intermetallics-aluminides-silicides

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Intermetallics: Crystal Structures

The search for new high-temperature structuralmaterials has stimulated much interest in ordered in-termetallic compounds (Westbrook and Fleischer1995, Williams 1997). Alloys based on the orderedintermetallic compounds constitute a unique class ofmetallic material that form a long-range orderedcrystal structure (as opposed to a random solidsolution) below a certain critical temperature, gener-ally referred to as the critical ordering temperature(Tc). For most of the aluminides of nickel, iron, andtitanium this Tc temperature is close to (usuallyabove) the melting temperature of an intermetallic. Anotable exception is Ti3Al, which disorders at 1100 1Cbut has a melting point around 16001C. Iron alum-inide of the Fe3Al type passes through two orderedstructures (D03 and B2) before becoming disordered(Vedula 1995). It must be pointed out that a perfectordered crystallographic structure of an intermetalliccompound exists only at the exact stoichiometry cor-responding to its stoichiometric formula. However,quite a number of intermetallics, including the alu-minides of nickel, iron, and titanium, exist over arange of compositions, but the degree of order de-creases as the deviation from stoichiometry increases.Additional alloying elements can be incorporated

without losing the ordered crystallographic struc-tures. The possibility of alloying the ordered struc-tures of intermetallic compounds opens up newavenues for development of novel high-temperaturealloys to improve or optimize properties for specificapplications. Therefore, a thorough understanding ofhow atoms are arranged in an ordered fashion in thecrystallographic lattices of intermetallic compoundsis a major prerequisite for further development ofnovel intermetallic-based high-temperature alloysand understanding their mechanical, physical, andchemical properties. This article presents a compre-hensive view of the crystal structures of intermetallic

compounds, which have some potential as structuralmaterials for commercial applications, mainly atelevated temperatures and hostile environments.

Ordered intermetallics based on aluminides andsilicides constitute a major group of intermetallics,

which has been extensively researched since the early1990s. Another important group of intermetallics arethe Laves phases. They represent a huge group ofbinary intermetallic compounds, many of which havegood combinations of high melting point, low den-sity, and good oxidation resistance. Many ternaryelements and, in particular, refractory elements, havelarge solid solubility in Laves phases for a potentialimprovement of mechanical properties. Last but notleast, the crystal structures of some non-commercialintermetallic compounds (important from the stand-point of fundamental studies) will also be discussed.Traditionally, the crystallographic lattices of inter-metallic compounds have been quoted according to

the Strukturbericht designation. However, becausethis system cannot be conveniently and systemati-cally expanded to cover the large variety of crystalstructures currently being encountered, the systemhas gradually fallen into disuse. Nowadays, the sys-tem ofPearson symbols (Villars and Calvert 1991) hasbecome more widely used. However, because theStrukturbericht designations are still quite popular, inthe present article each crystallographic lattice ofan intermetallic compound will be distinguished bythe Strukturbericht designation and accompanied bythe corresponding Pearson symbol in parentheses.

1. Aluminides and Silicides

1.1 Generic Cubic Crystal Structures

Figure 1 shows hard-sphere models of the genericcrystallographic unit cells of cubic aluminides andsilicides. The term generic means that these crystalstructures occur in aluminides and silicides naturally,for example upon congruent solidification from themelt without any change in composition.

Intermetallic compounds such as Ni3Al, Zr3Al, andNi3Si have an L12 (cP4) crystal structure (Fig. 1(a)), aderivative of the face-centered cubic (fcc) crystal

structure. This unit cell contains four atoms, i.e., thesame number as fcc. The nickel (Ni3Al, Ni3Si) or zir-conium (Zr3Al) atoms occupy face-centered positionsand the aluminum or silicon atoms occupy the cor-ners of the unit cell. Both NiAl and FeAl possess a B2(cP2) crystal structure a derivative of the body-cen-tered cubic (bcc) crystal structure (Fig. 1(b)). The unitcell contains two atoms, i.e., the same number as bcc.In reality, it consists of two interpenetrating primitivecubic cells and is sometimes referred to as the CsCltype. The aluminum atoms occupy the cube cornersof one sublattice and the nickel atoms occupy thecube corners of the second sublattice.



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Silicides such as Mg2Si, Co2Si, and Ni2Si possess amore complex C1 (cF12) crystal structure with 12atoms per unit cell (Fig. 1(c)). The archetype of thisstructure is the ionic ceramic compound CaF2 (cal-cium fluorite). This ionic structure can be viewed asbuilt on an fcc lattice of small cations (Ca2 ) witheight bigger anions (F) forming a simple cubicsublattice (at 1

414

14

etc. positions) inside the fccunit cell. There are (4Ca2 8F) ions per unit cell.

Such a crystal structure is termed a fluorite structure.If in some ionic compounds the fcc lattice is built onlarge anions, and smaller cations are located insidethe fcc cell, then the C1 crystal structure is termed ananti-fluorite structure. From the point of view ofatomic size, Mg2Si (and its companion intermetallicsMg2Ge, Mg2Sn, and Mg2Pb) is regarded as fluoritetype whereas CoSi2 and NiSi2 are regarded as anti-fluorite type.

The A15 (cP8) cubic structure with A3B stoic-hiometry (Cr3Si and Nb3Al) and eight atoms per unitcell consists of orthogonal chains of A (Cr or Nb)atoms along /100S directions criss-crossing the bcc

arrangement of B (Si or Al) atoms (Fig. 1(d)). TheA15 is considered the simplest of the so-called topo-logically close-packed (TCP) structures (Khanthaet al. 1989).

Finally, Fe3Al has the ordered cubic D03 (cF16)

crystal structure (Fig. 1(e)). This unit cell containseight bcc-type subcells and it may be thought of asbeing composed of four interpenetrating fcc lattices(notice that according to the system of Pearson sym-bols the D03 lattice is classified as cubic face-centeredcF). In each subcell the iron atoms occupycorners and as such each of them is shared witheight neighboring subcells, i.e., one eighth of the ironatom per subcell. Since there are eight subcells, thecorner atoms provide eight iron atoms per unit cell.In addition, four iron atoms occupy the centers offour subcells, so altogether there are twelve iron at-oms in the D03 unit cell. Including the four aluminumatoms, which also occupy the centers of four subcells,

the total number of atoms per unit cell comes to six-teen. Fe3Al transforms from the D03 structure to adefective ordered cubic B2 (cP2) crystal structure attemperatures above the critical ordering temperatureTc ofB814K (Vedula 1995).

1.2 Cubic Aluminides Stabilized by Alloying

Low-density and highly oxidation-resistant titaniumand zirconium trialuminide intermetallics, with re-spective stoichiometric formulas Al3Ti and Al3Zr(trialuminides), crystallize in tetragonal D022 (tI8)and D023 (tI16) crystallographic lattices, respectively(Fig. 2(b),(c)). Their crystallographic structures areclosely related to the L12 structure (Fig. 2(a)). Theycan be essentially derived from the former by intro-ducing an antiphase boundary (APB) with a dis-placement vector of a/2 /100S on every (001) planein D022 and every second (001) in the D023 lattice(Fig. 2(a),(b),(c)). It must, however, be pointed outthat a close examination of the close-packed planesand their respective order in the L12, D022, and D023structures reveals that the order in these planes isdistinctly different, being triangular (T) (Fig. 2(a)),rectangular (R) (Fig. 2(b)), and mixed (T-R) (Fig.2(c)) in the L12, D022, and D023 structures, respec-

tively. A close relationship between tetragonal andcubic structures in Fig. 2 allows transformation oftetragonal D022 Al3Ti into the L12 structure byalloying with approximately 810 at% copper, nickel,and zinc (Schubert et al. 1964a, Raman and Schubert1965), chromium and manganese (Zhang et al. 1990),iron (Kumar and Pickens 1988), and palladium(Powers and Wert 1990). Similar transformation ofthe low-symmetry D023 Al3Zr into the L12 structurecan be achieved by alloying with copper, nickel, andzinc (Schubert et al. 1964a, b), chromium and iron(Schneibel and Porter 1989), and manganese (Virkand Varin 1991, 1992). There is a general consensus

Figure 1

Generic cubic lattices of aluminides and silicides. (a) L12(cP4), (b) B2 (cP2), (c) C1 (cF12), (d) A15 (cP8), and (e)D03 (cF16).

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that the alloying element atoms substitute mostly forthe aluminum atoms. It is to be pointed out that thefirst case of systematic alloying-induced transforma-tion to the L12 structure was reported for D022-struc-tured Ni3V and hexagonal D019 (hP24)-structuredCo3V by Liu (1984) (so-called long range-ordered(LRO) alloys). Both low-symmetry intermetallics aretransformed to the L12 structure by partially substi-tuting iron for cobalt and nickel, respectively. Thecrystallographic transformation to the L12 structurein the LRO alloys results in a dramatic increase intensile ductility at room temperature as opposed tothe same transformation in Al3Ti, which does notlead to any improvement in tensile ductility.

1.3 Noncubic Aluminides and Silicides

Commercially important aluminides and silicides suchas D019 (hP8) Ti3Al, L10 (tP4) TiAl, C11b (tI6) MoSi2,

tetragonal tP32 (no Strukturbericht designation; PTi3prototype) Nb3Si, and its counterpart the tetragonalD8l(tI32) a-Nb5Si3 (or D8m (tI32) b-Nb5Si3), do notpossess cubic structures. Nb5Si3 belongs to a widergroup of intermetallic compounds, so-called 5:3 silic-ides, having a general stoichiometric formula M5Si3(MNb, Ta, Mo, V, Ti, Zr, W). These compoundstypically crystallize in either a tetragonal D8l (tI32)(e.g., Nb5Si3, Ta5Si3, Mo5Si3, V5Si3, W5Si3) or a hex-agonal D88 (hP16) structure (e.g., Ti5Si3). However,the tetragonal M5Si3 can also crystallize with the D88

structure if they contain small amounts of interstitialimpurities, in particular carbon (Sauthoff 1996). Fig-ure 3 shows the hard-sphere models of selected non-cubic crystallographic structures.

The crystallographic unit cell of TiAl (Fig. 3(a)) iscommonly referred to as a face-centered tetragonal(fct). The tetragonality is very small (c/a 1 02). Thestructure of MoSi2-type phases (Fig. 3(b)) has beendescribed as a superstructure of the bcc or CsClstructure with three subcells stacked along the [001]direction. The c/a ratio for MoSi2 is 2.45. Ti3Al has ahexagonal structure (Fig. 3(c)) with eight atoms perunit cell. Finally, the hexagonal structure of Ti5Si3 is

Figure 2The L12 structure in titanium and zirconiumtrialuminides, which can be stabilized by alloying oftetragonal D022 Al3Ti and D023 Al3Zr with Cr, Mn, Fe,

Ni, Cu, Zn, and Pd. A hard-sphere model of the close-packed planes in each crystal structure is also shown. (a)Stabilized L12 structure of Al3Ti (X) where X is theappropriate alloying element; triangular (T) ordering,(b) D022 Al3Ti; rectangular (R) ordering, and (c) D023Al3Zr; mixed triangularrectangular (T-R) ordering.

Figure 3Hard-sphere models of crystalline unit cells of non-cubicaluminides and silicides. (a) L10 (tP4), (b) C11b (tI16),

(c) D019 (hP8), and (d) D88 (hP16).



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built up by two atomic species which occupy threedifferent crystallographic positions (Fig. 3(d)). Thetitanium(I) atoms are arranged in chains parallel toeach other octahedrally surrounded by silicon atoms.In contrast, the titanium(II) atoms form columns ofoctahedrons connected along the c-axis. The octahe-dral interstices can be partially or completely filled byinterstitials.

2. Laves Phases

The binary Laves phases with an AB2 compositionbelong to the above-mentioned topologically close-

packed (TCP) structures. In the group of TCP inter-metallics, the Laves phases constitute the single larg-est group. They crystallize in the hexagonal C14(hP12; MgZn2 prototype), the cubic C15 (cF24;Cu2Mg prototype) (Fig. 4), or the dihexagonal C36(hP24; MgNi2 prototype) structures. The C14, C15,and C36 crystal structures differ only by the particularstacking of the same two-layered structural units,which allows structure transformations between thesestructures and twinning by synchroshear (Hazzle-dine et al. 1993). The stability of the three crystalstructures is controlled by both the atomic sizeratio of the A atoms and B atoms and by the valence

electron concentration of the Laves phase (Sauthoff1996). Some Laves phases have been regardedas promising for both functional and structural ap-plications, such as superconducting C15 (Hf, Zr)V2,magnetic C15 TFe2 (where TTi, Zr, Hf, Nb, andMo), and materials for hydrogen storage C15 ZrV2,ZrCr2, and C14 ZrMn2 materials (Sauthoff 1996).

Bibliography

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Khantha M, Vitek V, Pope D P 1989 Twinning in A15 com-pounds. Mat. Res. Soc. Symp. Proc. 133, 99104

Kumar K S, Pickens J R 1988 Ternary low-density cubic L12aluminides. In: Kim Y W, Griffith W M (eds.) DispersionStrengthened Aluminum Alloys. TMS, Warrendale, PA, pp.76386

Liu C T 1984 Physical metallurgy and mechanical properties ofductile ordered alloys (Fe, Co, Ni)3V. Int. Met. Rev. 29, 16894

Powers W O, Wert J A 1990 Microstructure, deformation andfracture characteristics of an Al67Pd8Ti25 intermetallic alloy.Metall. Trans. A 21A, 14551

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Schneibel J H, Porter W D 1989 Microstructure and mechanicalproperties of L12-structure alloys based on Al3Zr. In: Liu CT, Taub A I, Stoloff N S, Koch C C (eds). Mater. Res. Soc.Symp. Proc. 133, 33540

Schubert K, Meissner H G, Raman A, Rossteutscher W 1964aEinige Strukturdaten metallischer Phasen (9). Naturwissen-schaften 51, 287

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Vedula K 1995 FeAl and Fe3Al. In: Westbrook J H, Fleischer RL (eds.) Intermetallic CompoundsPrinciples and Practice.Wiley, Chichester, UK, Vol. 2, pp. 199209

Villars P, Calvert L D 1991 Pearsons Handbook of Crystallo-graphic Data for Intermetallic Phases. ASM International,Metals Park, OH

Virk I S, Varin R A 1991 Development of structure andporosity in cast Al5CuZr2 and Al66Mn9Zr25 intermetalliccompounds. Metall. Trans. A 22A, 254552

Virk I S, Varin R A 1992 Mechanical behavior and micro-

cracking of cubic ternary zirconium trialuminides. Metall.Trans. A 23A, 61725Westbrook J H, Fleischer R L (eds.) 1995 Intermetallic Com-

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potential, reality and the road ahead. In: Nathal M V, Dar-olia R, Liu C T, Martin P L, Miracle D B, Wagner R, Yam-aguchi M (eds.) Structural Intermetallics 1997. TMS,Warrendale, PA, pp. 38

Zhang S, Nic J P, Mikkola D E 1990 New cubic phases formedby alloying Al3Ti with Mn and Cr. Scr. Metall. Mater. 24,5762

R. A. VarinUniversity of Waterloo, Ontario, Canada

Figure 4The C15 unit cell characteristic of Laves phases.

Intermetallics: Laves Phases

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