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Exceptionally high glass-forming ability of an FeCoCrMoCBY alloy
Jun Shen,a ?Qingjun Chen,Jianfei Sun,Hongbo Fan,and Gang Wang
School of Materials Science and Engineering,Harbin Institute of Technology,Harbin 150001,
People’s Republic of China
?Received 22November 2004;accepted 17February 2005;published online 5April 2005?
It has been well documented that the maximum thickness of as-cast glassy samples attainable
through conventional metallurgical routes is the decisive criteria for measuring the glass-forming
ability ?GFA ?of bulk metallic glasses ?BMGs ?.Here we report the exceptionally high GFA of an
FeCoCrMoCBY alloy which can be fabricated in the form of glassy rods with a maximum sample
thickness of at least 16mm.It is demonstrated that,by substituting Fe with a proper amount of Co
in a previously reported Fe-based BMG alloy,the glass formation of the resultant new alloy can be
extensively favored both thermodynamically and kinetically.The new ferrous BMG alloy also
exhibits a high fracture strength of 3500MPa and Vickers hardness of 1253kg mm ?2.?2005
American Institute of Physics .?DOI:10.1063/1.1897426?
Over the last decade,consistently increasing interest has
been directed toward exploring and synthesizing bulk metal-
lic glasses due to the technological and scienti?c signi?-
cance.To date,BMGs have been developed in a variety of
multicomponent metallic systems based mainly on Zr,1–3
Pd,1Mg,1,4–7Ti,1,8,9Ni,1,10,11Fe,1,12–16Cu,17,18La,19and Y .20
Among these BMGs,Fe-based alloys are commercially the
most important due not only to the most plentiful natural
resources of iron element in the Earth’s crust and conse-
quently the less-expensive raw material,but also to the
unique combination of high physical,chemical,and me-
chanical properties.Since the ?rst Fe-based BMG was re-
ported in 1995,1various ferrous alloys with high GFA have
been developed.It has been demonstrated that Fe-based
BMGs have large glass-forming ability,combined with high
strength,high hardness,good corrosion resistance,and ex-
cellent soft magnetic properties.1,12–16Very recently,Lu et
al.14and Ponnambalam et al.15reported independently that
Fe-based BMGs can be cast in bulk form with a critical
sample thickness as large as 12mm in FeCr ?Co ?MoMnCB
system and FeCrMoCB system,respectively.The large GFA
for these alloys were realized by minor alloying with yttrium
or lanthanides.For the Fe 48Cr 15Mo 14C 15B 6Y 2alloy,the
maximum thickness of the as-cast glassy sample was re-
ported to be 9mm.15In this letter,we report that the GFA of
the Fe 48Cr 15Mo 14C 15B 6Y 2alloy ?hereafter denoted as base
alloy ?can be largely improved by partially replacing Fe with
Co.The alloy with the optimum composition containing
7%Co can be cast into a 16-mm-diameter amorphous ingot,
while the maximum thickness for glass formation for the
Co-free alloy ?base alloy ?is smaller than 8mm.The origins
of the exceptionally high GFA for the new alloy is discussed
in light of thermodynamic and kinetic factors.
Alloy button ingots having the target compositions were
prepared from the starting commercial-grade elements with
purities better than 99.0wt.%?the impurity levels of O and
C in the raw materials were 0.04and 0.05wt.%,respec-
tively ?,by nonconsumable arc melting under a Ti-gettered
argon atmosphere.For each ingot the melting was repeated
four times to ensure composition homogeneity.Glassy alloys
in cylinder form were prepared by remelting the button in-gots and drop casting the melts into a copper mold.Differ-ential thermal analysis ?DTA ?was conducted under ?owing argon using a Perkin-Elmer Model DTA7instrument to mea-sure the thermal parameters of the alloys.The amorphous nature of the as-cast samples were examined by x-ray dif-fraction ?XRD ?analysis using Cu radiation and transmission electron microscopy ?TEM ?.The specimens for TEM obser-vation were prepared by standard mechanical polishing and ion-beam thinning.Electron microscopic observations were conducted on a Philips EM430electron microscope operat-ing at 300kV.The Vickers hardness of amorphous samples was measured under a load of 300g by using the HVS-1000digital-display microhardness tester.The compression test at room temperature was performed on an Instron 5500me-chanical instrument with an initial strain rate of 1?10?3s ?1.The specimens for the compression tests were prepared by machining the as-cast amorphous rods to achieve a gauge of 3mm in diameter and 6mm in length.Figure 1shows the DTA thermograms obtained for the as-cast Fe 48?x Co x Cr 15Mo 14C 15B 6Y 2?x =0,3,5,7,and 9?al-loys at a heating rate of 0.33K/s ?all compositions herein are expressed in atomic percent ?.All alloys show distinct glass transition,followed by multiplestage crystallization ?Fig.1?a ??and then melting within a temperature interval ?Fig.1?b ??.The glass transition temperature T g ,onset tem-perature of crystallization T x ,onset melting temperature T m ,and offset melting temperature T l ?i.e.,the liquidus tempera-ture ?,and other deduced thermal properties are tabulated in Table I,where T rg is the reduced glass transition temperature,de?ned by the ratio of the glass transition temperature to the liquidus temperature,and ?H 1,?H 2,and ?H 3represent,re-spectively,the heats released during the ?rst,second,and third crystallization stage.From the DTA measurements,as shown in Fig.1as well as in Table I,one can see that,all characteristic temperatures do not change apparently as the Co addition increases,except for the liquidus temperatures.The alloy containing 7%Co shows the lowest T l ,and there-fore,the highest T rg .Although the alloys exhibit noneutectic melting,the proper addition of Co indeed brings the compo-sition closer to eutectic,leading to a lower T l for the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy.In view of a higher T rg ,
which is a basic criterion for measuring glass forming ten-
a ?Author to whom correspondence should be addressed;electronic mail:
junshen@https://www.wendangku.net/doc/ac16531104.html, APPLIED PHYSICS LETTERS 86,151907?2005?
0003-6951/2005/86?15?/151907/3/$22.50?2005American Institute of Physics 86,
151907-1
dency,it can be speculated that the alloy with 7%Co in place
of Fe has the highest GFA compared to other alloys with
different Co contents.
To examine the dependence of GFA on the Co addition,
we cast the Fe 48?x Co x Cr 15Mo 14C 15B 6Y 2?x =0,3,5,7,and 9?alloys into a group of rods with diameters ranging from
3to 16mm.Figure 2shows the XRD patterns of the repre-
sentative alloy samples.As can be clearly seen that,for all
10-mm-diameter samples,only the XRD pattern of the alloy
containing 7%Co exhibits a perfect halo peak,characteristic
of glassy material,indicating the sample of this alloy is
mostly amorphous.In contrast,other samples contain appre-
ciable amount of crystals,evidenced by the presence of sharp
Bragg peaks from the crystals superimposed on a broad halo https://www.wendangku.net/doc/ac16531104.html,bining the DTA and XRD results,we are con-vinced that the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy has the high-est GFA among the serial Fe 48?x Co x Cr 15Mo 14C 15B 6Y 2?x =0,3,5,7,and 9?alloys.It is evident,as shown in Fig.2,that the maximum thickness of the as-cast sample of the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy with mostly amorphous structure can reach at least 16mm,which is,to our knowl-edge,the largest critical thickness for glass formation,com-pared to those of any other Fe-BMGs reported to date.Figure 3presents an image showing the morphology of a rod-like glassy sample of 16mm in diameter,along with the selected area electron diffraction ?SAED ?pattern ?inset ?ob-tained from a 14-mm-diameter glassy sample,of the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy.A crack free glassy sample with a large thickness can be readily fabricated,indicating an excellent manufacturability of the new alloy.The surface of the sample shows metallic lustre and mirrorlike smoothness,which are typical of a BMG casting.The SAED pattern re-veals a full ring,which is inherent of an amorphous phase.This con?rms that the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy with a large sample thickness has a fully glassy structure.As can be seen in Table I,the heat of crystallization for the 16mm sample upon heating is equivalent to that of 3mm sample within experimental error.Assuming the 3mm sample is fully amorphous,it can be thus deduced that the sample with the maximum size also consist of a single amorphous phase,re?ecting that the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy has an ex-ceptionally high GFA.For the base alloy ?x =0?,the maxi-mum thickness achievable in this study is less than 8
mm FIG.1.DTA curves obtained for the as-cast Fe 48?x Co x Cr 15Mo 14C 15B 6Y 2?x =0,3,5,7,and 9?alloys at a heating rate of 0.33K/s,showing ?a ?glass transition and crystallization and ?b ?melting events.The samples are 3mm
in diameter.
TABLE I.Thermal properties,deduced from the DTA measurements at a heating rate of 0.33K/s,compressive fracture strength and Vickers hardness of the as-cast Fe 48?x Co x Cr 15Mo 14C 15B 6Y 2?x =0,3,5,7,and 9?alloys with a different sample size.Alloy
?x ?
Sample diameter ?mm ?T g ?K ?T x
?K ??H 1?J/g ??H 2?J/g ??H 3?J/g ???H a ?J/g ?T m ?K ?T l ?K ??H f ?J/g ?T rg ?f ?MPa ?H v ?Kg mm ?2?0
3839886…………138814642090.573320011463
3834880............13821446...0.577 (5)
3835872............13841442...0.579 (7)
38388759.735.613.358.6138814361700.583350012539
3838888............13891466...0.572 (7)
168388769.933.215.358.413871437…0.583……a 1+?H 2+?H 3.
FIG.2.XRD patterns of the as-cast samples of Fe 48?x Co x Cr 15Mo 14C 15B 6Y 2?x =0,3,5,7,and 9?alloys.
?see Fig.2?,which is slightly smaller than that reported by Ponnambalam et al.15The discrepancy in critical size for the same alloy is likely ascribed to the utilization of the raw material with different purities and the unlike casting condi-tions.
Undercooled melts can be frozen to be glassy as long as the formation of the competing crystalline phases can be sup-pressed.For the base alloy,as the casting size exceeds 6mm,crystalline phases tend to appear due to insuf?cient cooling rate to suppress the formation of the crystals.Addition of the proper amount of Co in the alloy can stabilize the supercooled melt by lowering the liquidus temperature,as shown in Table I,and destabilize the competing crystalline phases,as shown in Fig.2.To further elucidate the dramatic effect of Co addition on the stability of the alloy melt against crystallization on cooling,we estimate the Gibbs free energy difference between the undercooled melts and the corresponding crystalline solids,i.e.,?G
l →s ,for both Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy and base alloy,by using the expression:20
?G l →s =?H f ?T m ?T ?
T m ???H f T m ??T m ?T ??T ln ?T m
T ??,
?1?
where ?H f is the enthalpy of fusion,T the temperature of the undercooled melt,and ?the proportionality coef?cient.By taking ?=0.8for metallic glass forming liquids,20we can obtain the ?G l →s for Fe 41Co 7Cr 15Mo 14C
15B
6Y
2alloy
and base alloy at T =0.8T m to be 1.6and 2.0kJ/mol,respectively.It is apparent that the ?G l →s
for Fe 41Co 7Cr
15Mo 14C
15B 6Y
2alloy is lower than that
for base alloy.The relatively small ?G l →s for Fe 41Co 7Cr
15Mo
14C 15B 6Y 2alloy again provides an indication
that this alloy essentially possesses a stronger glass-forming ability.
From kinetic considerations,bulky glass formation in metallic systems requires a low cooling rate to avoid the nucleation and growth of detectable fraction of crystal in quenching molten alloys.Critical cooling rate is thus ac-cepted as a reliable reference for judging the GFA of BMGs.A low critical cooling rate naturally leads to ease of glass formation.The critical cooling rates for Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy and base alloy can be as-sessed by using the Barandiaran–Colmenero expression 21ln R =ln R c ?b ?T l ?T xc ?2,?2?where R c is the critical cooling rate,b the material constant,T l the liquidus temperature,and T xc the onset solidi?cation temperature of the melt at a cooling rate of R .By inserting the relevant data obtained through DTA measurements into Eq.?2?,the critical cooling rates of the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy and the base alloy are deter-mined to be 6.5and 11.2K/s,respectively.The signi?cantly lower critical cooling rate for Fe 41Co 7Cr 15Mo 14C 15B 6Y 2al-loy con?rms the remarkably improved GFA through alloying with Co in the base alloy.It is worthwhile to notice that the critical cooling rate of the new alloy is comparable to that of the so-called Vitreloy 1alloy ?R c =1.4K/s ?,22which is one of a few best glass formers in metallic systems.Since the mechanical properties of iron-based BMGs are the primary interest for engineering applications,we also measured the compressive strength at a strain rate of 1?10?3s ?1and Vickers hardness under a load of 300g for the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy and the base alloy.The results are also given in Table I.As can be seen the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy exhibits a high fracture strength of 3500MPa and hardness of 1253Kg mm ?2which are two to three times those of high strength steel.Moreover,the Co addition is also effective in further improving the mechanical properties of Fe-based BMGs.The enhancement in fracture strength and hardness is interpreted to result from the stronger atomic bonding nature among the constituent elements due to substitution of Co for Fe.16The unusual combination of high strength and hardness is consistent with those data reported in Refs.1and 12–16.1A.Inoue,Acta Mater.48,279?2000?.2A.Peker and W.L.Johnson,Appl.Phys.Lett.63,2342?1993?.3L.Q.Xing,P.Ochin,M.Harmelin,F.Faudot,and J.Bigot,J.Non-Cryst.Solids 205–207,597?1996?.4Y .Li,H.Y .Liu,and H.Jones,J.Mater.Sci.31,1957?1996?.5H.Ma,J.Xu,and E.Ma,Appl.Phys.Lett.83,2793?2003?.6H.Men and D.H.Kim,J.Mater.Res.18,1502?2003?.7X.K.Xi,R.J.Wang,D.Q.Zhao,M.X.Pan,and W.H.Wang,J.Non-Cryst.Solids 344,105?2004?.8X.H.Lin and W.L.Johnson,J.Appl.Phys.78,6514?1995?.9G.He,J.Eckert,and M.Hagiwara,J.Appl.Phys.95,1816?2004?.10S.Yi,T.G.Park,and D.H.Kim,J.Mater.Res.15,2425?2000?.11H.Choi-Yim,D.H.Xu,and W.L.Johnson,Appl.Phys.Lett.82,1030?2003?.12T.D.Shen and R.B.Schwarz,Appl.Phys.Lett.75,49?1999?.13Z.P.Lu,C.T.Liu,and W.D.Porter,Appl.Phys.Lett.83,2581?2003?.14Z.P.Lu,C.T.Liu,J.R.Thompson,and W.D.Porter,Phys.Rev.Lett.92,245503?2004?.15V .Ponnambalam,S.J.Poon,and G.J.Shi?et,J.Mater.Res.19,1320?2004?.16A.Inoue,B.L.Shen,and C.T.Chang,Acta Mater.52,4093?2004?.17T.Zhang,T.Yamamoto,and A.Inoue,Mater.Trans.,JIM 43,3222?2002?.18D.H.Xu,G.Duan,and W.L.Johnson,Phys.Rev.Lett.92,245504?2004?.19Y .Zhang,H.Tan,and Y .Li,Mater.Sci.Eng.,A 375–377,436?2004?.20F.Q.Guo,S.J.Poon,and G.J.Shi?et,Appl.Phys.Lett.83,2575?2003?.21J.M.Barandiaran and J.Colmenero,J.Non-Cryst.Solids 46,277?1981?.22T.A.Waniuk,J.Schroers,and W.L.Johnson,Appl.Phys.Lett.78,1213
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FIG.3.Image showing a 16-mm-diameter glassy sample and the selected area electron diffraction pattern ?inset ?obtained from a 14-mm-diameter glassy sample of the Fe 41Co 7Cr 15Mo 14C 15B 6Y 2alloy.