Microstructure and Property of Al?FeCoNiCrAl High Entropy Alloy Composite Coating on Ti?6Al?4V During Annealing Using MA Method

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College of Materials Science and Technology，Nanjing University of Aeronautics and Astronautics，Nanjing 211106，P.R.China

（Received 9 March 2020；revised 26 April 2020；accepted 18 May 2020）

Abstract: Al-FeCoNiCrAl high entropy alloy（HEA）composite coatings were prepared on Ti-6Al-4V via highenergy mechanical alloying（MA）. The microstructures and phase composition of the coatings were studied. A continuous and dense coating could be fabricated at a ratio of 35%（weight fraction）Al-FeCoNiCrAl after 4 h milling.The results showed that the thickness of the composite coatings increased first and then decreased with the increase of milling time. And the hardness of coating increased with the increase of milling time. The phase changed during the annealing process. Part of the initial body-centered cubic（BCC）phase of the composite coatings changed into the L12 phase，（Ni，Co）3Al4 and σ phase after annealing above 550 ℃. Ordered BCC was found in the coatings after annealing above 750 ℃. Only BCC and ordered BCC appeared in coatings after annealing above 1 050 ℃. The hardness of the coatings after annealing at 550 ℃and 750 ℃was higher than before because of spinodal decomposition and high hardness σ phase.The hardness of the coatings after annealing at 1 050 ℃decreased because residual stress released.

Key words：high entropy alloys；Ti-6Al-4V alloy；mechanical alloying；composite coating；annealing process；

0 Introduction

High entropy alloys（HEAs）are preferentially defined as alloys that composed of more than five principal elements，each with an atomic percentage between 5% and 35%［1］. Several important core ef?fects，such as high entropy，severe lattice distor?tion，sluggish diffusion and cocktail effect，were ex?hibited in HEAs［2］. With compositional complexity and rich microstructural variation，HEAs exhibit high hardness and other properties［3-5］.

Titanium and its alloys have been extensively applied in aeronautical manufacture，automotive and other fields due to their high specific strength and ex?cellent corrosion resistance［6-7］. Various surface mod?ification methods，such as the magnetron sputter?ing，laser cladding，plasma spraying，etc，have been widely applied to Ti-6Al-4V for improving their performance［8-12］，However，these methods have disadvantages，such as high cost，complex op?eration of equipment for experiment，unstable quali?ty of coating and low preparation efficiency，etc.

Mechanical alloying（MA）method is a solidstate and non-equilibrium processing technique［13］.The preparation of continuous coatings via MA relies on cold welding of solid-state powder particles. The ad?vantages of the MA method include the lower cost，high ambienttemperature，simpleequipment，high effi?ciency-preparation and favorable adhesion of coatings，etc.Considerable research efforts have been devoted to fabricating the composite coatings on bulk materials by MA［14-20］，such as Al-Ni，Al-Cr，Al-Si，Al-Ti，etc.

Therefore，it is imperative to investigate the preparation of HEA composite coatings on titanium alloys via MA method. In this paper，35%（weight fraction）Al-FeCoNiCrAl composite coatings have been fabricated on Ti-6Al-4V by MA. The Al-Fe?CoNiCrAl composite coatings belong to the ductilebrittle system，in which the ductile Al is a bonding agent and the FeCoNiCrAl particle could improve the performance of coatings. This work provides practi?cal preparation of HEA composite coatings by MA method. The microstructures，phases and composi?tions of the synthesized coatings were characterized.The phase transformation and micro-hardness of com?posite coatings during the annealing process were characterized as well.

1 Experimental Procedure

The Al-FeCoNiCrAl coatings were prepared using a Fritsch Pulverisettee6 planetary mono mill.The Ti-6Al-4V alloy was cut by laser into blocks with a dimension of 12 mm×12 mm×3 mm. The surface was polished by silicon carbide abrasive pa?per and then washed by ultrasonic cleaning with ab?solute ethyl alcohol.

The original powder used in this paper were ar?gon atomized FeCoNiCrAl（99%，500 mesh）pow?der and pure aluminum（99.9%，200 mesh）pow?der，and the chemical composition of FeCoNiCrAl powder is shown in Table 1. The coating preparing processes were conducted in a stainless steel grind?ing vial of 700 ml. 10.5 g Al powder，19.5 g Fe?CoNiCrAl HEA powder，4 blocks of substrates and 300 g grinding balls were put into the grinding vial before MA，and the schematic illustration is shown in Fig.1. The diameter of grinding balls is 6 mm and 8 mm for improving the efficiency of ball-powdersubstrate effects. The coatings prepared at 2 h，4 h，6 h and 8 h were used to compare the phase composi?tions and properties during milling duration. Other parameters including speed of 350 r/min and the ball-to-powder ratio of 10∶1 remained constant. The milling processes were performed in the ambient at?mosphere and no process control agent was added to the vial. In order to prevent an excessive tempera?ture rise occurred in the vial，the operation of 20 min ball milling was followed by 10 min of cooling. Then the specimens were annealed at 550 ℃，750 ℃，850 ℃and 1 050 ℃for 2 h under an argon atmo?sphere，to avoid samples oxidation.

Table 1 Chemical composition of FeCoNiCrAl powder%

Fig.1 Schematic of experimental apparatus

The cross-section of specimens were obtained by spark-erosion wire cutting method and then han?dled by sanding and polishing according to the stan?dard procedure. Microstructures and chemical com?positions of the coatings were observed by a scanning electron microscopy（FESEM，HITACHI S-4800 Field emission）equipped with an energy dispersive X-ray spectrometer（EDX，Bruker XFlash 5030）.

The phase structures of the coatings were iden?tified by X-ray diffraction（XRD，Bruker D8）with Cu Kα radiation（λ= 0.154 059 8 nm）at 40 kV and 40 mA，using a continuous scan mode over the wide angle 2θrange of 20°—90°.

The NETZSCH STA 449F3 instrument was used to get the TG-DSC curve at a constant heating rate of 10 K/min ranging from 30 ℃（303 K）to 1 200 ℃（1 473 K）under Ar atmosphere to check the phase changing temperature and thermal stabili?ty.Al2O3crucible was used in this experment.

A HXS-1000A micro-hardness tester was uti?lized to determine the Vickers hardness of the coat?ings and substrates，with a load of 0.1 kg and an in?dentation time of 15 s. The micro-hardness was test?ed along the profiles of the coated samples from the near-surface to the inner substrates.

2 Results and Discussion

2.1 Microstructures and composition

Fig.2 shows the cross-sectional microstructures of 35%（weight fraction）Al-FeCoNiCrAl coatings on Ti-6Al-4V substrate under different milling times. It can be seen that all coatings exhibit mottled structures and the gray areas represent the HEA par?ticles while the dark areas are the Al particles. Be?sides，FeCoNiCrAl particles are angular with nonuniformly dimensions. With the increase of milling time，the thickness of the coatings first increases and then decreases.The thickness of the coating after 4 h of ball milling reaches the maximum value.When the milling time prolongs to 8 h，the cracks and microholes are formed in the coatings as shown in Fig.2（d）.

Fig.2 Cross-section microstructures of 35% (weight frac?tion) Al-FeCoNiCrAl coatings under different mill?ing time

The outer light layer is hard-worked Al which will be analyzed later by energy dispersive spectrum（EDS）.The deposition of particles relies on the ballparticle-substrate effects，which could result in cold welding of particles and manufacture continuous coatings［13］. With the increase of milling time，the particles and the coating work hardened［21］. The flak?ing of coating and the deposition of work-hardened particles on the harder coatings would result in microholes and cracks，and the thickness of the coatings will reduce due to repeated collision of milling balls.

Fig.3 Corresponding EDS results of the coatings in Fig.2

Figs.3（a）—（c）are the EDS results of position 1―3 as shown in Figs.2（a）and（d）. The results show that the gray particles are FeCoNiCrAl HEA and the dark particles are Al particles. Fig.3（c）shows that the layer is composed of homogeneous Al-FeCoNiCrAl composite particles，which indi?cate the Al dissolved in the HEA. Fig.3（d）shows the EDS line scanning results obtained from the sur?face to the inner substrate of the coatings after 8 h milling and the EDS line scanning result is carried out from the surface to the inner substrate. Severe fluctuations of the main element contents indicate the composite structure in the coatings. The fluctua?tions of Fe，Co，Ni，and Cr in the coatings are simi?lar. The closer to the coating surface，the more uni?form the element distribution. The change of the proportion of Al is opposite to the change of Fe，Co，Ni and Cr.

Fig.4 shows the XRD patterns of the raw pow?der and the top surface of the composite coatings.The phase composition of the coatings is Al and Fe?CoNiCrAl HEA. Body-centered cubic（BCC）and face-centered cubic（FCC）structures of FeCoNi?CrAl powder transformed into BCC structure after 2 h milling duration，which is due to that Al dif?fused to the HEA during MA［22］.The peak of Ti dis?appearing in MA-4 h and MA-6 h indicates that the substrate has been covered completely，and continu?ous and dense coatings have formed on it. The dif?fraction peak of Al and HEA gradually decreases during the milling duration. However，the diffrac?tion peak of Al after 8 h milling disappears because the composite Al-FeCoNiCrAl particles have ad?hered to the top surface of the coatings.

Fig.4 XRD patterns of HEA and coatings using different milling time

2.2 Annealing process

The simultaneous thermal analysis method was used for 35%（weight fraction）of Al-FeCoNiCrAl powders after milling for 4 h. Fig.5 shows the differ?ential scanning calorimetry（DSC），thermogravime?try analysis（TG）and differential thermogravimetry analysis（DTG）curves of the powders. In the DSC curve，the endothermic long line from 50 ℃ to 365 ℃could be associated with thermal transients.Long endothermic curves can be associated with the gradual collapse of the crystalline structure. The phase change is characterized by the endothermic peaks of 387.8 ℃，629.0 ℃and 926.6 ℃.

The TG curve in Fig.5 indicates the weight change of 4 h mechanically alloyed 35% of Al-Fe?CoNiCrAl powder. The weight decreases first and then increases twice. 0.37% of weight gain after the first weight changing. 4.64% of total weight gain is achieved when the temperature reaches 1 200 ℃.Weight gain could be attributed to surface oxidation.The first peak of the DTG curve is 633.9 ℃ of 0.11%/min. Another peak is 926.9 ℃ of 0.24%/min. These peaks are related to the phase transfor?mation， which corresponds to the endothermic peaks of 629.0 ℃and 926.6 ℃in the DSC.

Fig.5 DSC, TG and DTG curves of Al-FeCoNiCrAl com?pound powder ball milled for 4 h

Fig.6 shows the XRD patterns of 35%（weight fraction）Al-FeCoNiCrAl coating surfaces after an?nealing at different temperatures. A super-saturated solid solution is easily formed during MA，and the annealing process makes phase precipitated in the coating. The mixing enthalpy between Ni and Al is relatively negative，so Ni and Al are easy to com?bine［23］. New diffraction peaks of Ni3Al prototype Ll2phase and Fe-Cr-Co type σ phase are formed af?ter annealing at 550 ℃along with the initial BCC phase［24］. The unstable precipitated Ll2phase is easy to transform to ordered BCC with the increase of an?nealing temperature. So the （Ni，Co）3Al4，BCC phase，ordered BCC and σ phase are found in the coating after annealing at 750 ℃. Diffraction peaks of the coating after annealing at 850 ℃are similar to 750 ℃. The phase of the coating after annealing at 1 050 ℃is BCC and ordered BCC. Other unstable phases like（Ni，Co）3Al4and σ phase are decom?posed after annealing at 1 050 ℃of 2 h.

Fig.6 XRD patterns of the top surface of the coating anneal?ing at different temperatures

Fig.7 is cross-section microstructures of Al-Fe?CoNiCrAl coatings after annealing at different tem?peratures. The coating after annealing at 550 ℃is composed of black Al particles and bright FeCoNi?CrAl particles，which are similar to those coating before annealing. Bright Al particles in the coating after annealing at 750 ℃are not obvious as the coat?ing before annealing. Narrow FeCoNiCrAl particles grow larger than those before annealing，because the Al particles are melted and dissolved into HEA particles during annealing. The particle in the coat?ing after annealing at 850 ℃is the same as the coat?ing after annealing at 750 ℃. The interface between the particles in the coating disappears after annealing at 1 050 ℃.

Fig.7 Cross-section of the sample after annealing for 2 h at different temperatures

The cracks between the coating and TC4 sub?strate are generated after annealing at 550 ℃ as shown in Fig.7（a）. That is attributed to the fact that the precipitated phase reduces the bonding between the coating and substrate. The cracks are found in the coatings after annealing at 750 ℃and 850 ℃as shown in Figs.7（b）—（c）. Fig.7（d） shows that there is no crack between the composite coating and the TC4 substrate in the coating after annealing at 1 050 ℃，because the precipitated phase is decom?posed after annealing at such high temperature.

Fig.8 shows the EDS line scanning results of the Al-FeCoNiCrAl coatings annealing at different temperatures. The EDS line scanning is carried out from the TC4 substrate to the coating surface. With the increase of annealing temperature，the distribu?tion of the Al element in the coating becomes more and more uniform，especially these areas close to the top surface. This reason is maihly that the annealing temperature is higher than the melting point of Al，which makes it easier for liquid Al to diffuse［25］.

The distributions of Fe，Co，Ni and Cr ele?ments of the coating after annealing at different tem?peratures shown by the EDS linear scan result are basically the same. However，the changing law of Cr in the coating annealing at 1 050 ℃is not similar to Fe，Ni and Co，but consistent with Al，because the enthalpy of Al and Ni is relatively negative，and Al，Ni-rich ordered BCC formed. The element con?tent of Ti in the interface of the coating is flatter with the annealing temperature increased，which proves that Ti diffuses from the TC4 substrate to the coating，and becomes more intense as the an?nealing temperature increases.

Fig.8 Corresponding linear scanning results of cross-sec?tion morphology of the coating annealing at different temperatures

2.3 Properties

Fig.9 shows the cross-section hardness of the Al-FeCoNiCrAl coating under different milling time. The maximum micro-hardness value is ob?tained near the surfaces of coatings. The micro-hard?ness values of the coating increase with the increase of milling duration. The maximum micro-hardness value of the coating after milling 8 h is 605.6 HV0.1，much higher than the maximum micro-hardness value of the coating after 2 h milling，which is 402.7 HV0.1.The hardness of the substrate increases with the in?crease of milling time，which is due to that the sub?strate，composite coating and powder gradually work harden under the collision of milling balls dur?ing MA and the grain of the coating and substrate is refined as well. The coating is denser under the colli?sion of the milling ball，and then the hardness of the coating and substrate increases. The hardness of the inner layer of the coating after milling for 2 h is low?er than the substrate because the Al cold-welded on the coating surface is soft due to lack of work hard?ening.

Fig.9 Variations of micro-hardness of the coating under dif?ferent milling duration along cross-section from the top surface to the inner substrate

Fig.10 shows the hardness of these Al-FeCoNi?CrAl coatings after annealing at different tempera?tures. Annealing makes the work hardening effect of the coating weaken. The grains in the coating recov?er to recrystallize，which reduces the hardness of the coating. However，the hardness of the coating surface after annealing at 550 ℃is higher than be?fore，because the hard σ phase could increase the hardness of coatings［26］. The maximum hardness of the coatings annealing at 750 ℃ is 559.9 HV0.1，which is much higher than before. Ordered BCC is discovered after the annealing process，spinodal de?composition could increase the hardness of the coat?ings as well［27］. The hardness of the coatings after annealing at 1 050 ℃is less than that before，be?cause the σ phase dissolves，and the grain grows af?ter annealing at such high temperature. Residual stress is released with the increase of the annealing temperature. As a result，the hardness of the coat?ing annealed at 1 050 ℃is less than that before.However，the hardness of the inner layer in the coat?ings annealed at 1 050 ℃is higher than before，be?cause Ti diffuses from the TC4 into the coating.

3 Conclusions

Al-FeCoNiCrAl composite coatings were suc?cessfully synthesized on the Ti-6Al-4V alloy sub?strate by MA. The effect of milling duration and ele?ment ratio for the Al-FeCoNiCrAl coating was stud?ied. The microstructure and phase of coatings after annealing were discussed. Based on the above re?sults，the following conclusions can be drawn：

（1）The TC4 substrate was fully covered with the Al-FeCoNiCrAl coating after MA treatment.The coatings were composite structure made up of FeCoNiCrAl particles and the ductile Al. The clos?er to the coating surface，the more uniform the structure of the coatings，and the less the Al con?tent.

（2）Milling time had a significant effect on the preparation of the Al-FeCoNiCrAl coating. The thickness of the composite coatings increased first and then decreased with the increase of milling time.The sample after milling for 4 h showed a better morphology and the thickest composite structure.Defects were generated in the coating after milling for 8 h.

（3）The TG-DSC curve of Al-FeCoNiCrAl powder indicated that the phase change occurred at 387.8 ℃，629.0 ℃and 926.6 ℃. The（Ni，Co）3Al4，L12phase and σ phase were found after annealing above 387.8 ℃. L12phase transformed into ordered BCC in the coating above 629.0 ℃. Only BCC and ordered BCC were found in the coating above 926.6 ℃.

（4）As the annealing temperature increased，the more uniformly the elements were distributed in the coating. Because Al diffused to the FeCoNiCrAl in a liquid state when the annealing temperature was higher than the melting point of Al. As the anneal?ing temperature increased，Ti diffused severer. The change law of Fe，Co，Ni and Cr was close after an?nealing at 550 ℃and 750 ℃.

（5）The maximum micro-hardness value of the Al-FeCoNiCrAl coatings was obtained near the top surfaces. The hardness of the coatings increased with the increase of milling duration. The coating af?ter annealing at 750 ℃had a higher hardness than before，which was due to the hardness σ phase and spinodal decomposition. The coating after annealing at 1 050 ℃was harder than before，because the re?sidual stress and the effect of work hardening were released.