Meteorites as specimens for microgravity research
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I.
INTRODUCTION
THE focus of microgravity materials research is to determine the role of gravity as a variable. Some extraterrestrial materials contain information which can assist in attaining this elusive goal. Nickel-iron and stony-iron (pallasite) meteorites were probably solidified from a melt under microgravity conditions, m The implications are as follows: 1. Meteoritic materials can be used to study microgravity solidification and phase separation phenomena. 2. A set of boundary conditions for meteorite formation is established. Fundamental to the work outlined herein is the now broad acceptance that meteorite phases are considered to be unchanged from their condition before entry into the Earth's atmosphere. The ablation of surface material during passage through the atmosphere serves to preserve the integrity of the internal material, with the exception of a narrow (10 mm deep) heat-affected zone. t21 II.
T H E WIDMANST.~TTEN STRUCTURE
The Widmanstiitten structure is a characteristic and identifying feature of nickel-iron meteorites-- the familiar octahedral pattern of cut and polished museum specimens, shown in schematic in Figure l(a). The octahedral Widmanstiitten structure is formed by body-centered cubic plates (lamellae) of low-nickel iron, called kamacite in meteoritics. Figure l(b) is an idealized nickel-iron meteorite microstructure showing the relationship of kamacite crystals to the other two major microstructural features, taenite and piessite (see Section V). Some meteoriticists consider kamacite to be alpha-ferrite, a solid state phase transformation product. An alternate interpretation is that kamacite plates are delta-ferrite dendrites, products of direct crystallization from a melt. Consider the possibility that nickel-iron and stony-iron (pallasite) meteorites began as weightless, perhaps multikilogram liquid masses which solidified in the near-vacuum P.Z. BUDKA is with Technical Communications Unlimited, P.O. Box 708, Schenectady, NY 12301-0708. This paper is based on a presentation made in the symposium "Experimen'~al Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Anzona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee. METALLURGICALTRANSACTIONS A
(a)
(b) Fig. 1--(a) Tschermak's schematics relating Widmanst~itten structure to the octahedron (1874; in Buchwald 1975). (b) Idealized mckel-iron meteorite rnicrostructure. VOLUME 19A, AUGUST 1988-- 1919
of space under nonequilibrium conditions. Heat transfer away from the mass was accomplished principally through radiative cooling. The thermal gradient near the melting point of iron was very shallow and, as solidification proceeded, not necessarily linear. Recalescence may have contributed to temperature fluctuations, and heat conduction through the high temperature solid phases may have accelerated solidification at low temperatures. Temperature gradients caused convective flow in a system otherwi
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