Surface precipitate analysis of gas atomized Ni-Base superalloy powders

Powders surface precipitate has strong relations with the microstructure and properties of powder metallurgy components. Morphology, element distribution and crystal structure of the precipitate at gas atomized Ni-base superalloy powders surface were investigated. Results show that the precipitate is lamelliform and prefers to form at grain boundaries and interdendrite regions than dendrite arms. With powders size decreasing, the precipitate shape observed from surface varies from strip to rectangular and to nearly round, and the precipitate density increases, measured 17.75 μm−2, 21.42 μm−2, 26.50 μm−2 at nearly 45μm, 30μm and 10μm powders surface respectively, but the precipitate average size changes conversely, that is 228nm, 193nm, 77nm respectively. The precipitate enriches in high melting point elements Ti, Nb, Mo, W, Zr and poor in low melting point elements Ni, Cr, Co, Al. Crystal structure analysis reveals that the precipitate is MC, M23C6 or M6C carbide.


Introduction
Ni-base superalloys are widely used in aerospace industry especially in hot section components of gas turbine engine, such as turbine disk, due to their superior strength, creep and oxidation resistance at elevated temperatures [1][2][3] .The development of gas turbine engine placed severe requirements on Nibase superalloys.Therefore, a dozen alloying elements added to optimize its properties [4][5][6] .However, these elements impose great barrier to chemical homogeneity and workability for conventional cast and wrought process [7,8] .Fortunately, powder metallurgy (PM) breaks the dilemma.When gas atomization, the rapid solidification process (10 4 -10 5 º C/s) achieves highly chemical homogeneity [9][10][11] .The application of hot isostatic pressing (HIP), extrusion and isothermal forging (ITF) lay a good foundation for components manufacturing [12][13][14] .While pre-alloyed powders with high quality are of prime importance for PM processing.
Precipitate at powders surface can evolute to as-HIPed compact, forming prior particle boundaries (PPB), which devastates microstructure homogeneity and deteriorates mechanical properties [15][16][17][18] .Therefore, powders surface characterization lay the foundation to clarify the mechanism of PPB and further optimize the microstructure.Previous studies have been carried out to address the powders surface microstructure, oxidation characteristics, chemical state, degassing behavior [19][20][21][22][23][24] .So far, little research has been published on surface precipitate analysis of Ni-base superalloy powders.In this work, a large quantity of precipitates was discovered on powders surface.Field emission scanning electron microscopy (FESEM) and quantitative analysis were applied to describe its shape, distribution, density and size.The high-angle annular dark field SEM (HAADF) was employed to characterize the precipitate elements distribution.Cystal structure was identified by high-resolution TEM (HRTEM) and Fast Fourier transform (FFT) images.Precipitates formation mechanism were also discussed.

Experiment
Ni-base superalloy with chemical composition of 16 Cr, 13 Co, 4 W, 4 Mo, 3.7 Ti, 2.1 Al, 0.7 Nb, Ni balance (all in wt.%) was investigated.The master ingot prepared by vacuum induction melting (VIM) was argon gas atomized to obtain Ni-based superalloy powders.
The powders morphology was observed by FESEM, Regulus8100, with an acceleration voltage of 3kV to ensure surface details.The density and size of powders surface precipitates were calculated by Image Pro Plus.Transmission electron microscopy (TEM), Talors F200X G2, equipped with a high angle annular dark field (HAADF) detector was carried out for elements distribution and phase identification.The way to TEM sample preparation was focused ion beam (FIB).Pt protection layer was deposited on the powder surface, and the thin foils obtained along the spherical powder radius direction.

Particle morphology
FESEM micrograph of Ni-based superalloy powders with particle size less than 63μm illustrated in Figure 1.shows the near spherical morphology.The presence of satellite and heteromorphic particles confirm the typical feature of gas atomized powders.1.The precipitate density at nearly 45μm, 30μm, 10μm powders surface are 17.75 μm -2 , 21.42 μm -2 , 26.50 μm -2 respectively.Conversely, precipitate average size decreases with particle size decreasing, which is 228nm, 193nm, 77nm respectively in nearly 45μm, 30μm, 10μm powder surface.During gas atomization, heat transferred by forced convection between gas and droplets.The average cooling rate ( T ̅ ) of the droplet can be described as [25] : T 1 and T g respectively represent the temperature of molten alloy and atomization gas.A and V are surface area and volume of the droplet respectively.C l means the droplet volume specific heat.∆H f is the latent heat of fusion per droplet volume.∆T s =( T 1 -T  ), in which T  corresponds to the temperature the moment droplet completely solidified.h is heat transfer coefficient, which is expressed as [25] : Where, K g is thermal conductivity of atomization gas, d means the diameter of droplet, v ' represents the relative speed of gas and droplet at the boundary, ρ g corresponds to the gas density, η is the gas dynamic viscosity, and   = ( )1/3, in which C g means the gas volume specific heat.
T 1 , T g , C l , ∆H f , ∆T s , K g , ρ g ,   remain constant.Since v ' , ρ g and η slightly changes during atomization, they can be proximately seen as constant.Combined equation ( 1) and ( 2), simplified expression is deduced as: Where A and B are constants.
During cooling process of melting droplets, element W, Mo, Ti, Nb, etc trend to segregate at grain boundaries and interdendrite regions for their relatively lower coefficient of solute distribution [25] .Therefore, new precipitate formed at these places of Ni-based superalloy powders when atomization.Meanwhile, the rapid solidification process of gas atomization hinders elements diffusion, thus some over-saturated elements also retained in dendrite arms and formed the precipitate.Based on equation (3), the droplet cooling rate increases with its size decreasing during gas atomization.For nearly 45μm powders, with lower degree of supercooling, longer growth time and relatively enough supplement of molten alloy, fewer precipitate nucleated and grew bigger.The finer droplets are, the more precipitate nucleation formed but grew with greater difficulty.Consequently, more and finer precipitate formed around finer powders surface.

Element distribution of the powders surface precipitate
The elements lining distribution analysis of precipitate at dendrite arm at nearly 45μm powder surface is in Figure 3.A slightly higher element Ti and lower element Ni intensity can be distinguished in the precipitate compared in the powder matrix.While the elements lining distribution of the precipitate at grain boundary in The cross section of precipitate at nearly 45μm powder surface was obtained by FIB. Figure 5 (a) and Figure 5 (a) present the TEM bright field images of the precipitate at dendrite arm and grain boundary.The thickness of the precipitate is approximately 20nm, suggesting that the precipitate is lamelliform.The voltage of FESEM accelerates higher, the electron penetrates deeper.Conventionally, high voltage, above 5kV, was applied in morphology observation, which is reason why the powders surface precipitate usually cannot be discovered.For element analysis, the voltage accelerates even higher to collect enough electron, and as a result, elements from deep layer interfere the element distribution of precipitate, which is shown in

Crystal structure of the powders surface precipitate
Since the precipitate is too thin to be identified by selected aera electron diffraction (SAED) patterns, HRTEM and FFT was adopted.[116] zone axis with lattice parameter measured 1.181nm, which can be inferred to be M23C6 or M6C.The type of carbide phases at Ni-base superalloy powders surface strongly relates with chemical composition [26] .Specifically, higher carbide formation alloy elements concentration promotes the carbide formation with higher ratio of M and C. Therefore, M23C6 or M6C carbide are prone to precipitate at grain boundaries where element W, Mo, Ti, Nb enriches.Interestingly, carbides were rarely found inner powders, because element C segregates around powders surface [27] .
Previous studies [28][29][30] revealed that the PPB of as-HIPed superalloys is comprised of mainly γʹ phase, MC carbides and minor oxides, and the degree of PPB increases with powders particle size decreasing.It can be referred that precipitate at powders surface evolutes to as-HIPed superalloys, facilitating the formation of PPB.Furthermore, since the precipitate density is higher at finer powders surface, more carbide evolutes to as-HIPed superalloys, forming more continuous PPB.

Conclusion
Precipitate at gas atomized Ni-based superalloy powders surface was investigated.Following conclusions can be drawn.
(1) The powders surface precipitate is lamelliform and prefers to form at grain boundaries and interdendrite regions than dendrite arms.With the powders size decreasing, precipitate shape observed from surface varies from strip to rectangular and to nearly round.
(3) The precipitate enriches in high melting point elements Ti, Nb, Mo, W, Zr, which are also carbide forming elements, and poor in low melting point elements Ni, Cr, Co, Al. (4) Crystal structure analysis reveals that the precipitate is MC, M23C6 or M6C carbide.

Figure 2 .
displays the single powder morphology with various sizes.Columnar dendrites can be observed around the powder surface nearly 45μm (Figure 2. (a)).The microstructure of nearly 30μm powder surface (Figure 2. (c)) is equiaxial dendrites.And the surface microstructure of nearly 10μm powder (Figure 2. (e)) is cellular-dendrite grains.Magnified graphs (Figure 2. (b)(d)(f)) indicate that small precipitate formed around the powders surface, and the precipitate prefers to form at grain boundaries and interdendrite regions than dendrite arms.The precipitate shape at powder nearly 45μm (Figure 2. (b)) is strip or rectangular.And the shape of precipitate at nearly 30μm powder surface (Figure 2. (d)) is mainly rectangular.At finer powder surface, nearly 10μm in Figure 2. (f), precipitate is nearly round.Density (number per square micron) and average size of these precipitate are measured in Table

Figure 1 .
Figure 1.Morphology of Ni-based superalloy powders with particle size less than 63μm.

Figure 4 .Figure 3 .Figure 4 .
Figure 3. Elements distribution analysis of the surface precipitate at dendrite arm: (a) FESEM image of the surface precipitate; (b) EDS lining analysis results.

Figure 3
(b) and Figure 4 (b).To clarify the elements distribution a step further, the precipitate cross section was analysis by HAADF in Figure 5 (b) and Figure 6 (b).Result indicates that both precipitate at dendrite arm (Figure 5 (b)) and grain boundary (Figure 6 (b)) are rich in high melting point elements Ti, Nb, Mo, W, which are also carbide forming elements, and poor in low melting point elements Ni, Cr, Co, Al.This phenomenon coincides with the solidification rule analysis above.Compared the elements distribution of the interface of precipitate and powder matrix, which are around 5nm in distance of Figure 5 (b) and 10nm in distance of Figure 6 (b), elements intensity changes more gradually at dendrite arm than at grain boundary, implying the less sufficient elements diffusion process.

Figure 5 .Figure 6 .
Figure 5. Elements distribution analysis of the precipitate cross section at dendrite arm: (a) TEM bright field image of the precipitate cross section; (b) HAADF lining analysis results.

Figure 7 .
is crystal structure analysis of the precipitate at powder surface dendrite arm.HRTEM image of the rectangular area in Figure 7. (a) displayed in Figure 7. (b), where A1 and A2 is the region of powder matrix and precipitate respectively.The diffraction spots of region A1 presented in Figure 7. (c) shows the typical face-centered cubic (FCC) structure under [011] zone axis with lattice parameter measured 0.364nm, which can be confirmed as γ phase.FFT image of region A2 illustrated in Figure 7.(d) also reveals FCC structure under [011] zone axis, while the lattice parameter is 0.433nm, which can be verified as MC carbide phase.

Figure 8 .-), ( 1 - 1 - 1 )
is crystal structure analysis of the precipitate at powder surface grain boundary.And Figure 8.(b) is the HRTEM image of the rectangular area in Figure 8. (a).The FFT image of region A1 in Figure 8.(b) presented in Figure 7. (c) shows mainly (000), (111 diffraction spots, which are completely coincident with those in Figure 7. (c).Therefore, the region A1 in Figure 8.(b) is also γ matrix.Figure 8.(d) is the FFT diffraction spots of precipitate in Figure 8.(b) region A2, indicating face-centered cubic (FCC) structure under

Figure 7 .
Figure 7. Crystal structure analysis of the precipitate at powder surface dendrite arm: (a) TEM bright field images of the precipitate cross section; (b) HRTEM image of the rectangular area in (a); (c) FFT diffraction spots of region A1 in (b); (d) FFT diffraction spots of region A2 in (b).

Figure 8 .
Figure 8. Crystal structure analysis of the precipitate at powder surface grain boundary: (a) TEM bright field images of the precipitate cross section; (b) HRTEM image of the rectangular area in (a); (c) FFT diffraction spots of region A1 in (b); (d) FFT diffraction spots of region A2 in (b).

Table 1 .
Density and average size of the precipitate at powders surface.