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Part 1. Fe-Ni Taenite Cooling Experiments

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Part 1. Fe-Ni Taenite Cooling Experiments Meteoritics & Planetary Science 38, Nr 11, 1669–1678 (2003) Abstract available online at http://meteoritics.org Ordinary chondrite metallography: Part 1. Fe-Ni taenite cooling experiments R. J. REISENER and J. I. GOLDSTEIN* Department of Mechanical and Industrial Engineering and Department of Geosciences University of Massachusetts, Amherst, Massachusetts 01003, USA *Corresponding author. E-mail: [email protected] (Received 16 January 2003; revision accepted 1 December 2003) Abstract–Cooling rate experiments were performed on P-free Fe-Ni alloys that are compositionally similar to ordinary chondrite metal to study the taenite → taenite + kamacite reaction. The role of taenite grain boundaries and the effect of adding Co and S to Fe-Ni alloys were investigated. In P-free alloys, kamacite nucleates at taenite/taenite grain boundaries, taenite triple junctions, and taenite grain corners. Grain boundary diffusion enables growth of kamacite grain boundary precipitates into one of the parent taenite grains. Likely, grain boundary nucleation and grain boundary diffusion are the applicable mechanisms for the development of the microstructure of much of the metal in ordinary chondrites. No intragranular (matrix) kamacite precipitates are observed in P-free Fe-Ni alloys. The absence of intragranular kamacite indicates that P-free, monocrystalline taenite particles will transform to martensite upon cooling. This transformation process could explain the metallography of zoneless plessite particles observed in H and L chondrites. In P-bearing Fe-Ni alloys and iron meteorites, kamacite precipitates can nucleate both on taenite grain boundaries and intragranularly as Widmanstätten kamacite plates. Therefore, P-free chondritic metal and P-bearing iron meteorite/ pallasite metal are controlled by different chemical systems and different types of taenite transformation processes. INTRODUCTION iron meteorites, i.e., by kamacite nucleation and growth within a taenite matrix (Wood 1967). However, important Fe-Ni metal composed of taenite (γ, face-centered-cubic) metallographic differences between iron meteorites and and kamacite (α, body-centered-cubic) is present in varying ordinary chondrites cast doubt on this assumption: amounts in ordinary chondrites, stony-iron, and iron 1. Iron meteorite microstructures developed from single meteorites. The metallography of meteoritic metal can be taenite crystals that are commonly >1 m across. Ordinary used to infer the thermal and physical history of meteorites chondrite metal occurs as 1 µm- to 1 mm-sized metal and their asteroidal parent bodies. troilite assemblages that are peppered throughout a Most meteoritic metal cooled from temperatures within matrix of olivine and pyroxene. Most of the metal the single-phase taenite field (Fig. 1) at rates of 1–100°C/Myr microstructures in ordinary chondrites developed from (Wood 1964; Wood 1967; Goldstein and Short 1967; Willis taenite particles that contain 1 or more grain boundaries and Goldstein 1981). In most iron and pallasite meteorites, a (polycrystalline taenite). Widmanstätten structure consisting of kamacite plates aligned 2. Most iron meteorites exhibit a well-developed along the octahedral planes of the taenite matrix is produced Widmanstätten structure. Widmanstätten structure is during cooling. rarely observed in ordinary chondrites, and taenite/ Type 4–6 ordinary chondrites experienced metamorphic kamacite interfaces are typically non-planar. heating to peak temperatures between 700 and 900°C 3. Iron meteorites contain up to 2 wt% P in alloy solid (McSween et al. 1988). At these temperatures, chondritic solution and as phosphide precipitates (Buchwald 1975). metal lies within the single-phase taenite field (or within the Ordinary chondrite metal contains <0.002 wt% P (Reed 2-phase taenite + troilite [FeS] field) (see Fig. 1). The taenite 1964). transformations, which occurred upon cooling, are commonly Ternary Fe-Ni-P alloy cooling experiments have been assumed to have been controlled by the same mechanism as used to investigate taenite transformations in P-bearing iron 1669 © Meteoritical Society, 2003. Printed in USA. 1670 R. J. Reisener and J. I. Goldstein and pallasite meteorites (Doan and Goldstein 1969; Goldstein and Doan 1972; Narayan and Goldstein 1984a, b). These experiments provide information about phase precipitation sequences and kamacite nucleation sites in iron and pallasite meteorites. Unfortunately, the Fe-Ni-P alloy cooling experiments are not useful for understanding the phase transformations that occurred in P-poor ordinary chondrite metal. In this study, P-free Fe-Ni alloy cooling experiments were performed to study the taenite → taenite + kamacite reaction in alloys that are compositionally similar to ordinary chondrite metal. Alloys were cooled from the taenite field into the taenite + kamacite field to identify the sites of kamacite nucleation and to describe the kamacite growth process. The cooling experiments investigated the effects of adding Co and S to P-free Fe-Ni alloys and the roles of interfaces such as taenite/taenite grain boundaries. The experiments provide information about the taenite transformations that occurred in ordinary chondrites as they cooled from peak metamorphic temperatures. The influence of taenite grain boundaries, which are common in ordinary chondrite metal but rare in iron meteorite metal, is investigated. The results of the cooling experiments provide a basis for interpreting ordinary chondrite metal microstructures and for understanding metallographic Fig. 1. Fe-Ni phase diagram showing the taenite field, kamacite field, differences between ordinary chondrites and iron meteorites. taenite + kamacite field (T + K), and region of spinodal The application of the experimental results to understanding decomposition. Line A → E indicates the composition of taenite at ordinary chondrite metallography is discussed in detail in a the taenite/kamacite interface during cooling for an alloy of 10 wt% Ni. Ms is the martensite-start line, γ is paramagnetic taenite, and γ companion paper (Reisener and Goldstein 2003). 1 2 is ferromagnetic taenite. TC is the Fe-50Ni critical ordering temperature at which taenite transforms to the ordered phase, EXPERIMENTAL PROCEDURE tetrataenite. The average metal Ni concentrations in H, L, and LL group ordinary chondrites, calculated from wet chemical data Alloy Preparation reported by Jarosewich (1990), are indicated. The peak metamorphic temperatures reached by type 3–6 ordinary chondrites (McSween et al. 1988) are indicated on the right hand side of the phase diagram. Two binary Fe-Ni alloys (alloys A1 and A2) and 2 Fe-Ni alloys (alloys A3–A4) containing variable amounts of Co and Table 1. Target alloy compositions. S were prepared to simulate ordinary chondrite metal Target composition of Target composition of (Table 1). The metal fraction of each alloy contains Alloy ingot (wt%) metal fraction (wt%) approximately 9 wt% Ni, typical of metal particles in H A1 Fe-9Ni (coarse) Same chondrites and in many iron meteorites. The alloys were A2 Fe-9Ni (fine) Same prepared from high purity elements (purities >99.99 wt%) and A3 Fe-9Ni-0.5Co Same contained ≤0.0015 wt% C. The pure elements were melted in A4 Fe-8Ni-0.5Co-2S Fe-9Ni-0.55Co a fused quartz melting chamber using a Lepel 30 kW high frequency induction furnace. The melting chamber was encapsulated inside a fused quartz capsule. The Ta foil flushed with Ar-H2 gas during melting to prevent alloy gettered oxygen and protected the alloys from Si vapor that oxidation and to suppress evaporative loss of volatile S. The was liberated from the quartz capsules during heat-treating. molten metal was held in the liquid state for 1–2 min to ensure The vacuum-encapsulated alloys were heat-treated in complete homogenization and was then cooled to room horizontal tube furnaces (Applied Test Systems). temperature. Alloy A1 (Fe-9Ni) was homogenized within the single- phase taenite field at 1100 ± 5°C for 20 days to eliminate Alloy Heat-Treatment elemental zoning formed during solidification. The alloy was cooled to the equilibrium kamacite precipitation temperature Each ingot of approximately 5 g was placed in an of 700°C over the course of 1 day and then cooled through the alumina crucible, wrapped in tantalum foil, and vacuum- taenite + kamacite field at a rate of 4°C/day. At 500°C, the Fe-Ni taenite cooling experiments 1671 alloy was removed from the furnace and quenched in liquid nitrogen. Alloys A2–A4 were heat treated at 900°C for 20 days to homogenize the alloys while avoiding eutectic melting in the sulfur-bearing alloy A4. The homogenization temperature placed alloys A2 and A3 within the single-phase taenite field, while the S-bearing alloy A4 was within the 2-phase taenite + troilite field. After homogenization, the alloys were cooled to the equilibrium kamacite precipitation temperature of approximately 700°C over the course of 1 day and then cooled from 700 to 550°C at a rate of 3°C/day. At 550°C, alloys A2–A4 were removed from the furnace and quenched in liquid nitrogen. The final heat-treatment temperature (500°C for alloy A1, 550°C for alloys A2–A4) was chosen so that the martensite phase transformation was not initiated during slow furnace cooling. The final furnace temperatures were at or above the Fe-9Ni martensite start temperature of ~500°C (Kaufman and Cohen 1956) (see Fig. 1), ensuring that any experimentally grown kamacite precipitates formed directly from taenite by the taenite → taenite + kamacite reaction.
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