Subnanometer-Scale Chemistry and Structure of a-Iron/Molybdenum Nitride Heterophase Interfaces

DIETER ISHEIM and DAVID N. SEIDMAN

The local chemistry and structure of a-iron/molybdenum nitride heterophase interfaces is studied ona subnanometer scale by atom-probe field-ion microscopy (APFIM), three-dimensional atom-probemicroscopy (3DAPM) and both conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). Molybdenum nitride precipitates are generated by annealingFe-2 at. pct Mo-X, where X 5 0.4 at. pct Sb or 0.5 at. pct Sn, at 550 8C or 600 8C, in an ammonia/hydrogen mixture. Internal nitridation at 550 8C produces thin, coherent platelet-shaped molybdenumnitride precipitates. Nitridation at 600 8C generates a much coarser structure with semicoherent thickplate-shaped and spheroidal precipitates in addition to the thin-platelet structure. The APFIM and3DAPM analyses of the heterophase interfaces show substantial segregation of the solute species Snand Sb only at the coarse precipitates, with Gibbsian interfacial excesses of up to 7 6 3 nm22, whereasthe broad faces of the thin platelets have no detectable segregation. The TEM and HREM analysesshow that the coarse precipitates are semicoherent, whereas the thin platelets are either coherent orhave much fewer misfit dislocations than geometrically necessary. This demonstrates that Sn and Sbsegregation is related to the presence of misfit dislocations at the interfaces of the coarse precipitates.

I. INTRODUCTION those estimated by a simply energetic criterion. The meanprecipitate size, in turn, depends on the aging conditions,THE importance of internal interfaces for the physical,and the coherency state, determining the atomic structure ofmechanical, and electrical properties of materials, as wellthe heterophase interfaces to a large degree, can be controlledas their microstructural evolution, has attracted a great dealby varying the decomposition temperature and aging time.of scientific and technological attention. Our focus concen-

Annealing Fe(Mo)-based alloys in an ammonia/hydrogentrates on the interdependence of solute segregation and localatmosphere produces molybdenum nitride precipitates byatomic arrangements at grain boundaries and heterophaseinternal nitridation.[7–10] The effective hardening of ironinterfaces, which has been studied experimentally[1] and byalloys by internal nitridation is well known and, dependingcomputer simulations.[2,3,4] As segregation proceeds towardon the nitridation temperatures and times, a variety of nitrideits equilibrium state, the local atomic arrangement may relaxphases and precipitates morphologies can be achieved. Also,due to the presence of segregated atoms. Of particular inter-internal nitridation can be performed to produce a high num-est is the influence of interfacial dislocations on the levelber density of precipitates, making their investigation byof segregation.high-resolution techniques achievable. The possibility ofInterfacial misfit dislocations are essential for the structureinteractions between solute atoms and the interfaces ofof heterophase interfaces separating two phases with a lat-nitride precipitates has attracted attention, and the segrega-tice-parameter mismatch. These interfaces are usually classi-tion of Sb and P has been reported.[11]

fied by their coherency state as coherent, semicoherent orThe present research characterizes both the structure ofpartially coherent, and incoherent.[5] For a given lattice-

and solute segregation of Sb and Sn at heterophase interfacesparameter mismatch or misfit, the coherency state of a pre-between molybdenum nitride precipitates and the Fe-richcipitate depends on the size of the precipitate and the thermo-bcc matrix in Fe-2 at. pct Mo-0.4 at. pct Sb and Fe-2 at.mechanical history of an alloy. During the course ofpct Mo-0.5 at. pct Sn alloys. Atomic-scale chemical analysishomogeneous nucleation of a decomposition process, theis performed by a combination of atom-probe field-ionheterophase interfaces associated with small second-phasemicroscopy (APFIM) and three-dimensional atom probeprecipitates are usually initially coherent.[6] If a growingmicroscopy (3DAPM). Conventional transmission electronprecipitate reaches a critical size, however, the incorporationmicroscopy (TEM) and high-resolution electron microscopyof misfit dislocations at the interface becomes energetically(HREM) are employed for microstructural characterizationfavorable. The critical size depends on the lattice-parameterand atomic-scale imaging.misfit between the precipitate and matrix. As there is a

Segregation of Sb or Sn is observed only at the hetero-nucleation barrier for the formation of misfit dislocations,phase interfaces of semicoherent coarse precipitates. It isprecipitates can remain coherent at even larger sizes thandemonstrated experimentally that this segregation is relatedto the loss of interfacial coherency upon growth of the precip-itates, that is, to the formation of interfacial misfitDIETER ISHEIM, Research Associate, and DAVID N. SEIDMAN, Pro-dislocations.fessor, are with the Department of Materials Science and Engineering,

Northwestern University, Evanston, IL 60208-3108.This article is based on a presentation made at the symposium entitled II. EXPERIMENTAL METHODS

“The Mechanisms of the Massive Transformation,” a part of the Fall 2000Alloys with a nominal composition of Fe-2 at. pct Mo-TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under

the auspices of the ASM Phase Transformations Committee. 0.5 at. pct Sb and Fe-2 at. pct Mo-0.5 at. pct Sn were prepared

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 33A, AUGUST 2002—2317

by arc melting the elements Fe, Mo, Sb, or Sn, with puritiesof 99.99, 99.95, 99.9999, and 99.999 wt pct, respectively.Chemical analyses of the alloys by a commercial laboratoryyielded compositions of Fe-2 at. pct Mo-0.4 at. pct Sb andFe-2 at. pct Mo-0.5 at. pct Sn. These values are used in thefollowing article as overall compositions. The ingots werecold rolled into sheets with a 0.1 mm thickness, and wireswere drawn to a 0.12 mm diameter. The sheets and wireswere annealed in vacuum for 2 hours at 800 8C to recrystal-lize them; they were subsequently nitrided in a flowing gasmixture of 10 pct NH3 and 90 pct H2 for 24 hours at 5508C or for 9 or 10 hours at 600 8C.

The FIM tips were prepared by electropolishing in a two-step procedure using a solution of 10 vol pct perchloricacid in acetic acid at 20 V d.c. at room temperature forprepolishing and a solution of 2 vol pct perchloric acid inbutoxyethanol at 15 to 12 V d.c. for the final tip preparation.The TEM foils were prepared using a STRUERS double-jet electropolisher and a solution of 10 vol. pct perchloricacid in acetic acid at 25 V d.c. at 11 8C. In order to obtainTEM foils of the coarser precipitation structures, an addi-tional ion milling step was necessary. The best results wereobtained by employing a precision ion polishing system withAr ions at 4 kV and a beam inclination angle of 5 deg for15 to 30 minutes.

Conventional TEM was carried out employing an HitachiH8000 microscope operating at 200 kV. The JEOL* 4000

*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

EXII electron microscope at Argonne National Laboratory,operating at 300 kV, was used for the HREM investigations.Atom-probe analyses were carried out employing a VacuumGenerator FIM100 at a pulse-to-d.c.-voltage ratio of 0.15, a50 Hz pulse repetition rate, and a tip temperature of 35 K,with the imaging gas present at a 1 ? 1023 Pa gauge pressure.Analyses in the 3 DAP[12,13] were carried out at a 1500 Hzpulse repetition rate in a vacuum, with a residual pressureof 3 ? 1028 Pa. Field-ion images were obtained with Ne asthe imaging gas, at a 1 ? 1023 Pa gauge pressure and aspecimen temperature of 35 K.

Fig. 1—(a) Bright-field TEM micrograph of an Fe- 2 at. pct Mo- 0.4 at.III. RESULTS pct Sb alloy after 10 h nitridation at 600 8C. Coarse molybdenum nitrideplatelets (A) and spheroids (B) are formed besides a much finer structureA. Microstructure of thin platelets (C) with a characteristics scale about 10 times smaller. (b)Selected area electron diffraction pattern taken from the central region ofA 10-hour nitridation treatment at 600 8C generates aFig. 1(a).duplex-type microstructure of nitride precipitates with two

distinct morphologies. The center region of Figure 1(a) isa coarse structure consisting of lamellae-like plate-shapedprecipitates (denoted A) and facetted spheroidal precipitates [100]M ,[14] where the subscripts P and M denote the precipi-

tate and matrix, respectively.(denoted B). In contrast, at the right-hand side of this micro-graph, a structure of much smaller and thinner platelets Figure 2(a) is a more highly magnified image of the C-

type thin-platelet structure, with the platelets in an edge-on(denoted C) is discernible. The A-type coarse platelets are0.1 to 1 mm in diameter and 2 to 10 nm in thickness, and projection orientation. The nitride platelets are about 40 to

100 nm in diameter and 1 nm or less in apparent thickness,the B-type coarse spheroidal precipitates are about 20 to 30nm in diameter. giving rise to a pronounced shape-induced streaking in the

diffraction pattern (Figure 2(b)). The streaking extends fromFigure 1(b) displays a diffraction pattern of the coarsestructure, obtained from the center of Figure 1(a). In addition the fundamental reflections parallel to the ^200&-type direc-

tions, corresponding to the {200} habit planes of theto the fundamental [001] zone reflections of the matrix,precipitate reflections are discernible, indicating the pres- nitride platelets.

Nitridation for 24 hours at 550 8C produces only precipi-ence of a cubic or tetragonal second phase with a lattice-parameter misfit of about 4 pct and a Baker–Nutting-type tates of the C-type thin-platelet structure. The field-ion

micrograph (Figure 3) exhibits nitride platelets of all threeorientation relationship (OR), (001)P//(001)M and [110]P//

2318—VOLUME 33A, AUGUST 2002 METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 3—Field-ion micrograph of small molybdenum nitride platelets(C-type microstructure) in Fe-2 at. pct Mo-0.5 at. pct Sn after 24 h nitridationat 550 8C.

Fig. 2—(a) Bright-field TEM micrograph of molybdenum nitride plateletsof the C-type microstructure in Fe- 2 at. pct Mo- 0.4 at. pct Sb nitridedfor 10 h at 600 8C, enlarged from region “C” in Fig. 1(a). Thin molybdenum Figure 4 is an FIM micrograph of the C-type microstruc-nitride platelets with the (200) habit plane are projected along the [002]- ture after a 10 hour nitridation treatment at 600 8C. In com-zone axis and appear as dark lines in the micrograph. (b) TEM diffraction

parison to Figure 3, it is evident that the platelets are muchpattern with a [001] zone axis. The shape effect causes strong streakinglarger in diameter. The structure of concentric rings andalong the ^200&-type directions.semicircles in Figure 4 indicates a tendency to form stacksof closely spaced parallel platelets. Their thickness, as deter-mined from the FIM micrographs, is three to four atomicplanes, or 0.6 to 0.8 nm.{100}-type orientation variants. The lines of bright dots are

the traces of platelets cutting through the surface of the Figure 5 displays an FIM micrograph of a B-type spheroi-dal precipitate after 10 hours of nitridation at 600 8C. Byapproximately hemispherical-shaped apex of the FIM tip.

For this geometry, platlelets of two variants appear as almost comparing this micrograph with Figures 3 and 4, the differentlength scales of the various types of precipitate microstruc-straight lines. The third variant, with a (100) habit plane,

appears as curved lines, concave with respect to the (100) tures become evident.pole in the micrograph. The details of this projection geome-try are discussed in Reference 9. The platelet traces close

B. Crystal Structure and Coherency of Nitrideto the pole allow for a determination of the precipitate thick-Precipitatesness, due to the resolution of individual atomic planes, pro-

ducing a pattern of concentric rings around this pole. Atom- Table I lists the stoichiometries and space groups of allknown intermetallic compounds in the ternary Fe-Mo-Nprobe analyses show that matrix adjacent to the platelets does

not give rise to additional bright rings. Also, the presence of system.[15–18] The compositional analyses of the nitride pre-cipitates by APFIM and 3DAPM, presented subsequently,darkly imaging planes between the bright ones can be

excluded by controlled evaporation sequences, carefully lay- clearly reveal the precipitates to be molybdenum-nitridebased with only a small Fe content. Hence, the possibleing bare atomic plane by atomic plane. Thus, the number

of bright rings visible in an FIM image is directly related crystallographic structures related to the square-shapedzone-axis patterns seen in the diffraction pattern (Figureto the platelet thickness. The platelets have a thickness of

typically two atomic planes ('0.4 nm) and an average diam- 1(b)) are either the fcc g-Mo2N[17] phase, the tetragonal b-Mo2N[17] phase, or the primitive cubic Mo3N2

[18] phase, buteter of about 20 nm.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 33A, AUGUST 2002—2319

Fig. 4—Field-ion micrograph of large molybdenum nitride platelets (C-typemicrostructure) in Fe-2 at. pct Mo-0.5 at. pct Sn after 10 h nitridation at600 8C.

Fig. 5—Field-ion micrograph of a coarse spheroid-shaped molybdenumnitride precipitate in Fe-2 at. pct Mo-0.5 at. pct Sn after 10 h nitridationat 600 8C.

not the Fe-rich cubic or tetragonal phases. The g-Mo2N, b- Table I. Intermetallic Compounds in the TernaryMo2N, and Mo3N2 structures are topologically closely Fe-Mo-N System,[12–15] Ordered by Increasingrelated in the sense that they can be thought of as being Metal-to-Nitrogen Ratio, M/Ngenerated from a bcc parent structure by filling the octahe-

Compound M/N Ratio Space Groupdral interstices with nitrogen atoms and, thereby, creating anMo4N5

a 0.8 P63/mmcafcc-based structure, according to Hagg’s rule[19] for transitionMoNb 1 P63/mmcbmetal–metalloid interstitial compounds. The tetragonality ofMo3N2

b,c 1.5 Pm3mb,cthe b-Mo2N phase arises from a preferential occupation

g-Mo2Nd 2 Fm3mdof a particular subset of interstices. Because these three

b-Mo2Nb 2 I41/amd b

crystallographic structures are so closely related, the atomicFe2Na 2 P63/mmca

structure of the precipitate heterophase interfaces should be Fe2Na 2 o**a

similar. Thus, a detailed distinction is not necessary for Fe2Nb 2 P31mb

the purposes of this article and, in the following text, the Fe5N2b 2.5 P63/mmcb

discussion uses the Mo3N2 structure as a prototype. Fe3Nb 3 P63/mmcb

Fe4Nb 4 Pm3mbFigure 6 is a schematic drawing of the unit cell of theFe4Nb 4 Fm3mbMo3N2 structure stacked on top of a unit cell of the bcc a-Fe7Mo13N4

a 5 P4t32airon matrix, representing the Baker–Nutting OR,[14] which

Fe3Mo3Nb 6 Fd3mbis frequently observed for fcc-based precipitates in a bcc

Fe8Nb 8 I4/mmmb

matrix, or vice versa. This OR gives rise to a tetragonal misfit Fe91N9a ,10 t*2a

geometry with a small interfacial lattice-paramter misfit inaRef. 15.two directions, usually identical with the habit plane of abRef. 16.plate-shaped precipitate, and a comparatively large lattice- cRef. 18.paramter misfit in the direction perpendicular to the habit dRef. 17.

plane. Table II lists the values of the lattice-paramter misfitwith respect to bcc a-Fe for the molybdenum nitride phasesunder consideration. The similarity of the three molybdenumnitride structures is reflected in the small differences of the varying from 2.05 to 3.60 pct and the large misfit in the

range from 44.3 to 46.5 pct.respective misfit tensors, with the small in-habit-plane misfit

2320—VOLUME 33A, AUGUST 2002 METALLURGICAL AND MATERIALS TRANSACTIONS A

Using these misfit values, the spacing of the misfit disloca- spacing of dav 5 8.8 nm results. This value is within therange of the calculated spacings of geometrically necessarytions geometrically necessary for a complete misfit accom-

modation in the respective crystallographic direction is misfit dislocations. Therefore, this precipitate’s lattice-parameter mismatch is fully accommodated by misfitcalculated and given in Table II. For a full accommodation

of the small misfit, misfit dislocations with an average spac- dislocations.Figure 8(a) displays a low-magnification HREM micro-ing of dav 5 5.6 to 9.9 nm are necessary.

The coarse A-type precipitate, framed in the lower left- graph of C-type precipitates in an edge-on projection alongthe ^001&-type direction, with a pair of two parallel nitridehand portion of Figure 1(a), is imaged in the weak-beam

dark-field micrograph, displayed in Figure 7, at a higher platelets spaced about 5 nm apart horizontally in the centerof the micrograph. The tendency to form stacks of closelymagnification. This type of imaging is particularly suitable

for detecting misfit dislocations, as only the strong elastic spaced parallel platelets has also been observed by FIM(Figure 4). The thickness of the nitride platelets is about 1strains close to a dislocation core cause significant contrast

effects.[20] In Figure 7, there are 29 dislocations along the nm, which is very similar to the value determined from theFIM micrographs. With the diameter of these platelets being127 nm interfacial length of the plate-shaped precipitate.

Taking into account that dislocations at both interfaces of at least about 100 nm, the aspect ratio (diameter divided bythickness) is greater than 100.the precipitate are imaged, an average misfit dislocation

Sections of the central region in Figure 8 are enlarged inFigures 8(b) and (c) to resolve the atomic structure of thetwo parallel precipitates and the surrounding matrix. Figure

Fig. 7—Weak-beam dark-field TEM micrograph of the A-type precipitatein the frame in the lower left-hand part of Fig. 1(a), showing misfit disloca-tions at the interface of the precipitate. Fe-2 at. pct Mo-0.4 at. pct SbFig. 6—Schematic of the simple cubic Mo3N2 structure and the Baker–

Nutting orientation relationship with the bcc a-iron matrix. nitrided for 10 h at 600 8C.

Table II. Space Group, Composition, Metal-to-Nitrogen Ratio (M/N), Lattice Parameters a and c for Mo3N2, g-Mo2N, andb-Mo2N, and Nonzero Components of the Misfit Tensor «ij with Respect to Bcc Fe (aFe 5 0.28865 nm) and a Baker–Nutting

Orientation Relationship; Also, the Average Spacing, dav , of the Geometrically Necessarily Misfit Dislocations for FullyAccommodating the Misfit «11 and «22 is Given.

Compound SpaceName Group Composition M/N a (nm) c (nm) «11 5 «22

e [%] «33f [%] dav

g [nm]

Mo3N2 Pm3ma Mo3N2 1.5 0.4165a c 5 a 2.74 45.3 7.4g-Mo2N Fm3mb Mo2N0.76 to Mo3N0.86

b 2.63 to 2.33 0.4137 to 0.4157b c 5 a 2.05 to 2.54 44.3 to 45.0 9.9 to 8.0b-Mo2N I4l/amd b Mo2N0.76

b 2.63 0.4200b 0.4005b,c 3.60 46.5 5.60.8010b,d

aRef. 18.bRef. 17.cFace centered tetragonal pseudo unit cell.dTetragonal unit cell.

e«11 5 «22 5a/!2 2 aFe

aFe.

f«33 5c 2 aFe

aFe.

gdav 5b

«11, with Burgers vector b 5 (aFe /!2)^110&.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 33A, AUGUST 2002—2321

Fig. 8—(a) Low magnification HREM micrograph of C-type molybdenum nitride platelets in Fe-2 at. pct Mo-0.5 at. pct Sn nitrided for 10 h at 600 8C.(b) and (c) Two areas of (a) enlarged sufficiently to resolve the crystallographic lattice. The incident beam is parallel to the [001] zone axis of the a-Fe matrix.

possibility of artifact dislocations being introduced duringspecimen preparation. Therefore, platelet 1 retains full coher-ency at the broad interfaces, while the number of dislocationsat the interfaces of platelet 2 is less than half the numberof geometrically necessary misfit dislocations.

The C-type platelets observed after nitridation for 24 hoursat 550 8C are thinner and much smaller than the plateletsobserved after a 10 hour nitridation at 600 8C. Because oftheir smaller size, the degree of coherency must be evenhigher than that for the platelets produced by a 10 hournitridation treatment at 600 8C, and they are most likelyfully coherent with the matrix.

Fig. 9—Schematic of Burgers circuit analysis of platelets 1 and 2 in Fig. In conclusion, the A-type precipitates of the coarse precip-8. Platelet 1 (top) is essentially coherent across 88-nm length of its broaditate structure are semicoherent and have achieved a near-interfaces. Platelet 2 (bottom) exhibits four dislocations at its top interfaceequilibrium accommodation of their misfit strains, while theand five dislocations associated with its bottom interface. The numbers in

parentheses are the uncertainties in identifying the lattice fringes. broad faces of the C-type platelets are either completelycoherent or have significantly fewer dislocations than geo-metrically necessary.

9 is a schematic of Burgers circuits encompassing the twoplatelets and their respective top and bottom interfaces over C. Composition of Precipitates and at their Interfacesa length of 88 nm. The Burgers circuits were carried outusing the {110} planes, enabling the detection of the ^110&- Figures 10(a) through (c) display one-dimensional APFIM

concentration profiles for Fe, Mo, and N across nitride plate-edge component of suspected misfit dislocations. Platelet 1did not reveal any interfacial dislocations, with a counting lets of the small-scale C-type microstructure obtained by

nitridation at 550 8C for 24 hours. This analysis was per-error of a single lattice plane, while four dislocations couldbe detected at the top interface of platelet 2, and five disloca- formed with the analysis cylinder close to a ^001&-type direc-

tion, producing two possible geometries for the cutting oftions could be detected at or close to its bottom interface.This results in an average dislocation spacing of dav 5 20 the analysis cylinder through a platelet with a {100} habit

plane. The vertical arrowheads below Figure 10(c) denotenm for platelet 2, which is an upper estimate because of the

2322—VOLUME 33A, AUGUST 2002 METALLURGICAL AND MATERIALS TRANSACTIONS A

closely spaced nitride platelets can be recognized by thenumerous Mo (shown in red) and N (shown in green) atoms.A large portion of the Mo and N atoms are detected as MoNcomplex ions, in the time-of-flight mass spectrum, and theyare represented in the reconstruction as circles filled half ingreen and half in red. Due to statistical limitations, the analy-ses of these larger C-type platelets do not, unfortunately,allow for a conclusive statement about a segregation effect.

Figures 12(a) through (d) display one-dimensionalAPFIM concentration profiles for Fe, Mo, N, and Sn,obtained from an A-type thick platelet with a 7 6 3 nmthickness in Fe-2 at. pct Mo-0.5 at. pct Sn after 10 hours ofnitridation at 600 8C. The analysis starts within the precipi-tate and crosses the rear interface into the matrix. This precip-itate’s composition is Fe3Mo62N35 (at. pct). The metal-to-nitrogen ratio is very close to the ideal stoichiometry ofMo2N. Directly after the precipitate’s interface, 10 Sn atomsare detected within a matrix region about 2 nm thick. Thisresults in an average Sn concentration of 6.0 6 1.8 at. pctin this region, which is a significant Sn enrichment. Thecorresponding Gibbsian interfacial excess of Sn is GSn 5 76 3 nm22, using an approach to determine Gi from atom-

Fig. 10—Atom-probe field-ion microscopy concentration profiles for (a) probe data described in Reference 21 for grain boundariesFe, (b) Mo, and (c) N across several C-type nitride precipitates in Fe-2 at. and in Reference 22 for heterophase interfaces.pct Mo-0.5 at. pct Sn after nitridation for 24 h at 550 8C. Profile (d )

Table III lists the composition, morphology, and segrega-displays the spatial distribution of individual Sn atoms. The concentrationtion characteristics of six individual coarse precipitates ofvalues are based on a calculation employing 25 atoms per data point.the A and B types. A significant enrichment in Sn is observedat four precipitate/matrix interfaces, with the values of

molybdenum nitride precipitates cut normal to their habit GSolute given in Table III. One analysis resulted in a singleplane (face-on geometry). This analysis geometry results in Sb atom as the second atom detected after crossing thesharp peaks of the Mo and N concentrations and a short interface. Because the solute concentration is only 0.4 at.plateau representing the precipitate’s composition. The aver- pct Sb, this observation is significant but not sufficient toage composition of the precipitates analyzed in the face-on establish a value of GSb. In comparison to the thin-plateletgeometry is Fe7.7Mo50.0Sn0.2N42.1 (at. pct). Assuming that all structure, the Fe content of the precipitates is significantlymetal atoms occupy sites of the metal lattice, this composi- lower.tion results in a metal-to-nitrogen ratio of 1.37 6 0.17.Within the error limits, this value is compatible with the

IV. DISCUSSIONideal metal-to-nitrogen ratio of 1.5 of the stoichiometricMo3N2 phase. The observation of excess nitrogen, however, The segregation characteristics of the various observed

nitride precipitate microstructures can be summarized asis not exceptional and has been reported previously.[10] Withthe observation of only a single Sn atom in all six precipitates follows: four out of six coarse precipitates of the A and B-

type microstructures formed during nitridation at 600 8Canalyzed in a face-on geometry combined, it can be con-cluded that Sn is not enriched in the nitride precipitates. revealed segregation of Sn or Sb, while none of the thin

platelet shaped C-type precipitates formed at 550 8C exhib-For the second analysis geometry, the analysis cylinderis in the plane of the platelet (edge-on geometry). If the ited an enrichment of the solute species at the investigated

broad interfaces. Even though the C-type microstructureprecipitate’s thickness is smaller than the diameter of thecylinder, atoms from both the precipitate as well as the offers much more interfacial area per unit volume than do

the coarse type-A and -B, structures, it is found that thematrix are detected, convoluting the measured composition.The section marked with a horizontal arrow and labelled solute atoms remain, to the largest part, in solid solution in

the matrix. Because of the presence of essentially the entire“edge on” in Figure 10 represents such an edge-on analysis.Figure 10(d) displays the distribution of individual Sn solute content in the matrix, the absence of substantial segre-

gation at the C-type precipitates cannot be due to an exhaus-atoms in this analysis. A correlation of the occurrence ofSn atoms with nitride precipitates and their interfaces could tion effect, that is, the total interfacial area is so large that

all solute atoms are adsorbed at the interfaces at a lowernot be established. It is concluded that segregation at theseinterfaces is either small or nonexistent. The average mea- segregation level.

We now discuss the possibility of kinetic effects, such assured Sn concentration of the matrix is 0.38 6 0.04 at. pct,demonstrating that a large part of the overall Sn content is a diffusion limitation or solute pileup at the moving interface

of a growing precipitate. The redistribution of the solutestill in solid solution in the a-Fe–rich matrix.Figure 11(b) displays a 3DAPM reconstruction of larger species Sb or Sn is controlled by diffusion in the Fe-rich

matrix away from a precipitate’s interface. As the thicknessC-type platelets obtained by a 10 hour nitridation treatmentat 600 8C. The reconstructed volume contains two edge-on of the enrichment zone is small with respect to the diameter

of the platelet- or spheroid-shaped precipitates, the hetero-platelets, from a stack of several precipitates, as shown bythe inset in the FIM micrograph (Figure 11(a)). The two phase interface can be treated, to first order, as being flat

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 33A, AUGUST 2002—2323

Fig. 11—(a) FIM micrograph of molybdenum nitride platelets in Fe-2 at. pct Mo-0.5 at. pct Sn after nitridation for 10 h at 600 8C and (b) 3DAPMreconstruction of the area marked by the white square in (a). The size of the box containing the reconstruction with 7198 atoms is 15 3 15 3 3 nm3. Thepositions of iron atoms are shown in blue, molybdenum in red, nitrogen in green, and tin in turquoise. The atoms detected as MoN complex ions arerepresented by circles one-half red and one-half green. Note well that there is not a quantitative relationship between atomic radii and marker sizes.

for all the precipitates observed. We, thus, use the one-dimensional root-mean-squared diffusion distance awayfrom the precipitate into the matrix half-space, !^x2& 5!2Dxt, to discuss a possible diffusion limitation effect,where Dx is the diffusivity of the solute species and t is thetime. The diffusion coefficients for impurity diffusion in Fehave been determined by References 23 and 24 for Sb andby References 25 through 27 for Sn. The one-dimensionalroot-mean-squared diffusion distance in the half space is,depending on the specific diffusion coefficients, in the rangefrom 100 to 234 nm for 24 hours of annealing at 550 8C,and in the range from 188 to 437 nm for 9 hours of annealingat 600 8C for Sb and in the range from 93 to 405 nm and210 to 648 nm, respectively, for Sn. This means that forboth heat treatments, the diffusion distance of both solutespecies is larger than the thickness of the segregation zoneby at least one order of magnitude, at each annealing temper-ature. This implies that solute diffusion in the matrix issufficient to establish at the very least local equilibrium, andthe solute enrichment cannot be due to a pileup effect infront of the interface of a growing precipitate.

Furthermore, it is stressed that solute segregation isobserved after annealing at the higher temperature, 600 8C.The Gibbs adsorption isotherm[28] implies that the Gibbsianinterfacial excess of solute, Gi , increases as the temperaturedecreases, due to the decreasing value of the entropic portion

Fig. 12—Concentration profiles for (a) Fe, (b) Mo, (c) N, and (d ) Sn across(-TS) of the Gibbs free energy of segregation. The lack of, a molybdenum nitride/a-iron heterophase interface in Fe-2 at. pct Mo-0.5or only a small amount of, solute segregation at the thin at. pct Sn after nitridation for 10 h at 600 8C. The concentration values

are based on a calculation employing 50 atoms per data point.platelets after annealing at 550 8C indicates that a different

2324—VOLUME 33A, AUGUST 2002 METALLURGICAL AND MATERIALS TRANSACTIONS A

Table III. Segregation Characteristics of Six Coarse Precipitates Formed in Fe-2 At. Pct Mo-X, where X 5 0.4 At. Pct Sbor 0.5 At. Pct Sn, after 10 Hours Nitridation at 600 8C (Precipitates 1 through 5) and 9 Hours of Nitridation at 600 8C

(Precipitate 6)

Morphology and Size Segregation CharacteristicsPptNumber X Compositiona M/Nb Shape d or tc (nm) G (nm22)d d (nm)e

1 Sb Mo56N50 1.12 plate t 5 12 6 3 — —2 Sb Fe1Mo27N22 1.23 plate d 5 25 — —

t 5 11 6 13 Sb Mo10N8 1.25 plate t 5 3.5 6 1 1 Sb atom at interface6 Sb Mo145N110 1.32 plate t 5 2 6 1 7 6 2 2 6 14 Sb Fe1Mo104N67 1.57 sphere d 5 50 1.5 6 0.6 1 nmf

5 Sn Fe5Mo97N55 1.85 plate t 5 7 6 3 7 6 3 2 6 1aNumber of detected atoms.bMetal-to-nitrogen ratio.cDiameter, d or thickness, t.dGibbsian interfacial excess of solute, G.eThickness of the segregation zone, d.fRadius of analysis cylinder in tangential analysis geometry.

mechanism is responsible for the attractive interaction and several nanometers thick, as well as spheroidal pre-cipitates about 25 to 50 nm in diameter.between solute atoms and interfaces of the coarser

precipitate. 3. The molybdenum nitride precipitates have metal-to-nitro-gen ratios around 1.5 or 2, corresponding to the stoichio-In this context, the structural differences between the inter-

faces of the coarse precipitates and the thin platelets need metric phases Mo3N2, b-Mo2N, or g-Mo2N, whosecrystallographic structures are closely related, with Moto be taken into account. The TEM and HREM observations

demonstrate that the coarse A-type precipitates are semico- atoms partially replaced by Fe atoms.4. The solute species Sn and Sb segregate at the heterophaseherent, while the small platelets of the C-type microstructure

most likely retain coherency with the matrix at their interfaces of the coarse precipitates, with Gibbsianinterfacial excesses of up to 7 6 3 nm22.broad interfaces.

Both Sb and Sn atoms exist in the Fe-rich matrix as 5. This segregation is related to interfacial misfit disloca-tions observed at the interfaces of the coarse precipitates,strongly oversized atoms with a volume mismatch of 36.40

pct (Sb) and 67.70 pct (Sn).[29] The large Sb and Sn atoms while the broad faces of the thin platelets remain coherentand do not exhibit segregation.cause lattice distortions in the Fe-rich matrix, which allow

for an interaction with the local elastic strains or the free 6. The elastic-strain energy of the oversized solute atomsSb and Sn (36.4 and 67.7 pct with respect to Fe, respec-volume associated with the cores of interfacial misfit disloca-

tions,[3] while the coherent interface of the thin platelets does tively) can be relaxed due to their segregation at themisfit dislocations of the semicoherent interfaces, whilenot provide attractive sites for the oversized atoms.

An interesting fact is that the thin platelets of the C-type segregation at coherent interfaces is difficult or the levelof segregation is far too low for detection by the atom-microstructure can grow to extraordinarily large aspect ratios

of 100 and higher, while remaining only a few atomic planes probe technique.7. The segregating Sn or Sb atoms reside, most likely, inin thickness. Similarly large aspect ratios have been reported

for molybdenum nitride precipitates produced by internal the elastic-strain fields or cores of the interfacial misfitdislocations.nitridation by other investigators also.[7] The large aspect

ratio indicates that the platelets grow primarily at their edgesin two dimensions, while the growth in thickness, controlledby nucleation and movement of growth ledges at the broad ACKNOWLEDGMENTSinterfaces of the platelets, is a much slower process.

This research was supported by the National ScienceFoundation (Bruce A. MacDonald, grant officer) under Grant

V. CONCLUSIONS No. DMR-9728986. The high-resolution electron micros-copy was performed at the Materials Science Division,Argonne National Laboratory, and Dr. Rosann Csencsits is1. Internal nitridation of Fe-2 at. pct Mo-X, where X 5 0.4

at. pct Sb or 0.5 at. pct Sn, at 550 8C results in small, thanked for cheerful assistance. DI received partial supportthrough the Max Planck Research Prize of D.N.S. and fromthin plate-shaped molybdenum nitride precipitates with

{100} habit planes, 40 to 100 nm in diameter and two the Deutsche Forschungsgemeinschaft. The authors thankMs. P. Epps, Mr. M. Potter, Mr. A. Pyzyna, Ms. E. Siem,atomic planes in thickness.

2. Internal nitridation at 600 8C produces, in addition to Mr. E. Spoerke, and Ms. I Uttayarat for their contributionsto this research as a result of a series of undergraduatea dispersion of larger platelets of the thin-platelet type

observed after nitridation at 550 8C, a much coarser struc- research projects, which were supported by the REU pro-gram of the National Science Foundation.ture of nitride platelets that are 0.1 to 1 mm in diameter

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 33A, AUGUST 2002—2325

Report No. 64, The Iron and Steel Institute, London, 1959, pp. 1-22.REFERENCES15. P. Villars and L.D. Calvert: Pearson’s Handbook of Crystallographic

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