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Phase Transformations in Iron Meteorites
J.
I. Goldstein
Department of Metallurgy and Materials Engineering Lehigh University Bethlehem, PA 18015
Iron meteorites (Fe—S to 25 wt% Ni alloys) were slow cooled from 1300°C at the slowest rates known to man C’— 1 to 100°C/million years). Among the phases which form on cooling are Widmansthtten ferrite, mar— tensite, decomposed martensite, Fe C, (FeNi) P and ordered FeNi at low These ferrous phase transformations have been studied by temperatures. Laboratory alloys have been optical microscopy, TEM, EPMA, AEM and SEM. In addition com heat treated to study most of these transformations. puter models of the growth of Widmansttten ferrite have been developed to explain the diffusion gradients which are still present in these The meteorites and to calculate cooling rates for these unique alloys. development of these phase transformations are discussed with relation to pertinent phase diagrams.
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Iron Meteorites
Iron meteorites are fragments of naturally produced solid material that have survived passage, from interplanetary space, through the Earth’s atmo sphere and have landed on the surface of the Earth. Meteoritic material probably originates in the belt of asteroids between Mars and Jupiter. The iron meteorites are composed mainly of iron and nickel with small amounts of cobalt, phosphorus, sulfur and carbon. The nickel content varies from 5 to 60 wt% although in the vast majority of cases it lies in the range between 5 The structures of these meteorites, Figures 1 and 2, were and 12%. developed during slow cooling, over millions of years, of these samples in their parent bodies. The regular octahedral pattern which is observed is called the Widmanstatten pattern. This microstructure was discovered first in iron meteorites in the early 1800’s and bears the name of one of the first scientists to study meteorites, namely WidmanstNtten. This pattern is found in hundreds of iron meteorites. The two photomicrographs (Figures 1 and 2) illustrate the ferrite, a, (bcc) precipitates which form in the (fcc). Almost all iron meteorite samples were single matrix austenite, crystal austenite before the Widmansttten pattern formed ( < 800°C) and some of the single crystals were more than 1 meter in size. ,
With the slow cooling which prevailed in asteroidal bodies, the phases which form in iron meteorites should be very close to their equilibrium The purpose of this paper is to examine the phase transforma composition. tions which occur in the iron meteorites. Particular attention will be paid to the pertinent phase diagrams and the process of diffusion controlled growth of the a phase. The effect of third elements such as P on the nucle ation and growth process will be discussed. We will also examine several low temperature phase transformations which also occur in ferrous materials such as the formation of martensite, and the decomposition of martensite. In addition we will discuss the ordering of alloy compositions close to 50—50 FeNi.
The WidmanstL’tten pattern Figure 1 in the Mt. Edith iron meteorite (9.6 It has a cooling rate of wt% Ni). 200°C/my. The large rounded pre cipitates are troilite, (FeS) and the lamellar precipitates are schreiber— The section is 20 cm site, (FeNi)3P. (Photograph long and 15 cm wide. courtesy of the Div. of Meteorites, U.S. National Museum). —
The Widmansttten pattern Figure 2 in the Bristol iron meteorite (8.1 wt It has a cooling rate of % Ni). Shocked deformation -‘-5000°C/my. bands are observed in the ferrite and rather extensive regions of trans formed austenite are found between the ferrite plates. The field of view is 0.8 x 0.6 cm. —
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Non Equilibrium in Iron Meteorites In three dimensions the a ferrite occurs as an interpenetrating ar rangement of plates that are oriented parallel to the faces of an octahedron and the Widmanstatten pattern arises because plates of a grow with habit planes approxinately parallel to the (111) octahedral planes of the parent An electron nicroprobe trace for Ni taken at austenite ‘y’ (Figures 1 and 2). right angles to the a growth front is illustrated in Figure 3 for the Grant Such a Ni profile with major Ni gradients in austenite shows that meteorite. the meteorite is not in total equilibrium at the final growth temperature. The observed diffusion profiles may be explained by the nucleation and diffusion—controlled growth of ferrite, a , when an originally homogeneous region of the Fe—Ni parent ‘i is slowly cooled through the two phase a + As the meteorite cools down to about equilibrium diagram (1) (Figure 4). 450°C, the Ni content of both the aand •y phases increases and the amount of Below about 450°C the aphase increases as the amount of V phase decreases. Since the rate of diffusion of Ni Ni content in the a phase will decrease. at any temperature is about two orders of magnitude faster in bcc a than in f cc ‘, chemical gradients in a will be much flatter than the gradients in v. Assuming that at any stage of growth, local equilibrium is maintained growth interface, one can understand how the Ni gradients in a at the a/v Ni is rejected by the growing a phase, which passes into the develop. and Because of the slow rates of diffusion the Ni gradient builds residual V. As the temperature decreases Ni builds up in the V near the up in the t. Because of faster diffusion in hcc a, the Ni gradient ma interface. a/v The Ni depletinn does not build up until low temperatures, less than 500°C. Ni the decreasing caused by is boundary the near a/v in the a phase Although iron meteorites solubility in the a phase below 450°C (Figure 4). cooled over a period of millions of years, total equilibrium was not However the Ni gradients which developed in a and v can be achieved. explained by considering the Widmanstatten pattern growth in terms of our understanding of diffusion controlled growth. We will return later in this paper to consider the microstructures which form in the V phase when the Ni concentration gradients in the v phase are frozen in and the meteorite is cooled to low temperatures.
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Fe-Ni binary phase diagram Figure 4 Figure 3 Concentration gradient determined by Romig and Goldstein (1). of Ni across a ferrite-austenite ferrite area in the Grant meteorite N5 is the martensite start temperature. taken with the electron microprobe. Note the Ni depletion, indicated by the arrow, in the a at the a/v boundary, and the Ni buildup, above the bulk composition, C09.4 wt%, Ni, in the austenite. -
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Iron Meteontes
Development of the Widmansttten Pattern in Iron Meteorites Most discussions of Widmanst&ten pattern nucleation and growth consider the reaction as occurring in a binary FeNi alloy. However it has been shown experimentally that ferrite, n , will not nucleate at either grain boundaries Goldstein and Doan (3) were (2). or in the matrix grains of austenite, able to produce experimentally, for the first time, a Widmansttten pattern in Fe—Ni alloys containing as little as 0.1 wt% P during cooling of these + V figure 5 shows the region of the phase diagram. alloys through the type of structure obtained in a 9.8 wt% Ni, 0.3 wt% P alloy slow cooled to The P present in the iron meteorite and in particular the phosphide, 650°C. (FeNi3)P, phase present in the iron meteorite (4), acts as the nucleating + phase boundaries of the phase. The effect of P on the agent for the binary diagram is to increase the Ni content of the n and to decrease the Ni content of the V (1).
A light micrograph showing the microstructure of an 89.9 wt% Figure 5 1% nital etch. Fe—9.8 wt% Ni—0.3 wt% P alloy slowly cooled to 650°C. Ferrite in a Widmanstatten pattern is formed within the austenite during cooling (Goldstein and Doan (3)). —
Narayan and Goldstein (4,5) have experimentally grown intragranular cy in Fe—Ni—P alloys containing between 5 and 10 wt% Ni and 0 and 1.0 wt% P and have examined the nucleation and growth process of these precipitates using Figure 6 shows a light analytical electron microscopy (AEN) techniques. precipitates in a 6.88 wt% Ni, 0.49 wt% P micrograph of intragranular Arrows point alloy which was cooled from 790 to 650°C at a rate of 5°C/day. precipitates and some grain boundary n is also visible. to some of the precipitates during growth can be described as The general shape of the Figure 7 is a TEN micrograph from the same alloy and shows an cylindrical. crystal in a matrix that has transformed to martensite intragranular during the quench to room temperature. Figure 8 shows the Ni profile The results of the obtained across an ni V interface in the same alloy. Narayan and Goldstein study (6) showed that n ferrite nucleates intragranu— In addition the measured concentration larly with little undercooling. profiles indicate diffusion controlled growth of n with interfacial equilibrium maintained at the n/V interface.
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Iron Meteorites
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,z.. A light micrograph show Figure 6 ing a typical microstructure for alloys exhibiting intragranular Arrows point ferrite precipitation. to some of the ferrite precipitates. Grain boundary ferrite is also vis The specific alloy described ible. here contains 6.88 wt% Ni, 0.49 wt% P and was cooled from 790 to 650°C at a rate of 5°C/day. —
A TEM micrograph showing a Figure 7 ferrite crystal which has nucleated intragranularly in a matrix that has The been transformed to martensite. alloy contains 6.88 wt% Ni and 0.49 wt% and was cooled from 790°C to 650°C at a rate of 5°C/day. —
A model to simulate the hulk diffusion controlled growth of m phase was A numerical method of lines (NNOL) technique was used also developed (6). The for the solution of the system of partial differential equations (7). transforma spacing grid variable (8) Murray—Landis the and NNOL technique tion were combined to solve the problem of diffusional growth of ferrite. The numerical model calculates the growth of ferrite in the ternary system Fe—Ni—P during continuous cooling and generates the concentration profiles of Ni and P in the ferrite and austenite phases as a function of temperature The computation was based on the following assumptions: and time. 1. 2. 3.
4.
The growth of ferrite is controlled by bulk diffusion of Ni in austenite. Interfacial equilibrium of Ni and P occurs at the a/V interface at all times during the growth process. Ferrite nucleates and grows as a cylindrical precipitate. The morphology of the precipitate chonges to platelike as impingement of the precipitates occurs. There is no P gradient in either phase and no Ni gradient in ferrite.
The computer model was applied to various samples including the alloy The bulk compositions of the alloys and the described in Figures 6 and 7. cooling rates measured in the laboratory were used as inputs to the computer Figure 8 shows the excellent agreement of the calculated Ni profiles model. with the measured data from the AEM.
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The same computer model should be applicable to the study of the cooling Instead of simulating growth of lp.m sized history of the iron meteorites. precipitates in’iron precipitates cooling at 5°C/day, one can consider meteorites in a size range from 10 p.m to 1 cii. Calculations or various iron years to lC/10 year (6). meteorites yield cooling rates from 500°C/b years. to 1°C/b Most of the iron meteorites cool in a range from 50°C/b These estimated cooling rates are very slow but are two orders of magnitude To faster than rates determined from previous computer models (9—12). accommodate the revised cooling rates predicted by this recent study, meteorite parent bodies need only be a few kilometers in diameter assuming that the iron meteorites are present in the center of the body. Microstnicture of Retained Austenite and Low Temperature Phase Transformations The characteristic M—shaped composition profile (Figure 3) arises because there is a higher concentration of Ni in the austenite adjacent to ferrite plates from which the Ni was rejected. This high Ni concen the tration in the austenite falls with distance from the C/v interface. Depending on the amount of diffusion that has occurred, the lowest value of the Ni content of the austenite varies from the bulk meteorite composition to almost the equilibrium composition predicted from the phase diagram. Figure 9 is an optical micrograph showing the microstructures developed in the Edmonton iron meteorite between 15 and 50 wt% Ni along the M shaped composition profile in austenite. Region 4 has the lowest Ni and region 1 has the highest Ni. Figure 10 shows the detailed Ni composition profile in regions k, 1, 2, and 3 of the taenite phase using the ARM from the Carlton iron meteorite. Up to - 20% Ni the untransformed ‘y’ will transform to (see the M curve in Figure 4). The microstructure of the martensite 2 l6
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Figure 8 Ni composition profile measured across a ferrite/austenite interface in a 6.88 wt% Ni and 0.49 wt% P alloy that was cooled from 790°C to 650°C at a rate of 5°/day. The solid line is the Ni profile calculated by the numerical model. -
Decomposition of v Figure 9 into distinct regions with decreasing Ni content away from the C/V boundary of the Edmonton k indicates iron meteorite (14). the C ferrite phase. Region 4 is martensitic. -
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martensitic area in the austenite of a similar meteorite is shown in Figure Over geological time the martensite, which may be either lath 9, region 4. as shown in Figure or lenticular, decomposes on a sub-micron scale to a + a + y. Region 3, from 25 to a2 The total reaction is given by y 11. 30 wt7, Ni, contains retained austenite, -.
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breaks down on In the composition range 30-4O Ni the untransformed invariably phases which Ni high and of low mixture duplex a to a fine scale border the Widmansttten a plates (region 2 in Figure 9). This phase mix ture etches deeply and is often termed “the cloudy zone” or “cloudy border.” Electron microscope observations (14,15) reveal that this region appears to consist of a structure of essentially single crystal austenite giobules sur rounded by a honeycomb network of single crystal ferrite, as shown in Figure 12 from the Estherville meteorite. As displayed in Figure 10 the composi tion profile oscillates wildly as the duplex cloudy border is encountered. Individual analyses taken in the two phases of the cloudy border of the Estherville meteorite are shown in Figure 13. As expected the high Ni 23 wt%. Ni and therefore cannot but the low Ni phase contains phase is be At this composition the low Ni phase would be expected to be marten Convergent beam patterns taken from individual phases in the sitic a2• cloudy border show that the high Ni y exhibits superlattice spots charac teristic of the Li0 structure. The mechanism for the formation of the cloudy zone is not known. Order-disorder or spinodal decomposition may be responsible for the formation of this unusual structure. ‘,
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Within the last few years there has been increasing evidence that the highest Ni regions (region 1 in Figure 9, with 48-52 wth Ni) close to the a/’y interface are ordered with the FeNi 110 superlattice structure. Initial observations were made in meteorites using M8ssbauer spectroscopy (16), and
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Distance (gm) Ni composition profile Figure 10 across regions 1, 2 and 3 of the taenite phase of the Canton iron The cloudy zone is meteorite. region 2. -
A SEM micrograph of Figure 11 decomposed austenite in the Weaver Mts. meteorite (13) showing the breakdown of lath martensite a2 to The platelets are ‘ phase and a + the highly etched black regions are a phase. -
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Iron Meteorites
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A ThM micrograph of Figure 12 cloudy border showing a honeycomb structure in the Estherville meteorite.
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polarized light microscopy (17) to observe the anisotropy associated with More direct evidence (18,19) has the tetragonality of the Li structure. also been produced in terms°of the observations of anti—phase boundaries and the observation of superlattice spots which permit dark field images to be formed in the TEM, as shown in Figure 14.
Superlattice-dark field Figure 14 image of the ordered FeNi superlattice in the cloudy border region The of the Estherville meteorite. superlattice reflection used is the arrowed reflection in the diffrac The superlattice tion pattern. image shows the fine scale of the phase. ordered regions within the -
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Neither the cloudy border, nor the order—disorder transformation are
reproducible in the laboratory, although the latter can be induced by irra Both microstructures are sensitive to reheating and are diation (20). rapidly destroyed above 300—400°C, emphasizing the metastability of the 320°C) of the ordering cloudy zone and the low critical temperature C required for both apparently times long of the view In n. transformatio these transformations, their absence (or presence on an exceptionally fine 10 urn) is an indication of substantial shock—induced heating, scale, followed by relatively rapid cooling to ambient temperatures.
Summary The iron meteorites are of much historical interest to the metallurgist This particularly because of the presence of the Widmansttten pattern. pattern is formed in the large FeNi single crystals and is often visible to However the iron meteorites yield unexpected information the unaided eye. about low temperature phase transformations and the major effect of minor Finally these elements in solid solution in the FeNi single crystals. ns well known transformatio phase several undergone samples have upon cooling to the ferrous metallurgist, namely——Widmanst’tten precipitation and growth, Studies martensite formation, and low temperature tempering of martensite. on cooling the information contributed only not have meteorites iron of the history of these unusual materials but have allowed the study of equilibrium and non—equilibrium phase transformations under slow cooling conditions which cannot be reproduced in the laboratory.
Acknowledgments The author wishes to thank Dr. A. D. Romig (Sandia Laboratories), Dr. C. Narayan (IBM Research Laboratories), Dr. D. B. Williams (Lehigh University), and Dr. R. C. Clarke (Smithsonian Institution) for their help and collaboration with the author on various phases of iron meteorite The research was supported by NSF Grant EAR 7900995 and NASA research. Grant NGR 39—007—043.
References 1.
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3.
4. 5. 6. 7. 8.
A. D. Romig, Jr. and J. I. Goldstein, “Determination of the Fe—Ni and Fe—Ni—P Phase Diagrams at Low Temperatures (700—300°C)” Met. Trans., hA (1980) p. 1151. Yin -“yand V N. P. Allen and C. C. Earley, “The Transformations Iron Rich Binary Iron—Nickel Alloys,” J. Iron Steel Inst., 166 (1950) p. 281. J. I. Goldstein and A. S. Doan, Jr., “The Effect of Phosphorus on the Formation of the Widmanstatten Pattern in Iron Meteorites,” Geochim. et Cosmochim. Acta, 36 (1972) p. 51. C. Narayan and J. I. Goldstein, “Nucleation of Intragranular Ferrite in Fe—Ni—P Alloys,” accepted Met. Trans. (1984). C. Narayan and J. I. Goldstein, “Growth of Intragranular Ferrite in Fe—Ni—P Alloys,” accepted Met. Trans. (1984). C. Narayan and J. I. Goldstein, “A Major Revision of Iron Meteorite Cooling Rates,” submitted Geochim. et Cosmochim. Acta (1984). W. E. Schiesser, “DSS Version 2, Introductory Programming Manual,” Lehigh University and Naval Air Development Center (1976). W. D. Murray and F. Landis, “Numerical and Machine Solutions of Transient Heat—Conduction Problems Involving Melting or Freezing. -.
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Iron Meteorites Method of Analysis and Sample Solutions,” Trans. ASME, 81 Part I (1959) p. 106. J. A. Wood, “The Cooling Rates and Parent Planets of Several Iron Meteorites,” Icarus, 3 (1964) p. 429. J. I. Goldstein and R. E. Ogilvie, “The Growth of the Widmanstatten Pattern in Metallic Meteorites,” Geochim. et Cosmochim. Acta, 29 (1965) p. 893. J. I. Goldstein and J. M. Short, “The Iron Meteorites, Their Thermal History and Parent Bodies,” Geochim. et Cosmochim. Acta, 31 (1967) p. 1733. J. Willis and J. T. Wasson, “Cooling Rates of Group IVA Iron Meteorites,” Earth and Plan. Sci. Lett., 40 (1978) p. 141. P. M. Novotny, J. I. Goldstein and D. B. Williams, “Analytical Electron Microscope Study of Eight Ataxites,” Geochim. et Cosmochim. Acta, 46 (1982) p. 2461. L. S. Liii, J. I. Goldstein and D. B. Williams, “Analytical Electron Microscopy Study of the Plessite Structure in Four III CD Iron Meteorites,” Geochim. et Cosmochim. Acta, 43 (1979) p. 725. E. R. D. Scott, “The Nature of Dark Etching Rims in Meteoritic Taenite,” Geochim. et Cosmochim. Acta, 37 (1973) p. 2283. J. F. Albertsen, G. B. Jensen and J. M. Knudsen, “Structure of Taenite in Two Iron Meteorites,” Nature, 273 (1978) p. 453. E. R. D. Scott and R. S. Clarke, Jr., “Identification of Clear Taenite in Meteorites as Ordered FeNi,” Nature, 281 (1979) p. 360. S. Mehta, P. N. Novotny, 13. B. Williams and J. I. Goldstein, “Electron— Optical Observations of Ordered FeNi in the Estherville Meteorite,” Nature, 284 (1980) p. 151. P. M. Novotny, “An Electron Microscope Investigation of Eight Ataxite and One Mesosiderite Meteorites,” MS Thesis, Lehigh University (1981). K. Benusa, E. P. Butler, D. B. Williams and J. I. Goldstein, “Ordering in Fe—Ni Alloys Induced by Irradiation in the High Voltage Electron Microscope,” 7th International Conference on lIVEN (1983), in press. —
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Phase Transformations in Iron Meteorites
J.
I. Goldstein
Department of Metallurgy and Materials Engineering Lehigh University Bethlehem, PA 18015
Iron meteorites (Fe—S to 25 wt% Ni alloys) were slow cooled from 1300°C at the slowest rates known to man C’— 1 to 100°C/million years). Among the phases which form on cooling are Widmansthtten ferrite, mar— tensite, decomposed martensite, Fe C, (FeNi) P and ordered FeNi at low These ferrous phase transformations have been studied by temperatures. Laboratory alloys have been optical microscopy, TEM, EPMA, AEM and SEM. In addition com heat treated to study most of these transformations. puter models of the growth of Widmansttten ferrite have been developed to explain the diffusion gradients which are still present in these The meteorites and to calculate cooling rates for these unique alloys. development of these phase transformations are discussed with relation to pertinent phase diagrams.
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Iron Meteorites
Iron meteorites are fragments of naturally produced solid material that have survived passage, from interplanetary space, through the Earth’s atmo sphere and have landed on the surface of the Earth. Meteoritic material probably originates in the belt of asteroids between Mars and Jupiter. The iron meteorites are composed mainly of iron and nickel with small amounts of cobalt, phosphorus, sulfur and carbon. The nickel content varies from 5 to 60 wt% although in the vast majority of cases it lies in the range between 5 The structures of these meteorites, Figures 1 and 2, were and 12%. developed during slow cooling, over millions of years, of these samples in their parent bodies. The regular octahedral pattern which is observed is called the Widmanstatten pattern. This microstructure was discovered first in iron meteorites in the early 1800’s and bears the name of one of the first scientists to study meteorites, namely WidmanstNtten. This pattern is found in hundreds of iron meteorites. The two photomicrographs (Figures 1 and 2) illustrate the ferrite, a, (bcc) precipitates which form in the (fcc). Almost all iron meteorite samples were single matrix austenite, crystal austenite before the Widmansttten pattern formed ( < 800°C) and some of the single crystals were more than 1 meter in size. ,
With the slow cooling which prevailed in asteroidal bodies, the phases which form in iron meteorites should be very close to their equilibrium The purpose of this paper is to examine the phase transforma composition. tions which occur in the iron meteorites. Particular attention will be paid to the pertinent phase diagrams and the process of diffusion controlled growth of the a phase. The effect of third elements such as P on the nucle ation and growth process will be discussed. We will also examine several low temperature phase transformations which also occur in ferrous materials such as the formation of martensite, and the decomposition of martensite. In addition we will discuss the ordering of alloy compositions close to 50—50 FeNi.
The WidmanstL’tten pattern Figure 1 in the Mt. Edith iron meteorite (9.6 It has a cooling rate of wt% Ni). 200°C/my. The large rounded pre cipitates are troilite, (FeS) and the lamellar precipitates are schreiber— The section is 20 cm site, (FeNi)3P. (Photograph long and 15 cm wide. courtesy of the Div. of Meteorites, U.S. National Museum). —
The Widmansttten pattern Figure 2 in the Bristol iron meteorite (8.1 wt It has a cooling rate of % Ni). Shocked deformation -‘-5000°C/my. bands are observed in the ferrite and rather extensive regions of trans formed austenite are found between the ferrite plates. The field of view is 0.8 x 0.6 cm. —
Authors Copy Iron Meteorites
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Non Equilibrium in Iron Meteorites In three dimensions the a ferrite occurs as an interpenetrating ar rangement of plates that are oriented parallel to the faces of an octahedron and the Widmanstatten pattern arises because plates of a grow with habit planes approxinately parallel to the (111) octahedral planes of the parent An electron nicroprobe trace for Ni taken at austenite ‘y’ (Figures 1 and 2). right angles to the a growth front is illustrated in Figure 3 for the Grant Such a Ni profile with major Ni gradients in austenite shows that meteorite. the meteorite is not in total equilibrium at the final growth temperature. The observed diffusion profiles may be explained by the nucleation and diffusion—controlled growth of ferrite, a , when an originally homogeneous region of the Fe—Ni parent ‘i is slowly cooled through the two phase a + As the meteorite cools down to about equilibrium diagram (1) (Figure 4). 450°C, the Ni content of both the aand •y phases increases and the amount of Below about 450°C the aphase increases as the amount of V phase decreases. Since the rate of diffusion of Ni Ni content in the a phase will decrease. at any temperature is about two orders of magnitude faster in bcc a than in f cc ‘, chemical gradients in a will be much flatter than the gradients in v. Assuming that at any stage of growth, local equilibrium is maintained growth interface, one can understand how the Ni gradients in a at the a/v Ni is rejected by the growing a phase, which passes into the develop. and Because of the slow rates of diffusion the Ni gradient builds residual V. As the temperature decreases Ni builds up in the V near the up in the t. Because of faster diffusion in hcc a, the Ni gradient ma interface. a/v The Ni depletinn does not build up until low temperatures, less than 500°C. Ni the decreasing caused by is boundary the near a/v in the a phase Although iron meteorites solubility in the a phase below 450°C (Figure 4). cooled over a period of millions of years, total equilibrium was not However the Ni gradients which developed in a and v can be achieved. explained by considering the Widmanstatten pattern growth in terms of our understanding of diffusion controlled growth. We will return later in this paper to consider the microstructures which form in the V phase when the Ni concentration gradients in the v phase are frozen in and the meteorite is cooled to low temperatures.
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Fe-Ni binary phase diagram Figure 4 Figure 3 Concentration gradient determined by Romig and Goldstein (1). of Ni across a ferrite-austenite ferrite area in the Grant meteorite N5 is the martensite start temperature. taken with the electron microprobe. Note the Ni depletion, indicated by the arrow, in the a at the a/v boundary, and the Ni buildup, above the bulk composition, C09.4 wt%, Ni, in the austenite. -
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Iron Meteontes
Development of the Widmansttten Pattern in Iron Meteorites Most discussions of Widmanst&ten pattern nucleation and growth consider the reaction as occurring in a binary FeNi alloy. However it has been shown experimentally that ferrite, n , will not nucleate at either grain boundaries Goldstein and Doan (3) were (2). or in the matrix grains of austenite, able to produce experimentally, for the first time, a Widmansttten pattern in Fe—Ni alloys containing as little as 0.1 wt% P during cooling of these + V figure 5 shows the region of the phase diagram. alloys through the type of structure obtained in a 9.8 wt% Ni, 0.3 wt% P alloy slow cooled to The P present in the iron meteorite and in particular the phosphide, 650°C. (FeNi3)P, phase present in the iron meteorite (4), acts as the nucleating + phase boundaries of the phase. The effect of P on the agent for the binary diagram is to increase the Ni content of the n and to decrease the Ni content of the V (1).
A light micrograph showing the microstructure of an 89.9 wt% Figure 5 1% nital etch. Fe—9.8 wt% Ni—0.3 wt% P alloy slowly cooled to 650°C. Ferrite in a Widmanstatten pattern is formed within the austenite during cooling (Goldstein and Doan (3)). —
Narayan and Goldstein (4,5) have experimentally grown intragranular cy in Fe—Ni—P alloys containing between 5 and 10 wt% Ni and 0 and 1.0 wt% P and have examined the nucleation and growth process of these precipitates using Figure 6 shows a light analytical electron microscopy (AEN) techniques. precipitates in a 6.88 wt% Ni, 0.49 wt% P micrograph of intragranular Arrows point alloy which was cooled from 790 to 650°C at a rate of 5°C/day. precipitates and some grain boundary n is also visible. to some of the precipitates during growth can be described as The general shape of the Figure 7 is a TEN micrograph from the same alloy and shows an cylindrical. crystal in a matrix that has transformed to martensite intragranular during the quench to room temperature. Figure 8 shows the Ni profile The results of the obtained across an ni V interface in the same alloy. Narayan and Goldstein study (6) showed that n ferrite nucleates intragranu— In addition the measured concentration larly with little undercooling. profiles indicate diffusion controlled growth of n with interfacial equilibrium maintained at the n/V interface.
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Iron Meteorites
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,z.. A light micrograph show Figure 6 ing a typical microstructure for alloys exhibiting intragranular Arrows point ferrite precipitation. to some of the ferrite precipitates. Grain boundary ferrite is also vis The specific alloy described ible. here contains 6.88 wt% Ni, 0.49 wt% P and was cooled from 790 to 650°C at a rate of 5°C/day. —
A TEM micrograph showing a Figure 7 ferrite crystal which has nucleated intragranularly in a matrix that has The been transformed to martensite. alloy contains 6.88 wt% Ni and 0.49 wt% and was cooled from 790°C to 650°C at a rate of 5°C/day. —
A model to simulate the hulk diffusion controlled growth of m phase was A numerical method of lines (NNOL) technique was used also developed (6). The for the solution of the system of partial differential equations (7). transforma spacing grid variable (8) Murray—Landis the and NNOL technique tion were combined to solve the problem of diffusional growth of ferrite. The numerical model calculates the growth of ferrite in the ternary system Fe—Ni—P during continuous cooling and generates the concentration profiles of Ni and P in the ferrite and austenite phases as a function of temperature The computation was based on the following assumptions: and time. 1. 2. 3.
4.
The growth of ferrite is controlled by bulk diffusion of Ni in austenite. Interfacial equilibrium of Ni and P occurs at the a/V interface at all times during the growth process. Ferrite nucleates and grows as a cylindrical precipitate. The morphology of the precipitate chonges to platelike as impingement of the precipitates occurs. There is no P gradient in either phase and no Ni gradient in ferrite.
The computer model was applied to various samples including the alloy The bulk compositions of the alloys and the described in Figures 6 and 7. cooling rates measured in the laboratory were used as inputs to the computer Figure 8 shows the excellent agreement of the calculated Ni profiles model. with the measured data from the AEM.
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The same computer model should be applicable to the study of the cooling Instead of simulating growth of lp.m sized history of the iron meteorites. precipitates in’iron precipitates cooling at 5°C/day, one can consider meteorites in a size range from 10 p.m to 1 cii. Calculations or various iron years to lC/10 year (6). meteorites yield cooling rates from 500°C/b years. to 1°C/b Most of the iron meteorites cool in a range from 50°C/b These estimated cooling rates are very slow but are two orders of magnitude To faster than rates determined from previous computer models (9—12). accommodate the revised cooling rates predicted by this recent study, meteorite parent bodies need only be a few kilometers in diameter assuming that the iron meteorites are present in the center of the body. Microstnicture of Retained Austenite and Low Temperature Phase Transformations The characteristic M—shaped composition profile (Figure 3) arises because there is a higher concentration of Ni in the austenite adjacent to ferrite plates from which the Ni was rejected. This high Ni concen the tration in the austenite falls with distance from the C/v interface. Depending on the amount of diffusion that has occurred, the lowest value of the Ni content of the austenite varies from the bulk meteorite composition to almost the equilibrium composition predicted from the phase diagram. Figure 9 is an optical micrograph showing the microstructures developed in the Edmonton iron meteorite between 15 and 50 wt% Ni along the M shaped composition profile in austenite. Region 4 has the lowest Ni and region 1 has the highest Ni. Figure 10 shows the detailed Ni composition profile in regions k, 1, 2, and 3 of the taenite phase using the ARM from the Carlton iron meteorite. Up to - 20% Ni the untransformed ‘y’ will transform to (see the M curve in Figure 4). The microstructure of the martensite 2 l6
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3 2 II
y
ID a
a
59
S
f DISTANCE
Figure 8 Ni composition profile measured across a ferrite/austenite interface in a 6.88 wt% Ni and 0.49 wt% P alloy that was cooled from 790°C to 650°C at a rate of 5°/day. The solid line is the Ni profile calculated by the numerical model. -
Decomposition of v Figure 9 into distinct regions with decreasing Ni content away from the C/V boundary of the Edmonton k indicates iron meteorite (14). the C ferrite phase. Region 4 is martensitic. -
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martensitic area in the austenite of a similar meteorite is shown in Figure Over geological time the martensite, which may be either lath 9, region 4. as shown in Figure or lenticular, decomposes on a sub-micron scale to a + a + y. Region 3, from 25 to a2 The total reaction is given by y 11. 30 wt7, Ni, contains retained austenite, -.
-.
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breaks down on In the composition range 30-4O Ni the untransformed invariably phases which Ni high and of low mixture duplex a to a fine scale border the Widmansttten a plates (region 2 in Figure 9). This phase mix ture etches deeply and is often termed “the cloudy zone” or “cloudy border.” Electron microscope observations (14,15) reveal that this region appears to consist of a structure of essentially single crystal austenite giobules sur rounded by a honeycomb network of single crystal ferrite, as shown in Figure 12 from the Estherville meteorite. As displayed in Figure 10 the composi tion profile oscillates wildly as the duplex cloudy border is encountered. Individual analyses taken in the two phases of the cloudy border of the Estherville meteorite are shown in Figure 13. As expected the high Ni 23 wt%. Ni and therefore cannot but the low Ni phase contains phase is be At this composition the low Ni phase would be expected to be marten Convergent beam patterns taken from individual phases in the sitic a2• cloudy border show that the high Ni y exhibits superlattice spots charac teristic of the Li0 structure. The mechanism for the formation of the cloudy zone is not known. Order-disorder or spinodal decomposition may be responsible for the formation of this unusual structure. ‘,
.
Within the last few years there has been increasing evidence that the highest Ni regions (region 1 in Figure 9, with 48-52 wth Ni) close to the a/’y interface are ordered with the FeNi 110 superlattice structure. Initial observations were made in meteorites using M8ssbauer spectroscopy (16), and
50
40
30
z
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10 -Th
(a) 0
2
D
I I I 34567
Distance (gm) Ni composition profile Figure 10 across regions 1, 2 and 3 of the taenite phase of the Canton iron The cloudy zone is meteorite. region 2. -
A SEM micrograph of Figure 11 decomposed austenite in the Weaver Mts. meteorite (13) showing the breakdown of lath martensite a2 to The platelets are ‘ phase and a + the highly etched black regions are a phase. -
.
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Iron Meteorites
ii H tr I
48
-J Li
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0
z
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A ThM micrograph of Figure 12 cloudy border showing a honeycomb structure in the Estherville meteorite.
I
-
I
I
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0.0
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DISTANCE (jun) Ni composition profile Figure 13 regions of the within the 2 and cloudy border in the Estherville meteorite. -
‘
polarized light microscopy (17) to observe the anisotropy associated with More direct evidence (18,19) has the tetragonality of the Li structure. also been produced in terms°of the observations of anti—phase boundaries and the observation of superlattice spots which permit dark field images to be formed in the TEM, as shown in Figure 14.
Superlattice-dark field Figure 14 image of the ordered FeNi superlattice in the cloudy border region The of the Estherville meteorite. superlattice reflection used is the arrowed reflection in the diffrac The superlattice tion pattern. image shows the fine scale of the phase. ordered regions within the -
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Neither the cloudy border, nor the order—disorder transformation are
reproducible in the laboratory, although the latter can be induced by irra Both microstructures are sensitive to reheating and are diation (20). rapidly destroyed above 300—400°C, emphasizing the metastability of the 320°C) of the ordering cloudy zone and the low critical temperature C required for both apparently times long of the view In n. transformatio these transformations, their absence (or presence on an exceptionally fine 10 urn) is an indication of substantial shock—induced heating, scale, followed by relatively rapid cooling to ambient temperatures.
Summary The iron meteorites are of much historical interest to the metallurgist This particularly because of the presence of the Widmansttten pattern. pattern is formed in the large FeNi single crystals and is often visible to However the iron meteorites yield unexpected information the unaided eye. about low temperature phase transformations and the major effect of minor Finally these elements in solid solution in the FeNi single crystals. ns well known transformatio phase several undergone samples have upon cooling to the ferrous metallurgist, namely——Widmanst’tten precipitation and growth, Studies martensite formation, and low temperature tempering of martensite. on cooling the information contributed only not have meteorites iron of the history of these unusual materials but have allowed the study of equilibrium and non—equilibrium phase transformations under slow cooling conditions which cannot be reproduced in the laboratory.
Acknowledgments The author wishes to thank Dr. A. D. Romig (Sandia Laboratories), Dr. C. Narayan (IBM Research Laboratories), Dr. D. B. Williams (Lehigh University), and Dr. R. C. Clarke (Smithsonian Institution) for their help and collaboration with the author on various phases of iron meteorite The research was supported by NSF Grant EAR 7900995 and NASA research. Grant NGR 39—007—043.
References 1.
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4. 5. 6. 7. 8.
A. D. Romig, Jr. and J. I. Goldstein, “Determination of the Fe—Ni and Fe—Ni—P Phase Diagrams at Low Temperatures (700—300°C)” Met. Trans., hA (1980) p. 1151. Yin -“yand V N. P. Allen and C. C. Earley, “The Transformations Iron Rich Binary Iron—Nickel Alloys,” J. Iron Steel Inst., 166 (1950) p. 281. J. I. Goldstein and A. S. Doan, Jr., “The Effect of Phosphorus on the Formation of the Widmanstatten Pattern in Iron Meteorites,” Geochim. et Cosmochim. Acta, 36 (1972) p. 51. C. Narayan and J. I. Goldstein, “Nucleation of Intragranular Ferrite in Fe—Ni—P Alloys,” accepted Met. Trans. (1984). C. Narayan and J. I. Goldstein, “Growth of Intragranular Ferrite in Fe—Ni—P Alloys,” accepted Met. Trans. (1984). C. Narayan and J. I. Goldstein, “A Major Revision of Iron Meteorite Cooling Rates,” submitted Geochim. et Cosmochim. Acta (1984). W. E. Schiesser, “DSS Version 2, Introductory Programming Manual,” Lehigh University and Naval Air Development Center (1976). W. D. Murray and F. Landis, “Numerical and Machine Solutions of Transient Heat—Conduction Problems Involving Melting or Freezing. -.
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Iron Meteorites Method of Analysis and Sample Solutions,” Trans. ASME, 81 Part I (1959) p. 106. J. A. Wood, “The Cooling Rates and Parent Planets of Several Iron Meteorites,” Icarus, 3 (1964) p. 429. J. I. Goldstein and R. E. Ogilvie, “The Growth of the Widmanstatten Pattern in Metallic Meteorites,” Geochim. et Cosmochim. Acta, 29 (1965) p. 893. J. I. Goldstein and J. M. Short, “The Iron Meteorites, Their Thermal History and Parent Bodies,” Geochim. et Cosmochim. Acta, 31 (1967) p. 1733. J. Willis and J. T. Wasson, “Cooling Rates of Group IVA Iron Meteorites,” Earth and Plan. Sci. Lett., 40 (1978) p. 141. P. M. Novotny, J. I. Goldstein and D. B. Williams, “Analytical Electron Microscope Study of Eight Ataxites,” Geochim. et Cosmochim. Acta, 46 (1982) p. 2461. L. S. Liii, J. I. Goldstein and D. B. Williams, “Analytical Electron Microscopy Study of the Plessite Structure in Four III CD Iron Meteorites,” Geochim. et Cosmochim. Acta, 43 (1979) p. 725. E. R. D. Scott, “The Nature of Dark Etching Rims in Meteoritic Taenite,” Geochim. et Cosmochim. Acta, 37 (1973) p. 2283. J. F. Albertsen, G. B. Jensen and J. M. Knudsen, “Structure of Taenite in Two Iron Meteorites,” Nature, 273 (1978) p. 453. E. R. D. Scott and R. S. Clarke, Jr., “Identification of Clear Taenite in Meteorites as Ordered FeNi,” Nature, 281 (1979) p. 360. S. Mehta, P. N. Novotny, 13. B. Williams and J. I. Goldstein, “Electron— Optical Observations of Ordered FeNi in the Estherville Meteorite,” Nature, 284 (1980) p. 151. P. M. Novotny, “An Electron Microscope Investigation of Eight Ataxite and One Mesosiderite Meteorites,” MS Thesis, Lehigh University (1981). K. Benusa, E. P. Butler, D. B. Williams and J. I. Goldstein, “Ordering in Fe—Ni Alloys Induced by Irradiation in the High Voltage Electron Microscope,” 7th International Conference on lIVEN (1983), in press. —
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