Authors Copy EXPERIMENTAL STUDY OF SEGREGATION IN PLANE FRONT SOLIDIFICATION AND ITS RELEVANCE TO IRON METEORITE SOLIDIFICATION

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1 PROCEEDNGS OF THE FOURTEENTH LUNAR AND PLANETARY SCENCE CONFERENCE, PART 1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 88, SUPPLEMENT, PAGES B343-B352, NOVEMBER 15, 1983 EXPERMENTAL STUDY OF SEGREGATON N PLANE FRONT SOLDFCATON AND TS RELEVANCE TO RON METEORTE SOLDFCATON R. Sellamuthu and J.. Goldsten Lehgh Unversty Abstract. A drectonal soldfcaton technque was developed and appled to the problem of fractonal crystallzaton of an ron meteorte parent body. Samples of Fe-N alloys close to meteorte compostons and contanng S, P, and C were made. The soldfed structures contan secondary phases such as sulphdes wthn the proeutectc sngle crystal austente (taente). As a result of these experments, we propose that the secondary phases observed n ron meteortes were formed durng prmary soldfcaton of austente (taente). The measured composton profles of N, P and C n the alloys were used to explan the elemental dstrbuton wthn a chemcal group of ron meteortes. An analytcal procedure was appled to determne the equlbrum dstrbuton coeffcents as a functon of fracton soldfed for N and P from the composton profles. The dstrbuton coeffcents of N and P agree wth prevous values. These dstrbuton coeffcents are of partcular nterest n the determnaton of the elemental dstrbutons n ron meteortes. ntroducton t s of consderable nterest to explan the elemental dstrbuton wthn a chemcal group of ron meteortes and the mode of soldfcaton whch controls the process. f a defnte trend n the elemental dstrbuton can be establshed by assumng a certan model for the fractonaton process, then the bulk elemental composton for each ron meteorte group can be estmated. The knowledge of the bulk compostons may lead to greater understandng of the formaton hstory of the ron meteortes and ther parent bodes. n order to explan the elemental dstrbuton, Scott [1972] has proposed a fractonal crystallzaton model n whch elements preferentally segregate to the coexstng lqud or sold phase. Fractonal crystallzaton at the sold-lqud nterface of a growng crystal can occur by ether a planar or dendrtc mode. Scott [1972] has appled the planar growth model to explan the elemental dstrbuton wthn a chemcal group of meteortes. Narayan and Goldsten [1982] have appled a dendrtc fractonal crystallzaton model to explan the varaton of Ge wth N n AB and VA ron meteorte groups. To apply the fractonal crystallzaton model to the soldfcaton of the ron core of a parent body, the dstrbuton coeffcents of varous elements are requred. These ds- Copyrght 1983 by the Amercan Geophyscal Unon Paper number 3B /83/003B trbuton coeffcents are nfluenced by elemental nteractons whch occur durng soldfcaton. Several studes have reported such coeffcents [Narayan and Goldsten, 1981; Narayan and Goldsten, 1982; Wlls and Goldsten, 1982; Jones and Drake, 1982]. However, the complextes arsng from elemental nteractons wth mnor elements n Fe-N alloy systems have not been clearly characterzed. n addton to varatons n composton, the ron meteortes show consderable varaton n ther mcrostructure. These mcrostructural varatons are n many cases determned n the soldfcaton process. Durng the soldfcaton process, the frst sold phase to form from the lqud s the austente (taente), n the Fe-N system. The austente may subsequently transform to other consttuents such as kamacte durng coolng. Drect metallographc and chemcal examnaton has been employed to trace the orgn of the sulphdes present n the ron meteortes [Esbensen et al., 1982; Esbensen and Buchwald, 1982]. Recently, Esbensen and Buchwald [1982] have proposed that the elongated sulphde nodules (trolte-fes) found n the sngle-graned, Cape York meteorte develop from entrapped eutectc lqud formed durng the last stage of dendrtc soldfcaton. n summary, consderable questons stll exst as to the orgn of ron meteortes wth large varatons n composton and structure. The scope of ths study s (1) to qualtatvely explan the elemental dstrbuton whch occurs durng soldfcaton wthn a chemcal group of ron meteortes, (2) to determne equlbrum dstrbuton coeffcents for the soldfcaton process, and (3) to understand the development of secondary phases durng the soldfcaton of ron meteortes. Theory 1. Soldfcaton of the ron Core of a Parent Asterodal Body n the parent body core soldfcaton model, t has been assumed that the molten ron core s encased n a slcate mandrel as llustrated n Fgure 1. n the planar growth model of Scott [1972], the frst sold formng on the nner surface of the slcate mandrel would grow towards the center of the core wth a planar nterface as the body cools. n the case of dendrtc soldfcaton [Narayan and Goldsten, 1982], the nterface growth would be dendrtc, and a soldfyng regon consstng of dendrtes and parttoned lqud would exst as llustrated n Fgure 1. n ths study, no attempt has been made to determne whch soldfcaton mode, planar or dendrtc, would B343

2 B344 Sellamuthu and Goldsten: Expermental Study of Segregaton //,,/" SLCATE MANDREL / \"'"s compostons. n ths technque, a column of molten lqud s undrectonally soldfed wth a planar nterface between the coexstng lqud and sold phases (refer to Expermental Procedure secton). The segregaton of solute elements n many cases determnes the soldfcaton mcrostructure. n addton, the plane front soldfcaton technque has been employed to determne the equlbrum dstrbuton coeffcents n the present laboratory Fe-N alloys. These alloys contan mnor elements such as P, S, and C whch are present n ron meteortes. / SOLD + LQUD REGON DENDRTE... g.. Schematc lusc aco o dend cc soldfcaton model o the on co e o a pa e c as e odal body, sho g a sold (soldfed egon), a sold- qud egon (soldfyng egon), and a lqud egon. Deadlnes o several szes a e andom- 7 dstrbuted. The sze dsc buco a ses snce he deadlnes age d e en s ages o soldfcaton. be applcable to the parent body core soldfcaton. However, the followng dscusson s amed at showng the smlarty between planar and dendrtc soldfcaton processes. Fgures 2a and 2b llustrate a smplfed model of planar and dendrtc soldfcaton processes, respectvely. Dendrte growth s llustrated by an array of ndvdual dendrtes. A convenent way to look at the soldfcaton process for a sngle dendrte s to assume a volume element (shown n Fgure 2b) of the order of one-half the dstance between two dendrtes. Wthn ths volume element, a small amount of sold forms as the dendrte tp grows. Subsequently, the nterface growth occurs by a planar mode snce the sold thckens laterally wthout formng any other dendrtes wthn the volume element (Fgure 2b). Durng soldfcaton by ether the planar or dendrtc mode, the parttonng of solute (fractonal crystallzaton) occurrng between sold and lqud phases gves rse to a compostonal varaton (segregaton) n the soldfed alloy. n the planar growth model [Scott, 1972] for ron meteortes, the fractonal crystallzaton s operatve over the entre ron core of the parent body. n the dendrtc soldfcaton model for ron meteortes, the fractonal crystallzaton s operatve over a scale of one-half the dendrte arm spacng (refer to Fgure 2b). n addton, the extent of segregaton due to fractonal crystallzaton may not be the same n both cases. 2. Plane front soldfcaton n ths study, a plane front soldfcaton technque s used to study the segregaton behavor of Fe-N alloys close to meteorte PLANAR SOLD GROWTH PROCESS LQUD LQUD- DSTANCE, X (A) SOLD NTERFACE DENDRTC GROWTH PROCESS SOLD SOLD AND LQUD LQUD x--. l DSTANCE, LQUD X = O (DENDRTE VOLUME ELEMENT CORE) X = X (LQUD-SOLD NTERFACE) k =L (MDPONT BETWEEN TWO DENDRTES) Fg. 2. Smplfed llustraton of planar (A) and dendrtc (B) growth models. n the planar mode, fractonal crystallzaton occurs along the X drecton. n dendrtc mode, fractonal crystallzaton occurs along the X drecton.

3 Sellamuthu aad Goldsten: Expermental Study of Segregaton B Determnaton of dstrbuton coeffcents n a fully soldfed sample, the solute elements are nonunformly dstrbuted along the growth drecton. Fgure 3 llustrates a schematc composton profle n an eutectc formng alloy system. The eutectc refers to the last sold to form n a soldfcaton process. The proeutectc regon (Fgure 3) refers to the frst or prmary sold to form durng soldfcaton. The austente or taente phase s the prmary sold n Fe-N alloy systems, whch may contan some nonmetallc nclusons such as sulphdes. Assumng no sold-state dffuson and complete mxng of the solute elements n the lqud durng soldfcaton, the dstrbuton coeffcent, K, at any fracton soldfed, fs'' s computed from Fgure 3 as: where: CL(fS') = C S K(fs') =CL(fS, ) (1) 1-f E C df S + C E f fs' (1 - f$ ') C S = the nterface composton n the sold at a gven value of fs' CL(f S ) = the composton of he lqud at a gven value of f ' df S = the dfferental e{ement of the C C E fracton soldfed = the composton = the composton of eutectc f = the fracton eutectc E n addton, the composton of the lqud, CL(fS'), ahead of the sold-lqud o cs PROEUTECTC REG -- f FRACTON SOLDFED, fs CE.,-- fe Fg. 3. llustraton of an assumed composton profle whch may occur n plane front soldfcaton. (2) '- --METAL ROD CONNECTED TO ARGON NLET MOTOR ORVE ASSEMBLY TO VACUUM GLASS =- CYLNOER - ALUMNA TUBE [ 1 '-M :TAL BOTTOM ELECTRCAL FEED THROUGH ALLOY L= LQUD 2 HEATNG ELEMENT S=SOLD PLATE Fg. 4. Schematc drawng of the apparatus for plane front soldfcaton. nterface and the fracton soldfed, f_', are related by (2). Therefore, the dstrbuton coeffcent can be expressed as a functon of the lqud composton. Expermental Procedure The apparatus used for the plane front soldfcaton experments s shown schematcally n Fgure 4. t conssts of a resstance heated hgh temperature furnace mounted nsde a vacuum tght glass chamber. The glass chamber s attached to a vacuum system and contans an nlet for argon. A metal rod, attached to a motor drve assembly, functons as the crystal pullng mechansm. n a typcal expermental run, the alumna tube contanng the alloy charge s suspended nsde the furnace by means of the metal rod as shown n Fgure 4. The alloy s frst melted by supplyng power to the heatng element under a partal argon atmosphere of about 600 mcrons. Then, the alumna tube contanng the molten alloy s wthdrawn from the furnace nto a metal chll block by means of the crystal pullng mechansm. The growth rate employed was about 10 cm/s and the thermal gradent n the furnace was about 170øC/cm. Ths procedure was employed for three alloys of the followng composton: (1) Fe wt % N-1.64 wt % S-1.04 wt % C, (2) Fe wt % N-l.07 w t% P, and (3) Fe-6.93 wt % N-0.78 wt % S-0.3 wt % P. Samples made by the above process were about 5 mm n dameter and 15 mm n length. A metallographc specmen was prepared by mountng the whole sample longtudnally n lucte. The specmen was ground on the belt grnder to reveal suffcent area, and was subsequently mcropolshed n an alumna slurry. Optcal examnaton showed that the samples were soldfed wthout formng any cells or dendrtes. The absence of cells and dendrtes

4 B346 Sellamuthu and Goldsten; Expermental Study of Segregaton 0 S 2 PROEUTECTC REGON 5 E.UTECTtC 7.1MM 5.6MM 1.4MM fs=o... '... '... ß,OZ.50.55,90.94 FRACTON SOLDFED (fs) Fg. 5. llustraton of the mcrostructure of a plane front sample (Fe wt % N-1.64 wt % S-1.04 wt % C) showng the morphology and dstrbuton of sulphdes (S) n the prmary austente or taente (¾) phase at three locatons (1,2,3). The eutectc contans ntertwnned hgh-n ( > 12%) ¾ and FeS. The dstance covered by each electron mcrograph s much smaller than the length of the sample. The whte arrows ndcate the growth drecton. The prmary ustente/eutectc nterface s ndcated by. confrmed that the samples were grown wth a planar nterface durng soldfcaton. n order to determne the dstrbuton coeffcents usng (1) and (2), solute composton profles are requred. The composton profles were determned usng a JEOL 733 electron probe mcroanalyzer. X ray data were obtaned along the growth drecton (the longtudnal drecton n the specmen) at ntervals of about 1 mm usng a 50 m x 50 m raster scan. An acceleratng voltage of 15 kv and a beam current of 50 nanoamperes were employed. To analyze carbon, a defocussed beam 20 m n dameter and an acceleratng voltage of 5 kv were used. Usng ths procedure, each sample was analyzed for all the elements present n the alloy. Results and Dscusson 1. Mcrostructural aspects Fgure 5 llustrates the mcrostructural features of an Fe-N-S-C alloy that was grown by the plane front soldfcaton technque. Scannng electron mcrographs of selected areas representng approxmately the ntal, mddle, and fnal stages of the soldfcaton process are shown n Fgure 5. The structure of the plane front sample (Fgure 5) conssts of a proeutectc regon (prmary austente, ¾, and FeS) and an eutectc. Fgure 5 shows that the sulphdes (label S) are dstrbuted nonunformaly wthn the proeutectc regon, and exhbt algned cylndrcal and ellpsodal morphologes. The last porton of the proeutectc regon shows a larger proporton of sulphdes (trolte). The eutectc conssts of a metallc (hgh-n austente) and a nonmetallc (FeS) phase. Fgures 6 to 9 llustrate the mcrostructural features of an Fe-N-S-P alloy that was grown by the plane front soldfcaton technque. The sulphdes (trolte) are dstrbuted wthn the proeutectc regon wth globular and ellpsodal morphologes. Durng the last stages of the proeutectc soldfcaton of the Fe-N-S-P alloy, certan other secondary phases form; namely, phosphde, phosphde eutectc, and sulphde eutectc. The morphologes of these secondary phases are shown n Fgures 7 to 9. The sulphde eutectc (SE) (Fgure 7) has been dentfed qualtatvely by ts characterstc hgh-nckel austente rm [Buchwald, 1975]. A focussed electron beam analyss showed that the rm contans about 17% nckel. The morphology of the sulphde eutectc s ellpsodal (Fgure 7). The phosphde eutectc (Fgure 8) s globular and contans three consttuents: (FeN) P, ¾, and FeS. The phosphde phase (FeN) P s own n Fgure 9 has an elongated morphology and some sulphdes have formed adjacent to t. These secondary phases--phosphde, phosphde eutectc, and sulphde eutectc--are not as abundant as the sulphde (FeS) phase observed n the proeutectc regon (Fgure 6). The complextes of elemental nteracton effects on the structure of the secondary phases can be noted from Fgures 6 to 9. The addton of P to the Fe-N-S system changes the cylndrcal morphology of sulphdes (Fgure 5) to a globular or ellpsodal morphology (Fgure 6). Addtonally, the addton of P ntro-

5 Sellamuthu and Goldsten: Expermental Study of Segregaton B347 '".. ':S... j ' ' *.",l ----' - ;... :... *:.. ;.. : :. *: :'%:. :..... e.: :.:;:...:::;:::.; :;:½. :%..' PROEUTECTC REGON EUTECTC,--r'....SMM... 'T" "'T"" 14'8 :0.'; MM ' -'.....:4 MM...'-r' ', "T' MM :5.02MM.SMM.5MM '!... ' ' '... '... ß ,.97 FRACTON SOLDFED (fs) " Fg. 6. llustraton of the mcrostructure of a plane front sample (Fe-6.93 wt % N-.78 wt % S-.3 wt % P) showng the morphology and dstrbuton of sulphdes (S) n the prmary austente (¾) phase at three locatons (1,2,3). The phosphde eutectc (A) contans: (1) (FeN) P (grey) (2) ¾ (lght), and (3) FeS (dark). The sulphde eutectc (B) conssts ofl (1) ¾ lght), and (2) FeS (dark). The whte arrows ndcate the growth drecton. The prmary austente/eutectc nterface s ndcated by. duces other secondary phases such as phosphde (schreberste), phosphde eutectc, and sulphde eutectc nto the proeutectc regon (Fgures 7-9). Snce a slow growth rate s used n ths study for the plane front soldfcaton, the amount of undercoolng requred to nucleate certan secondary phases on the austente-lqud nterface s mnmal. Any small thermal fluctuaton ahead of the nterface facltates the nucleaton event. The sulphdes and the phosphdes form by such a nucleaton process and grow along wth the prmary austente (taente). We postulate that the formaton of sulphde and phosphde eutectcs n the prmary austente (taente) s a two-step process. n the frst step, an enrchment of P and S n the bulk lqud occurs as soldfcaton progresses [Wlls and Goldsten, 1982; Jones and Drake, 1982]. Lqud droplets of sulphde and phosphde eutectcs form wthn the lqud phase when the enrchment of P and S exceeds the saturaton level. n the second step, the sold-lqud nterface moves across the mmscble lqud eutectc droplets and ncorporates some of these droplets nto the sold-austente phase. n the soldfcaton of the ron core of a parent body, the secondary phases such as sulphde, phosphde, sulphde eutectc and phosphde eutectc form wthn the prmary austente (taente) by the above supersaturaton process. Based on mcrostructural observatons (Fgures 5 and 6), we propose that the sulphdes (trolte), ncludng the algned sulphdes observed n ron meteortes such as Cape York, were formed along wth the prmary austente (taente) phase durng the proeutectc soldfcaton. Based on the mcroscopc observatons of Fgures 6 to 9 and the postulated formaton process for secondary phases as above, the phosphde eutectc observed n hgh-p, low-n rons such as Sao Julano, Santa Luca, etc. [Buchwald, 1975], and the sulphde eutectc wth ts characterstc austente (taente) rm observed n Cape York [Esbensen and Buchwald, 1982], Nordhelm [Buchwald, 1975], etc., were formed durng proeutectc soldfcaton. Some meteortes such as Mundrablla, Sorot, and Ptts, whch contan an ntertwnned metal/sulphde structure, may have orgnated from the eutectc regon of the soldfed structure (note Fgures 5 and 6). 2. Elemental Dstrbuton The measured compostons of two soldfed Fe-N alloys are plotted n Fgures 10 and 11 as a functon of the fracton soldfed. Usng a statstcal method [Goldsten et al., 1981], the relatve error n the measured chemcal compostons was estmated. The relatve errors for N and S are below 3%. For P, the relatve error s below 5%. However, a 10% relatve error was estmated for P contents below 0.1 wt %. The relatve error for C s below 15%. Fgure 10 shows that the composton of N n the Fe-N-C-S alloy remans almost constant through most of the proeutectc regon, and ncreases slghtly n the end porton. Ths end porton approxmately corresponds to the locaton 3 of the proeutectc regon n Fgure 5. The composton of N ncreases sgnf-

6 B348 Sellamuthu and Goldsten- Expermental Study of Segregaton Fg. 7. Scannng electron mcrograph showng the morphology of a sulphde eutectc (SE) present n locaton 3 of Fgure 6. The sulphde eutectc conssts of: (1) FeS (dark), and (2) ¾ (whte rm). The rm s hgher n N (417%). The whte arrow ndcates the growth drecton. Scale bar: 10 m cantly n the eutectc regon of the Fe-N-C-S alloy. The dscontnuty n the composton profle of N occurs at the nterface between the proeutectc and eutectc regons because the solublty of N n the proeutectc austente s dfferent from that n the eutectc. The above argument s vald for varous other elements such as P, S, and C. However, t s mperatve to understand that the solublty of a gven element n the eutectc may not be constant n multcomponent systems. The composton of N vares systematcally n the ntal porton of the eutectc regon of the Fe-N-C-S alloy, and thereafter vares randomly due to certan lack of algnment of the metal phase and FeS present n the eutectc. Therefore, Fgure 10 shows a constant lne representng the average composton of the last porton of the eutectc regon. Carbon s parttoned to the proeutectc wth a consderable composton gradent. The exstence of the composton gradent of C shows that dffuson n the sold-state s neglgble n the plane front soldfcaton experments. The composton of C does not show any sgnfcant gradent close to the end of proeutectc soldfcaton, approxmately correspondng to locaton 3 n Fgure 5. The solublty of C n the eutectc s consderably lower. Fg. 8. Scannng electron mcrograph showng the morphology of a phosphde eutectc (PE) present n locaton 3 of Fgure 6. The phosphde eutectc conssts of: (1) (FeN) P (grey), (2) ¾ (lght) and (3) FeS (dark). 3The labels P and S ndcate (FeN) P and FeS, respectvely. The whte arrowvndcates the growth drecton. Scale bar: 10 m.p Fg. 9. Scannng electron mcrograph showng the morphology of a phosphde (P) present n locaton 3 of Fgure 6. The label S ndcates FeS. The whte arrow ndcates the growth drecton. Scale bar: 10 m y

7 ...._ - Authors Copy Sellamuthu and Goldsten: Expermental Study of Segregaton B z 20.0 ß 12.0 o.o FRACTON SOLDFED ', '--.-'.PROEUTECTC N ß %, C ß 1.04 %, S-1.64 %,,L-1 BAL. F$ T_ REGON (X + FeS) --- EUTECTC REGON.--- ß ] DSTANCE (MM) Fg. 10. Composton profles of N and C n Fe wt % N-1.64 wt % S-1.04 wt % C alloy sample grown by the plane front soldfcaton technque. n the Fe-N-S-P alloy (Fgure 11), the N and P compostons ncrease slghtly wth fracton soldfed n the ntal porton of the proeutectc regon, and show a consderable gradent close to the end of proeutectc soldfcaton. The plane front sample of the Fe-N-S-P alloy shows two types of eutectcs (Fgure 6): phosphde (label A) and sulphde (label B). Snce the fracton soldfed s very small for each eutectc, the average compostons of N and P n phosphde (label A) and sulphde (label B) eutectcs are plotted n Fgure 11. The average composton was determned for each eutectc from 20 to 40 random measurements. The composton of S was measured only n the eutectc regon snce the amount of sulphdes present n the proeutectc regon s DSTANCE (MM) o NTAL COMPOSTON OF MELTf -..; ß - 12.o N %, P-1.07%, BAL. Fe f z 0.8 O.O PROEUTECTC REGON, 7'--- REGON FRACTON SOLDFED Fg. 12. Composton profles of N and P n Fe wt % N-l.07 wt % P alloy sample grown by the plane front soldfcaton technque. neglgble. The average composton of S s 25.6% n the eut ctc of the Fe-N-C-S alloy. The average compostons of S are 3.1% and 29.2% n the phosphde and sulphde eutectcs, respectvely, of the Fe-N-S-P alloy. The average compostons were determned from 20 to 40 random measurements. 3. Formaton of ron Meteortes Wthn a Chemcal Group The structure of the plane front samples (Fgures 5 and 6) can be related to the structure of an ron core of the parent body that! z O SO 70 pl _--- NTAL COMPOSTON OF MELT H - A- PHOSPHDE N- 6.95, S- EUTECTC.78, P-.S, BAL F, / 1 - B- SULPHDE EUTECTC o d'ød'ø... oooo. o-ø / :_... A-H '- = PROEUTECTC REGON, 7' 76 Z 1.0 N E. o. 8 0 '. z ß p 0.3 o. Fe-10.54%N-1.O7%P ALLOY 02, go FRACTON SOLDFED Fg. 11. Composton profles of N and P n Fe-6.93 wt % N-0.78 wt % S-0.3 wt % P alloy sample grown by the plane front soldfcaton technque.! FRACTON SOLDFED Fg. 13. Expermentally -determned dstrbuton coeffcents for N and P n Fe wt % N-l.07 wt % P alloy plotted as a functon of the fracton soldfed. The dstrbuton coeffcents termnate at the start of eutectc soldfcaton.

8 B350 Sellamuthu and Goldsten: Expermental Study of Segregaton TABLE 1. Fe-N-P Alloy System Present Study Narayan and Goldsten [ 1981] Narayan and Present Study Goldsten [1981] soldfed by ether a planar or dendrtc process. n the case of planar soldfcaton [Scott, 1972], the ntal structure shown n Fgures 5 and 6 represents the surface of the ron core and the eutectc represents the center. Accordng to the planar model, a sngle-graned soldfed body wth structures smlar to that shown n Fgures 5 and 6 would be several klometers n sze. n the case of dendrtc soldfcaton [Narayan and Goldsten, 1982], the structure (Fgures 5 and 6) represents that of a sngle dendrte (essentally a sngle crystal) of several meters to klometers n sze. The ntal porton of Fgures 5 and 6 represents the dendrte center and the eutectc represents the regon between two dendrtes (nterdendrtc regon). n ether case, the consttuents of the soldfed structure of the ron core of the parent body are of consderable sze. Fgures 5 and 6 show that the ntal porton of the soldfed structure s vrtually free of sulphdes. Another area such as locaton 3 of Fgures 5 and 6 shows the presence of consderable amount of sulphdes. Thus, the soldfed structure of a meteorte parent body can be as heterogeneous as the structure shown n Fgures 5 and 6. Fgures 10 and 11 show the segregaton pattern measured n laboratory alloys. The composton of N ncreases wth that of P n the Fe-N-S-P alloy. Ths segregaton behavor of N and P s smlar to that observed n several ron meteorte groups [Scott, 1972]. ron meteortes are consdered to be fragments of soldfed parent bodes. Fragmentaton of the soldfed parent body wth such heterogeneous structures (Fgures 5 and 6) would gve rse to meteortes close n N and P contents but contanng varyng amounts of sulphdes. We can therefore nfer that a number of meteortes wth large varatons n structure and mnor varatons n composton can be produced from a sngle soldfed parent body, orgnally contanng molten lqud of unform composton. A focussed electron beam analyss of the austente phase n the sulfde eutectc (Fgure 5) ndcates that the N composton vares from 12% to about 60%. n a slowly-cooled body, the austente wthn the eutectc would be of consderable sze. The austente fragments from the eutectc porton of the structure may gve rse to hgh-n meteortes. A more detaled study of ths process may provde a classfcaton scheme for several hgh-n anomalous meteortes such as Santa Catharna and Twn Cty. 4. Dstrbuton Coeffcents Fgures 11 and 12 show the composton profles for N and P n two Fe-N alloys. The lqud composton was computed from the data plotted as composton profles usng (2), The equaton was evaluated usng Smpson's numercal ntegraton technque [Nelsen, 1964]. The sold nterface composton was approprately nterpolated at a gven fracton soldfed from Fgures 11 and 12. The dstrbuton coeffcents were calculated from the computed lqud composton and the sold nterface composton usng (1). The same procedure was repeated for dfferent amounts of the fracton soldfed. Smlarly, the dstrbuton coeffcent of N n the Fe-N-C-S alloy was determned from the composton profle plotted n Fgure 10. The dstrbuton coeffcents are plotted n Fgures 13 and 14. n the determnaton of the dstrbuton coeffcents, the sulphde nclusons present n the proeutectc austente phase were not ncluded snce the amount of sulphdes was much less than 1.0 vol % for the proeutectc regon. Only a few partcles of other secondary phases (phosphde, phosphde eutectc, and sulphde eutectc) were observed wthn the proeutectc austente phase. Therefore, the excluson of such phases does not result n sgnfcant error n the computed dstrbuton coeffcents. Fgure 13 llustrates the varaton of K and wth fracton soldfed n an Fe-N-P alloy. The K and,. values of ths study are n accord wt the v ues of Narayan and Goldsten [1981] reported n Table 1. The varaton of from s due to the K. to elemental nteractons of Fe and N. The nonlnear varaton of llustrates the z Fe-695%N- 78%S- $%P ALLOY A Fe-1045%Nt-164%S-104%C ALLOY C B o o o..o, o, - ø oo NTER CE COMPOSTON OF LQUD (WT % S) Fg. 14. Expermentally-determned dstrbuton coeffcents for N and P plotted as a functon of the nterface composton of the lqud, wt % S. The dstrbuton coeffcent termnates at the start of eutectc soldfca- ton.

9 Sellamuthu and Goldsten: Expermental Study of Segregaton B351 TABLE 2. Fe-N-S-P Alloy System wt%s n Jones and lqud Present Study Drake [1982] Present Study K N Jones and Drake [1982] complextes of the elemental nteractons. n study the case are of sl g, tly the hgher values than obtaned that of Narayan ths and Goldsten [1981] reported n Table 1. The dscrepancy s due to the dffculty n accu- rately evaluatng a dstrbuton coeffcent close to unty. Fgure 14 llustrates the varatons of Kp and.. wth the lqud composton of S n wo alloy systems. t s mperatve to understand that the lqud composton of S ncreases contnuously for every ncrement of soldfcaton. n the case of the Fe-N-C-S alloy, the lqud composton reaches about 26%. n the case of the Fe-N-S-P alloy, the proeutectc soldfcaton has termnated when the lqud composton of S reached about 16%. Therefore, the dstrbuton coeffcents of N and P n the Fe-N-S-P alloy are plotted up to 16% S n Fgure 14. The values for. and t n the Fe-N-S-P alloy are compared wth values of Jones and Drake [1982] n Table 2. The values are reported at two dfferent lqud compostons of sulfur representng approxmately the lqud composton at the ntal and fnal stages of soldfcaton n an Fe-N-S-P alloy. The results of ths study are n accord wth those of Jones and Drake [1982]. We therefore conclude that the plane front soldfcaton technque can be used to determne the dstrbuton coeffcents n Fe-N alloys contanng small amounts of S, P, and C. Usng ths technque, these dstrbuton coeffcents can also be determned as a contnuous functon of fracton soldfed or lqud composton. 5. Characterzaton of Elemental nteractons For all practcal purposes, S can be assumed ndependent of Fe, P, C, or N. The self-nteracton of N s also mnmal [Wlls and Goldsten, 1982]. nkn. s greatly reduced by sulphur enrchment he lqud as shown n Plot (A) of Fgure 14. Also, K.. s moderately reduced by P as shown n Fgure 13. However, as shown n Plot (B) of Fgure 14, the nteractve effect of S and P s not addtve, but rather produces a complex varaton n K... Assumng that N does not nfluence K [Wllms and Goldsten, 1982], the self-nteralton effect of P on Kp produces a nonlnear varaton n Kp (Fgure 13). The addton of S to the Fe-N=P system moderately changes ths behavor as shown n plot (C) of Fgure 14. Snce meteortes contan N, S, and P as alloy consttuents, the characterzaton of ther nteractons s essental to further applcaton of dstrbuton coeffcents to a fractonal crystallzaton model. Although a mathematcal expresson can be wrtten for K as a functon of the lqud compostons of S and P, the termnal (eutectc) composton of S n an Fe-N-S-P alloy system s about 16% as compared to about 26% n an Fe-N-S-C system. Hence, more work needs to be done to extend curve (C) to the termnal (eutectc) composton of S, shown n curve (A) of Fgure 14. Ths extenson would allow the development of a relatonshp between Kp and the lqud compostons of S and P. Onc such mathematcal relatonshps are establshed for both mnor and trace elements, the fractonal crystallzaton model can be appled to determne the elemental dstrbuton wthn a chemcal group of ron meteortes. Conclusons 1. The secondary phases observed n meteortes, such as trolte nodules,form along wth the prmary austente (taente) phase durng the proeutectc soldfcaton. 2. Meteortes wth slght varatons n N content but exhbtng large varatons n sulphde content have orgnated from the same parent melt composton. 3. Plane front soldfcaton can be used to determne equlbrum dstrbuton coeffcents n multcomponent systems. 4. The dstrbuton coeffcents for N and P determned n ths study are n accord wth exstng values n the lterature. However, the effect of elemental nteractons on the dstrbuton coeffcents s hghly complex. Acknowledgments. The authors would lke to thank Mr. Jm Kerner (Lehgh U.), Mr. Bll Mohylsky (Lehgh U.), and Mr. John Gasper (U. of Pttsburgh) for techncal assstance. Specal apprecaton s extended to Professor H. D. Brody (U. of Pttsburgh) for offerng unlmted use of the skull meltng faclty n the Department of Metallurgy. Ths research was supported by NASA Grant NAG References Buchwald, V. F., Handbook of ron Meteortes, vol., p. 87, Unv. of Calf., Berkeley, Esbensen, K. H., and V. F. Buchwald, Planet- (od) core crystallzaton and fractonaton--evdence from the Agpallk mass of the Cape York ron meteorte shower, Phys. Earth Planet. nter., 29, , Esbensen, K. H., V. F. Buchwald, D. J. Malvn, and J. T. Wasson, Systematc compostonal varatons n the Cape York ron meteorte, Geochm. Cosmochm. Acta, 46, 1913, Goldsten, J.., D. E. Newbury, P. Echln, D. C. Joy, C. For, and E. Lfshn, Scannng Electron Mcroscopy and X-ray Mcroanalyss, 432, Plenum, New York, 1981.

10 B352 Sellamuthu and Goldsten: Expermental Study of Segregaton Jones, J. H., and M. J. Drake, Expermental nvestgatons of trace element fractonaton n ron meteortes, : The nfluence of sulfur, Geochm. Cosmochm. Acta, n press, Narayan, C., and J.. Goldsten, Expermental determnaton of ternary partton coeffcents n Fe-N-X alloys, Met. Trans., 12A, 1883, Narayan, C., and J.. Goldsten, A dendrtc soldfcaton model to explan Ge-N varatons n ron meteorte chemcal groups, Geochm. Cosmochm. Acta, 46, 259, Nelsen, K. L., Methods n Numercal Analyss, pp , Macmllan New York, Scott, E. R. D., Chemcal fractonaton n ron meteortes and ts nterpretaton, Geochm. Cosmochm. Acta, 36, , Wlls, J., and J.. Goldsten, The effects of C, P, and S on trace element parttonng durng soldfcaton n Fe-N alloys, Proc. Lunar Planet. Sc. Conf. 13th n J. Geophys. Res., 87, A435-A445, R. Sellamuthu and J.. Goldsten, Department of Metallurgy and Materals Engneerng, Lehgh Unversty, Bethlehem, PA ( Receved Aprl 18, 1983; revsed August 3, 1983; accepted August 12, 1983.)