| In the uranium-niobium alloy system,U-5.4 wt.%Nb(U-5.4Nb)alloy has attracted much attention due to its excellent corrosion resistance and special mechanical behavior.U-5.4Nb alloy is mainly processed by the wrought processing and the near-net-shape(NNS)casting processing to obtain the product.The solidification process in these two forming methods has an important effect on the as-cast microstructure of the product.Therefore,understanding and mastering the relationship between process variables and the as-cast microstructure of the alloy helps to accurately locate key influencing factors and optimize control parameters.Thus,this article focuses on the change of the cooling rate during solidification,which can comprehensively reflect the characteristics of temperature gradient and solidification rate,and the relationship between the structure and performance by using optical microscope,scanning electron microscope,X-Ray diffraction,nanoindentation,and time of flight secondary ion mass spectrometry.Finally,this paper discusses the response of grain morphology,solute distribution and inclusions in the as-cast microstructure of U-5.4Nb alloy to changes in cooling rate during solidification.In this paper,we have obtained the as-cast samples of U-5.4Nb alloy with various cooling rates during solidification by using the sample preparation techniques such as wedge casting,gas-cooled melt,arc remelting,vacuum suction casting and melt spinning with different cooling media and cooling methods,combining with finite element calculation and analysis.The results indicated the cooling rate during solidification increased from 2.3℃/s of wedge casting to 5 × 102℃/s of arc remelting samples when the mould changes from graphite to copper;with the improvement of the cooling capacity,the cooling rate rose from 3.2 x 104℃/s of the suction casting to the 1.5 × 106℃/s of the melt-spun ribbons.The as-cast microstructure of U-5.4Nb alloy exhibits a transition from the lamellar microstructure→the dendrite→the fine grains with the increase of the cooling rates,because the as-cast microstructure is controlled by competition of the cellular decomposition,the dendrite growth and the melt nucleation.Additionally,the average grain size in the fine grains decreases with the increase of cooling rates.When the cooling rate is less than 2.3℃/s,the high-temperature y phase decomposed into Nb-poor α and Nb-rich γ1-2 due to the lower cooling rate of the casting after the solidification is completed.When the cooling rate is 90℃/s～5 ×102℃/s,the number of nucleus is small,the dendrite is well developed,and the cellular decomposition of the γ phase is suppressed.Thus,the as-cast microstructure consists of well-developed dendrites,and some dendrites show a six-fold symmetrical structure.When the cooling rate is 3.2 × 104℃/s～1.5×106℃/s,the undercooling and nucleation rate increase sharply resulting a large number of nuclei,therefore,the growth of dendrite is suppressed,the as-cast microstructure is composed of the fine grains,and the average grain size decreased from 16.8 μm to 1.8 μm,and some grains were refined to 0.75 μm.Under the solidification conditions without solute trapping,the increase of the cooling rate reduces the local solidification time,inhibits the diffusion of solute Nb during the solid-liquid interface,and increases the variation of Nb concentration in the grain.Furthermore,the difference in Nb concentration leads to different micro-mechanical behaviors.When the cooling rate increases from 90℃/s to 9.9 ×105℃/s,the segregation ratio(SR)of solute Nb increases from 3.06 in the well-developed dendrites to 4.74 in the fine equiaxed grains,and the dimensionless coefficient for back-diffusion(α)reduced from 6.2 ×10-3 to 9.8 × 10-6.The variation of Nb has a significant effect on the mechanical properties of the dendrite.The elastic modulus and hardness increase from 61.38 GPa and 3.64 GPa of the primary dendrite arm to 113.10 GPa and 6.48 GPa of the secondary dendrite arm,respectively.The nucleation and growth of the inclusions(Nb2C and U(C,N))is suppressed and the size becomes smaller as the cooling rate increases.When the cooling rate increases from 2.3℃/s to 105℃/s,the size distribution of inclusions decreases from 10 μm～20 μm to 1μm～2 μm;when the cooling rate is 1.5 ×106℃/s,no inclusions were observed in the as-cast microstructure.The structure and composition of the inclusions indicate that U(C,N)can form a coherent interface with Nb2C and the y phase,respectively,during the precipitation and growth,and form a multi-phase inclusion to reduce the interface energy.In multi-phase inclusions,UO2 or Nb2C is located in the core;and U(C,N)forms a shell on the UO2(or Nb2C)and connects to the γ,where UO2 and Nb2C are mostly globular morphology,while U(C,N)shows a angular morphology.Based on the structural characteristics of multi-phase inclusions,combined with the phenomenon that some of the inclusions appear in the core of the dendrite of U-5.4Nb alloy,referring to the relationship between the thermodynamic parameters of UO2,Nb2C,UN and UC with temperature,it is speculated that the nucleation and growth of some inclusions includes following steps:(1)When the melt temperature(T1)is higher than the liquidus(TL)(TL<T1),some small UO2 and Nb2C particles presented in the melt.(2)When the melt temperature(T2)is between TL and Ts(solidus)(Ts<T2<TL<T1),the U-5.4Nb alloy start solidifying.Some nuclei of U-Nb alloy form preferentially on the UO2 or Nb2C particles.At the same time,some nuclei of Nb2C form on the UO2 particles,and form the multi-phase inclusions.Furthermore,some individual Nb2C particles precipitate and grow to form the single-phase inclusions.(3)When the melt temperature(T3)continues to decrease,Ts<T3<T2,the multi-phase inclusions appear since some nuclei of U(C,N)form on the UO2 or Nb2C particles.In addition,some individual U(C,N)particles precipitate because of the decrease in solubility of carbon and nitrogen in the melt.(4)When the temperature of the alloy(T4)is lower than(TS),the solidification process is accomplished and the multi-phase inclusions,the single-phase inclusions and grains coexist in the matrix.In summary,the systematic research results of this paper show that the as-cast microstructure of U-5.4Nb alloy with various cooling rates during solidification can be obtained by using different cooling media,cooling methods and methods of adjusting the total amount of metal melt.The grain growth of U-5.4Nb alloy and the nucleation and growth of the inclusions are suppressed,and the average grain size and the inclusion size are reduced with the increase of the cooling rate during solidification,thus,the microstructure is refined. |
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