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The high-entropy alloys with high hardness and soft magnetic

property prepared by mechanical alloying and high-pressure sintering

P.F.Yu a ,L.J.Zhang a ,H.Cheng a ,b ,H.Zhang a ,M.Z.Ma a ,Y.C.Li b ,G.Li a ,c ,*,P.K.Liaw c ,R.P.Liu a ,**

State Key Laboratory of Metastable Materials Science and Technology,Yanshan University,Qinhuangdao,066004,PR China b

Beijing Synchrotron Radiation Facility,Institute of High Energy Physics,Chinese Academy of Sciences,Beijing,100039,PR China c

Department of Materials Science and Engineering,The University of Tennessee,Knoxville,TN,37996-2200,USA

a r t i c l e i n f o

Article history:

Received 10September 2015Received in revised form 6November 2015

Accepted 24November 2015Available online 7January 2016Keywords:

High-entropy alloy Mechanical alloying High-pressure sintering High hardness

Magnetic property

a b s t r a c t

The equiatomic multiprincipal CoCrFeCuNi and CoCrFeMnNi high-entropy alloys (HEAs)were consoli-dated via high pressure sintering (HPS)from the powders prepared by the mechanical alloying method (MA).The structures of the MA'ed CoCrFeCuNi and CoCrFeMnNi powders consisted of a face-centered-cubic (FCC)phase and a minority body-centered cubic (BCC)phase.After being consolidated by HPS at 5GPa,the structure of both HEAs transformed to a single FCC phase.The grain sizes of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were about 100nm.The alloys keep the FCC structure until the pressure reaches 31GPa.The hardness of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were 494Hv and 587Hv,respectively,much higher than their counterparts prepared by casting.Both alloys show typical paramagnetism,however,possessing different saturated magnetization.The mechanisms responsible for the observed in ?uence of Cu and Mn on mechanical behavior and magnetic property of the HEAs are discussed in detail.

1.Introduction

The high-entropy alloys (HEAs)consisted with a minimum of ?ve principal elements each of them with an atomic concentration between 5and 35at%were ?rstly proposed by Yeh et al.,in 2004[1].The alloy design [2],properties optimization [2,3],the atomic distributions of HEA [4],phase formation rules [5],and the sample preparation method were widely studied.At present,typical pro-cessing routes for HEAs can be summarized according to the starting states for the alloy preparation [6],mainly (1)from the liquid state:the arc melting [7],(2)from the solid state:the high-energy ball mill [8,9],(3)from the gas state:the sputtering method [10,11],and (4)from electrochemical process [12].For the same components,different methods produce distinct micro-structures especially for grain sizes,leading to different properties.A small grain size will result in high hardness and high strength according to Hall e Patch relationship [13].The grain size usually

reaches several hundred micrometers even several millimeters by the arc melting and then casting.However,the mechanically alloying (MA)could prepare alloy powders with nanocrystalline particles [14].Then,the powders could be sintered by spark-plasma sintering (SPS).Fu et al.fabricated the inequi-atomic Co 0.5Fe-NiCrTi 0.5[15]and CoNiFeCrAl 0.6Ti 0.4[16]HEAs by MA e SPS.Zhang et al.synthesized the equiatomic multicomponent CoCrFeNiCuAl [17]and CoCrFeNiTiAl [18]HEAs with the same method.The grain size after SPS is nearly 300e 400nm [16]hard to reach less than 100nm.Even though the grain size doesn't reach less than 100nm,the yield stress of Co 0.5FeNiCrTi 0.5alloy has been measured to be 2.65GPa while the compressive strength reaches 2.69GPa higher than most of the HEAs prepared by casting [16,19,20].

When the HEAs prepared by SPS,the pressure is usually dozens of MPa.In order to gaining high density,the sintering temperature will be high leading to a large grain size.When the pressure applied to sintering comes to several GPa,not only restraining the grain growth to acquiring a small grain size,but also increasing the density of the alloy bulk.Therefore,mechanical alloying (MA)and then sintered by high pressure sintering (HPS)is an ef ?cient way to produce ?ne-grain alloys.

There has been expectation that some HEAs may also possess

*Corresponding author.State Key Laboratory of Metastable Materials Science and Technology,Yanshan University,Qinhuangdao 066004,PR China **Corresponding author.

E-mail addresses:gongli@https://www.sodocs.net/doc/b7900482.html, (G.Li),riping@https://www.sodocs.net/doc/b7900482.html, (R.P.

Liu).Contents lists available at ScienceDirect

Intermetallics

journal homepage:www.elsev https://www.sodocs.net/doc/b7900482.html,/locat

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Intermetallics 70(2016)82e 87

excellent magnetic properties for several ferromagnetic elements with high magnetic moments known to form HEAs[21,22].The magnetic properties is strongly composition dependence.The saturation magnetization Ms drops almost monotonically upon the additions of Al and Si in FeCoNi(AlSi)x(0x0.8)[22].Therefore, magnetic properties of the alloys were tested in the present work.

In the present work,with the aim to obtain excellent mechanical properties of HEAs with the nanoscaled grain size,equi-atomic CoCrFeCuNi and CoCrFeMnNi HEAs powders were fabricated by MA and then sintered by HPS.The structure of the powders pre-pared by MA and subsequent consolidation bulks were carefully investigated by X-ray diffraction(XRD),scanning electron micro-scopy(SEM),and transmission electron microscopy(TEM).The compression behavior of the samples was investigated by in situ high-pressure synchrotron diffraction.The hardness and magnetic properties of the HEAs after HPS were investigated by a Vickers hardness instrument and vibrating sample magnetometer(VSM). This study provides a valuable method to prepare nanocrystalline HEAs with excellent properties.

2.Experimental

The CoCrFeCuNi and CoCrFeMnNi HEAs were prepared by MA and HPS with elemental powders of Co,Cr,Fe,Ni,Cu,Mn,with diameter45m m and99.7wt.%purity.The elemental powders were milled in a planetary ball mill(Fritsch Pulverisette P-5)with the tungsten-carbide grinding media in toluene.The ball-to-powder weight ratio is10:1,and a450rpm speed was used un-der the argon atmosphere.The powders were dry milled for25h. Then,the ethanol was used as a process-controlling agent in order to avoid excessive cold welding and also act as a reducing medium to avoid oxidation of the alloy within25h e30h.The CS-1B type hexahedron anvils press was utilized for sintering30h ball-milled alloy powders at1273K and5GPa for15min.A graphite tube and pyrophyllite were taken as the heating device and pressure-transmitting medium.The size of the bulk alloys we produced by this equipment is about F6?2mm.

The structures of the samples were characterized by XRD using the D/MAX-2500/PC diffractometer with Cu K a radiation.The microstructure of the powders and the bulk samples were studied by SEM using Hitachi-S4800and TEM using JEM-2010.

Some powders were carefully scraped from the bulks of HEAs with a4Cr13stainless-steel scalpel for pressure experiments.In-situ high-pressure XRD experiments with a wavelength of 0.6199?and a focused beam size of about26?8m m2were per-formed at the beamline4W2of Beijing Synchrotron Radiation Fa-cility in China.The powders were loaded into diamond anvil cells with the sample chamber about180m m in diameter,drilled in a T301-stainless-steel gasket.Silicone oil was used as a pressure-transmitting medium,while for the pressure calibration ruby pieces were dispersed inside.The pressure applied to the sample was calculated from the positions of ruby-?uorescence levels.The XRD patterns were acquired in the pressure range of0e31GPa upon compression with a transmission mode through the di-amonds with a20-min interval for each pressure point to allow for the stress relaxation.Debye rings were recorded using an image plate in a transmission mode,and the XRD patterns were integrated from the images using the FIT2D software[23].

Hardness measurements were conducted employing a Vickers hardness tester with an applied load of300g for10s.At least ten tests were conducted to obtain the average value.The hysteresis loops of the HEAs at room temperature were measured by using the Lakeshore7407VSM.

3.Results and discussion

3.1.Crystal structures and morphologies of powders and bulks

The XRD patterns shown in Fig.1reveal the phase of the MA powders and HPS bulks of both CoCrFeCuNi and CoCrFeMnNi HEAs. From the XRD results it is clear that the structure of the HEAs powders is a main face-centered-cubic(FCC)phase and a minor body-centered cubic(BCC)phase.After HPS,the BCC phase disap-pears for the bulk CoCrFeCuNi(lattice parameter a?2.87?)and CoCrFeMnNi(lattice parameter a?2.88?)HEAs.Therefore,phase transition from BCC to FCC occurred during sintering of the HEAs powders.Phase evolution during sintering may be attributed to the metastable supersaturated solid-solution phases transforming to equilibrium phases[8,24].In the milled powders,a substantial amount of energy is stored because of the high dislocation density and the signi?cant volume fraction of grain boundaries[25,26]. Accordingly,the excess energy may reduce the activation energy for phase evolution and,therefore,in favor of its occurrence during sintering[8,27].Thus,metastable supersaturated solid-solution BCC phases transform to equilibrium FCC phases under the effect of temperature and https://www.sodocs.net/doc/b7900482.html,pared with the powders XRD,the diffraction peaks of FCC phases shift to high angles.It means that the lattice parameters become smaller.The lattice parameters of the FCC phases transform from3.59?to3.54?for the CoCrFeCuNi HEA,and from3.61?to3.56?for the CoCrFeMnNi HEA shown in Table1,respectively.And they are smaller than their counterparts prepared by casting.The reason is similar to the phase transition from BCC to FCC.The substantial amount of stored energy induce the lattice of the FCC phase highly distortion with a larger

lattice Fig.1.XRD patterns of powders and bulk alloys;(a)As-mill'ed and HPS'ed CoCrFeCuNi,(b)As-mill'ed and HPS'ed CoCrFeMnNi.

P.F.Yu et al./Intermetallics70(2016)82e8783

parameter during milling.While sintering,the temperature and pressure lead to the FCC phase transforming from a larger lattice parameter in the metastable state to a smaller lattice parameter in the stable state.In addition,the diffraction peaks narrow after HPS,which indicates that the grains growth during sintering.

The morphology and size of powder particles for milled pow-ders and grain sizes for the bulk samples were investigated using SEM and TEM shown in Fig.2.In Fig.2(a)and (b),the powder particles of the milled HEAs exhibit a lamellar morphology with a particle size of 20m m.The particle sizes have a homogeneous distribution.Fig.2(c)and (d)show the SEM micrographs of the bulk HEAs after HPS.The sintering bulks possess a homogeneous,?ne,

and dense structure.The grain sizes could be characterized by TEM in Fig.2(d)and (f).For the bulk CoCrFeCuNi and CoCrFeMnNi HEAs,the grain sizes of about 100nm have a homogeneous distribution.The-select-area-electron diffraction patterns inserted in Fig.2(d)and (f)indicate the FCC structures.

3.2.The structure of the HEAs at high pressures

The structures of the CoCrFeCuNi and CoCrFeMnNi HEAs under high pressure were investigated by in situ high-pressure synchro-tron diffraction.The diffraction spectra of the alloys were displayed in Fig.3from ambient pressure to more than 30GPa.With the increased pressure,the (111)and (200)diffraction peaks of FCC visibly shifted to a high angle,which shows the compression behavior of HEAs.Under the testing pressures,no new diffraction peaks were detected from the curves,indicating that the structures of the alloys were stable at high pressures.

In order to have a better understanding of their compression behavior of the alloys,it is necessary to obtain the equation of state (EOS).Bridgman presented the EOS as follow [28]:àD V /V 0?a 0ta P tb P 2tc P 3t…

(1)

Table 1

The lattice parameters of the CoCrFeCuNi and CoCrFeMnNi HEAs prepared by the as-milled,HPS'ed,and as-cast (?)conditions.Alloy

As milled HPS'ed As cast FCC

BCC FCC BCC FCC BCC CoCrFeCuNi 3.59 2.87 3.54e 3.57[1]e CoCrFeMnNi

3.61

2.88

3.56

3.59[31]

Fig.2.SEM micrographs of powders and bulk alloys after sintering;(a),(c)CoCrFeCuNi and (b),(d)CoCrFeMnNi;TEM micrographs of bulk alloys after HPS,(e)CoCrFeCuNi and (f)CoCrFeMnNi.

P.F.Yu et al./Intermetallics 70(2016)82e 87

where V 0is the volume at zero pressure,coef ?cients a 0,a ,b ,and c can be determined,using the least-squares ?tting method.One can estimate the relative volume change àD V /V 0?V P àV 0at a given pressure (V P )to that at zero pressure (V 0).The experimental àD V /V 0data were shown in Fig.4.When ?tted by the Bridgman equation,EOS can be expressed as Equations (2)and (3)for CoCrFeCuNi and CoCrFeMnNi,respectively:

àD V /V 0?8.51?10à3P à2.73?10à4P 2t4.38?10à6P 3(2)àD V /V 0?7.35?10à3P à1.80?10à4P 2t2.52?10à6P 3

(3)

The bulk modulus,K,can be obtained according to the rela-tionship [29],K ?1/a.The bulk modulus K of the HEAs are 117.5GPa and 136.1GPa for CoCrFeCuNi and CoCrFeMnNi,respectively.The result of the CoCrFeMnNi HEA is in good agreement Laplanche's result (137GPa)testing in bending and torsion modes [30].

3.3.Mechanical and magnetic properties

Fig.5shows the hardness of the CoCrFeCuNi and CoCrFeMnNi HEAs fabricated by HPS and their counterparts by casting.The hardness of the CoCrFeCuNi HEA produced by casting is 133Hv [1].While,the hardness increases to 494Hv for the HEA by MA-HPS.For the CoCrFeMnNi HEA,the hardness increases from 300Hv by casting [31]to 587Hv by MA-HPS.The hardness of the HEAs pre-pared by MA-HPS was improved greatly.The great increase of HEAs hardness prepared by MA-HPS is the reason of decreasing of grain size.The relationship between hardness and grain size of the CoCrFeMnNi HEA was found to follow the classical Hall e Petch relationship,strengthening with a relatively high hardening coef-?cient of 677MPa m m à0.5[32].According to the Hall e Petch rela-tionship for the CoCrFeMnNi HEA,the hardness of the CoCrFeMnNi HEA with a grain size of 100nm in the present work is about 336Hv.However,it is 587Hv higher than the value calculating by the formula.This trend indicates that grain re ?nement plays more evident strengthening effects when the grain size is reaching the nanometer.

The CoCrFeCuNi and CoCrFeMnNi HEAs were consolidated with the same procedures including MA and HPS except for elements difference.Because of their similar grain sizes,the difference of grain-boundary strengthening effect on bulk modulus and hard-ness could be ignored.The atomic radii of Cu and Mn are 1.27?and 1.32?,respectively.The HEAs are different from traditional alloys because of different-sized atoms randomly distributed in the crystal lattice with the same probability to occupy the lattice sites [6].Mn atoms with a larger atomic radius lead to larger lattice distortion and more evident solution strengthening.Therefore,the CoCrFeMnNi HEA will show a higher resistance to compression than the CoCrFeCuNi HEA during high pressure procedure.In addition,the higher hardness of CoCrFeMnNi HEA is also likely responsible for more evident solution strengthening.

The magnetic hysteresis loops of the CoCrFeCuNi and CoCr-FeMnNi HEAs were presented in Fig.6measured by VSM at room temperature.The CoCrFeCuNi HEA show excellent soft magnetic property.The saturated magnetizations (Ms),remanence ratio (Mr/Ms),and coercivity (Hc)of the CoCrFeCuNi HEA are estimated to

Fig. 3.Angle-dispersion X-ray diffraction patterns;(a)CoCrFeCuNi and (b)

CoCrFeMnNi.

Fig.4.Pressure dependence of relative volumetric change at room temperature;(a)CoCrFeCuNi and (b)CoCrFeMnNi.

P.F.Yu et al./Intermetallics 70(2016)82e 8785

53.41emu/g,58.09%,and 166Oe,respectively.The Ms of the CoCrFeCuNi HEA is higher than the CoCrFeNiCuAl HEA (38.18emu/g)by casting [33].However,the Ms of the CoCrFeMnNi HEA is 1.34emu/g,similar to the value of CoCrCuFeNiTi alloy (1.368emu/g)[34].Compared with the magnetic properties of CoCrFeMnNi HEA prepared by SPS in Ji's paper [35],the HEA in this paper show lower saturated magnetizations for structure coarsening.The structure coarsening and phase transformation could result in magnetic property changes [6].On the other hand,according to our results,the elements play a crucial role in the magnetic properties of HEAs.The CoCrFeCuNi and CoCrFeMnNi HEAs with a similar grain size,however,their saturated magnetizations vary widely.Therefore,the magnetic property of HEAs could be designed by choosing different elements.4.Conclusions

In this study,two equiatomic CoCrFeCuNi and CoCrFeMnNi HEAs were prepared by MA-HPS.The bulk alloys exhibits a simple FCC solid solution structure with a grain size of about 100nm.The HEAs remained stable up to 31GPa,and the bulk modulus are

117.5GPa and 136.1GPa for CoCrFeCuNi and CoCrFeMnNi,respec-tively.The CoCrFeCuNi and CoCrFeMnNi HEAs show high hardness of 494Hv and 587Hv,respectively.Meanwhile,the CoCrFeCuNi HEA exhibits a high Ms value of 53.41emu/g,whereas the CoCr-FeMnNi HEA shows a low Ms value of 1.34emu/g.This study reveals that MA-HPS can be used as an effective way to design HEA ma-terials with the high hardness,high bulk modulus,and controllable magnetic property.Acknowledgments

The research was supported by the National Science Foundation of China (Grant No.51271161/51171163/51121061).The present work was performed at the 4W2HP-Station,Beijing Synchrotron Radiation Facility (BSRF).Gong Li would like to acknowledge the Specialized Research Fund for the Doctoral Program of Higher Ed-ucation (Grant No.20131333110019).Peter K.Liaw would like to acknowledge the U.S.Army Research Of ?ce project (W911NF-13-1-0438),the National Science Foundation (CMMZ-1100080)(Dr.S.N.Mathaudhu,and Dr.D.Stepp,Dr.C.Cooper)and the Department of Energy,Of ?ce of Fossil Energy,National Energy Technology Labo-ratory (DE-FE-0008855,DE-FE-0011194,and DE-FE-0024054),with Mr.V.Cedro,Mr.S.Markovich,Mr.R.Dunst,and Dr.J.Mullen as program managers,respectively.References

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The high-entropy alloys with high hardness and soft magnetic

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