’-Mg₂Si and AlLiSi precipitates can hinder GND slip and alleviate the occurrence tendency of local stress concentration.The intergranular AlLiSi phase in micron level near continuous distribution along grain boundary can hinder GND transfer between adjacent grains and provide"skeleton"strengthening for the alloy.Doth of them improve the deformation resistance of the alloy,and significantly improve the high-temperature strength of the alloy.ALCM-1.6Si alloy obtained the best mechanical properties at 300℃.The tensile strength reached143.2 MPa and the strain reached 15.2%.