Study On The Morphologies And Effects Of Trace Scandium In Al-Sc-(Zr) And Al-Zn-Mg Alloys

Adding trace scandium（Sc）to aluminum alloy can effectively refine grain and inhibit recrystallization,thus improving the microstructure and properties.Therefore,Sc-containing aluminum alloys have been widely used in aerospace and other fields.However,there are still some outstanding problems in related research:（1）Al-Sc master alloys have different grain refining effect on aluminum alloy,which may be related to the characteristics of primary Al₃Sc phase,but the influence melting and casting conditions on the morphology of primary phase and the growth mechanism of primary phase are not clear.（2）The phenomenon of annealing hardening of scandium-containing aluminum alloys is related to the precipitation behavior of secondary Al₃Sc/Al₃（Sc,Zr）phase,but the strengthening mechanism and the relationship between the coarsening behavior of the secondary phase and the change of alloy hardness are not clear.（3）Some studies have shown that when the scandium content exceeds the eutectic point composition,rare Sc-containing discontinuous precipitates will appear in the organization.However,the studies on the morphology,formation conditions,evolution law and crystallographic characteristics of the discontinuous precipitates of Al₃Sc are not in-depth.（4）High-strength Al-Zn-Mg alloy has a prominent problem that strength and corrosion resistance are mutually restricted and difficult to coordinate and match.Adding scandium to Al-Zn-Mg alloy can simultaneously improve the strength and corrosion resistance.It is of great significance to study the role of trace scandium in Al-Zn-Mg alloy because the addition of trace scandium can improve the strength and corrosion resistance at the same time.This thesis combined the National Key Science and Technology Research and Development Plan and the Major Science and Technology Projects of Hunan Province for ultra-high strength corrosion resistant aluminum alloy.The morphology and growth mechanism of primary Al₃Sc phase,the annealing precipitation behavior and strengthening mechanism of secondary Al₃Sc phase,the discontinuous precipitation behavior and crystallography characteristics of Al₃Sc phase,the effect of trace scandium on the microstructure and properties of Al-Zn-Mg alloy and the mechanisms have been studied.The effects of casting conditions on the morphologies and growth mechanisms of primary Al₃Sc phases have been studied by controlling the melting temperature,holding time and cooling rate.The results showed that:All the primary Al₃Sc phases have L1₂ type face-centered cubic structure,and which preferentially grow in<111>,<011>and<001>directions.When the crystal nucleus was not destroyed,the fastest growth rate was in the<111>direction,and the slowest growth rate was in the<001>direction.Increasing the melting temperature,prolonging the holding time and decreasing the cooling rate would lead to the dendrite of the primary phase.When the melting temperature was 760°C,holding for15 min,the cooling rate was 100～500 K s^-1,the morphologies of the primary phases were diversified,mainly cube and star shapes.The dendrites with symmetric morphology grew from cuboidal primary Al₃Sc phase,and the growth process conformed to the periodic chain bonding theory.When the melting temperature was 820°C,holding for 72 h,and the cooling rate was 30～70 K s^-1,the morphologies of the primary phases were mainly broken cube and butterfly shapes.The butterfly shape primary Al₃Sc phase had multilayer structure,and its growth mechanism was repeated eutectic reaction.When the melting temperature was 860°C,holding for 72 h,and cooling rate was about 0.02～0.05 K s^-1,the crystal nucleus of the primary phase was destroyed,and the primary phases were mainly thick strip dendrites.The grain refinement effects of Al-2.2Sc master alloys prepared under different casting conditions were obviously different.The larger the size of the primary phase was,the more obvious the dendrite was,and the worse the grain refinement effect was.The annealing hardening behavior of Al-0.25Sc and Al-0.25Sc-0.12Zr alloys annealed at 260～390°C and the continuous precipitation behavior and strengthening mechanism of secondary Al₃Sc/Al₃（Sc,Zr）phases have been investigated.The Al-0.25Sc alloy hardened first and then weakened when annealed above 290°C.The hardness transition was directly related to the size of the secondary phase.When the average radius of secondary Al₃Sc phase was less than 1.5 nm,the yield increment was close to the ordered strengthening increment,the cutting mechanism was dominant,and the hardening phenomenon occurred.When the average radius of Al₃Sc phase was larger than 2.5 nm,the yield increment was close to the strengthening increment of bypassing mechanism,and the bypassing mechanism was dominant,and the alloy appearred weakening phenomenon.During the annealing process,the hardness of Al-0.25Sc-0.12Zr alloy did not weaken after reaching the plateau zone.The coarsening behavior of secondary Al₃（Sc,Zr）phase was inhibited by its core-shell structure.The average radius of secondary Al₃（Sc,Zr）phase was always lower than 2.5 nm,the strengthening mechanism did not change from cutting mechanism to bypassing mechanism,and the alloy exhibited good thermal stability.The discontinuous precipitation behavior of Al₃Sc phase,the diffusion behavior of scandium in scandium-containing phase and the influence of scandium inhomogeneous distribution on the core-shell structure of Al₃（Sc,Zr）phase in Al-0.50Sc and Al-0.50Sc-0.25Zr alloys have been studied.The results showed that:The formation of discontinuous Al₃Sc phase in the as-cast structure was related to the inhomogeneous distribution of scandium,and the discontinuous precipitates of Al₃Sc gradually decreased or even disappeared during the annealing process.The atomic distributions of Sc-containing phases in each alloy were reconstructed by Three-Dimensional Atomic Probe（3DAP）.It was found that the segregation behavior of scandium affects the atomic concentration ratio of Sc and Zr in the shell aera of the core-shell structure of Al₃（Sc,Zr）phase.When the Sc/Zr value（>8）was too high,the diffusion inhibition effect of Zr on Sc decreased,and the anti-coarser ability of Al₃（Sc,Zr）phase decreased.Al-7.0Zn-3.0Mg-0.30Cu-（Sc）-0.12Zr ultra-high strength alloys with different scandium content have been designed and prepared to study the effect of scandium on the microstructure and properties of the alloys and their mechanisms.The results showed that:The dendrite in the microstructure was significantly reduced after the addition of scandium,the microstructure was fine equiaxed crystals,and the strength and corrosion resistance of the alloys were improved by adding scandium when the addition amount of scandium reached 0.15%.The increase in strength is due to the grain refine strengthening by scandium and the substructural strengthening of Al₃（Sc,Zr）phase.The reason for the improvement of corrosion resistance was that the addition of scandium increased the number of grain boundaries of the alloy,and decreased the area ratio of grain boundary precipitates,which effectively reduced the potential difference between grain boundaries and transgranular areas,increased the discontinuity of grain boundary precipitates,and failed to form continuous corrosion channels,resulting in the decrease of anodic dissolution rate of grain boundary precipitates.The properties of aluminum alloy with scandium content of 0.15%were obviously better than the alloy with Sc content of 0.07%.Compared with the alloy without scandium,the tensile strength of aluminum alloy with 0.15%scandium content increased by 8.6%,the yield strength increased by 8%,the hardness increased by 23%,and the corrosion resistance of the alloy was obviously improved.90 Figures;18 Tables;186 References...