| The nickel based superalloys contain a variety of alloy elements,which makes it difficult to control the composition,and in order to ensure the performance during the high-temperature and long-term service,there are also strict requirements for the controlling of trace impurity elements and inclusions in the alloy.At present,a double or triple melting process is mainly employed for the domestic preparation of superalloys,but problems such as high trace impurity content,severe segregation of alloy elements and long process are still exist.Electron beam refining is a process in which an electron beam with high energy density is used to bombard the surface of the material thus melts and purifies the material,and because it breaks through the problem of the driving force weakening during migration of impurity elements at the liquid-solid interface in the traditional metallurgical process,it shows unique advantages in the removal of impurity elements and inclusions,and thus the preparation of superalloy with ultra-purity and low segregation degree can be achieved.However,some basic problems during electron beam smelting of superalloys have not been completely solved,for example,with the removal of impurity elements and non-metallic inclusions,alloy elements also show different degrees of volatilization loss,besides,the microstructure characteristics of the prepared high-purity superalloy and its relationship with the properties are still unclear.The clarification of these problems is conducive to realize the control of microstructure and thus improving the performance of superalloys,and is of great significance for the industrialization of electron beam smelting for superalloys.The preparation of high purity Inconel 718 alloy by electron beam smelting was focused in this research,and the volatilization behavior of elements during electron beam smelting was studied,and the smelting process was directed according to the volatilization loss of each element to achieve precise control of alloy composition.In the meantime,the microstructure characteristics and mechanical properties of electron beam smelted Inconel 718(EBS 718)alloy under different heat treatment conditions were studied to establish the relationship between the microstructure and precipitation behavior and the mechanical properties.On the basis of the research above,the strengthening mechanisms of γ’/γ" phase in EBS 718 alloy were discussed,and a quantitative relationship between precipitation behavior and strengthening effect was acquired,which could provide a theoretical support for optimizing the microstructure and mechanical properties EBS 718 alloy.In addition,the corrosion resistance of EBS 718 alloy was analyzed,and the key factors affecting the electrochemical corrosion performance of EBS 718 alloy was explored.The results show that it is feasible to calculate the activity and activity coefficients of Fe,Ni and Cr elements in Inconel 718 alloy by using Miedema model,and the activity coefficient compensation factor ωAl of Al element is calculated to be 0.0023 by the relationship between the actual evaporation rate and the theoretical evaporation rate.As the smelting temperature and time increase,the mass fraction of Ni,Ti,Nb,Mo,Fe elements increases,while the content of Cr and Al elements decreases,and as the quality of the raw material increases,the mass fraction of Ni,Nb,Mo,Ti,Fe elements decreases first and then remains stable,while the mass fraction of Cr and Al elements increases first and then remains stable.After optimization of the smelting parameters,the alloy composition can be precisely controlled,and the mass fractions of the most volatile Cr and Al elements only reduced by 0.42%and 0.01%,respectively.The secondary dendrite space in the as-cast micro structure of EBS 718 alloy is about 28μm,and the cooling rate during the end of solidification is about 279.2 ℃/min.After standard heat treatment,the main precipitation phase in the alloy is the plate-like γ" phase with size of 12～15 nm,and the ratio of γ"toγ’ precipitates is about 2.92.With the increasing of solution temperature,the content of δ phase in EBS 718 alloy decreases exponentially,and the lower solution temperature favors the precipitation of γ" phase,while the higher solution temperature favors the precipitation of y’ phase.When the solution temperature is 1150 ℃,the γ’ phase is uniformly distributed in the matrix,whose size is about 8.43 nm,and the smallest mismatch between the precipitation phase and the matrix is observed.After the forging treatment of EBS 718 alloy,fine(Nb,Ti)C carbides are dispersively distributed in the matrix,and the δ phase in the matrix basically disappears.After standard solution treatment,the recrystallization and grain growth happens,the size of grain boundary 8 phase is about 1.5μm after aging treatment.As the solution temperature increases,the grain size of the forged EBS 718 alloy increases,while the amount of δ phase along the grain boundary decreases,and when the solution temperature is higher than 980 ℃,only the dispersively distributed MC carbides are observed.The Vickers hardness of EBS 718 alloy after standard heat treatment is about 418(HV0.1),and the compressive strength and tensile strength are 1169.5 MPa and 981.9 MPa,respectively,and the creep life is 327.3 h with the minimum creep rate of 3.90×10-5/h under the condition of 680 ℃/500 MPa.As the solution temperature increases,the Vickers hardness,tensile strength and compressive strength of the aged EBS 718 alloy are all increased.The creep results show that the EBS 718 alloy solution treated at 1150 ℃ and aged has lower steady state creep rate and higher creep resistance compared with the standard heat treated alloy.After standard heat treatment of the forged EBS 718 alloy,the Vickers hardness is 458(HV0.1),and the compressive strength is 1434.6 MPa,which is higher than the conventional forged fine-grained Inconel 718 alloy,besides,its creep life at 680 ℃/500 MPa is 1266.2 h,which is 2.71 times of the standard Inconel 718 alloy,and its minimum creep rate is 7.50×10-7/h,which is about 1/20 of the traditionally forged Inconel 718 alloy,showing excellent creep resistance.The compressive strength and Vickers hardness decreases with the increasing of solution temperature for the aged alloy,and similar rule is found for the high temperature compression tests.When the solution temperature is 1150 ℃,the creep life and minimum creep rate of the forged EBS 718 alloy are 798.3 h and 1.78×10-6,respectively,and the creep performance is lower than that of the forged and standard heat treated EBS 718 alloy,but superior than the traditionally forged Inconel 718 alloy.The main strengthening mechanism of γ’ precipitates in EBS 718 alloy is stacking fault strengthening,and for γ"phase,modulus and ordering strengthening are the main strengthening mechanisms.The strengthening effect of EBS 718 alloy is composed of Ογ" andγ’ precipitation strengthening,and the critical shear stress caused by precipitation strengthening can be expressed as a function related to the precipitation behavior of γ’’ and yγ’,in which the value of precipitation strengthening index factor q of γ" and γ’phase are 1.9 and 1.8,respectively.The theoretically calculated critical shear stress maintains a linear relationship with the yield strength and Vickers hardness,and the y’’ and y’ dual-phase strengthening models can well predict the precipitation strengthening effect of EBS 718 alloy.EBS 718 alloy shows a higher self-corrosion potential and a lower self-corrosion current density than the induction melted Inconel 718 alloy,exhibiting excellent electrochemical corrosion performance.The pitting potential of EBS 718 alloy increases with the increasing of solution temperature,the effect of solution temperature on the electrochemical behavior is mainly manifested by that the size and volume fraction of δ phase decrease with increasing solid solution temperature,thus reducing the density of the pitting initial zone.After aging treatment,the y’ and γ" phases are precipitated in the alloy,and apart from the increasing of the density of pitting initial zone,the lattice misfit caused by the Nb-rich γ" phase results in a degradation of the corrosion resistance.When the solution temperature is 1150 ℃,the y’precipitates with size of about 10 nm are uniformly distributed in the aged alloy,and the electrochemical activation energy of the alloy is 52.86 kJ/mol,which shows a high electrochemical corrosion resistance... |
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