| Inconel718（IN718）alloys have been largely applied in turbine blades of aircraft engines and heat exchangers of nuclear reactors,valves with excellent high-temperature strength and corrosion resistance.In recent years,additive manufacturing has provided solutions for the preparation of hot end components with complex structures.With the rapid development of nuclear reactors,aircraft engines and gas turbines,it is urgent to develop and use high-temperature structural materials with higher temperatures and better comprehensive performance.At the same time,additive manufacturing of IN718 alloys still faces problems such as anisotropy and insufficient high-temperature strength.The introduction of oxide particles is considered to be an important way to solve anisotropy and improve properties.In this paper,the rare earth element Sc is introduced into IN718alloy to explore the microstructure regulation mechanism of rare earth element Sc on IN718 alloy by laser powder bed fusion（LPBF）.The existence form of Sc elements in the thermal cycle process of additive manufacturing was revealed.The effect of Sc element addition on the strengthening mechanism of IN718 alloy was analyzed,and the evolution law of Sc-particles during heat treatment and the influence on the microstructure,mechanical properties and corrosion resistance of IN718alloy were explored,which provided theoretical support for the control of IN718 alloy composition,microstructure control and performance improvement of IN718 alloy added by Sc in additive manufacturing.The main research conclusions of this paper are as follows:（1）IN718 alloy formed by LPBF has high density,multi-level microstructure structure.The grain morphology is columnar grains parallel to the built direction,and the columnar grains grow across multiple molten pools,showing a strong（100）texture.The inside of the columnar grains are submicron-level cellular structure,and the cell wall has the segregation of Cr,Fe,Nb.A small amount of Laves phase exists inside the as-deposited IN718 alloy,and theγ’andγ"phases have not been significantly precipitated due to the rapid solidification process under the high temperature gradient.As-built IN718 alloy exhibits good mechanical properties at room temperature,with the tensile strength of 1177 MPa,and the yield strength of 864 MPa and elongation of 24%.（2）In the solidification process of Sc element added with IN718,Sc2O3 particles are formed in situ at the front of the solid-liquid interface and serve as heterogeneous nucleation points to promote grain nucleation,showing remarkable grain refinement effect.The average grain size and aspect ratio decrease from 51.6μm and 4.3 to 28.6μm and 2.4μm,respectively.In IN718 added by 0.1wt.%Sc（IN718-0.1Sc）,Sc element was mostly occupied by element segregation and solution,and no other form of Sc element was found.In IN718 added by 0.3 wt.%Sc（IN718-0.3Sc）,the Sc2O3 particles generated in situ can nail grain boundaries,hinder dislocation movement,and produce good fine grain strengthening and dislocation strengthening effects.The yield strength of as-deposited IN718-0.3Sc at 25°C and 650°C was 1030 MPa and 837 MPa,respectively,which were 19.2%and 21.7%higher than that of IN718.（3）Solution treatment is beneficial to improve elemental segregation and promote recrystallization.IN718-0.3Sc after solution treatment at980°C has a minimum average grain size（26.8μm）.With the increase of solution temperature,columnar grains transform to equiaxed grains,the cellular structure dissolves,the recrystallized grain grows,and the proportion of low-angle grain boundary and grain aspect ratio decreases.The double aging promoted the precipitation ofγ’andγ"phase in IN718alloy,but did not change the grain morphology.The orientation relationship between theγ"phase and the matrix phaseγis{100}γ"∥{100}γ,[001]γ"∥<100>γ.In IN718-0.3Sc of direct double aging treatment,Cr2O3 particles and Sc-Ti（Al）-O particles were present at the cell wall of the cellular structure,which reduced the segregation of Nb elements and facilitated the precipitation ofγ"phases.（4）A small amount of needle-likeδwas precipitated withγ’andγ"in IN718 after 980℃solution and double aging,which can nail the grain boundaries.and the orientation relationship betweenδphase and matrix is as follows:[102]δ//[011]γ,（010）δ//（11-1）γ.Sc atom is easy to crowd out Al atom with Ti atom,forming Ni3（Al,Ti,Sc）particles precipitation,in IN718-0.3Sc after 980℃solution and double aging,but there is a small amount of submicron size distribution in the precipitatedγ’andγ"phases.Columnar grains undergo complete recrystallization,exhibiting a uniform equiaxed grains morphology,but the grains have grown in IN718-0.3Sc after solution at 1040℃and double aging.Ni3（Al,Ti,Sc）particles were also distributed,andγ’andγ"phases were all fine nanophase diffusion distributions in IN718-0.3Sc after solution at 1040℃and double aging.The comprehensive mechanical properties of IN718-0.3Sc after solution at1040℃and double aging were the best,the yield strength was 1278 MPa,the tensile strength was 1538 MPa,and the elongation was 6.7%at room temperature（25°C）;The yield strength was 1173 MPa,the tensile strength was 1359 MPa,and the elongation was 13.2%at high temperature（650°C）.（5）As-built IN718-0.3Sc has better corrosion resistance,its corrosion rate is only half that of IN718,but the corrosion resistance is three times that of IN718.After 1040℃high temperature solution and double aging,the corrosion rate of IN718-0.3Sc is 5.9×10-3 mm/a,which is lower than that of IN718(6.3×10-2 mm/a)by an order of magnitude.The self-corrosion potential of IN718-0.3Sc alloy after heat treatment is still the same as that of the deposition state,and although the corrosion rate increases,the corrosion resistance does not change significantly,so it can still maintain good corrosion resistance.The corrosion mechanism of IN718:during the corrosion process,Ni,Cr and Fe elements with negative potential dissolve firstly form hydroxide,forming the outer layer of the corrosion product film.These hydroxides undergo dehydration reaction to oxides and become the inner layer of corrosion products.These oxides constantly react with chloride ions in the solution.The corrosion interface is pushed into the matrix,and finally the matrix is corroded. |
PDF Full Text Request