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参考文献

1 Introduction▲
2 Experimental▲
3 Results and discussion▲
3.1 Microstructure of high-entropy alloy coating
3.2 Microhardness and wear resistance properties of high-entropy alloy coating
4 Conclusion▲
图表▲

Rare Metals2019年第12期

Microstructure and wear resistance of CoCrNbNiW high-entropy alloy coating prepared by laser melting deposition

Yan-Jun Jia Han-Ning Chen Xiao-Dan Liang

School of Mechanical Engineering,Tianjin Polytechnic University

School of Computer Science and Technology,Tianjin Polytechnic University

作者简介：*Yan-Jun Jia e-mail:allanjia@qq.com;*Han-Ning Chen e-mail:perfect_chn@hotmail.com;

收稿日期：12 July 2019

基金：financially supported by the National Key R&D Program of China (No.2017YFB1103604);

Microstructure and wear resistance of CoCrNbNiW high-entropy alloy coating prepared by laser melting deposition

Yan-Jun Jia Han-Ning Chen Xiao-Dan Liang

School of Mechanical Engineering,Tianjin Polytechnic University

School of Computer Science and Technology,Tianjin Polytechnic University

Abstract：

The defect-free CoCrNbNiW high-entropy alloy coating was successfully prepared by laser melting deposition,and its microstructure and wear resistance were investigated.The results showed that the microstructure of CoCrNbNiW high-entropy alloy coating consisted of fee phase rich in Nb and fee phase including un-melted W particles and rich in Cr.Moreover,an amount of fee phase was formed at the middle and top of coating,while the bcc phase rich in Cr was formed at the bottom.Meanwhile,the un-melted W particles were diffusely distributed in the coating.Therefore,the microhardness of CoCrNbNiW high-entropy alloy coating was improved and was 2.78 times as high as that of substrate.The wear loss and wear rate of coating were 0.26 and 0.23 times higher than those of substrate,respectively.The wear resistance of substrate was obviously improved due to the preparation of CoCrNbNiW high-entropy alloy coating.

Keyword：

Microstructure; High-entropy alloy; Microhardness; Wear resistance;

Received： 12 July 2019

1 Introduction

A plenty of traditional metaallic materials,such aas carbon steel,some pure metals and alloys,are used in the field of aaerospaace,automobile die,marine,military,even in civilian life due to their excellent abrasion resistance,corrosion resistaance,high-temperature resistance,ductility and so forth [ 1, 2, 3, 4, 5, 6, 7, 8, 9] .However,they always present poor properties in extreme conditions (complex corrosive medium or worse wear environment),and then,their service lives are decreased.To overcome these problems,all kinds of coatings were prepared by many researchers [ 10, 11, 12, 13] .

Among the above-mentioned materials,the bulk amorphous coatings contained one principal element and some small amounts of minor elements have been extensively used to improve the properties of substrate because of their excellent corrosion resistance,wear resistance,high strength and clastic strain limits [ 14, 15] .However,the completely amorphous phase and higher dimension are gained at the same time.Moreover,the catastrophical failure under loading at room temperature has taken place in these metallic materials.The high-entropy alloys(HEAs) as new kinds of developed materials are different from these materials.They not only consist of at least five major elements,but also have the concentration of each element in the range of 5 at%-35 at% [ 16] .Therefore,their properties depend on their multiple principal compositions.Generally,the HEAs are mainly composed of solid solution phases due to high entropies of mixing in liquid or solid solution state [ 17] .This indicates the higher work hardening and plasticity at room temperature compared to the amorphous alloys.Therefore,an increasing number of researchers paid attention to the development and application of HEAs.For example,Shu et al. [ 18] studied the microstructure and wear and corrosion resistance of CoCrBFeNiSi high-entropy alloy amorphous coating on H13 steel prepared by laser cladding,and the results showed that the coatings contained the bottom dendritic layer,the upper amorphous layer and the transition layer.Moreover,the microhardness of coating increased with the increase in amorphous phase and could reach～5 times that of substrate.However,the poor wear and corrosion resistance of coating were caused by the decrease in amorphous phase.Shang et al. [ 19] studied the microstructure and properties of CoCrFeNi(W_1-xMo_x)high-entropy alloy coatings.They found that CoCrFeNi(W_1-xMo_x) high-entropy alloy powders were the mixture of bcc and fcc solid solution phases,while the VHPS-ed CoCrFeNiW and CoCrFeNiW_0.5Mo_0.5 high-entropy coatings had two fcc phases and a small amount ofδ-NiW andσ-CoCr phases.The microhardness and wear resistance of the former were better than that of the latter,but the corrosion resistance was poorer due to the addition of Mo.Therefore,the researches of wear and corrosion resistance of high-entropy alloys prepared by different methods have been conducted.

Recently,the laser melting deposition technology as one of the additive manufacturing methods that are employed to prepare the high-entropy alloys has possessed excellent mechanical properties,corrosion resistance and high strength due to its high hating speed and cooling rate [ 20, 22] .For example,Dobbelstein et al. [ 23] prepared the refractory element TiZrNbHfTa high-entropy alloy by laser melting deposition and found that the microstructure of alloy has bcc crystal and its microhardness was～HV 509.Therefore,in this study,the CoCrNbNiW high-entropy alloy coating possessed elements of higher melting point and hardness was successfully prepared on medium carbon steel by laser melting deposition,and then,the microstructure and wear property of coating were studied.

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Table 1 Chemical component of primary powder (wt%)

2 Experimental

The mechanical mixture powders of Co-Cr-Nb-NiWwere used as the primary powders (Table 1),and the 45steel (Table 2) was employed as substrate.Before testing,the mixture powders were mixed for 2 h by TJ-2L planetary ball mill.The ratio of mixture powders to balls was5:1,and the diameter of stainless steel was～8 mm.Figure 1 shows the microscopic morphology of primary powders.Obviously,the powders of size～116μm presented the sphere or polygonal shapes.In this paper,the1 kW fibre laser (RFL-C1000,China) with the wavelength of 1080 nm was used to prepare the high-entropy alloy coating.The laser power was 8.00 W,scanning speed was8 mm·s^-1 and overlapping ratio was 50%.

The phase composition of the high-entropy alloy coating was analysed by D/MAX-2500 X-ray diffractometer(XRD,target:Cu,40 kV,40 mA).Microstructure was characterized by ZEISS Sigma 300 field emission scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS).Microhardness was measured by HV-1000 microhardness tester with a load of 1.96 N and a dwelling time of 10 s.The block-on-ring M-2000 with the pressure of 190 N and a sliding speed of 200 r·min^-1 was employed to measure the wear resistance of high-entropy alloy coating.The GCr 15 steel (its diameter was 30 mm)was used as counterpart ring,and the experimental time was 1 h.In addition,the wear morphology of high-entropy alloy coating was analysed by SEM.

Fig.1 SEM image of primary powders

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Table 2 Chemical component of substructure (wt%)

3 Results and discussion

3.1 Microstructure of high-entropy alloy coating

Figure 2 shows XRD pattern of CoCrNbNiW high-entropy alloy coating.Obviously,the phase composition of coating is composed of bcc and fcc phases.Intermetallic or Laves phase are not formed in coating.This indicates that highentropy effect significantly decreases the Gibbs free energy of CoCrNbNiW high-entropy alloy coating,and thus,the solid solutions are more likely to form during the rapid solidification [ 24, 25] .Figure 1 shows that the strongest peak located at 20 of～40.1°is identified as bccα-W phase;some weaker peaks with bcc phases are rich in Cr and Ni with a small amount of Fe.The existence of Fe in coating is caused by the fact that substrate is melted by power from laser and melting primary powders.However,fcc phases are composed of Nb-rich elements.

Fig.2 XRD pattern of CoCrNbNiW high-entropy alloy coating

Figure 3 shows the microstructure of CoCrNbNiW highentropy alloy coating.A large amount of irregular lamellar or block grey microstructureⅡ,black phaseⅢII and some un-melted white particlesⅠwith the average diameter of～75μm in coating are observed in Fig.3.Moreover,at the top and middle of coating,a lot of finer and denser grey phaseⅡ(～4.1μm) is formed in black phaseⅢII and around the white particlesⅠ(Fig.3a,b).What is more,Fig.3c shows that some grey phasesⅡpresent flocculent shape and interconnect with each other.On the contrary,a small amount of slender grey phaseⅡis produced with much more black phasesⅢat the bottom of coating(Fig.3d) and the average dendrite arm of these grey structures is～8.2μm.This shows that black phaseⅢis easy to be formed at the bottom of coating and grey phaseⅡtends to be produced at the top and middle of coating.In addition,it is note that un-melted white particles are diffusely distributed in coating.

Fig.3 SEM images of CoCrNbNiW high-entropy alloy coating:a,b grey phaseⅡin black phaseⅢand around white particlesⅠ;c flocculent-shaped grey phasesⅡ,d slender grey phaseⅡat the bottom of coating

Fig.4 a SEM image of CoCrNbNiW high-entropy alloy coating;elemental distributions of regions:b Co,c W,d Ni,e Nb,f Fe and g Cr

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Table 3 EDS analysis of CoCrNbNiW high-entropy alloy coatings (at%)

To further investigate the phase composition of coating except for white particlesⅠ,Fig.4a shows SEM image of the middle of coating and Fig.4b-g gives the EDS results of region in Fig.4a.Obviously,grey phaseⅡis rich in Nb and black phaseⅢis rich in Cr and Ni.All elements evenly distribute in phases,and there is no segregation of components [ 26] .Therefore,according to EDS analysis(Table 3),the results of XRD (Fig.2) and Fig.4,the grey phase is identified as fcc phase,the black phase is bcc phase and the white phase is in accordance with un-melted W particles.Meanwhile,as shown in Fig.2,some Fe is also found in coating by EDS.This is because high laser power is absorbed by substrate and some power is transformed into substrate from melting primary powders,leading to that the substrate is melted,and then,some Fe from substrate enters the molten pool.Thus,some elements,such as Cr and Ni,interact with Fe.

According to the theory of solidification [ 27, 28, 29] ,the temperature gradient (G) and solidifying forefront rate(R) decide the microstructure of materials.When laser beam interacts with primary powders and substrate,the molten pool is rapidly formed and some components of CoCrNbNiW high-entropy alloy interact with each other to form solid solution.At the bottom of the molten pool,the largest G and the smallest R are gained,resulting in the planar crystal of size～6μm,and the molten powder can transfer the partial heat to melt the surface of substrate,resulting in metallurgical bonding.With the increase in the distance from the interface of coating and substrate,the G is reduced and R is increased,leading to that the columnar dendrites are produced (Fig.3d).Therefore,the black bcc matrix presents coarse columnar shapes and the grey fcc phase presents long shapes.The finer and dense equiaxed crystals are formed due to the smallest G and largest R with the further increase in the distance from the interface (Fig.3a).Owing to the smaller enthalpy of mixing of Ni with Nb (～-30 kJ·mol^-1),Co with Nb(～-25 kJ·mol^-1) and Cr with Nb (～-7 kJ·mol^-1) [ 10, 30] than that of other solid solution,the grey fcc phase is preferred nucleation and growth.Subsequently,black bcc rich in Cr and Ni is formed.Additionally,the dislocation motion of grains decides the growth of grains.The higher content of W doping in CoCrNbNiW high-entropy alloy results in impeding the dislocation motion,further leading to that the growth of grains is impeded [ 24] .Furthermore,un-melted W particles inhibit grain growth and thus become finer microstructures around W precipitates.

3.2 Microhardness and wear resistance properties of high-entropy alloy coating

Figure 5 shows the microhardness of CoCrNbNiW highentropy alloy coating and substrate.With the increase in the distance from the surface of coating,the microhardness value of coating presents the evidently fluctuating variation.The average microhardness value and the maximum value of coating are～HV_0.2 515.4 and～HV_0.2 613(～200μm),respectively.However,the average microhardness value (～HV_0.2 185.6) of substrate is about 0.36times as high as that of coating.Moreover,the microhardness of CoCrNbNiW high-entropy alloy coating at the bottom is higher than that at the top.This is because the unmelted white W particles diffusely distributed in coating play a role of reinforced phase,and black bcc phase (rich in Cr) further increases the microhardness of coating.Meanwhile,the grey fcc phase rich in Nb is easier to be formed at the top and middle of coating,and black bcc phase rich in Cr tends to be produced at the bottom of coating.Therefore,the microhardness of CoCrNbNiW high-entropy alloy coating can obviously increase the microhardness of the substrate and prolong its service life.

Fig.5 Microhardness curves of CoCrNbNiW high-entropy alloy coating and 45 steel

Figure 6a gives the curves of frication coefficient of CoCrNbNiW high-entropy alloy coating and substrate with sliding cycles.All friction coefficients represent the unstable fluctuation that is accompanied with sharp decline in both the CoCrNbNiW high-entropy alloy coating and substrate at the earlier stage of the dry sliding wear.This is caused by the incomplete occlusal between sample and ring [ 31] .With the sliding cycles increasing,the friction coefficients of coating and substrate tend to stalely change.Although the average friction coefficient of coating(～0.306) is slightly higher than that of substrate(～0.291),Fig.6b shows that the wear loss (0.039 g) and wear rate (0.3) of the latter are 3.8 times and 4.3 times higher than those of the former.This indicates that the wear resistance of CoCrNbNiW high-entropy alloy is significantly higher than that of substrate.

Figure 7 shows the worn morphologies of CoCrNbNiW high-entropy alloy coating and substrate.Figure 7a,b(Fig.7b is the magnification of Fig.7a) shows that the apparently ploughed furrows with the maximum width of～18.9μm are produced with some pits of size～1.2μm and little debris is observed in substrate,indicating that the uniform abrasion and abrasive wear take place due to its homogeneous microstructure during the wear test.However,the worn surface of the CoCrNbNiW high-entropy alloy coating contains a lot of irregularly layered and powdered structure.More importantly,the depth of ploughed furrows is sharply reduced,and the extent of damage of partial grey phases is the largest compared with black and white phases in coating.Moreover,the white and black phases present the excellent wear resistance because there are no evident changes in their worn surface.Therefore,the whole wear resistance of the CoCrNbNiW high-entropy alloy coating is higher than that of the substrate.This coincides with the above-mentioned results in Fig.6.

At the initial stage of dry sliding wear,the block/ring counter surfaces exhibit asperity-to-asperity contact.A relatively sharp surface makes it move relatively violently [ 32] .The high-hardness ring (60 HRC) leads to high stress on the surface of the CoCrNbNiW high-entropy alloy coating,accompanied by microgrinding microploughing action on the surface of the CoCrNbNiW high-entropy alloy coating [ 33] .As a result,black phaseⅢin the lamellar form is removed from the surface of the coating due to adhesive wear.As the wear test progresses,the white W particles are protruded from the black matrix;then,the contact between the block/ring can be considered as the white W particles to the ring.These white W particles are worn preferentially to protect the black matrix from direct contact with the surface of the ring.This can play an important role in protecting the black matrix due to“particle enhancement effect”of white W particles in Fig.7c [ 34] ;therefore,it will have a protective effect on the surface of CoCrNbNiW high-entropy alloy coating.Moreover,the width of ploughs on the surface of CoCrNbNiW high-entropy alloy coating is noticeably less than that of the substrate,further confirming that CoCrNbNiW high-entropy alloy exhibits much higher wear resistance in comparison with substrate.

Fig.6 Relationships between a friction coefficient and distance and b wear loss and wear rate of CoCrNbNiW high-entropy alloy coating and 45steel

Fig.7 SEM images of a,b 45 steel and c,d CoCrNbNiW high-entropy alloy coating after wear measurement

By comparison,the texture of substrate is much softer than that of the high-hardness ring.Thus,the ring can grind and cut deeply into the surface of substrate,resulting in relative severe plastic deformation on the surface of substrate and in large amounts of debris (Fig.7d).As a result,the lamellar substrate debris is peeled continuously from the worn surface of substrate,whose wear mechanism is mainly characterized by adhesion wear [ 35] .

4 Conclusion

The CoCrNbNiW high-entropy alloy coating on 45 steel was prepared by laser melting deposition,and the wear resistance of coating was mainly studied.An amount of fee phase was easier to be formed at the top and middle of coating,while black bcc phase rich in Cr inclined to be produced at the bottom.Meanwhile,some un-melted W particles were diffusely distributed in coating.Therefore,the microhardness of CoCrNbNiW high-entropy alloy coating was higher than that of substrate (45 steel),and the microhardness of coating at the bottom was slightly higher than that at the top.The wear loss and wear rate of CoCrNbNiW high-entropy alloy coating were 0.26 and0.23 times as high as those of substrate,respectively,leading to that the wear resistance of CoCrNbNiW highentropy alloy coating was significantly higher than that of substrate.