Charpy V-notch impact specimens prepared from the high nickel ataxite Hoba have been tested at 77° and 195°K. The specimen tested at 77°K was fully brittle, and the one tested at 195°K was fully ductile. It is concluded that the transition to complete brittleness occurs below about 140°K or 150°K. Thus, high nickel material such as this is ductile at the ambient temperature of an asteroid, which is about 150°K, and does not fragment in a brittle manner when meteoritic material is formed by the collision of asteroids. This confirms a hypothesis put forward by Remo and Johnson [1974] according to which this failure of high nickel material to fragment explains the large average size of high nickel meteorites. The microstructure of Hoba was studied using optical microscopy, scanning electron microscopy, and electron beam microprobe analysis. It was found, in agreement with the results of earlier workers, that Hoba has a fine plessitic structure with prolific small mineral inclusions. Some troilite inclusions were shown to have transverse bands of daubreelite within them.
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