Design of high entropy alloys based on the experience from
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Philosophical Magazine Letters
ISSN: 0950-0839 (Print) 1362-3036 (Online) Journal homepage: https://www.360docs.net/doc/fc1540340.html,/loi/tphl20Design of high entropy alloys based on the experience from commercial superalloys
Z. Wang, Y. Huang, J. Wang & C.T. Liu
To cite this article: Z. Wang, Y. Huang, J. Wang & C.T. Liu (2015) Design of high entropy alloys based on the experience from commercial superalloys, Philosophical Magazine Letters, 95:1,1-6, DOI: 10.1080/09500839.2014.987841
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Design of high entropy alloys based on the experience from
commercial superalloys
Z.Wang a ,b ,Y .Huang b ,J.Wang b and C.T.Liu a *
a Center of Advanced Structural Materials,Department of Mechanical and Biomedical Engineering,City University of Hong Kong,Kowloon,Hong Kong,P .R.China;
b State Key Laboratory of Solidi ?cation Processing,Northwestern Polytechnical University,Xi ’an 710072,P .R.China (Received 1July 2014;accepted 9November 2014)High entropy alloys (HEAs)have been drawing increasing attention recently and gratifying results have been obtained.However,the existing metallurgi
c rules of HEAs coul
d not provid
e speci ?c information o
f selectin
g candidate alloys for structural applications.Our brief survey reveals that many commer-cial superalloys have medium and even to hig
h con ?gurational entropies.The experience of commercial superalloys provides a clue for helping us in the development of HEAs for structural applications.Keywords:high entropy alloys;superalloys As a new class of metallic alloys,high entropy alloys (HEAs)have been drawing an increasing attention in recent years [1–4].HEAs have several attractive characteristics:high con ?guration entropy,sluggish diffusion and severe lattice distortion.Considering these aspects,HEAs are expected to be good candidates for structural applications.Although HEAs exhibit attractive mechanical and metallurgical properties for potential applications,they have not been widely applied as structural material and the criteria of selecting HEAs for structural applications are also absent.In principle,numerous kinds of HEAs can be assembled based on the periodic table of atomic elements.Several parameters have been proposed to successfully classify the phases in HEAs;however,the criteria of screening potential candidates of HEAs for structural application have not been well developed yet [4].The design of HEAs with desirable ductility and strength is one of the key chal-lenges to the materials community.On the one hand,the available experimental results have con ?rmed that HEAs with body-centred cubic (bcc)solid solutions or intermetallic phases have excellent hardness and strength,but most of them lack suf ?cient room tem-
perature ductility [5].On the other hand,the HEAs with face centered cubic (fcc)solid solutions have proper ductility,but their strength needs to be improved [6,7].Appar-ently,there is no easy solution to design HEAs with desirable mechanical properties at the present time.Because of the lack of a distinct strategy,it is desirable for us to ?nd the clues from the mature multi-component alloy systems.Here,we attempt to ?nd the design criteria of HEAs for structural applications from well-established superalloys.*Corresponding author.Email:chainliu@https://www.360docs.net/doc/fc1540340.html,.hk
?2014Taylor &Francis
Philosophical Magazine Letters ,2015
V ol.95,No.1,1–6,https://www.360docs.net/doc/fc1540340.html,/10.1080/09500839.2014.987841
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There are many kinds of commercial superalloys available,and some of their entro-pies are in the same range of HEAs.Superalloys have been developed for more than half of a century [8–10].The superalloys exhibit an excellent combination of the mechanical strength and resistance to creep,resulting in their wide applications in the aeronautical and aerospace industries [8].The examination of the trend in the develop-ment of superalloys reveals that,with the increased number of alloying elements,the content of the principle element decreases continuously,even less than 40%in the molar fraction.As a result,some superalloys have a high con ?gurational entropy.The con ?gurational entropy is de ?ned as
D S conf ?R X n i ?1c i ln ec i T;(1)where c i is the atomic concentration of i th atom,R is the gas constant.D S conf is smaller than 1R in most of the traditional alloys.In equiatomic multi-component alloys,D S conf =1.10R,1.39R and 1.61R for n =3,4and 5,respectively.The con ?gurational
2Z.Wang et al.
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entropy distributions in solid-solution strengthened superalloys and precipitation-strengthened superalloys are presented in Figure 1a and b,respectively.It is indicated that most of the con ?gurational entropies of selected superalloys are around 1.3R.There are several superalloys with the con ?gurational entropy larger than 1.5R.This indicates that a part of the superalloys belongs to HEAs.Considering that superalloys with medium-to-high con ?gurational entropy are widely used for structural applications,it is feasible to see that a careful analysis of these commercial superalloys will provide useful information for the development of HEAs for structural applications.Therefore,a summarization of the phase selection in commercial superalloys with medium-to-high con ?gurational entropy will be helpful for us to search for HEAs suitable for structural applications.Although the critical value of the entropy commonly accepted is 1.5R [1,4],in our study,the entropy of 1.1R,corre-sponding to the entropy of equiatomic ternary alloys,is selected as the critical medium con ?gurational entropy for analysis.This selection has already excluded most traditional alloys.The superalloys with D S conf >1.39R are also highlighted to show the effect of increased con ?gurational entropy.In HEAs,the most important phase selection parameters are the mixing enthalpy [11,12],D H mix ?X n i ;j ?1;i ?j a ij c i c j ;(2)atomic size difference [11,12]d ??????????????????????????????????X n i c i e1àr i = r T2s ; r ?X n i c i r i ;(3)and valence electron concentration [3
]
Figure 2.The distribution region of commercial superalloys with high entropy in d àD H mix plot,which is only a small part in the d àD H mix plot of HEAs.IM represents the intermetallics.The solid lines delineate the boundary of solid solutions in rapid solidi ?ed HEAs.The data of HEAs is from Ref.[12].
Philosophical Magazine Letters 3
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VEC ?X
n i c i eVEC Ti (4)
where a ij ?4D H mix AB and D H mix AB is the mixing enthalpy of binary AB alloys,r i and eVEC Ti are the atomic radius and valence electron concentration of i th atom.
Figure 2compares the plot of D H mix àd for superalloys (with D S conf [1:10R)and HEAs.It indicates that the commercial medium-entropy and high-entropy superalloys only distribute in a small region in the D H mix àd plot for HEAs.In the solid-solution strengthened superalloys,there are only fcc solid solutions.In the precipitation-strengthened superalloys,there are fcc solid solutions as the matrix and intermetallic compounds as precipitations.Figure 2also marks out the superalloys with D S conf [1:39R.There is no signi ?cant difference in phase selection for superalloys with D S conf [1:10R and D S conf [1:39R.In Figure 2,the solid-solution strengthened superalloys are within the range of 0kJ/mol !D H mix !à7kJ/mol and 2% d 6%,a subset of the region of solid-solution phases within the range of 5kJ/mol !D H mix !à15kJ/mol and d 6%[12].This implies that the candidates of structural HEAs are in a more narrow region of D H mix .In the solid-solution superalloys,the atomic size difference d ,on the one hand,should be large enough (>2%)to increase the solid-solution strengthening.On the other hand,the atomic size difference is smaller than 6%to avoid the formation of large amount of harmful intermetallic compounds.For this reason,the atomic size difference of alloying elements should be in a ?nite range used for the development of solid-solu-tion HEAs for structural applications.For the mixing enthalpy,the solid-solution strengthened superalloys are with the range of 0kJ/mol !D H mix !à7kJ/mol.In Figure 2,precipitation-strengthened superalloys are within the range of à3kJ/mol !D H mix !–18kJ/mol and 3% d 8%.In the precipitation-strengthened superalloys,the phases are composed of fcc matrix and intermetallic precipitations.The phase selection of HEAs with heat treatments indicates that intermetallics appear in some HEAs in the region D H mix \à3kJ/mol and d [3%after heat treatments [13].Most of the commercial precipitation-strengthened superalloys are in the aging state and they are also in the region D H mix \à3kJ/mol and d [3%.Up to now,we do not have commercial superalloys in the region D H mix \à18kJ/mol and d [8%.Figure 3presents the VEC of commercial superalloys and HEAs.The VEC rule has been successfully used to distinguish the fcc and bcc solid solutions in the as-cast HEAs [3].The fcc is favoured at VEC >7.8while bcc is favoured at VEC <6.8.The mixed fcc and bcc appear in the interval of 6.8 D o w n l o a d e d b y [U n i v e r s i t y o f S c i e n c e & T e c h n o l o g y B e i j i n g ] a t 23:00 12 J a n u a r y 2016 mechanical properties for structural applications in superalloys [15].The excellent high-temperature mechanical properties of precipitated superalloys come from the combina-tion of ductile primary fcc matrix and the precipitated nanoparticles.The precipitated phases for strengthening are essentially c 0fcc ordered Ni 3(Al,Ti),c 00bct ordered Ni 3Nb,g hexagonal ordered Ni 3Ti and d orthorhombic Ni 3Nb intermetallic compounds in the commercial superalloys [10].All of these intermetallics are geometrically close-packed phases,where c 0and c 00phases are optimal because of the coherent domain boundary between precipitations and the matrix.The commercial precipitation-strengthened superal-loys containing mostly of Ni,Co,Fe and Cr,are favour for forming the fcc phase satisfy-ing the VEC rules,while geometrically close-packed phases are favoured by adding small amounts of Ti,Al and Nb.Accordingly,in the design of HEAs for structural application,the VEC could be larger than 7.8in order to form the fcc matrix and to avoid the forma-tion of topologically close-packed intermetallic phases after heat-treatment.In conclusion,although the criteria of HEAs for structural applications are lacking at the present time,the commercial superalloys with a medium-to-high entropy provide a clue to con ?rm the potential of HEAs for structural applications.Considering the mix-ing enthalpy,atomic size difference and valence electron concentration are commonly used as the design strategy of HEAs for structural applications;the phase selection rule used for superalloys has the same trend as for that of HEAs.The superalloys with solid-solution structures are in the range of 0kJ/mol !D H mix !à7kJ/mol and 2% d 6%,while precipitation-strengthened superalloys are in the range of 3kJ/mol !D H mix !à18kJ/mol and 3% d 8%,and these ranges are relatively smaller as compared with those for previous HEAs.All the superalloys are in the range of VEC >7.8.Therefore,based on the experience obtained from commercial superal-loys,the HEAs for structural applications should have a ?nite atomic size difference and mixing enthalpy but larger VEC.Moreover,the geometrically close-packed precipi-tations should be preferred than the topologically close-packed precipitations for structural use. Figure 3.The valence electron concentration distribution in the superalloys with D S conf ?1:1R.GCP and TCP represents geometrically close-packed intermetallic phases and topologically close-packed intermetallic phases respectively.The data of HEAs is from Ref.[13].Philosophical Magazine Letters 5 D o w n l o a d e d b y [U n i v e r s i t y o f S c i e n c e & T e c h n o l o g y B e i j i n g ] a t 23:00 12 J a n u a r y 2016 Funding This research is supported by the Research Grant Council (RGC),the Hong Kong Government,through the General Research Fund (GRF)under the project numbers CityU/521411.Zhijun Wang is also supported by,the Hong Kong Scholar Program and the National Science Foundation for Post-doctoral Scientists of China under the project number 2013M542385,the Natural Science Basis Research Plan in Shaanxi Province of China under the project number 2014JQ6207.Supplemental data Supplemental data for this article can be accessed here.[https://www.360docs.net/doc/fc1540340.html,/10.1080/09500839.2014.987841]References [1]J.-W.Yeh,JOM 65(2013)p.1759–1771.[2]Y .Zhang,T.T.Zuo,Z.Tang,M.C.Gao,K.A.Dahmen,P.K.Liaw and Z.P.Lu,Prog.Mater.Sci.61(2014)p.1–93.[3]S.Guo,C.Ng,J.Lu and C.T.Liu,J.Appl.Phys.109(2011)p.103505.[4]D.Miracle,https://www.360docs.net/doc/fc1540340.html,ler,O.Senkov,C.Woodward,M.Uchic and J.Tiley,Entropy 16(2014)p.494–525.[5]Y .Zhang,X.Yang and P.K.Liaw,JOM 64(2012)p.830–838.[6]A.Gali and E.P.George,Intermetallics 39(2013)p.74–78.[7]F.Otto,A.Dlouhy,C.Somsen,H.Bei,G.Eggeler and E.P.George,Acta Mater.61(2013)p.5743–5755.[8]R.C.Reed,The Superalloys ,Cambridge University Press,New York,2006.[9]B.Geddes,Superalloys:Alloying and Performance ,ASM International,Materials Park,OH,2010.[10]M.J.Donachie,Superalloys:A Technical Guide ,ASM International,Materials Park,OH,2002.[11]Y .Zhang,Y .J.Zhou,J.P.Lin,G.L.Chen and P.K.Liaw,Adv.Eng.Mater.10(2008)p.534–538.[12]S.Guo,Q.Hu,C.Ng and C.T.Liu,Intermetallics 41(2013)p.96–103.[13]Z.Wang,S.Guo and C.T.Liu,JOM 66(2014)p.1966–1972.[14]C.T.Liu,Int.Met.Rev.29(1984)p.168–194.[15]B.Seiser,R.Drautz and D.G.Pettifor,Acta Mater.59(2011)p.749–763.6 Z.Wang et al. 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