Effect Of Sc, V, Fe Microalloying On The Microstructure Evolution And Service Performance Of 2519 Aluminum Alloy

2519 aluminum alloy was a widely used material in military and aerospace fields due to its light weight,high strength,weldability,excellent thermal stability and corrosion resistance properties.In recent years,driven by the demand for light weight of structural parts and energy saving and emission reduction,aluminum alloy sheet has become the development trend to replace steel as a new material.In this paper,2519 aluminum alloy is used as the research object,and experiments are conducted to analyze the precipitated phases by using optical micrograph（OM）,scanning electron microscope（SEM）,electron probe X-ray microanalyzer（EPMA）,X-ray diffraction（XRD）,transmission electron microscopy（TEM）and other analytical techniques in combination with thermodynamic phase diagram and first principle calculations.The paper investigates the effects of microalloying on the evolution of microstructure,thermal stability,and corrosion properties,as well as the related mechanisms.Additionally,a new heat treatment process applicable to the microalloying of 2519 alloy is proposed,which improves the service performance to a new level.The paper concludes with the main findings:（1）The solidification process of 2519（Sc）aluminum alloy was analyzed through phase diagram calculation,which revealed that intermetallics precipitate in the order of Al₃Sc,W,andθ.The formation of the W phase occurs through two different types of reactions:peritectic eutectic reaction（L+Al₃Sc→α-Al+W）and eutectic reaction（L→α-Al+W,L→α-Al+θ+W）.By combining first principle calculations,it was discovered that the interfacial energy of the precipitated phase is the primary factor affecting grain refinement.And the grain size of the alloy was reduced by 70%with the addition of 0.3%Sc,and its yield strength,tensile strength and elongation were increased by 43.5%,35.1%and 50.0%compared with that of 2519 alloy.On this basis,a double-stage homogenization method was designed to control the W phase formation and improve the mechanical properties.（2）During homogenization process,the GP zone/θ’’nucleates on Al₃Sc and quickly transforms into theθphase.Then Sc atoms replace Al atoms at the Al/θinterface,and Sc atoms moving to the octahedral position are bound.Cu atoms occupy some of the Al atoms by diffusion and become displaced,ultimately transforming theθphase into the W phase in situ.The precipitation sequence of the W phase is proposed to be GP zone/θ’’→θ→W.XRD refinement results show that the W phase has a Th Mn₁₂ type structure with cell parameters a=b=0.86069 nm,c=0.50892nm,α=β=γ=90°,and a space group of I4/mmm.The DFT calculation determined the most stable crystal structure of the W phase based on the enthalpy of generation.（3）The Sc microalloying method was used to improve the microstructure of 2519 aluminum alloy,and it was found that Sc segregation at theθ’/Al matrix interface could inhibit the coarsening of the precipitated phase,and a coarsening model of theθ′phase was proposed.Based on this,an isolate with stabilized interfacial structure was designed using elements with lower diffusion coefficients（Fe,V）,which strongly inhibited the interfacial precipitation and migration of theθ′phase.Furthermore,the addition of element V was found to introduce a new grain boundary nano strengthening phase(V₂Mg₃Al₁₈)to hinder the migration of grain boundaries and substantially improve the thermal stability of the alloy.（4）The multiple microalloyed corrosion studies showed that V-containing alloys had the lowest corrosion current density and the most positive corrosion potential,exhibiting the best corrosion resistance.In contrast,the combined V/Sc-added had the highest corrosion current density and more negative corrosion potential,indicating higher chemical activity.Meanwhile,the addition of Sc elements transformed the corrosion mode of the alloy from pitting to intragranular corrosion.The electron work functions of the W,Al₂Cu,and Al₇Cu₂Fe phases were in the range of 4.16-4.59 e V,4.06-4.53 e V,and 4.09-4.35 e V,respectively,which were much larger than that of the Al（111）surface（4.01 e V）.The intrinsic potential differences were between 150-580 m V,50-520 m V,and80-340 m V,respectively.When exposed to a corrosive environment,most of the end surfaces had higher electronic work functions than the Al matrix.The second phase was less prone to lose electrons in electrochemical corrosion and acted as the cathodic phase.（5）Slow strain rate tensile tests showed that V-containing alloys had the lowest SCC sensitivity with an I_SSRT of 3.5%.In contrast,the Fe/Sc combined addition alloy exhibited the highest SCC sensitivity with an I_SSRT of 17.1%.It is related to the following three factors:（a）The grain deflection in the crack propagation path has an axial angle relationship of<XXY>/48°-58°,the proportion of 48°-58°grain boundaries of V-containing alloy is the highest,and the rotation axis is almost distributed near the<101>-<111>line,the crack deflection is the most significant,and the SCC resistance is the best.（b）The coarse and discontinuous grain boundary precipitation phase in V-containing alloys inhibited the anodic dissolution of grain boundaries,reduced H⁺aggregation near grain boundaries,and effectively hindered SCC crack extension.The ability of intermetallic compounds to capture hydrogen is Al₇Cu₂Fe>S>Al₂Cu.There are many strong hydrogen capture sites in the alloy with the addition of Fe/Sc,which increases the dissolution rate of hydrogen at the grain boundary and leads to the enhancement of SCC sensitivity.（c）The accumulation of H⁺and Cl^-at the crack deflection decreased ion concentration and crack tip driving force,effectively hindering crack extension.Conversely,if H⁺and Cl^-filled the entire crack and were adsorbed on the crack surface,which would reduce surface energy.Under tensile stress,the driving force would concentrate at the crack tip,accelerating crack extension.