稀有金属(英文版) 2020,39(10),1174-1180

Microstructure and microhardness of directionally solidified NiAl-W eutectic alloy

Jian-Jun Gao Zhi-Long Zhao Lu-Feng Wei Kai Cui Lin Liu

School of Mechanical Engineering and Automation,Fuzhou University

School of Mechanical Engineering,Northwestern Polytechnical University

State Key Laboratory of Solidification Processing,Northwestern Polytechnical University

作者简介：*Zhi-Long Zhao,e-mail:zhaolong@nwpu.edu.cn;

收稿日期：14 January 2018

基金：financially supported by the National Natural Science Foundation of China (No.51374173);

Microstructure and microhardness of directionally solidified NiAl-W eutectic alloy

Jian-Jun Gao Zhi-Long Zhao Lu-Feng Wei Kai Cui Lin Liu

School of Mechanical Engineering and Automation,Fuzhou University

School of Mechanical Engineering,Northwestern Polytechnical University

State Key Laboratory of Solidification Processing,Northwestern Polytechnical University

Abstract：

The microstructure and microhardness of directionally solidified NiAl-W eutectic alloys at growth rates of 2-25 μm·s^-1 were investigated by a Bridgman crystal growing facility at a temperature gradient of 300 K·cm^-1.In view of the competitive growth between W dendritic and eutectic phases,W dendritic phase was eliminated,whereas the fully eutectic phase was prominent in the steady progress of the directionally solidified NiAl-W eutectic alloys.As the growth rate(V) increased,both the structure and solid/liquid interface of the directionally solidified NiAl-W eutectic alloys changed from planar to cellular.Both the fibrous spacing(d) and the diameter(a) decreased with increase in growth rate(V).The Vickers microhardness(H) of the directionally solidified NiAl-W eutectic alloys decreased with fibrous spacing(d) or diameter(a) increasing.The relationships of H to d and a were H=371.58 d^-0.09 and H=297.70 a^-0.09,respectively.

Keyword：

Microstructure; Directional solidification; Eutectic alloy; Microhardness;

Received： 14 January 2018

1 Introduction

NiAl intermetallic alloy has attracted considerable attention because of its low density,high melting point,good oxidation resistance,and high thermal conductivity [ 1] .However,the wide application of NiAl alloys is hindered by its low ductility and toughness below the brittle-toductile transition temperature and its poor creep strength at high temperatures [ 2, 3] .

The mechanical properties of NiAl alloys can be improved using different methods,such as alloying [ 4] ,compositing with other materials [ 5] ,and directional solidification (DS) [ 6] .DS eutectic alloys exhibit an improved room temperature fracture strength relative to that of binary NiAl alloys [ 7, 8] .DS method combined with second-phase strengthening has been widely used in fabricating B2-ordered NiAl-X (W,V,Re,Cr,Mo) alloys [ 9, 10, 11, 12, 13] .Several studies have investigated the morphologies of directionally solidified eutectic alloys and the application of W fibers.Milenkovic et al. [ 9] confirmed the eutectic reaction temperature and the composition of NiAlW eutectic alloys,and they processed the eutectic cells through DS.Hassel et al. [ 14, 15] combined DS with selective dissolution to fabricate W nano wires with high aspect ratios,and they proved that these W nano wires were single crystals [ 16] .W fibers have been used as pH-sensitive electrodes in the selective etching of directionally solidified NiAlW eutectic alloys [ 17] and as substrates for forming gold nanoelectrode arrays by electrodepositing gold metal on W nanowires in directionally solidified NiAlW eutectic alloys [ 18] .

However,the growth rate greatly influences the morphologies of directionally solidified eutectic alloys.As the growth rate increased,the microstructure of directionally solidified NiAl-Cr (Mo) hypereutectic alloys changed from planar eutectic to cellular eutectic and then dendritic eutectic [ 19] ,and both the fibrous spacing and the diameter decreased in directionally solidified NiAl-Re eutectic alloys [ 11] .Moreover,the mechanical properties of eutectic alloys depend on the eutectic spacing and the morphology,which are determined by DS parameters,such as growth rate and temperature gradient.To date,limited works have investigated the influence of growth rate on the microstructure and solid/liquid (S/L) interface morphology of directionally solidified NiAl-W eutectic alloys in a wide range of growth rate.A few works have explored the parameters affecting the relationship between the microhardness and the solidified microstructure,such as fibrous spacing and diameter of directionally solidified NiAlWeutectic alloys.In this work,the microstructure,S/L interface morphology,and microhardness of directionally solidified NiAl-W eutectic alloys grown at different growth rates at a constant temperature gradient were investigated in detail.

2 Experimental

The starting materials used in the experiments were Ni(99.99%),Al (99.99%),and NiW (99.9%) master alloys.NiAl-1.5 W (at%) alloys were prepared by induction melting and casting in a water-cooled mold.Dry argon was introduced to prevent oxidation during the process.The samples were processed in a Bridgman-type directional solidification furnace at a heating temperature of 1700℃,a temperature gradient of approximately 300 K·cm^-1,and growth rates of 2,4,6,8,15,and 25μm·s^-1.Once the solidification distance reached 30 mm,the samples were rapidly quenched into a liquid Ga-In-Sn alloy to retain their S/L interface morphologies.The directionally solidified samples were cut along the longitudinal direction by using a wire electro-discharged machine.The location of the cross section for metallographic analysis was 5 mm below the S/L interface.

After grinding and polishing,the specimens were etched with a solution of 3.0 vol%HCl and 3.2 vol%H₂O₂.The etched specimens were analyzed by using a Lecia DM4000 M optical microscope (OM).The W fibrous spacing was measured using line-intercept method,and the W fibrous diameter was quantitatively analyzed based on the scanning electron microscope (SEM) images.Microstructural analyses were performed using a Quanta600FEG SEM and a FEI-Tecnai G2 F20 transmission electron microscope (TEM) with energy-dispersive spectra(EDS) analysis.

3 Results and discussion

3.1 Competitive growth in DS process

Figure 1 shows microstructural evolution of the directionally solidified NiAl-W eutectic alloy grown at 8μm·s^-1.Figure la-d presents SEM images of the as-cast,initial stage,transition zone,and steady zone of the alloy,respectively.As shown in Fig.la,the microstructure of the as-cast alloy consisted of dendritic W and eutectic phases.As illustrated in Fig.1b,the W dendrites phase appeared at the initial stage of the directional solidification process.Bits of the eutectic phase were situated near the W dendritic phase.As the directional solidification progressed,the W dendritic phase gradually vanished due to the competitive growth between the W dendritic and eutectic phases (Fig.1c).The W dendritic phase was eliminated once the directional solidification reached a steady condition,and the full cellular eutectic microstructure was obtained (Fig.1d).The same phenomenon has also been found in previous works [ 9, 19] .The W fibrous phase is parallel to the growth direction,and these fibers were dense in the middle and sparse at the edges of the cellular eutectic microstructure.The cellular eutectic microstructure was also obtained at growth rates of 15 and 25μm·s^-1.As illustrated in Fig.le,the cellular eutectic phase was distributed uniformly along the front edge of the S/L interface,and the W fibrous phase was distributed along the border of the cellular eutectic phase,indicating that the fibrous W phase preferred the growth direction rather than lateral growth in the mushy zone.By contrast,in the solid zone,the W fibrous phase grew parallel to the growth direction was distributed uniformly in the middle of the cellular eutectic phase.The W fibrous phase was radial at the edge of the cellular eutectic microstructure.

3.2 Morphologies of directionally solidified NiAl-W eutectic alloys at different growth rates

Figure 2 shows the microstructures of directionally solidified NiAl-W eutectic alloys grown at different growth rates.The growth rates in the range of 2-25μm·s^-1resulted in a regular fibrous eutectic microstructure.W fibers were distinctly present on the surface of the samples after the NiAl matrix was partially dissolved in 3.0 vol%。HCl and 3.2 vol%H₂O₂ mixed solution.When the growth rate was 2-6μm·s^-1,an inpidual W fiber displayed a hexagonal shape and was distributed uniformly on the surface of the alloys,as shown in Fig.2a-c.The shape of W fibers at growth rates of 2-6μm·s^-1 was hexagonal in the cross section,suggesting that the directionally solidified NiAl-W eutectic alloy had an anisotropic interfacial energy at low growth rates.When the growth rate was8-25μm·s^-1,the cellular micros truc ture can be observed,and it exhibited a radial pattern from the cell interior outward to the cell boundary.As shown in Fig.2d-f,an inpidual W fiber presented an elliptical shape,implying that the directionally solidified NiAlW eutectic alloys had an isotropic interfacial energy at high growth rates.

Fig.1 SEM images of transverse sections showing microstructures of alloy grown at 8μm·s^-1:a as-cast,b initiation zone,c transition zone,d steady-state zone,and e solid/liquid interface

Fig.2 SEM images of transverse microstructure of alloys grown at different growth rates insert with enlarged pictures:a 2μm·s^-1,b 4μm·s^-1,c 6μm·s^-1,d 8μm·s^-1,e 15μm·s^-1,and f 25μm·s^-1

The W fibrous spacing (d) was determined as the distance between adjacent W fibers,and the W fibrous diameter (a) referred to the average length of the cross section of fibers.The measured data of fibrous spacing and diameter are presented in Table 1,and corresponding linear analysis is shown in Fig.3.Given that the fibers bend at the cellular boundary,the fibers at the center of the cellular eutectic structure was utilized to determine the fibrous spacing.As shown in Table 1,both the fibrous spacing and the diameter decreased with growth rate increasing.The NiAl and W phases rejected each other during the eutectic growth of the NiAl-W eutectic alloys.Therefore,the lateral diffusion between NiAl and W phases determined the fibrous spacing.A higher growth rate corresponded to a more difficult lateral diffusion and a lower fibrous spacing.As illustrated in Fig.3,the variations in the maximum,mean,and minimum fibrous spacing were linearly related to the growth rate,and the linear fitting results were as follows:d_max=9.00V-0.38,d_mean=7.34V^-0·38,and dmin=5.93V^-0.38,all of which satisfied the Jackson-Hunt model because the fibrous spacing displayed a nonlinear relationship with the growth rate [ 20] .As listed in Table 1,the mean fibrous diameter decreased from 0.484 to0.190μm as the growth rate increased from 2 to25 The fibrous diameter and the growth rate also presented a linear relationship as follows:a=0.65 V^-0.40.

3.3 S/L interface morphologies at different growth rates

Figure 4 shows SEM images of S/L interface morphologies of the alloys grown at 2,4,6,8,and 25μm·s^-1.The S/L interface morphologies were planar at growth rates of2-6μm·s^-1.The microstructure of the alloys exhibited a fully eutectic structure in the steady-state zone and consisted of W and NiAl phases in the steady-state zone.Figure 4d-e depicts the morphologies of the S/L interface at growth rates of 8 and 25μm.s^-1.When the growth rate was 8μm·s^-1,the S/L interface was cellular,as shown in Fig.4d.The average cell size decreased at the growth rate of 25μm·s^-1 (Fig.4e).Both the temperature gradient and the Qrowth rate determined the S/L interface features of a binary eutectic alloy.If an alloy was directionally solidified at a certain temperature gradient,the stability of the S/L interface was determined by G/V [ 21] ,where G is the temperature gradient ahead of the S/L interface.When the growth rate of the directionally solidified alloys was low,the S/L interface was planar.By contrast,at high growth rates,the S/L interface was unstable and became cellular.A similar phenomenon has also found in other directionally solidified eutectic alloys [ 22] .

Fig.3 Variations of W fibrous spacing (d) as function of growth rate(V)

3.4 Effect of growth rates on microhardness of DS NiAl-W eutectic alloys

The mechanical properties of solidified materials are typically characterized by a hardness test,which can be used to predict the yield strength of directionally solidified samples [ 23, 24] .The microhardness of directionally solidified NiAlW eutectic alloys was assessed under a load of 19.61 N,as shown in Fig.5.Microhardness decreased with fibrous spacing increasing,as shown in Fig.5a.The absolute value of the exponent representing the phase spacing was 0.09,which was smaller than that for THi-49Al (0.23) [ 25] .These differences may be due to different alloy compositions and different microstructures of the solidification conditions.Microhardness also decreased with fibrous diameter increasing (Fig.5b).Microhardness decreased with an increase in fibrous spacing or diameter,which is similar to that of other directionally solidified Al-based alloys [ 26] .

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Table 1 Experimental data of DS NiAl-W eutectic alloy

Fig.4 SEM images of solid/liquid interface morphologies of alloys grown at different growth rates:a 2μm·s^-1,b 4 1m·s^-1,c 6μm·s^-1,d 8μm·s^-1,and e 25μm·s^-1

Fig.5 Microhardness of NiAl-W alloy under a load of 19.61 N:a variation of microhardness (H) with fibrous spacing and b variation of microhardness with diameter

Figure 6 shows the indentations on the directionally solidified samples under a load of 19.61 N.Micro-cracks were not observed at the acute angle of indentation in Fig.7,indicating that the directionally solidified eutectic sample possessed a good toughness.The microhardness value of pure W measured in this work was 0.405-0.425GPa,and that of pure NiAl was～0.254 GPa [ 27] .This finding confirmed that the microhardness of directionally solidified NiAl-W eutectic alloys exceeded that of pure NiAl probably because of the dispersion strengthening mechanism.The dispersion strengthening of W atoms in the NiAl matrix is an important strengthening mechanism in NiAl-W eutectic alloys.TEM analysis of the micro structure revealed that some W nanoparticles with a size of 20-50 nm were present in the NiAl matrix,as shown in Fig.7a.Thus,the dispersion strengthening of W in the NiAl matrix contributed to the increase in the strength of directionally solidified NiAlW eutectic alloys.The same dispersion strengthening mechanism can be found in other DS alloys [ 22] .EDS analysis in Fig.7b showed that the W nanoparticle was composited by Ni,Al,and W.Ni and Al were detected because the detection signal was applied to the NiAl matrix.Moreover,the solution strengthening of the W element in the NiAl matrix may also contribute to the increase in the microhardness of directionally solidified NiAlW eutectic alloys.A TEMimage of NiAl matrix is shown in Fig.8.The lattice fringes possessed a spacing of 0.208 nm in the (110) NiAl plane,as shown in Fig.8b.The SAED pattern of NiAl in Fig.8c indicated that the NiAl matrix was a cubic lattice.The NiAl alloy had a lattice parameter of 0.294 nm,which was higher than that measured by Milenkovic (0.289 nm) [ 9] .As substitutional solutes,large W atoms can increase theNiAl lattice parameters [ 28] ,implying that W had a solidsolution in the NiAl matrix.

Fig.6 a OM image of inverted pyramidal indentations under a load of 19.61 N on DS NiAl-W eutectic sample grown at a growth rate of8μm·s^-1 and b a magnified image of local indentation as a white box shown in a

Fig.7 Fine W nanoparticles in NiAl phase at a growth rate of 8μm·s^-1:a TEM image and b EDS analysis of W nanoparticle

4 Conclusion

In this study,the microstructure and microhardness ofdirectionally solidified NiAlW eutectic alloys grown at growth rates of 2-25μm·s^-1 were investigated.The experimental results revealed that because of the competitive growth between W dendritic and eutectic phases,the dendritic W phase was eliminated,and the directionally solidified microstructure transformed from the initial dendritic W and eutectic phases to fully eutectic phases in the steady process of directionally solidified NiAl-W eutectic alloys.

Fig.8 a TEM image,b HRTEM image in Region A and c S AED pattern of NiAl matrix in Region B for W fiber of DS NiAl-W eutectic alloy grown at a growth rate of 8μm·s^-1

The S/L interface of the eutectic alloy changed from planar to cellular as the growth rate increased.When the growth rate was 2-6μm·s^-1,the S/L interfaces of the directionally solidified NiAl-W eutectic alloys were planar.When the growth rate was greater than 8μm·s^-1,the S/L interfaces of the directionally solidified NiAlWeutectic alloys were cellular.Both the fibrous spacing(d) and the diameter (a) decreased with growth rate(V) increasing,and the relationships among d,a and V were given as follows:d_max=9.00V^-0.38,d_mean=7.34V^-0.38,d_min=5.93V^-0.38 and a=0.65V^-0.40.

The Vickers microhardness (H) of the directionally solidified NiAl-W eutectic alloys decreased with fibrous spacing (d) or diameter (a) increasing.The relationships among d,a and V were given as H=371.58d^-0.09 and H=297.70a^-0.09.

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