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Crystallography of low alloy iron martensites Jeffrey, Paul William

Abstract

The morphology and crystallography of the martensite transformation in pure iron and low alloy Fe-C, Fe-Mn, Fe-Mn-Si, Fe-Ni steels which contain no retained austenite was studied. Single surface trace analyses by transmission electron microscopy on directions parallel and normal to the martensite substructure were found to be consistent with the martensite crystals having the form of plates rather than needles. The directions normal to the martensite substructure plates were consistent with a {145}M habit plane. Optical studies of the martensite surface shears within prior austenite grains revealed that 4 shear variants generally occur. However grains containing 5 or more shear variants could be found which appears to suggest an austenite habit plan close to but different from {111}A. A single surface trace analysis of the martensite surface shears using the austenite annealing twin vestiges to orient the grain was consistent with an austenite habit plant ~ 7° from {111}A. Two inhomogeneous shear systems were found to predict the experimental results when applied to the Wecheler, Lieberman, Read theory of martensite transformations. They are (100)A[010]A and (111)A[112]A. The (111)A[112]A system is to be generally preferred as its predicted habit planes ( 7.12° from {111}A and 5.03° from {145}M are more consistent with the trace analyses. Preliminary work included an investigation of the maraging properties of the Fe-Mn-Si system.

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