Growth Behavior and Interfacial Character of Ir3Y Precipitates

in the Ir2Y Lave Phase Matrix

N. Sekido* and Y. Yamabe-Mitarai

National Institute for Materials Science, Tsukuba 305-0047, Japan

A C15 Laves phase Ir2Y that forms in the Ir-Y binary system develops off-stoichiometry toward the Ir-rich composition at hightemperatures, and its range of homogeneity becomes narrow with decreasing temperatures. One consequence of this solubility behavior is theformation of Ir3Y precipitates within the Ir2Y matrix of an Ir-30mol%Y alloy. The orientation relationship between Ir3Y and Ir2Y has beenidentified as: ð0001ÞIr3Y == ð111ÞIr2Y and ½2�11�110�Ir3Y == ½�1110�Ir2Y. The similarity in the crystal structures of the two phases yields a plate-like Ir3Yprecipitate showing a typical Widmanstatten structure. The growth of the precipitates has been suggested to follow the ledge mechanism. Theinterfacial character between the two phases has been identified in this study. [doi:10.2320/matertrans.MBW200901]

(Received October 20, 2009; Accepted December 9, 2009; Published January 27, 2010)

Keywords: Laves phase, precipitation, transmission electron microscopy, orientation relationship, ledge growth

1. Introduction

Laves phases are among the most common intermetalliccompounds that appear in a number of binary and multi-component systems.1) Laves phases have been considered asdeleterious products that degrade the mechanical propertiesof structural materials, however their remarkable mechanicaland physical properties have been recognized and haveinduced much research interest in recent years. Some of thepromising application fields include high temperature struc-tural materials,2–6) precipitation strengthening phases for heatresistant steels,7,8) magnetoelastic materials,9) and hydrogenstorage materials.10)

A Laves phase is characterized by an A2B-type compound,where the smaller A atoms and the larger B atoms formtetragonal clusters that become arranged into topologicallyclose-packed forms.11–13) Three crystal structures that com-monly appear are cubic C15, and hexagonal C14 and C36.They are all comprised of the same basic structure unit withdifferent stacking sequences. Although a lot of effort hasbeen made into the elucidation of factors that control thestability of the Laves phases,14–16) there still seems to be lackin comprehensive understanding between the atomic sizeand the attributes of the electronic structure. In addition, thefactors that determine the range of homogeneity are stillunclear. The stability of Laves phases is shown to be sensitiveto the size of constituent atoms,16–18) in which the ideal ratiofor the closest packing is 1.225 in the binary alloys systems.Many Laves compounds do not exhibit a large range ofhomogeneity. Among the 219 of the C15 Laves phases in thebinary systems, 73% of them do not exhibit a definedsolubility.18) Moreover, the homogeneity ranges of the Lavesphases most often exhibit a negligible change with temper-ature. This solubility behavior provides a limited opportunityfor second phase precipitation within the Laves phase matrix.On the contrary, we have demonstrated that a C15 Lavesphase of Ir2Y is one of the exceptions that form precipitateswithin the Lave phase matrix.19) An Ir-rich portion of the Ir-Y

binary phase diagram (Fig. 1) shows that the Ir2Y phasedevelops off-stoichiometry toward the Ir-rich composition athigh temperatures, and its range of homogeneity becomesnarrow with decreasing temperature. One consequence ofthis solubility behavior is the formation of Ir3Y precipitateswithin the Ir2Y matrix in an Ir-30mol%Y alloy.19) In thisstudy, the precipitation of Ir3Y in Ir2Y is investigated with aspecial focus on the growth behavior and the interfacialstructures. The orientation relationship between the twophases, as well as their interface characters, are examined bytransmission electron microscopy. The lattice correspond-ence and the phase transformation behavior are discussed.

2. Experimental Procedures

Ir-30mol%Y alloys were prepared by arc-melting highpurity raw materials under an Ar atmosphere. Hereafter the

Fig. 1 An Ir rich portion of the Ir-Y binary phase diagram.19)

*Corresponding author, E-mail: sekido.nobuaki@nims.go.jp

Materials Transactions, Vol. 51, No. 3 (2010) pp. 447 to 454#2010 The Japan Institute of Metals

alloy composition is given in mole (atomic) percent. The arc-melted Ir-30Y alloys were annealed in a tungsten heaterfurnace under vacuum (10�3 Pa) at 1973K, followed byfurnace cooling. Phase identification and lattice parametermeasurements were conducted by X-ray diffractometry. TheRietveld refinement for the XRD profiles was made using theRIETAN-2000 software.20) The microstructures of the alloyswere characterized by scanning electron microscopy (SEM)and transmission electron microscopy (TEM). The foils forTEM were prepared by an ion-miller with an accelerationvoltage of 4 kV. The TEM examination was conducted withJEM-2000FX and TECNAI-G2-F30 microscopes. The imagesimulation for high-resolution electron microscopy was doneusing a commercial software.21)

3. Results

3.1 Microstructures and constitutional defect mecha-nism

Figure 2 shows the SEMmicrographs of the Ir-30Y alloys.Ir2Ywas formed as the primary solidification phase, followedby the formation of an (Ir)/Ir3Y two-phase microstructure inthe as-cast Ir-30Y alloy (Fig. 2(a)). The precipitation of Ir3Yhad already started during cooling following arc-melting.Thin plates of Ir3Y precipitates formed at the grain interior ofthe Ir2Y matrix. After annealing at 1973K for 100 hours, theprecipitation of Ir3Y progressed further to exhibit a typicalWidmanstatten structure as shown in Fig. 2(b).

Image analyses on the SEM micrographs have revealedthat the volume fractions of Ir3Y precipitates are about 8%for the as-cast alloy and 44% for the annealed alloys. Thissuggests that a fairly large degree of supersaturation has beenretained in the Ir2Y phase in the as-cast condition. To clarifythe defect mechanism of an Ir-rich Ir2Y phase, the Rietveldanalysis was conducted on the XRD profiles of an as-castIr-30Y alloy. Upon Rietveld refinement, two models, (i)anti-site substitution of Ir on Y sites and (ii) constitutionalvacancy development on Y sites, were examined under theconditions of the fixed atom positions that are given in thereference.22) The peaks from Ir and Ir3Y are also includedin the analysis. A much better fitting was obtained in the

XRD profiles in the anti-site substitution model than in theconstitutional vacancy development. Figure 3 shows theresult of the Rietveld refinement. The structure parametersare summarized in Table 1. The site occupancy of Y on the8b site was evaluated to be 0.8423, and the rest of the 8b sitewas occupied by Ir. Thus, the Y concentration of the Ir-richIr2Y phase is calculated to be 28.1mol%, which is inreasonable agreement with the terminal composition of

Fig. 2 Scanning electron micrographs of Ir-30Y alloys: (a) as-cast and

(b) annealed at 1973K for 100 h.

Fig. 3 A Rietveld-refined XRD profile of an as-cast Ir-30Y alloy. The determined structure parameters are summarized in Table 1.

448 N. Sekido and Y. Yamabe-Mitarai

29.2mol% at the peritectic temperature as shown in Fig. 1.The present result also agrees with the tendency for thepresence of a number of binary Laves compounds, where theanti-site substitution is likely to occur for the extensions ofthe homogeneity ranges to the smaller atom rich side.16,18)

3.2 Orientation relationship between Ir3Y precipitatesand the Ir2Y matrix

Figure 4(a) shows a bright field image of the Ir2Y phasein an as-cast Ir-30Y alloy. Many stacking faults and Ir3Yprecipitates have formed in the Ir2Y matrix. Since thesestacking faults are on the f111gIr2Y planes, most of them arenearly edged-on and show a faint contrast in Fig. 4(a). Bytilting the specimen, the stacking faults bounded by partialdislocations are clearly visible as shown in Fig. 4(b).Figure 4(c) shows a high-resolution transmission electronmicroscope (HRTEM) image of an intersection of two Ir3Yplates. The interfaces between Ir3Y and Ir2Y are shown to becoherent. A selected area diffraction pattern taken from thetwo phases and its indexing are shown in Fig. 4(d) and (e).

The crystal structures of Ir2Y and Ir3Y are cF24 (Fd-3mMgCu2-type, C15) and hR36 (R-3m, PuNi3-type), respec-tively.22) The orientation relationship (OR) between the twophases has been identified as:

ð0001ÞIr3Y == ð111ÞIr2Y and ½2�11�110�Ir3Y == ½�1110�Ir2Y: ð1Þ

The interfaces of the two phases coincide with the aboveplanar parallelism.

Based on the crystal structure examinations of Ir2Y andIr3Y, the sectioning of the {0001} planes of Ir3Y was foundto have rather similar atomic arrangements and layer stackingsequences to those of the {111} planes in Ir2Y. Figure 5shows the atomic arrangements of the layers for f111gIr2Yand f0001gIr3Y. Here the layers are assigned as: (a) K-layer(Kagome) where two triangles and two hexagons of Ir atomsmeet at each net, (b) C-layer (Capping) where the Y atomsform triangle nets, (c) I-layer (Interstitial) where the Ir atomsform triangle nets, and (d)M-layer that consists of both Ir andY atoms with a triangular arrangement. Each of these layershas three different sites, where the relative displacementsare equivalent to the Shockley partials in an fcc lattice,1=6h112iC15, represented by the arrows in Fig. 5(e). The Ir2Yis comprised of K, C, and I layers, while Ir3Y contains Mlayers in addition to these three layers. The layer stackingsequences are quite similar in both structures except for thesite difference and the periodic appearance of theM layers inIr3Y, as shown in Figs. 5(f) and (g). The OR between the twophases expressed by eq. (1) implies that they satisfy a face-on-face matching by either of the identical layers at theinterface. From the XRD measurements on an Ir-30Y alloy

Table 1 The structure parameters of the Ir2Y phase formed in an as-cast

Ir-30Y alloy refined under the condition of fixed atom positions and

substitutional defects on Ir on the Y site. (Fd-3m (227), a ¼ 0:7497ð3Þ nm,

Rwp ¼ 8:88, Rp ¼ 6:14, S ¼ 1:87)

Site Wyck. g x y z U (nm2)

Ir1 16c 1 0 0 0 0.000315

Y 8b 0.8423 3/8 3/8 3/8 0.001496

Ir2 8b ¼1� g(Y) ¼x(Y) ¼y(Y) ¼z(Y) ¼U(Y)

Fig. 4 (a) A bright field image of an as-cast Ir-30Y alloy taken near ½�1110�Ir2Y, (b) near ½001�Ir2Y showing stacking faults in the Ir2Y phase,

(c) an HRTEM image showing an intersection of two Ir3Y precipitates, (d) a selected area diffraction pattern and (e) its key diagram.

Growth Behavior and Interfacial Character of Ir3Y Precipitates in the Ir2Y Lave Phase Matrix 449

annealed at 1973K for 100 h, the lattice parameter of cubicIr2Y was determined to be a ¼ 0:7516ð4Þ nm, and those forIr3Y in the hexagonal coordinate to be a ¼ 0:5305ð3Þ nm andc ¼ 2:6022ð8Þ nm. The mismatch in the neighboring atomicdistances along the close-packed directions (h110i for Ir2Yand h11�220i for Ir3Y) of the two phases are calculated to be0.18%. Thus, the OR expressed by eq. (1) is reasonable interms of atom matching on the habit planes.

3.3 Character of the interface between Ir3Y and Ir2YFigure 6 shows the TEM images for two characteristic

Ir3Y/Ir2Y interfaces taken by HRTEM and high-angleannular dark field scanning transmission electron microscopy(HAADF-STEM). The atom positions are superimposed onthe micrographs, where the black circles represent the Ircolumns and the white circles represent the Y columns.Under the defocused condition in Fig. 6(a), the bright spotscorrespond to the close-packed Ir columns in the K-layers,which agrees with the image simulation shown in the rightside of the micrograph. Since the contrast in HAADF-STEMimages depends strongly on the atomic number (Z-contrast),the close-packed Ir columns appear as bright spots, the grayregions to the Ir columns with a lower atom density, and thedark regions to the Y columns, respectively in Fig. 6(b).According to the OR expressed by eq. (1), two orientationvariants are present on the basis of the ð0001ÞIr3Y == ð111ÞIr2Yparallelism as follows:

Variant 1 (V1):

ð0001ÞIr3Y == ð111ÞIr2Y and ½2�11�110�Ir3Y == ½�1110�Ir2Y ð2ÞVariant 2 (V2):

ð0001ÞIr3Y == ð111ÞIr2Y and ½�22110�Ir3Y == ½�1110�Ir2Y ð3Þ

The OR in Fig. 6(a) and (b) is V2 and V1, respectively. Inboth cases, the atom positions of Ir2Y and Ir3Y coincideat their K-layers. Similar observations by HRTEM and

HAADF-STEM have been made on several Ir3Y/Ir2Yinterfaces in this study, and four types of interfaces havebeen found to be present, which are schematically shown inFig. 7. The -Ir-Y- atom chains are represented by lines, bywhich the topologies of the two orientation variants can beclearly visualized. C andM layers are the two possible layersthat are in contact with the interface layer K. The atomarrangements shown in Fig. 7(a) and (c) are substantially thesame in the vicinity of the interface. That is, the layerstacking equivalent to that of Ir2Y continues across theinterface for a couple of layers in Ir3Y, and vice versa. In sucha case, the definition of the interface between the two phasesis ambiguous; however, at the same time the interfacialenergy would be low.

3.4 Growth mechanismThe development of a Widmanstatten structure indicates

that a large difference in the relative mobility of growinginterphase boundaries yields a highly elongated plate-likemorphology for the Ir3Y precipitates. The barrier to thegrowth along the thickness direction of the plate-likeprecipitates is the low energy interfaces. In such case, thegrowth of the precipitates is often governed by the ledgewisegrowth mechanism.23,24) Figure 8 shows an evidence for theledge growth of the Ir3Y precipitates observed in an annealedIr-30Y alloy. The bright field image was taken near anintersection of two Ir3Y precipitates, and the ledges areindicated by the triangles. These ledges are believed to begrowth ledges since much fewer ledges were observed awayfrom the intersection of the two precipitates. One reason forthe accumulation of growth ledges near the intersection oftwo precipitates is that the mobility of these ledges werelowered by the reduced driving force for precipitation in thevicinity of another precipitate variant. The ledge spacing inFig. 8 is believed to be dominated by the elastic energiesassociated with the ledge, as well as the degree of soft

Fig. 5 (a)–(d) Atom arrangement in the layers that appear in the sectioning of f0001gIr3Y and f111gIr2Y planes, (e) relative translation

vectors for sites 1, 2, and 3, and (f),(g) their stacking sequences for Ir2Y and Ir3Y.

450 N. Sekido and Y. Yamabe-Mitarai

impingement; however, the evaluation of these factors isbeyond the scope of this paper. The height of the ledges is notclear, but appears to be a couple of nm. A number of stakingfaults are also visible within the Ir3Y precipitates.

Figure 9 shows an HRTEM image of a ledge on the face ofthe Ir3Y precipitates viewed edge-on along the ½2�11�110�Ir3Y ==½�1110�Ir2Y direction in an as-cast Ir-30Y alloy. The riser of theledge is not clear in the micrograph, probably because it is notedged-on from this direction. The height of the ledge denotedby ‘‘h’’ in the micrograph shows an agreement with the c-axisof the Ir3Y unit cell (2.6022 nm).

3.5 Heterogeneous nucleation of Ir3Y precipitatesMicrostructure observation clearly depicts that a number

of stacking faults have developed within the Ir2Y matrix ofthe as-cast Ir-30Y alloys shown in Figs. 4(a) and (b). Thereason for the planar fault development is not clear, but theirformation has been speculated to occur from the thermalshock during cooling following arc-melting. Planar faultshave been known to become potent heterogeneous nucleationsites for a subsequent precipitation reaction.25) Some traces ofthe heterogeneous nucleation of Ir3Y precipitates on planarfaults have been observed in this study. Figure 10 exhibits anHRTEM image showing a thin plate of Ir3Y precipitate thatformed in the Ir2Y matrix, together with the simulationimages on the right side of the micrograph. Under thedefocused condition, the Y columns appear as the brightestspots. The interface is characterized by the same type as that

Fig. 7 Four types of the interface characters identified in the present study. (a),(c) ½2�11�110�Ir3Y == ½�1110�Ir2Y, and (b),(d) ½�22110�Ir3Y == ½�1110�Ir2Ybased on the ð0001ÞIr3Y == ð111ÞIr2Y parallelism. (a),(b) C-layer and (c),(d) M-layer of Ir3Y are in contact with a K-layer as the interface

plane.

Fig. 6 Micrographs of two types of Ir2Y/Ir3Y interfaces: (a) an HRTEM

image and a simulated image, and (b) an HAADF-STEM image. The

bright spots correspond to the closed-packed Ir columns, and the dark

spots to the Y columns in both images.

Fig. 8 A bright field image showing an intersection of two Ir3Y

precipitates in an Ir-30Y alloy annealed at 1973K for 100 h. Ledges are

indicated by triangles.

Growth Behavior and Interfacial Character of Ir3Y Precipitates in the Ir2Y Lave Phase Matrix 451

in Fig. 7(a) or (c), which are substantially indistinguishableas mentioned in Section 3.3. By defining their interface asshown by the broken lines, the thickness of the precipitate isevaluated to be 2/3 of the c-axis of the Ir3Y unit cell. InFig. 10, the dotted lines are drawn on the micrograph tovisualize the translation between the upper and lower Ir2Ymatrix across the Ir3Y precipitate. The upper and lower Ir2Ycrystals are translated by �2�=3, which is identical to thetranslation by the synchroshear15,26–29) in C15 compounds,i.e. either of the vectors shown in Fig. 5(e). If an Ir3Y platewith its thickness being equal to the c-axis of the unit cellwere to form on this planar fault, the K-layers of the twophases would never coincide at the interface. Therefore,the formation of Ir3Y precipitates that is smaller than its unitcell can be regarded as a consequence of maintaining thecoherency between the precipitate terrace and the Ir2Ymatrixat the interface.

4. Discussions

4.1 Kinetics of ledge growthThe Ir3Y precipitates have been demonstrated to exhibit a

ledgewise growth. Since the rate of lateral migration of a

growth ledge is governed by the diffusivity of atoms ahead ofthe ledge, ledges with a smaller height can migrate faster.25)

Generally, the height of growth ledges is discrete; rangingfrom a single atom layer to, sometimes, hundreds of atomlayers high.25,30) Such a widely ranging ledge height invarious systems is considered to result from the elasticstrain effects associated with the riser.30,31) In this study, theobserved ledge height is equivalent to the c-axis of the Ir3Y(2.6022 nm), or 2/3 of the c-axis (1.733 nm) when nucleatedon stacking faults. Although these ledge heights may not besmall in general, the precipitation has already progressed tosome extent in the as-cast condition. Thus, a discussionon the kinetics of the ledge growth for Ir3Y precipitates in anas-cast Ir-30Y alloy is worthwhile.

Assuming that the rate of ledge nucleation is constant, therate of lateral migration of a ledge u, and the thickening rate vof a plate-like precipitate are given by:25)

u ¼D�

khð4Þ

v ¼D�

k�ð5Þ

where D is the interdiffusion coefficient, � is the degree ofsupersaturation, h is the ledge height, � is the ledge spacing,and k is a numerical constant close to unity. From the SEMand TEM micrographs shown in Fig. 2(a) and 4(a), thedimensions of the largest Ir3Y precipitates formed in an as-cast Ir-30Y alloy are about 50 nm in thickness and 20 mm inlength. Although the cooling following arc-melting is acontinuous cooling process, we assume here that this coolingcorresponds to the isothermal annealing at temperature withan interdiffusion coefficient D for 10 s, followed by rapidquenching. Based on this assumption, the growth ratesalong the lateral and thickness directions are defined to beu ¼ 2� 10�6 (m/s), and v ¼ 2:5� 10�9 (m/s), respectively.The degree of supersaturation can be evaluated as 0.45from the phase diagram shown in Fig. 1. By substitutingthese values, as well as h ¼ 2:6022� 10�9 (m) and k ¼ 1 toeq. (4) and (5); D and � are deduced as 1:1� 10�14 (m2/s)and 2 (mm), respectively.

Upon examining an as-cast Ir-30Y alloy, we found that theinterfaces between Ir3Y and Ir2Y are very smooth for several

Fig. 9 An HRTEM image showing a growth ledge formed in an as-cast Ir-30Y alloy. The interface is viewed edge-on along

½2�11�110�Ir3Y == ½�1110�Ir2Y, and the ledge height, h, is equivalent to the c-axis of the Ir3Y unit cell.

Fig. 10 A HRTEM image and the corresponding simulated image viewed

from ½2�11�110�Ir3Y == ½�1110�Ir2Y, where an Ir3Y precipitate with a height

smaller than its unit cell has formed in the Ir2Y matrix.

452 N. Sekido and Y. Yamabe-Mitarai

hundreds of nm, and such growth ledges shown in Fig. 9 arenot often observed. Considering that the above estimationis quite rough and that the ledge spacing often exhibits alarge scatter,30) the estimation of a ledge spacing of 2 mmseems to be acceptable. At the same time, the aboveestimation suggests that atomic diffusivity that is equivalentto 1:1� 10�14 (m2/s) for 10 (s) is required during coolingfrom the melt. There are no data available for the meltingtemperature and diffusion coefficients of Ir2Y, but some areavailable for other Laves compounds. The extrapolation ofthe diffusion coefficients at the melting temperatures ofCo2Nb, Zn2Mg, and Fe2Ti has been reported to be rangingfrom 10�11 to 10�13 (m2/s),32,33) which is in reasonableagreement with that of the bcc and fcc metals.34) Thediffusion coefficient of 10�14 (m2/s) is attained at around0:75Tm � 0:95Tm, where Tm is the melting temperature.Moreover the interdiffusion coefficient of Co2Nb shows astrong compositional dependence, where the defect mecha-nism in a Co-rich Co2Nb is the anti-site substitution, as in theIr2Y phase, and the diffusion is more enhanced with a largerdegree of off-stoichiometry.33) Therefore, the atomic diffu-sivity that is equivalent to 10�14 (m2/s) for 10 (s) seems to beattainable during cooling from the melt. These considerationssupport the feasibility of the kinetics of microstructuredevelopment by ledgewise growth with a 2.6 nm ledge heightin the as-cast condition.

4.2 Nucleation of growth ledgesIn the previous section, the ledge spacing at the early stage

of precipitation was estimated to be roughly around 2 mm.This large ledge spacing suggests that ledge nucleation is therate controlling mechanism for the growth of Ir3Y precip-itates. In general, new ledges are often nucleated heteroge-neously at the grain boundaries, the corner of the precipitate,and the point of contact with another precipitate.25) Anotherheterogeneous nucleation site for Ir3Y precipitation is aplanar fault that developed within the Ir2Y matrix, as shownin Fig. 10.

One of the important results in this study is that the ledgeheight that secures the coherent interface has been chosen forthe growth ledges. This criterion is derived from Fig. 9,which shows the formation of a new growth ledge on aterrace, and from Fig. 10 showing an initial nucleation of aledge on a planar fault. The ledge height is often subordinatedby the elastic strain normal to the precipitate habit planearound the riser.30,31) Based on the lattice parameters of theannealed alloy determined by XRD, the misfit between thetwo phases along the normal direction to the precipitate habitplane can be evaluated to be 0.05%. This small misfit impliesthat the elastic strain effects associated with the riser on thefavored ledge height may not be significant.

Figure 11 shows an HRTEM image taken in the vicinity ofan Ir3Y/Ir2Y interface. Some stacking faults have been foundto develop within the Ir3Y precipitate, and the thickness ofsome precipitate variants is smaller than the Ir3Y unit cell.The reason for the stacking fault development is not clear, buttheir formation is speculated to be related with the kineticsof the nucleation of growth ledges. In Section 3.3, twoorientation variants are shown to be present on the fixedplanar parallelism. When the stacking sequence of the K-

layers in the Ir2Ymatrix is fixed to be K1K3K2��, the stacking

in the Ir3Y precipitate of the V1 variant is K1K3K3K2K2K1��,

while K1K2K2K3K3K1�� in the V2 variant. This means that

the ledge height that secures the coherent interface at the K-layers can be reduced by the introducing a new ledge ofanother orientation variant with 1/3 or 2/3 the height of thec-axis on the terrace. The reduction in the height of a newledge would be kinetically advantageous, since the requiredsize for the ledge nuclei becomes smaller. The counterpartbarrier, however, is the stacking fault energy in the Ir3Yphase. No information is available on the stacking faultenergies of Ir3Y and Ir2Y; however, some is available forother C15 Laves compounds: Cr2Zr, Cr2Nb, and Cr2Ta. Theirstacking fault energies range from 25 to 120mJ/m2,35–40)

which is comparable to that of Cu, and lower than that of Niand Al.41) Therefore, the stacking fault energies of Ir2Y andIr3Y are not speculated to be very high. These facts suggestthat the development of multiple staking faults within theIr3Y precipitates may be related to the degree of the ease ofledge nucleation on the basis of the coherent interfacesecuring on the terrace. Future work should be addressed fora further elucidation of the nucleation event.

5. Conclusions

A Laves phase Ir2Y that appears in the Ir-Y binary systemexhibits a homogeneity range where the solubility limitextends toward the Ir-rich composition at high temperatures.Anti-site substitution has been shown to occur as thedefect mechanism in an Ir-rich Ir2Y phase. The orientationrelationship between the Ir3Y precipitates and the Ir2Ymatrix has been identified as: ð0001ÞIr3Y == ð111ÞIr2Y and½2�11�110�Ir3Y == ½�1110�Ir2Y. The Ir3Y precipitates exhibits plate-like morphology, of which habit planes agree with f0001gIr3Yand f111gIr2Y. The interface between the two phases iscoherent and occurs at the K-layers. The growth of Ir3Y

Fig. 11 Development of multiple stacking faults within the Ir3Y precip-

itate. The heights of the precipitate variants bounded by the stacking faults

are smaller than its unit cell.

Growth Behavior and Interfacial Character of Ir3Y Precipitates in the Ir2Y Lave Phase Matrix 453

follows the ledge mechanism, in which the ledge heightis usually equivalent to the length of the Ir3Y unit cell.Preexisting stacking faults in the Ir2Y matrix are heteroge-neous nucleation sites against the Ir3Y precipitation. Thenucleation on the staking faults yields Ir3Y precipitates withthickness shorter than the unit cell. By introducing a newledge of another orientation variant, the ledge height can alsobe reduced without violating the coherency at the interface,which at the same time leaves a stacking fault in Ir3Y. Aledge height with specific multiples of the 1/3 Ir3Y unit cellis required to secure the coherency on the terraces of thegrowth ledges.

Acknowledgement

This work was supported by Grant-in-Aid for YoungScientists (B) 20760477. The revision in English by Ms.Hono is greatly acknowledged.

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