LOW-TEMPERATURE PHASE-DECOMPOSITION IN METAL FROM IRON, STONY-IRON, AND STONY METEORITES

The microstructure of the outer taenite rim and cloudy zone of meteori tic metal was investigated using high resolution scanning electron mic roscopy and analytical electron microscopy. The microstructure is form ed by a series of complex phase transformations at less than or equal to 400 degrees C. These transformations are interpreted using the Fe-N i phase diagram. The outermost subzone (zone 1) of the outer taenite r im is composed of two single-phase layers, Ni3Fe and FeNi (tetrataenit e). The Ni3Fe phase appears at the border between kamacite and the out er taenite rim. The inner two subzones (zones 2 and 3) in the outer ta enite rim are two-phase with a matrix of tetrataenite and low Ni bce p recipitates. The three zone structure is a general feature in the oute r taenite rim of metal particles in all major types of meteorites. The cloudy zone is composed of an island region and a bcc single phase ho neycomb region containing similar to 9.0 wt% Ni. The island region is a two-phase mixture of a L1(0) ordered fee tetrataenite matrix and low -Ni bcc precipitates. The high-Ni taenite region, gamma(2), resulting from the monotectoid reaction, gamma(1) --> alpha + gamma(2), at 400 d egrees C remains as a supersaturated solid solution to lower temperatu res and corresponds metallographically to the outer taenite rim. The w idth of the outer taenite rim varies inversely with the cooling rate o f the meteorite. The gamma', Ni3Fe, phase is formed when the high-Ni g amma(2) phase containing similar to 53 wt% Ni undergoes an eutectoid r eaction, gamma(2) --> alpha + gamma' at about 345 degrees C. The cloud y zone develops by spinodal decomposition below 350 degrees C. Metasta ble gamma(2) island region of high Ni content and metastable gamma(1) honeycomb region of low Ni content are formed. The size of the constit uents of the cloudy zone increases with increasing Ni content because the higher Ni regions enter the spinodal region at higher temperatures . The precipitates in the island region of the cloudy zone and in zone s 2 and 3 of the outer taenite rim have essentially the same Ni conten t (14.2-14.7 wt%) and form at lower temperatures when the Ni content o f gamma(2) is located in the gamma(1) + gamma(2) miscibility gap but o utside the spinodal. If cooling rates or metamorphic reheating is to b e studied at or below 400 degrees C, the low temperature phase transfo rmations must be considered. Improvement in simulation models will be difficult to make, however, because the diffusion rates in ordered FeN i, tetrataenite, and ordered Ni3Fe are not known. Copyright (C) 1997 E lsevier Science Ltd.