| The Al-Mg-Si alloys are widely used in production and life-related fields due to their light weight,good thermal and electrical conductivity,reasonable mechanical properties,formability,and weldability.The increasing actual demands have put forward more stringent requirements for the comprehensive performance of the alloys.Micro-alloying is one of the main strategies to improve the properties of alloys effectively.As industrial vitamins,rare earth elements have received widespread attention in improving the properties of aluminum alloys.The diversity of rare earth elements provides more possibilities for the preparation of aluminum alloys with excellent performance.It has been relatively clearly studied about the effects of elements with smaller atomic radii on the properties and microstructures of Al-Mg-Si alloys,but not those of the elements with larger atomic radii,especially lanthanide rare earth elements.In this dissertation,focusing on the microalloying effects of lanthanide rare earth elements on Al-Mg-Si alloys and their mechanisms,the impacts of two typical lanthanide rare earth elements,La and Er,on the mechanical properties,electrical conductivity,corrosion resistance,and multi-scale microstructures of the Al-Mg-Si alloys have been investigated using some advanced eletron microscopy characterization techniques and performance tests,and the relationship between the property enhancements and the microstructural changes has been clarified.It has also been elucidated the mechanism by which micro-alloying La-and Er-addition changes the types of precipitates in Al-Mg-Si alloys based on firstprinciples calculations.The following innovative conclusions could be drawn:(1)Micro-alloying La-and Er-addition changes the types of aging precipitates of AlMg-Si alloys.Most of β" precipitates formed in the peak-aged alloy with La-addition are polycrystalline β" precipitates.In the over-aging stage of this alloy,the dominant precipitates formed are the β"/U2/β′,β"/U2,β"/β’ and β’/U2 composite precipitates,rather than the expected β’ precipitates.Apart from a small amount of the β" precipitates,a large number of β’’/LDC precipitates and some disordered β’’ precipitates are precipitated in the Er-added alloy in the under-aging condition,and the β"/U2/β′/B′,β"/U2,U2/B’ and β′/B′ composite precipitates are precipitated in the peak-aging condition.First-principles calculations and atomic-resolution transmission electron microscopy characterizations show that the formation of the polycrystalline β"precipitate and composite precipitates is related to the increased elastic strain at the precipitate/Al interfaces resulted from the low-level substitution of La.The mechanism of Er-addition on the change of precipitate types is the same as that of La-addition.Meanwhile,the microalloying La-and Er-addition promotes the B’ phase precipitating along the grain boundary during aging process.(2)The precipitation sequence of Al-Mg-Si alloys is changed by microalloying Laand Er-addition.The new precipitation sequence in the La-added alloy should be: supersaturated solid solution(SSSS)→ clusters/GP zones → β " → polycrystalline β" →β "/β’/U2,β" /U2,β "/β’,β’/U2 → U2,β′.And the new precipitation sequence in the Eradded alloy should be: super-saturated solid solution(SSSS)→ clusters/GP zones → β" → disordered β ",β" /LDC → β "/U2/β′/B′,β" /U2,U2/B’,β′/B′ → U2,β′,B′.(3)Microalloying La-and Er-addition effectively improves the strength and electrical conductivity of Al-Mg-Si alloys.La-addition induces the formation of a portion of Al Si La secondary phases,which have a space group F-43 m and lattice parameter a=0.64 nm and could be slipped and sheared by dislocations.As the La-content increases,the strength of the alloy first increases due to the increase in the secondary-phase strengthening contribution of the Al Si La phases and the grain refinement contribution,and then decreases owing to the decrease in the solid-solution strengthening contribution;and the electrical conductivity of the alloy gradually increase s due to the increase in the fraction of the Al Si La phases and the amount of Si solute atoms consumed by these phases.The improvement of strength in the Er-added alloy is also attributed to the secondary-phase strengthening contribution of the Al Si Er phase s induced by the Er-addition and the grain refinement.The formation of Al Si Er phases also consumes a certain amount of Si atoms,and increases the volume fraction of precipitates of the alloy,which effectively improves the electrical conductivity of the alloy.(4)Microalloying La-and Er-addition improves the ductility of Al-Mg-Si alloys.There would be two reasons for the improved ductility: firstly,the grain size is refined;secondly,the secondary phases in the matrix provide more dislocation sources,and these secondary phases could be easily sheared by dislocations during the deformation process to avoid stress concentration,making the alloy more prone to transgranular fracture(ductile fracture).(5)The microalloying La-addition reduces the pitting corrosion resistance of Al-MgSi alloys,but improves the intergranular corrosion(IGC)resistance of the alloy s.With the La content increasing,the pitting corrosion resistance of the alloys gradually decreases,and the IGC resistance shows a contrary tendency.The Al Si La secondary phases with better corrosion resistance have a more positive corrosion potential than the Al-matrix,meanwhile a certain width of PFZ is formed around the Al Si La phase s.Hence,pitting corrosion would preferentially occur around the Al Si La phase s.The higher the La content,the higher the Al Si La phase content,and the more obvious the coarsening of the precipitates,and therefore the worse the pitting corrosion resistance of the alloys.As corrosion occurs along the grain boundaries,the discontinuously distributed Mg-rich B’ phases at the grain boundaries are preferentially corroded,and some coarse Al Si La phases located at the grain boundary would hinder the propagation of the corrosion medium,and La-addition refines the grain size,which improves the IGC resistance of the La-added alloys.The B’ phase distributed discontinuously is the main factor affecting the IGC resistance of the alloys.With the increase of La content,the number of B’ phase at the grain boundary increases,and the IGC resistance then improves.With prolonged aging time,the pitting corrosion resistance of both La-free and La-added alloys gradually decreases,and the IGC resistance of the La-free alloy gradually decreases,but the IGC resistance of the La-added alloy increases first and then decreases.The coarsening of the precipitates during the aging process will lead to the deterioration of the pitting corrosion resistance of the alloys.As the aging time increases,the width of grain boundary PFZ increases,resulting in a gradual decrease in the IGC resistance of the La-free alloy.For the La-added alloy,the width of PFZ does not change significantly in the early-aging stage,and the full precipitation of grain boundary B’phase leads to the enhancement of the IGC resistance.As the aging time extend s,the intergranular corrosion resistance of the alloy would be deteriorated with the wide ning of PFZ,and with the coarsening and reduction of the B’ phase. |
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