简介概要

In situ TiN-reinforced CoCr₂FeNiTi_0.5 high-entropy alloy composite coating fabricated by laser cladding

来源期刊：Rare Metals2020年第10期

论文作者：Ya-Xiong Guo Qi-Bin Liu Xiao-Juan Shang

文章页码：1190 - 1195

摘要：The fcc structural CoCr₂ FeNiTi_0.5 high-entropy alloy（HEA） composite coating with TiN particles reinforced was acquired by laser cladding on the commercial904 L stainless steels.The results show that phase structure is mainly composed of fcc solid solution and TiN phases.The coating exhibits excellent structural stability below850℃.The microstructure consists of irregular dendrite and TiN particles.Transmission electron microscopy（TEM） results reveal that the close-packed plane of fcc phase is（111） with interplanar spacing of ～ 0.208 nm.The interface between TiN and fcc matrix is semi-coherent.And the angle of boundary between dendrite and matrix is ～ 65°.The hardness and corrosion resistance of coating have much improvement compared with those of substrate.

详情信息展示

稀有金属(英文版) 2020,39(10),1190-1195

In situ TiN-reinforced CoCr₂FeNiTi_0.5 high-entropy alloy composite coating fabricated by laser cladding

Ya-Xiong Guo Qi-Bin Liu Xiao-Juan Shang

College of Materials and Metallurgy,Guizhou University

Guizhou Province Key Laboratory of Materials Structure and Strength

作者简介：*Qi-Bin Liu,e-mail:qbliugzu@163.com;

收稿日期：6 July 2017

基金：financially supported by the National Natural Science Foundation of China (No.51671061);the High-Level Innovative Talents Plan of Guizhou Province(No.(2015)4009);the Industrial Research Project of Guizhou Provincial Science and Technology Department (No.(2015)3022);

In situ TiN-reinforced CoCr₂FeNiTi_0.5 high-entropy alloy composite coating fabricated by laser cladding

Ya-Xiong Guo Qi-Bin Liu Xiao-Juan Shang

College of Materials and Metallurgy,Guizhou University

Guizhou Province Key Laboratory of Materials Structure and Strength

Abstract：

The fcc structural CoCr₂ FeNiTi_0.5 high-entropy alloy(HEA) composite coating with TiN particles reinforced was acquired by laser cladding on the commercial904 L stainless steels.The results show that phase structure is mainly composed of fcc solid solution and TiN phases.The coating exhibits excellent structural stability below850℃.The microstructure consists of irregular dendrite and TiN particles.Transmission electron microscopy(TEM) results reveal that the close-packed plane of fcc phase is(111) with interplanar spacing of ～ 0.208 nm.The interface between TiN and fcc matrix is semi-coherent.And the angle of boundary between dendrite and matrix is ～ 65°.The hardness and corrosion resistance of coating have much improvement compared with those of substrate.

Keyword：

TiN particle reinforced; High-entropy alloy; Semi-coherent interface; Laser cladding;

Received： 6 July 2017

1 Introduction

High-entropy alloys (HEAs) with excellent mechanical properties have received great research interest recently in the field of materials community [ 4] .Theoretically,the HE A systems are in favor of forming single fcc [ 5] ,bcc [ 6, 7] and hcp [ 8, 9] solid-solution phases in virtue of highentropy effect.Evidently,the bcc structural HEAs usually exhibit extremely high hardness and brittleness;however,fcc structural HEA s possess opposite characteristics,both of them cannot be available for engineering application [ 10, 11, 12, 13] .Normally,CoCrFeNi-M (where M could be Al,Ti and other transition metal elements) alloys have been extensively researched by scholars in domestic and abroad [ 5, 14, 15, 16, 17, 18] .For instance,fcc structural HEAs exhibit pronounced tensile properties and corrosion resistance.However,HEAs with bcc structure exhibit outstanding compressive strength [ 10, 12] .HEAs possess many properties with obvious advantages over tradition alloys [ 19, 20, 21] .Among these HEAs,the four elements Co,Cr,Fe and Ni with similar crystal structures,atomic radius and enthalpies of mixing (ΔH_mix) close to 0 kJ·mo^-1 could be prior to form infinitely substitutional solid solutions.Segregation of Ti,Cu and other transition metals could be appeared in varying degrees.

The AISI 904L stainless steels with prominent corrosion resistance to strong acids have been widely applied in agitator blades of phosphoric acid reactors in chemical enterprises.Here phosphoric acid is fabricated by the reaction of sulphuric acid with phosphate ore.However,in the process of mechanical agitation,the 904L stainlesssteel blades with poor hardness and wear resistance collide violently with phosphate ores,causing severe corrosion wear.Therefore,coating technology on agitator blades to prolong the service life could be necessary [ 22, 23] .Presently,detailed observations on the microstructure and mechanical properties of TiZrNbWMo [ 24] and 6FeNiCoSiCrAlTi [ 25] coating by laser cladding indicate that the coating is composed of bcc and fcc phases with high hardness and thermal stability.In addition,TiCrAlSiV [ 26] with promising performances was deposited by laser alloying on Ti alloys.Consequently,Laser cladding with its rapid heating and solidification could fabricate millimeter-level thickness and uniform coating with high corrosive wear resistance,which can meet our requirement [ 27, 28, 29] .

The fcc crystals have low package energy,exhibiting stable structure without decomposition at low temperature [ 30] .Moreover,the self-corrosion potential of fcc structures could be more positive than that of bcc structure.However,single fcc solid solution without any strengthening approaches could not improve wear resistance.Therefore,hard ceramic particles enhanced fcc matrix high-entropy composite alloys may be an adjustable approach to obtain fine coating materials with outstanding performance.Generally,Cr is the most effective element to improve the corrosion resistance.Addition of higher content of Cr is expected to improve the corrosion resistance of the coating.Simultaneously,Ti atoms with larger atomic size and negative enthalpy of mixing can be effectively dissolved into fcc matrix to enhance the effect of solidsolution strengthening.Moreover,in the process of laser cladding,Ti atom has strong affinity with N atoms in the atmosphere,and in situ TiN particles could be synthesized in the coating.Therefore,in this paper,CoCr₂FeNiTi_0.5HEA was designed and anticipated to achieve novel coating with fine microstructure and properties.The microstructure,phase interfaces and properties were carefully investigated.

2 Experimental

The 904L stainless steels with size of 50 mm×30mm×10 mm were used as substrate.The surfaces of steel plates were polished and cleaned by alcohol.The raw materials of coating were composed of pure metal powders of 18.2 at%Fe,36.3 at%Cr,18.2 at%Co,18.2 at%Ni and 9.1 at%Ti with particle sizes of～74μm Subsequently,the weighed powders were fully blended in the stainless-steel ball milling pots for 8 h.The YLS-6000fiber laser system produced by IPG Corporation was used for laser cladding.Before experiment,the alloy powders with thickness of 1.5 mm were pre-placed on the substrate.The specific processing parameters were output power of1.8 kW,spot diameter of 6 mm,scanning rate of 7 mm·s^-1and overlap ratio of 40%.

Subsequently,the prepared coating samplers were linecut into the size of 10 mm×5 mm×5 mm,then polished and etched by aqua regia for microstructural observation using Spura-40 scanning electron microscope(SEM).The phase structures and interfaces were measured by FEI Tecnai G2 F20 S-TWIN transmission electron microscope (TEM).The phase structure of coating was characterized by X'pert Powder X-ray diffractometer(XRD).The structure stability and phase transition of HEA composite coating were investigated by differential thermal analysis (DTA) and thermogravimetry (TG) method using TG/DTA7300 instrument.

The microhardness of HEA composite coating was measured by JMHV-1000AT hardness test with the load of1.98 N for 10 s.Three coating samples were used to characterize and the average values were used.The coating soaked into the 35 wt%H₃PO₄ and 40 wt%H₂SO₄ simulated phosphoric acid reactor solution was measured by VSP-300 electrochemical workstation with a scanning rate of 1 mV·s^-1,to evaluate corrosion resistance.

3 Results and discussion

3.1 Phase structure and thermal stability

XRD and DTA analyses were used to verify the in situ reaction of TiN particles,single solid solution matrix and thermal structure stability in CoCr₂FeNiTi_0.5 HEA composite coating.Figure la shows XRD pattern of as-cladding HEA composite coating.The phase structure is composed of fcc structural phase plus few TiN phases,which have achieved the anticipated results.Figure lb shows DTA and TG curve of the coating.Only two endothermic peaks appear near 450 and 875℃.In consideration of XRD results,the weak endothermic peak could be regarded as decomposition of fcc phase at the temperature of 450℃.Meanwhile,when the experimental temperature reaches 875℃,an intensive endothermic peak could be related to the decomposition of TiN phases,which could be indeed consistent with Arslan and Efeoglu's experimental results [ 31] .

3.2 Microstructure and phases interface

Figure 2 shows the microstructure of HEA composite coating.Figure 2a shows the whole cross-sectional morphology.The HEA composite coating is of excellent castability without any other defects,greatly more predominant than that of as-cast bulk HE As.The microstructure of 904L substrate consists of equiaxed grain,and the angles of grain boundaries are～120°.The microstructure of coating is composed of fine and irregular dendrite (marked as DR in Fig.2b) plus dispersed particle phases.To confirm the structures of particle phases,selected area electron diffraction (SAED) was chosen,as shown in Fig.2b.The particles are further identified as TiN phase with polycrystalline.Figure 2c shows TEM image of the microstructure of fcc matrix with numerous microdefects.To further explain the structure of HEA matrix,highresolution transmission electron microscopy (HRTEM)image is shown in Fig.2d.Lattice distortion appears,which could be caused by Ti atom dissolved into fcc phase.Further observation of atomic arrangement could indicate that crystal plane spaces of (111) and (200) are 0.208 and0.180 nm,respectively,much corresponding to fcc struc-tural y-Fe (01-089-4185).

Fig.1 a XRD pattern and b DTA curves of HEA composite coating

Fig.2 Micro structure of HE A composite coating:a SEM image of whole morphology,b SEM image and S AED pattern of TiN particles,c TEM image of fee matrix,and d HRTEM image of fee matrix

Figure 3 shows phase interfaces of HEA composite coating.Figure 3a shows the interface of TiN particle and fcc matrix.Here excellent metallurgical bonding between TiN phase and fce matrix is achieved.There exists an elastic strain zone in the matrix around the TiN particle.Furthermore,the interplanar spacing of (220) is 0.157 nm,the angle between interface and (220) plane is about 41.5°,and the projected length of (220) plane on the interface is calculated to be 0.253 nm.Similarly,in fce phase,the interplanar spacing of (111) is 0.208 nm,the angle between interface and (111) plane is about 55.0°,and the projected length of (220) plane on the interface is also calculated to be 0.267 nm,following the formula of mismatch,

Fig.3 TEM images of interface of HE A coating:a,b interface between TiN and FCC matrix and c,d grain boundary of matrix and dendrite

whereδis the mismatch,andα_βandα_αare the atomic spacing on the interface of TiN and fcc phases.Therefore,the value of mismatch can be easily calculated to be 0.078.Generally,the interface transforms from coherent one(whileδlower than 0.05) to semi-coherent interface (whileδfrom 0.05 to 0.25),and then to be incoherent,whileδmore than 0.25.As a result,the interface of TiN and fcc phase belongs to semi-coherent one.The interface between dendrite and matrix is shown in Fig.3d,and interplanar spacing of (111) in the dendrite is about 0.208 nm.The intersection angle of (111) planes between matrix and dendrite is 65°,indicating that the grain boundary is of high angle.

3.3 Microhardness

Figure 4 shows the microhardness of CoCr₂FeNiTi_0.5 highentropy alloy coating.The thickness of HEA coating is～1.6 mm,the hardness distribution is uniform,and the average hardness of coating is about HV_0.2 403 which could be over 2 times that of 904L steel substrate.Two reasons could contribute to the hardness increasing.One is solution strengthening of Ti atomics dissolving into fcc lattice.Another is the combination of dispersion strengthening that TiN particles well form and semi-coherence interface between TiN and matrix.

Fig.4 Microhardness of HEA composite coating

3.4 Corrosion resistance

To characterize the corrosion resistance of HEA composite coating,the electrochemical polarization curves in simulated phosphoric acid reactor solution were measured,and the results are shown in Fig.5.Compared with that of904L stainless steel,the self-corrosion potential of HEA composite coating is little negative,which could be caused by high alloying and second phases of TiN producing.However,the self-corrosion current density of HEA composite coating is lower by about two orders of magnitude than that of substrate.Homogeneous microstructure and high contents of Cr,Ti dissolving into fec lattice could be conducive to the formation of dense oxide film,hindering the corrosion rate.

Fig.5 Electrochemical curve of HEA composite coating and 904L steel substrate

4 Conclusion

The structure of a CoCr₂FeNiTi_0.5 HEA composite coating successfully fabricated by laser cladding is composed of fcc solid solution and TiN phase.The HEA composite coating exhibits excellent thermal stability until the decomposition of TiN above 850℃.The coating and substrate are of well metallurgical bonding.The TiN particles are uniformly distributed and dispersed in fcc matrix.The lattice constant of fcc matrix is about 0.392 nm.Moreover,the interface between TiN and fcc matrix belongs to semi-coherent one and the value of mismatch is about 0.078.The hardness and corrosion resistance of coating have much improvement compared with those of904L stainless-steel substrate.