简介概要

Microstructure and dry sliding wear behavior of laser clad AlCrNiSiTi multi-principal element alloy coatings

来源期刊：Rare Metals2017年第7期

论文作者：Can Huang Yi-Zhou Tang Yong-Zhong Zhang An-Ping Dong Jian Tu Lin-Jiang Chai Zhi-Ming Zhou

文章页码：562 - 568

摘要：The approximately equimolar ratio AlCrNiSiTi multi-principal element alloy（MPEA） coatings were fabricated by laser cladding on Ti-6A1-4V（Ti64） alloy.Scanning electron microscopy（SEM）,equipped with an energy-dispersive spectroscopy（EDS）,and X-ray diffraction（XRD） were used to characterize the microstructure and composition.Investigations show that the coatings consist of（Ti,Cr）₅Si₃ and NiAl phases,formed by in situ reaction.The phase composition is initially explicated according to obtainable binary and ternary phase diagrams,and the formation Gibbs energy of Ti₅Si₃,V₅Si₃ and Cr₅Si₃.Dry sliding reciprocating friction and wear tests of the AlCrNiSiTi coating and Ti64 alloy substrate without coating were evaluated.A surface mapping profiler was used to evaluate the wear volume.The worn surface was characterized by SEM-EDS.The hardness and wear resistance of the AlCrNiSiTi coating are well compared with that of the basal material（Ti64）.The main wear mechanism of the AlCrNiSiTi coating is slightly adhesive transfer from GCr15 counterpart,and a mixed layer composed of transferred materials and oxide is formed.

稀有金属(英文版) 2017,36(07),562-568

Microstructure and dry sliding wear behavior of laser clad AlCrNiSiTi multi-principal element alloy coatings

Can Huang Yi-Zhou Tang Yong-Zhong Zhang An-Ping Dong Jian Tu Lin-Jiang Chai Zhi-Ming Zhou

School of Materials Science and Engineering,Chongqing University of Technology,Chongqing 400054,China

Chongqing Municipal Key Laboratory of Institutions of Higher Education for Mould Technology

Center for Composites,General Research Institute for Nonferrous Metals

Shanghai Key Laboratory of Advanced High-temperature Materials and Precision Forming,Shanghai Jiao Tong University

收稿日期：26 November 2016

基金：supported by the Chongqing Research Program of Basic Research and Frontier Technology(No.CSTC2013jcyjA50016);the National Natural Science Foundation of China(Nos.51401039,51571037 and 51204110);the Scientific and Technological Research Program of Chongqing Municipal Education Commission(No.KJ1709204).;

Microstructure and dry sliding wear behavior of laser clad AlCrNiSiTi multi-principal element alloy coatings

Can Huang Yi-Zhou Tang Yong-Zhong Zhang An-Ping Dong Jian Tu Lin-Jiang Chai Zhi-Ming Zhou

School of Materials Science and Engineering,Chongqing University of Technology,Chongqing 400054,China

Chongqing Municipal Key Laboratory of Institutions of Higher Education for Mould Technology

Center for Composites,General Research Institute for Nonferrous Metals

Shanghai Key Laboratory of Advanced High-temperature Materials and Precision Forming,Shanghai Jiao Tong University

Abstract：

The approximately equimolar ratio AlCrNiSiTi multi-principal element alloy(MPEA) coatings were fabricated by laser cladding on Ti-6A1-4V(Ti64) alloy.Scanning electron microscopy(SEM),equipped with an energy-dispersive spectroscopy(EDS),and X-ray diffraction(XRD) were used to characterize the microstructure and composition.Investigations show that the coatings consist of(Ti,Cr)₅Si₃ and NiAl phases,formed by in situ reaction.The phase composition is initially explicated according to obtainable binary and ternary phase diagrams,and the formation Gibbs energy of Ti₅Si₃,V₅Si₃ and Cr₅Si₃.Dry sliding reciprocating friction and wear tests of the AlCrNiSiTi coating and Ti64 alloy substrate without coating were evaluated.A surface mapping profiler was used to evaluate the wear volume.The worn surface was characterized by SEM-EDS.The hardness and wear resistance of the AlCrNiSiTi coating are well compared with that of the basal material(Ti64).The main wear mechanism of the AlCrNiSiTi coating is slightly adhesive transfer from GCr15 counterpart,and a mixed layer composed of transferred materials and oxide is formed.

Keyword：

Laser cladding; Multi-principal element alloy; Microstructure; Wear behavior;

Author： Can Huang e-mail:cquthc@cqut.edu.com;

Received： 26 November 2016

1 Introduction

Owing to the advantages of low density,high specific strength,excellent corrosion resistance and biocompatibility,titanium alloys are widely used in many fields such as aviation,navigation,biomedicine.However,because of the weak oxidation resistance at temperature higher than600℃and the unsatisfactory anti-wear ability,the broader use of titanium alloys is limited.Among different attempts to solve the problem,laser surface treatment is an effective tool for protecting materials against wear and high-temperature oxidation [ 1, 2] .It is of great significance to find new cladding materials that can simultaneously enhance the resistance of both wear and high-temperature oxidation.

Multi-principal element alloys (MPEAs),containing more than 5 elements in equiatomic or near-equiatomic,accelerate the development of new materials to break through the conventional approach that alloys have one or two base elements as major component [ 3, 4, 5] .In the initial developed stage of MPEAs,high configurational entropy was emphasized,and the formation of a random solid solution attracted great attention [ 6, 7] .Admittedly,high configurational entropy weighs heavily on the phase formation in MPEAs.However,the recent researches suggest that high configurational entropy is not a sufficient condition for the formation of random solid solution [ 8] .Most studies focus on those alloy systems whose major phases are solid solutions rather than intermetallic compounds.If the major phases formed in an alloy are intermetallics,the room temperature brittleness of the alloy will prohibit the widespread application,especially for bulk materials.Nevertheless,many intermetallics or intermetallic-based alloys with tolerable toughness are ideal coating materials,because of their high hardness,good wear resistance and good high-temperature oxidation resistance [ 9, 10] .Some researchers report that decreasing the degree of ordering is beneficial to toughen the intermetallics [ 11, 12, 13] .

Based on the above analysis,it may be interesting if the alloying design strategy of multi-principal elements can be used in the development of intermetallic-based alloys.Firstly,the degree of ordering of the intermetallics can be reduced because other elements can dissolve in the intermetallic phase.Secondly,the cocktail effect offers the possibilities to enhance different properties simultaneously [ 14] .In order to enhance the wear property and hightemperature oxidation resistance of titanium components,the authors want to develop MPEA coating,forming intermetallic in situ composite prepared by laser deposition.In previous papers,the alloy system is chosen as AlCr-Si-Ti-V,considering the cocktail effect of the constitute elements and preliminary thermodynamic calculation.The AlCrSiTiV MPEA coating fabricated on Ti-6Al-4V(Ti64) substrate by laser surface engineering had better wear property and superior high-temperature oxidation resistance than the substrate [ 15, 16] .It is our aim to further improve the wear property and high-temperature oxidation resistance.The studies demonstrate that the formation of V₂O₅ is harmful to the high-temperature oxidation resistance [ 16] .Researches show that some alloys of the Ni-CrTi,Ni-Cr-Al and Ni-Cr-Al-Ti-Si alloy system have good high-temperature oxidation resistance [ 17, 18, 19] .Therefore,Ni was used to replace V in AlCrSiTiV MPEA coating in the upcoming research.However,the effect of Ni replacement on wear resistance is still in doubt.In this paper,a new AlCrNiSiTi MPEA coating was prepared on the surface of Ti64 alloy by laser cladding,and its microstructure and dry sliding wear behavior were investigated.

2 Experimental

The experimental starting material was commercially available Al,Cr,Ni,Si and Ti elemental powders with particle size in the range of 45-100μm.The purity of each powder was above 99.5%.The powders were milled by ball milling.Owing to the evaporation of some element,the dissimilar catchment efficiency of different element powders by the molten pool and the diffusion of the atoms from the basal material (Ti64) melted by laser,there is a gap between the composition of the mixed powders and the coating.Based on some previous experiments,the loss rate of each element was calculated.The composition of the powder mixture was selected in the weight ratio of Al/Cr/Ni/Si/Ti=11:11.5:13:8:6,so as to obtain a near equimolar ratio coating.Ti64 titanium plates with thickness of 6 mm were used as substrates and initially sandblasted to increase the absorption of laser energy.Laser multilayer cladding on Ti64 titanium alloy surface was performed with a 5 kW CO₂ laser system.The laser cladding parameters were as follows:output power of 2 kW,scanning speed of 3.0 mm·s^-1,powder feeding rate of2 g-min^-1,laser beam diameter of 2.5 mm and overlapping ratio of 50%.During the cladding process,argon gas was employed to carry the mixed powder and to prevent oxidation.

Samples for microstructural characterization were ground,polished and etched in an HNO₃ and HF water solution.Scanning electron microscope (SEM,S-4800,equipped with an energy-dispersive spectroscopy (EDS,Emax-350) and X-ray diffractometer (XRD,D/max-2200PC) were used to characterize the microstructure and composition.HX-1 hardness meter was used to measure the microhardness,with 15 s load holding time and under load of 1.96 N.To obtain the reliable hardness data,the values at ten various locations of the same depth were measured and then averaged.

Dry sliding reciprocating friction and wear tests of the AlCrNiSiTi MPEA coating and Ti64 alloy substrate without coating were evaluated on a UMT-2 tribometer.The counterbodies were GCr₁₅ steel balls (about HV_0.27700),4 mm in diameter.The specimens for the wear test were cut into dimension of about 15 mm×15 mm×6 mm,and their surfaces were polished.The tests were conducted using an applied load of 9.8 N,a testing time of 60 min for each specimen,amplitude of 1 mm and frequencies of 5,10 and 15 Hz,respectively.The friction coefficients were continually obtained by the tester system.After the test,a surface mapping profiler was used to evaluate the wear volume.The worn surface was characterized by SEM equipped with EDS.More information about the experimental procedure can be found in our previous papers [ 15, 16] .

3 Results and discussions

During the fabrication of AlCrNiSiTi MPEA coatings,the laser cladding processing parameters have great influence on the sensitivity of crack formation.With the aforementioned optimized parameters,AlCrNiSiTi MPEA coatings with no macrocracks could be achieved on Ti64 alloy,as is shown in Fig.1.

Fig.1 Surface macro-morphology of laser clad AlCrNiSiTi coating

Fig.2 XRD patterns of laser clad AlCrNiSiTi alloy coating

XRD pattern of AlCrNiSiTi coating is shown in Fig.2.The analyzed plane is approximately 0.7 mm away from the boundary between Ti64 substrate and AlCrNiSiTi MPEA coating.It indicates that the AlCrNiSiTi coating is composed of (Ti,Cr)5Si₃ (PDF No.09-0242 for Cr₅Si₃,PDF No.78-1429 for Ti₅Si₃) and NiAl (PDF No.20-0019).Figure 3a shows representative microstructure of AlCrNiSiTi MPEA coating.It can be seen that there are mainly two phases formed in the coating.Table 1 displays EDS results of different phases and the average constituent of the sample.Four different spots were measured for both phases,and the average values are shown.The phase labeled as A is rich in Si,Ti and Cr,and the phase labeled as B is rich in Al and Ni.There is some difference between the constituent of the mixing powder and the average constituent of the laser clad coating,because during the preparation of the coating,some elements such as Al are easily volatilized,the powder catchment efficiency of different elements by the molten poor is not the same and some atoms are diffused from the titanium substrate.By optimizing the proportion of different elements in the precursor materials,a near-equiatomic coating is obtained.Figure 3b shows pure metallurgical bonding between AlCrNiSiTi MPEA coating and Ti64 substrate.

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Table 1 EDS analysis results of AlCrNiSiTi coating shown in Fig.3a (at%)

Phase diagrams are very useful tools in analyzing micros true tural evolution of materials,but there are little experimental phase diagrams containing more than three elements.Nevertheless,the phase composition of laser clad AlCrNiSiTi alloy can be initially explicated according to the obtainable binary and ternary phase diagrams.B ased on the liquidus surface of Cr-Si-Ti system,when the proportion of Cr/Si/Ti is closed to 1:1:1 in a Cr-Si-Ti ternary system,the first precipitation during cooling is (Ti,Cr)5Si₃phase [ 20] .Although there are two other elements in AlCrNiSiTi alloy coating,there is no other phase whose melting point is higher or the free energy of formation is more negative than that of (Ti,Cr)5Si₃.During the preparation process of AlCrNiSiTi coating,a molten pool containing Ni,Cr,Ti,Al and Si forms resulted by laser irradiation.With the moving of the motion platform,the molten pool is away from the laser spot,and is cooled by the titanium alloy substrate and air convection.During cooling,the (Ti,Cr)5Si₃ silicide solidifies first because of its high melting point.When the silicide precipitates over,the remaining elements of the molten pool are mainly Al and Ni,and the atomic ratio of Al and Ni is close to 1:1.According to the Ni-Al binary phase diagram [ 21] ,it can be inferred that after solidification of the silicides during the cooling of the AlCrNiSiTi alloy,the left elements tend to form NiAl phase with further cooling.Thus,the intermetallic composites composed of (Ti,Cr)5Si₃ and NiAl are formed by in situ reaction.

Fig.3 SEM images of laser clad AlCrNiSiTi coating:a typical microstructure of coating and b interface

The crystal structure of Cr₅Si₃ at high temperature is D8₈,the same with that of Ti₅Si_3.Therefore,Cr₅Si₃ and Ti₅Si₃ often form (Ti,Cr)5Si₃ by mutual dissolution.But there is almost no Cr existing in M₅Si₃-type phase in the AlCrSiTiV coating in previous work [ 15] ,which can be preliminarily explained by the formation Gibbs energy of Ti₅Si₃,V₅Si₃ and Cr₅Si₃.Figure 4 shows the variation of Gibbs energy of formation versus temperature for the three M₅Si₃-type phases [ 22] .At each temperature,Gibbs energy of formation of Ti₅Si₃ is most negative,and it is less negative for Cr₅Si₃ than for V₅Si₃.According to the thermodynamic theory,V has priority than Cr to dissolve in Ti₅Si₃ and form (Ti,V)5Si₃.

NiAl is one of the most common B2 compounds,an ordered structure of bcc structure.The B2 compound is formed in a lot of MPEAs [ 23, 24, 25] .It can be found that almost all the B2 compounds are resulted from the interaction of A1 with some other 3d transition elements [ 26] .Nevertheless,the order degree of B2 phase in MPEAs is decreased,since other elements dissolve into B2 structure.Therefore,it is hard to distinguish the ordered B2 structure from the disordered A2 structure in some research.The B2phase was declared as bcc solid solution in some reports [ 27] .

Over the past ten years,research in the field of MPEAs has demonstrated that high configurational entropy is not a sufficient condition for the formation of a random solid solution.Nevertheless,high configurational entropy has a great influence on the phase evolution in MPEAs,and the number of phase formed in MPEAs is much less than the maximum number of phase predicted by Gibbs phase rule.

Fig.4 Variation of Gibbs energy of formation versus temperature for Ti₅Si₃,Cr₅Si₃ and V₅Si₃

Figure 5 presents Vickers hardness change along depth of AlCrNiSiTi MPEA coating.AlCrNiSiTi coating is much harder than basal material (Ti64) and is also harder than AlCrSiTiV coating prepared in previous work [ 15] ,which is due to the formation of intermetallic-based phase constitute.In most cases,materials with higher hardness have superior abrasive and adhesive wear resistance.Formation of AlCrNiSiTi MPEA coatings on Ti64 substrate increases the hardness,and therefore,the wear performance is expected to be elevated.The wear volume loss (WVL) of AlCrNiSiTi MPEA coating and basal material at different frequencies are exhibited in Fig.6.As the frequency grows,the WVL of every sample goes up.At every frequency adopt in the present study,the WVL of Ti64 basal material is greater than that of AlCrNiSiTi MPEA coating and the WVL of basal material increases more quickly than that of AlCrNiSiTi coating.The specific wear rate (SWR)can be reckoned with the data derived from Fig.6.At various frequencies,the SWR of AlCrNiSiTi coating and basal material are demonstrated in Fig.7.It can be found that the SWR of AlCrNiSiTi coating decreases while that of Ti64 basal material increases with the increase of frequency.At each frequency adopt,the SWR of AlCrNiSiTi coating is less than that of basal material.

The typical worn surfaces of the uncoated Ti64 and AlCrNiSiTi MPEA coating are displayed in Fig.8.Chemical compositions of the worn surface of uncoated Ti64 and AlCrNiSiTi MPEA coating after wear test using frequency of 15 Hz were measured by EDS,and the data are exhibited in Table 2.With the increase of the alternating movement frequency,there is no obvious change in the morphologies for the uncoated Ti64 and AlCrNiSiTi MPEA coating,respectively,indicating little difference in wear mechanism at perse frequencies.The main wear mechanism of the uncoated Ti64 is severe abrasive wear combined with adhesive wear and oxidation,which was discussed in detail in the previous paper [ 15] .

Fig.5 Microhardness profile along cross section of laser clad AlCrNiSiTi coating on Ti-6Al-4V substrate

Fig.6 Wear loss of Ti-6A1-4V substrate and laser clad AlCrNiSiTi coating after dry sliding wear test under different frequencies

Fig.7 Specific wear rate of Ti-6Al-4V substrate and laser clad AlCrNiSiTi coating after dry sliding wear test under different frequencies

Table 2 EDS results of worn surface of Ti-6Al-4V and laser clad AlCrNiSiTi coating after dry sliding wear test under frequency of15 Hz (at%)

Plowing grooves and plastic deformation are hardly seen for the worn surface of the laser clad AlCrNiSiTi MPEA coating,indicating good resistance to abrasive wear.The worn surface morphologies of the laser clad coating show that slight adhesive transfer is the primary wear mechanism.According to the adhesive theory,the real contact area is only a small proportion of the apparent area.Therefore,the actual force is very high,and cold weld junctions form during sliding.When the junction breaks at one side of the asperities with the relative motion,materials are transferred from one surface to the other.EDS results of the worn surface can be used to identify the materials transfer direction.As is shown in Table 2,Fe can be detected on the surface of the worn surface of AlCrNiSiTi MPEA coating,proving that the material transfer is mainly from laser clad coating to GCr15 counterpart (in contrast,EDS proves the absence of Fe on the worn surface of Ti64basal material,demonstrating the inverse material transfer direction).Moreover,large oxygen amount is detected on the worn surfaces of the coating,demonstrating obvious oxidation during sliding.In the wear process,the starting transferred materials are repeatedly rolled and deformed.

Fig.8 Typical worn surface morphology of Ti-6Al-4V alloy substrate under frequencies of a 5 Hz,b 10 Hz and c 15 Hz and of laser clad AlCrNiSiTi coating under frequencies of d 5 Hz,e 10 Hz and f 15 Hz

Meanwhile,oxidation occurs due to local friction heat.The combined effects result in the formation of a mixed layer consisting of transferred fragments and oxides.The mixed layer prevents the direct contact between the surface of the couterbody and AlCrNiSiTi MPEA.With the repeated deformation,work-hardened effect of the mixed layer results in local cracking.Partial cracking contributes to a little fragmentation and spalling of the mixed layer.But severe spalling and delamination do not occur throughout the wear process.These features imply the relatively slight wear morphology characteristics.

Compared with the Ti64 basal material,the improvement of the wear behavior is primarily due to the high hardness of AlCrNiSiTi MPEA coating.The (Ti,V)₅Si₃/NiAl intermetallic-based phase constitutes result in the high hardness and therefore good adhesive and abrasive resistance of the coating.Besides,the similarity of the friction pair materials and the surface energy also greatly affect the adhesion [ 28] .The degree of dissimilarity between (Ti,V)₅Si₃/NiAl intermetallic-based phase and GCr15 metallic counterpart is very high.Intermetallic phases generally have lower surface energies,and the formation of oxide also reduces surface energy.Therefore,the dissimilar pair materials and low surface energy contribute to low adhesion.Moreover,the mixed layer prevents direct contact between the surface of the coating and the counterpart,thereby minimizing wear.In addition,the composite toughening effect by in situ formation of (Ti,V)₅Si₃/NiAl intermetallic composite [ 29, 30] ,and the decrease of ordering degree by dissolving large amount of different elements [ 11, 12, 13] increase the toughness to some extent,preventing the coating materials from serious brittle fracture.

Figure 9 shows the friction coefficient of the laser clad AlCrNiSiTi coating.With the increase of frequency,the friction coefficient of the coating decreases.It may be resulted from the formation of oxide.The frictional heat increases with the increase of dry sliding frequency,and speeds up the formation of oxide,which tends to reduce the strength of the bonds at junctions.

Fig.9 Variation of friction coefficients versus test time for laser clad AlCrNiSiTi coating under different frequencies during dry sliding wear test

4 Conclusion

Approximately equimolar ratio AlCrNiSiTi MPEA coatings were prepared on Ti64 alloy by laser cladding.The microstructure of the coating consists of (Ti,Cr)5Si₃ and NiAl phases.The phase composition can be initially explicated according to obtainable binary and ternary phase diagrams,and the formation Gibbs energy of Ti₅Si₃,V₅Si₃and Cr₅Si₃.The hardness and wear resistance of the AlCrNiSiTi coating are well compared with those of Ti64titanium alloy and also higher than those of AlCrSiTiV coating prepared in our previous work.The main wear mechanism of AlCrNiSiTi coating is slightly adhesive transfer from GCr 15 counterpart,and a mixed layer composed of transferred materials and oxide is formed.Partial cracking contributes to a little fragmentation and spalling of the mixed layer.But severe spalling and delamination do not occur throughout the wear process.

Acknowledgements This study was financially supported by the Chongqing Research Program of Basic Research and Frontier Technology (No.CSTC2013jcyjA50016),the National Natural Science Foundation of China (Nos.51401039,51571037 and 51204110) and the Scientific and Technological Research Program of Chongqing Municipal Education Commission (No.KJ1709204).