TEM study of an analogue of the Earth’s inner core ε-Fe

Abstract

Metallurgists know that a hexagonal close-packed phase (ε-martensite) is formed by straining at low temperature or pressurization of nickel–chromium stainless steels. In many respects, the ε-martensite is similar to the hexagonal phase of iron thought to constitute the Earth’s inner core (ε-Fe), but, contrary to ε-Fe, it is metastable at atmospheric pressure. Observation of dislocations in ε-martensite in transmission electron microscopy (TEM) may thus provide information on the deformation mechanisms of the inner core alloy. Samples of 18 Cr–8 Ni austenitic stainless steel have been pressurized at 13 GPa and 800 °C, in a multi-anvil press and 30 GPa and RT, in a diamond-anvil cell. Subsequent TEM study shows that thin lamellae of ε-phase form on {1 1 1} planes of the fcc austenite. They were accompanied by crystals of bcc α′-martensite. In one case, a few dislocations were observed in a glide band of the ε-phase, compatible with second order pyramidal slip {1 1 2 ̄ 2}〈1 1 2 ̄ 3〉 . The observations suggest that ε-martensite, hence presumably ε-Fe, deforms by slip on the basal plane and second order pyramidal planes, as hexagonal cobalt.