00, Intermetallic phase selection in 1XXX Al alloys

Intermetallic phase selection in 1XXX Al alloys
C.M.Allen a,*,K.A.Q.O'Reilly a ,B.Cantor a ,P.V.Evans b a Oxford Centre for Advanced Materials and Composites,Department of Materials,University of Oxford,Parks Road,Oxford,OX13PH,UK b Alcan International Limited,Banbury Laboratory,Southam Road,Banbury,Oxon,OX167SP,UK
Accepted 1October 1998
CONTENTS
1.INTRODUCTION 90
2.BINARY Al±Fe PHASES 922.1.The equilibrium Al±Fe 4AL 13eutectic 922.2.Metastable Al±Fe eutectic phases 932.2.1.Metastable Al±FeAl 6eutectic 952.2.2.Metastable Al±FeAl m eutectic 962.2.
3.Metastable Al±FeAl x eutectics 972.2.
4.Metastable Al±Fe 2Al 9eutectic 1012.2.
5.Metastable Al±FeAl p eutectic 1012.2.
6.The e ect of Si addition on the formation of Al±Fe phases 1013.TERNARY Al±Fe±Si PHASES 1023.1.The equilibrium a -AlFeSi and b -AlFeSi phases 1023.2.Metastable a -AlFeSi and b -AlFeSi phases 1053.2.1.Metastable cubic a -AlFeSi phase 1053.2.2.Metastable a v -AlFeSi phase 1063.2.3.Metastable a 0or q 1-AlFeSi phase 1063.2.4.Metastable a T -AlFeSi phase 1063.2.5.Metastable q 2-AlFeSi phase 1093.2.6.Metastable b H -AlFeSi phase 1114.FACTORS GOVERNING PHASE SELECTION IN 1XXX ALLOYS petitive growth 1124.1.1.Transition from Fe 4Al 13to FeAl 61124.1.2.Transition from Fe 4Al 13to FeAl x 1184.1.3.Transition from FeAL 6to FeAl m 1184.1.4.Transitions in Al±Fe±Si alloys petitive nucleation 1204.2.1.The transition from Fe 4Al 13to FeAl 61204.2.2.Promotion of nucleation of other phases 1224.3.Suppression of equilibrium solidi®cation reactions 1234.4.Metastable phase diagrams and solidi®cation microstructure selection maps 124Progress in Materials Science 43(1998)89±170
0079-6425/99/$-see front matter #1999Elsevier Science Ltd.All rights reserved.PII:S 0079-6425(98)00003-
6PERGAMON *Corresponding author:Tel.:+44-1865-273774;fax:+44-1865-273764;e-mail:chris.allen@.
5.FIR TREE FORMATION IN DC CASTS 1275.1.Fir tree zones 1275.2.Cooling rate 1285.3.Fir tree nucleation 1305.4.Fir tree nucleation and growth 1325.5.E ect of solid fraction 135
6.FIR TREE PHASES IN DC CASTS 137
7.TRANSFORMATION OF METASTABLE PHASES 1407.1.The FeAl 6À4Fe 4Al 13transformation 1417.1.1.Transformation mechanism and activation energy 1417.1.1.1.Dissolution±precipitation mechanism and net activation energy 1417.1.1.2.Formation of acicular Fe 4Al 13precipitates 1437.1.1.3.Continuous heating transformation 1437.1.1.4.Two step ageing 1457.1.1.5.Isothermal transformation 1487.1.2.Transformation rate 1517.1.2.1.Microstructural scale 1517.1.2.2.E ect of cold work 1547.1.2.3.Presence of pre-existing nuclei 1547.2.The FeAl m À4Fe 4Al 13transformation 1567.3.E ect of Si on transformations of metastable Al±Fe phases 1577.4.Transformations involving ternary Al±Fe±Si phases 158
8.EFFECT OF IMPURITIES ON PHASE FORMATION IN Al±Fe AND Al±Fe±Si ALLOYS 1599.EFFECT OF GRAIN REFINER ADDITIONS ON Al±Fe AND Al±Fe±Si ALLOYS 1649.1.Proposed mechanisms of primary Al grain re®nement 1649.2.The ro Ãle of grain re®ners on secondary/ternary phase selection 16610.SUMMARY 167REFERENCES
1681.Introduction
Flat rolled aluminium products account for approximately 40%of the 24million tonnes annual world production of aluminium.These products are commonly used for packaging and canning,in electrical applications (e.g.capacitor electrodes),architectural cladding,cable wrap,lithographic printing and automotive sheet.About 90%of ¯at rolled products are produced from the melt by the following manufacturing route:the melt is degassed,®ltered and grain re®ned,then direct chill (DC)cast into rectangular or cylindrical water cooled ring moulds with removable bases.Fig.1shows a schematic of the DC casting process.The removable bases are withdrawn at a controlled rate as the metal solidi®es,resulting in the semicontinuous casting of rectangular ingots or cylindrical billets,typically 0.5±1m in diameter and 5±10m in length.The cast surface is often uneven,and the outermost H 0.2m of the cast in from surface is often of a coarser grain structure than the interior and can contain higher levels of segregates.The cast surface is commonly scalped o therefore,as discussed in Section 5.1,and the remainder heat treated in the temperature range 450±6008C,in the form of a pre-heat in order to e ect microstructural homogenization prior to rolling.Homogenization reduces segregation,encourages the transformation of metastable secondary and ternary phases into equilibrium phases,and acts to equilibrate solid
C.M.Allen et al./Progress in Materials Science 43(1998)89±17090
solution levels of soluble elements,resulting in certain cases in the precipitation of dispersoids.A series of both hot and cold rolls with intermediate annealing treatments are then applied to produce a foil or sheet of the required ®nal gauge,typically in the range 6±150m m (foil)or 150±3000m m (sheet),which is then commonly subjected to a ®nal anneal.A range of di erent aluminium alloys are DC cast and processed by the above route.The exact compositions depend upon the ®nal application of the casting,but Cu,Zn,Mg,Mn,Si and Fe are common alloying additions.The alloys that are the subject of this review are those designated AA1xxx by the International Alloy Designation System (IADS).Commercial 1xxx series Al alloys contain typically 0.5wt%Fe and 0.2wt%Si,sometimes present as deliberate alloying additions,but also as impurities.Other common impurities are Cu,Cr,Mn,Mg,V and Zn.Al±Ti±B additions are frequently used to promote primary Al grain
re®nement.
Fig.1.Schematic of the vertical DC casting process (after Maggs).
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The identity,size and distribution of the secondary and ternary inter-metallic phases are critical in¯uences on the material properties of the alloy [74],including strength,toughness,formability,fatigue resistance,corrosion resistance and anodizing response [58].Anodizing quality and etching response are especially important in surface critical products such as lithographic printing sheet,as well as in sheet used in architectural applications.The solid solution content is particularly important in controlling properties such as electrical conductivity and recrystallization characteristics.Thermodynamic consider-ations often fail to predict correctly the phase content and solid solution content of the as-cast microstructure because of the non-equilibrium nature of solidi®cation during DC casting.The key alloy properties are controlled by solid solution levels and secondary and ternary phase crystallography and morphology,which in turn are dependent on complex kinetic competitions for nucleation and growth.In Sections 2and 3the wide range of both equilibrium and metastable secondary Al±Fe and ternary Al±Fe±Si phases reported in 1xxx alloys are examined.
2.Binary Al±Fe phases
The maximum equilibrium solid solubility of Fe in Al is very low,at H 0.05wt%Fe,and Fe is usually present therefore in the form of secondary Fe aluminide phases [74].The maximum equilibrium solid solubility of Si in Al is higher at H 1.6wt%,and low levels (H 0.1±0.2wt%)of Si in the bulk are readily accommodated therefore by dissolution in the Al matrix and in the Fe aluminides.Consequently,the phase contents of DC cast Al±Fe and Al±Fe±Si alloys with 0.1wt%Si are similar,although in the latter case the so called `binary'Fe aluminides often contain dissolved Si.Ternary Al±Fe±Si phases,as reviewed in Section 3,only form at higher bulk concentrations of Si,typically >0.1wt%Si in 0.2wt%Fe containing alloys,and >0.2wt%Si in 0.3±0.4wt%Fe containing alloys.
2.1.The equilibrium Al±Fe 4Al 13eutectic
Fig.2shows the equilibrium Al±Fe binary phase diagram.As shown in Fig.2,the ®rst secondary phase to form on solidi®cation of dilute Al±Fe alloys under equilibrium conditions is given by the eutectic reaction;
Liquid À4a ÀAl Fe 4Al 13 also denoted as FeAl 3Y or the y phase
The exact temperature and composition of the invariant point is of some debate,but Liang and Jones [65]have recently reported 655.120.18C at 1.8wt%Fe,respectively.The eutectic temperature of 655.18C has subsequently been con®rmed
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by Allen [4]using calorimetric methods.At the eutectic temperature the a -Al matrix has the maximum equilibrium solid solubility of Fe,at H 0.05wt%[74].The equilibrium secondary phase exists over a range of compositions,and is often denoted as having the stoichiometry of either FeAl 3or Fe 4Al 13.Black [15]determined from X-ray di raction studies that Fe 4Al 13has a c-face centred monoclinic structure containing 100atoms per unit cell [74].Fig.3a and b shows a typical TEM micrograph of needle shaped Fe 4Al 13particles at the grain boundaries in a DC cast ingot and a typical [100]zone axis selected area di raction pattern (SADP)from a crystal of Fe 4Al 13extracted from the Al matrix,respectively.Fe 4Al 13commonly forms relatively large angular precipitates in as-cast microstructures (Fig.3a),which increase hardness but lead to embrittlement,reducing formability and fatigue resistance.As shown in Fig.3b,Fe 4Al 13exhibits spot streaking in certain zone axis di raction patterns parallel to the (00l)reciprocal lattice vector,although it is not clear whether this is due to stacking faults or microtwinning on (001)[96,97].Fe 4Al 13can also form pseudo 10-fold twins,resulting from alternate repetition of (100)and (201)twins [56,57].Fig.4shows a bright ®eld TEM micrograph of a 10-fold branched dendritic particle present in a melt-spun Al±20at%Fe alloy.
2.2.Metastable Al±Fe eutectic phases
As long ago as 1925Dix [2]noted that fully eutectic microstructures could be attained in rapidly cooled alloys of Fe content well in excess of that of the equilibrium eutectic,1.8wt%.This indicated that large undercoolings are required to nucleate and/or grow the Al±Fe 4Al 13eutectic under certain
solidi®cation
Fig.2.Al rich corner of the equilibrium Al±Fe binary phase diagram.
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conditions.The nucleation and growth requirements of both the Al±Fe 4Al 13and metastable Al±Fe eutectics is discussed in further detail in Section 4.2and 4.1,respectively.Under non-equilibrium solidi®cation conditions a range
of
Fig.3.(a)Fe 4Al 13at grain boundaries in cast ingot.After Skjerpe [97].Reproduced by kind permission of Blackwell Science Ltd.(b).Typical [110]di raction pattern of a faulted Fe 4Al 13crystal.Faults on {001}planes produce lines parallel to the {001}direction in reciprocal space.After Skjerpe [96].Reproduced from Metallurgical and Materials Transactions by kind permission of TMS and ASM international.
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94
thermodynamically metastable Al±Fe eutectic phases that have smaller undercoolings for nucleation and growth than Al±Fe 4Al 13can form in addition to Al±Fe 4Al 13.These are summarized in Table 1.The composition ranges of some of the metastable Al±Fe eutectics are shown in Fig.5.
2.2.1.Metastable Al±FeAl 6eutectic Hollingsworth et al.[42]were the ®rst to identify one of the metastable Al±Fe eutectics displacing Al±Fe 4Al 1
3.They observed the displacement of Al±Fe 4Al 13by Al±FeAl 6in continuously cast Al±2wt%Fe.The exact solidi®cation conditions (cooling rate and solidi®cation velocity)at which this displacement occurs have since been characterized by Ba ckerud [12],Adam et al.[1,2],Hughes and Jones [45],Liang and Jones [85],Gilgien et al.[35],Evans et al.[32]and Thomas et al.[105]and will be discussed further in Section
4.Liang and Jones [65]report the eutectic temperature as 652.920.28C,with a eutectic composition of 3.0wt%Fe.The crystal structure is c-face centred orthorhombic [96],with 28atoms per unit cell [74].FeAl 6is a common constituent of DC cast ingots and billets [112].Fig.6a and b show a typical TEM micrograph of FeAl 6eutectic embedded in an Al matrix and the corresponding [110]zone axis SADP.FeAl 6is also an important phase in Mn-containing alloys.MnAl 6and FeAl 6are isomorphous,and consequently Mn can substitute freely for Fe in the FeAl 6lattice,lowering its free energy.This raises the thermodynamic stability of the FeAl 6phase in Mn containing Al alloys (see Section 8and Table
6).
Fig.4.TEM micrograph of 10-fold branched dendritic Fe 4Al 13particle.After Kim and Cantor [56,57].Reproduced from Phil.Mag.A by kind permission of Taylor and Francis Ltd.
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2.2.2.Metastable Al±FeAl m eutectic Kosuge and co-workers [58]report that a metastable Al±FeAl m eutectic appears at higher cooling rates (e.g.>10K s À1in wedge-shaped moulds of Al±0.6wt%Fe)than those at which Fe 4Al 13and FeAl 6form.This phase has also been observed in the more rapidly cooled zones of DC cast billets [96,112].Fig.7a and b shows a typical TEM micrograph of a dendritic like FeAl m particle extracted from the Al matrix and the corresponding [110]zone axis SADP,respectively.As shown in Fig.7b,FeAl m exhibits incommensurate re¯ections in certain zone axis di raction patterns parallel to the (hh0)reciprocal lattice vector,due to stacking faults on (110)planes [96,98].The eutectic temperature and composition for FeAl m in Al±Fe binary alloys have not been determined.Allen et al.[5]have determined a eutectic temperature of 649.58C for FeAl m in Al±0.3wt%Fe-0.1wt%Si±0.05wt%V,1.7K lower than the eutectic temperature of 651.28C for Fe 4Al 13measured in the same alloy.The FeAl m phase exists over a range of compositions,with the value of m quoted in the range from 4.0to 4.4.The crystal structure is body centred tetragonal [96],the unit cell containing in the region of 110to 118atoms [98].
Table 1Al±Fe phases formed in dilute Al±Fe alloys
Phase
Bravais lattice Lattice parameters References Fe 4Al 13c-Centred monoclinic a =15.49A ,Skjerpe [96,97]b =8.08A ,c =12.48A ,b =107.758
FeAl 6c-Centred orthorhombic a =6.49A ,b =7.44A c =8.79A
Hollingsworth et al.[42],Ba ckerud [12],Jones [51],Adam and Hogan [1],Hughes and Jones [45]FeAl x (I)c-Centred orthorhombic a I 6A ,b I 7A ,Westengen [112],Skjerpe [96](x I 5.7±5.8)c I 4.7A
FeAl x ??
Young and Clyne [113](x I 5),Evans et al.[31](x I 4.5),Wang et al.[111]FeAl m Body centred tetragonal a =8.84A ,b =c =31.6A
Young and Clyne [113],Westengen [112],Skjerpe [96],Skjerpe [98](m I 4.0±4.4)Fe 2Al 9Monoclinic a =8.90A ,b =6.35A ,Simensen and Vellasamy [94],Brobak and Brusethaug [16],Griger et al.[38]c =6.32A ,b =93.48
FeAl p Body centred cubic a =b =c =10.3A Ping et al.[80]
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96
2.2.
3.Metastable Al±FeAl x eutectics Westengen [112]discovered another Al±Fe eutectic in DC cast material,denoted Al±FeAl x ,determining from EDX a stoichiometry of x I 5.8,with traces (<1wt%)of Si and Ni also being detected.Westengen was unable to determine the crystal structure of FeAl x from the irregular nature of its di raction patterns,and suggested that it was heavily faulted.Skjerpe also detected this phase,with a similar stoichiometry of x I 5.7,and containing 1.9wt%Si and 0.3wt%Ni.Fig.8a and b show a typical TEM micrograph of an FeAl x particle and a typically irregular SADP,respectively [96,97].Skjerpe indexed the strongest intensity spots of his di raction patterns to ®t a c-face centred orthorhombic unit cell of cell parameters very similar to that of FeAl 6(Table 1).HREM lattice imaging of FeAl x revealed that the di raction patterns arise from a complex stacking sequence in real space.Given also the similarity in stoichiometries (Fig.5),Skjerpe suggested that FeAl x was a Si modi®ed version of FeAl 6.The eutectic temperature and composition of this phase have not been reported.Another phase also denoted FeAl x has been reported by Young and Clyne [113].Young and Clyne determined x I 5from EDX data,and tentatively proposed a monoclinic crystal structure to ®t the XRD data they obtained from this phase.If this structure is correct then this is not therefore the same FeAl x as observed by Westengen and Skjerpe.Young and Clyne's FeAl x displaced Fe 4Al 13at cooling rates below that of FeAl 6during unidirectional solidi®cation experiments.Evans et al.[31]have similarly observed produced FeAl x at cooling rates intermediate between Fe 4Al 13and FeAl 6during unidirectional solidi®cation,with a value
of
positions of the common binary and ternary compounds found in dilute Al±Fe±Si alloys.After Langsrud [61].
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Fig.6.(a)FeAl 6.After Westengen [112].Reproduced by kind permission of Carl Hauser Verlag,Munich,Germany.(b)[110]zone axis selected area di raction pattern of FeAl 6.After Westengen [112].Reproduced by kind permission of Carl Hauser Verlag,Munich,Germany.C.M.Allen et al./Progress in Materials Science 43(1998)89±17098
x I 4.5.The XRD trace from this phase could not be made to ®t the monoclinic structure proposed by Young and Clyne however.Wang et al.[111]have since produced a fully eutectic microstructure of Al±FeAl x in Al±3wt%Fe±0.1wt%V alloys directionally solidi®ed at velocities in the range 0.09±1.03mm s À1,with the same XRD trace as Evans's FeAl x ,and have shown that Young and Clyne's structure determination was incorrect [33].The FeAl x eutectics produced by Young and Clyne,Evans et al.and Wang et al.are the same phase therefore,with x I 4.5±5.0,and in turn are di erent to the FeAl x eutectics produced by Westengen and Skjerpe,with x I
5.7±5.8.
Fig.7.FeAl m and corresponding [110]di raction pattern.Stacking faults on {hh0}planes lead to incommensurate re¯ections along the {hh0}direction in reciprocal space.After Skjerpe [96].Reproduced from Metallurgical and Materials Transactions by kind permission of TMS and ASM International.
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Fig.8.(a)FeAl x .After Skjerpe [96].Reproduced from Metallurgical and Materials Transactions by kind permission of TMS and ASM International.(b)Di raction pattern from FeAl x showing incommensurate nature of re¯ections.After Skjerpe [96].Reproduced from Metallurgical and Materials Transactions by kind permission of TMS and ASM International.
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2.2.4.Metastable Al±Fe 2Al 9eutectic Fig.9shows a typical TEM micrograph of Fe 2Al 9eutectic embedded in an Al matrix,obtained in strip cast Al±0.5wt%Fe±0.2wt%Si alloy [94].The stoichiometry of this phase was determined by EDX.Analysis of electron di raction patterns indicated a monoclinic crystal structure.The solidi®cation conditions under which this phase form are unclear.Tezuka and Kamio [104]noted that in DC cast Al±0.5wt%Fe,additions of >0.075wt%Co promoted the formation of the (Fe,Co)2Al 9phase.Fe 2Al 9and Co 2Al 9are isomorphous,and consequently Co can substitute freely for Fe in the Fe 2Al 9lattice,lowering its free energy.This raises the thermodynamic stability of the Fe 2Al 9phase in Co containing Al alloys (see Section 8and Table 6).
2.2.5.Metastable Al±FeAl p eutectic Ping et al.[80]reported the formation of a metastable body centred cubic phase FeAl p (where p I 4.5)in directionally chill cast Al±(0.25±0.50)wt%Fe±0.125wt%Si.This phase has yet to be observed independently.
2.2.6.The e ect of Si addition on the formation of Al±Fe phases As stated in Section 2,small quantities of Si (typically <0.1wt%Si bulk composition in 0.2wt%Fe containing alloys,and 0.2wt%Si in 0.3±0.4wt%Fe containing alloys)can be dissolved into the `binary'Fe aluminides.However,these aluminides have di erent degrees of Si solubility (Fig.5).Consequently,
the Fig.9.Fe 2Al 9.After Simensen and Vellesamy [94].Reproduced by kind permission of Carl Hauser Verlag,Munich,Germany.
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occurrence of FeAl 6which can only dissolve up to 0.5wt%Si in its lattice is restricted in Al±Fe±Si alloys.FeAl 6is replaced by Fe aluminides that can incorporate Si,such as Fe 4Al 13or FeAl m [61].
3.Ternary Al±Fe±Si phases
3.1.The equilibrium a -AlFeSi and b -AlFeSi phases
Three ternary phases form under equilibrium solidi®cation conditions in dilute Al±Fe±Si alloys of su ciently high bulk Si content,>0.1wt%Si in 0.2wt%Fe containing alloys,and >0.2wt%Si in 0.3±0.4wt%Fe containing alloys,at temperatures below that of the liquid 4Al +Fe 4Al 13eutectic reaction.Fig.10shows the liquidus projection and associated equilibrium solidi®cation reactions in the Al corner of the Al±Fe±Si ternary phase diagram.The three equilibrium ternary phases produced by either of two ternary peritectic reactions followed by a ternary eutectic reaction are;i.Liquid +Fe 4Al 13À4Al +Fe 2SiAl 8(also denoted as the a phase);ii.Liquid +Fe 2SiAl 8À4Al +FeSiAl 5(also denoted as the b phase);and/or iii.Liquid À4Al +Si +FeSiAl 5.A range of temperatures have been measured for these three invariant points in the ternary phase diagram:620±6388C for the a peritectic,611±6158C for the b peritectic and 576±5778C for the ternary eutectic [14,74,85].These ranges may re¯ect a di culty in nucleating or growing one or more of these phases during solidi®cation.This point is discussed in Section 4.3.Both phases exist over a range of compositions,as shown in Fig.5.The accepted stoichiometries as given above are those of Mondolfo [74].Both the a and b phases can adopt a number of di erent crystal structures.Table 2summarizes the structural variants of these two phases.Munson [75]determined from X-ray di raction studies that a -AlFeSi has a hexagonal crystal structure (Table 2),in agreement with earlier single crystal studies performed by Robinson and Black [86],and this was con®rmed in the same year by Sun and Mondolfo [102].The hexagonal crystal structure of a is also known in the literature as a H [86]or a 2[9,10].Fig.11a and b show a typical TEM micrograph of an a H particle in DC cast 1050alloy and corresponding [100]zone axis SADP,respectively [112].Dons [29]observed that a H survived heat treating in >99.9wt%pure Al based DC cast commercial purity alloys,to progressively higher temperatures with increasing bulk Si content of the alloy,from 4508C in Al±0.6wt%Fe±0.15wt%Si,to 6008C in Al±0.6wt%Fe±0.6wt%Si.Phragmen [79]determined that b -AlFeSi has a monoclinic crystal structure (Table 2).b -AlFeSi is an important ternary phase in wrought aluminium alloys.Fig.12shows a typical SEM micrograph of b platelets on a deep etched surface of a DC cast alloy,showing their characteristic long thin curving morphology,which can dramatically reduce ductility.
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Table 2Common structural variants of the ternary phases Fe 2SiAl 8(a )and FeSiAl 5(b )
Phase
Bravail lattice Lattice parameters References a (a 1)
Cubic a =12.56A (Im3)a =12.52A (Pm3)Cooper [23],Munson [75],Sun and Mondolfo [102],Westengen [112],Griger et al.[37],Turmezey et al.[109]a H (a 2)
Hexagonal a =b =12.3A c =26.2A Dons [29],Munson [75],sun and Mondolfo [102],Griger et al.[37],Thoresen et al.[106]a v Monoclinic a =8.90A ,Dons [29]b =6.35A ,c =6.32A ,b =93.48a 0(q 1)c-Centred orthorhombic a =12.7A ,b =26.2A Westengen [112],Skjerpe [96],Ping et al.[80],Ping [82]c =12.7A q 2Monoclinic a =12.50A ,Ping et al.[80]b =12.30A ,c =19.70A ,b =1118a T c-Centred monoclinic a =27.95A ,b =30.62A ,Dons [29],Skjerpe [96],Jensen and Wyss [50],Turmezey et al.[109],Ping [82]c =20.73A ,b =97.748b Monoclinic a =6.12A Skjerpe [96]b =6.12A ,c =41.5A ,b =918b H Monoclinic a =8.9A ,Westengen [112],Skjerpe [96]
b =4.9A ,
c =41.6A ,b =92
8Fig.10.Liquidus surface and associated equilibrium phase ®elds in the Al corner of the ternary Al±Fe±Si phase diagram.After Skjerpe [96].Reproduced from Metallurgical and Materials Transactions by kind permission of TMS and ASM International.
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103
Fig.11.(a)Hexagonal a H Al±Fe±Si.After Westengen [112].Reproduced by kind permission of Carl Hauser Verlag,Munich,Germany.(b)[100]zone axis selected area di raction pattern from a H .After Westengen [112].Reproduced by kind permission of Carl Hauser Verlag,Munich,Germany.
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104
3.2.Metastable a -AlFeSi and b -AlFeSi phases
Under non-equilibrium solidi®cation conditions,the liquid becomes enriched in Si,due to partitioning of the Si to the interdendritic liquid,and hence there is a greater tendency for ternary phases to form [61].In addition,a number of metastable structural variants of both the a -and b -AlFeSi ternary phases are commonly observed in commercial DC cast alloys,and are summarized in Table 2.
3.2.1.Metastable cubic a -AlFeSi phase Munson [75]and Sun and Mondolfo [102]determined that the equilibrium hexagonal form of a -AlFeSi was only thermodynamically stable in high purity Al±Fe±Si alloys.Additions of V,Cr,Mn,Cu,Mo and W all promote a body-centred cubic structure for the a -AlFeSi phase,also known in the literature as c [79],a 1[9,10]or simply a [78].Additions of Ti,Ni,Zn and Mg do not promote the cubic structure [102].The cubic structure had also been previously observed by Phragmen [79]and Cooper [23],but had incorrectly been assumed to represent the equilibrium crystal structure for the a -AlFeSi phase.The cubic structure is isostructural with a -AlMnSi.Only 0.1wt%Mn is required therefore to stabilize the cubic form during solidi®cation at a cooling rate of 0.75K min À1[75].The stabilization of the cubic form by trace elements common to commercial purity alloys results in the cubic form being the one which is most commonly observed in commercial alloys.Westengen [112]observed cubic a in DC cast AA1050alloy in the more rapidly cooled outer zone of the billet.Weak h +k +l =odd integer spots were observed in the di raction patterns,indicating that the structure may not have been body centred but primitive cubic.Dons [29]observed cubic a in both as-cast and heat-treated DC cast commercial
purity Fig.12.SEM micrograph of b Al±Fe±Si platelets,in surface etched DC cast alloy.After Griger et al.[37].Reproduced by kind permission of Aluminium .
C.M.Allen et al./Progress in Materials Science 43(1998)89±170105
aluminium of Fe/Si ratio <1,which survived to progressively higher heat treatment temperatures with increasing bulk Si content.Fig.13a and b shows a typical TEM micrograph of a cubic a particle extracted from a DC cast Al±0.25wt%Fe±0.13wt%Si alloy,with a partly dendritic morphology,and a corresponding [111]zone axis SADP,respectively [97].Skjerpe observed cubic a in the more rapidly cooled outer 50mm of the billet.Griger et al.[37]observed cubic a in semicontinuously cast Al±0.5wt%Fe±0.2wt%Si,across the entire cross-section of the billet.Turmezey et al.[109]observed that the Si content of the cubic a phase was directly proportional to the bulk Si content,suggesting that direct Al t Si substitution can take place in the cubic a lattice.Thoresen et al.[106]investigated Al±4wt%(Fe,Mn)±7.5wt%Si alloys,with varying Fe:Mn ratios,in which the primary phase was a -AlFeSi,either in its cubic or hexagonal form,depending upon the bulk Mn content of the alloy [75].The total transition metal content (Fe +Mn)of the cubic a phase in the Al±(Fe,Mn)±Si alloy was less than the cubic a phase in the Al±Mn±Si alloy,indicating that vacancies stabilise the cubic a structure when both Fe and Mn are present.
3.2.2.Metastable a v -AlFeSi phase Dons [29]observed a monoclinic structural variant of a -AlFeSi in DC cast commercial purity Al±0.2wt%Fe±0.2wt%Si,denoted a v .Dons stated that a v was structurally related to the Fe 2Al 9phase [94],the a-axis being 2.6%shorter and the c-axis 3.6%shorter than the monoclinic structure of Fe 2Al 9.However,the Si content of a v was in the range
4.5±10.5wt%(corresponding to the Si content range typically observed in a H and a ),signi®cantly higher than the maximum Si content of H 2wt%seen in Fe 2Al 9.
3.2.3.Metastable a 0or q 1-AlFeSi phase Fig.14a and b show a typical TEM micrograph of a 0particles embedded in a DC cast AA1050alloy and a corresponding [100]zone axis SADP,respectively [112].Westengen observed that a 0was closely related to the cubic a form,indexing the a 0di raction patterns as originating from a tetragonal unit cell.EDX data showed that a 0had a lower Si content than cubic a .Westengen therefore suggested that a 0was a low Si modi®cation of cubic a ,as illustrated in Fig. 5.Fig.15summarizes the subsequent EDX measurements made by Skjerpe [96]on particles in DC cast Al±0.25wt%Fe±0.13wt%Si,which supported the idea that a 0was a low Si modi®cation of cubic a .Ping et al.[80]also observed the a 0phase in DC cast Al±0.28wt%Fe±0.13wt%Si,but denoted it q 1,which formed at a cooling rate of H 10K s À1.Detailed convergent beam electron di raction analysis by Ping and co-workers [80,81]of a 0revealed a c-face centred orthorhombic structure,which was also con®rmed by [96].
3.2.
4.Metastable a T -AlFeSi phase Dons [29]observed a further structural variant of a -AlFeSi in DC cast commercial purity Al±0.2wt%Fe±0.2wt%Si,denoted a T ,whose crystal
C.M.Allen et al./Progress in Materials Science 43(1998)89±170106。