The 18R martensite phase was observed in the b.c.c. matrix of an Fe-25.8 wt% Mn-7.4 wt% Al-0.11 wt% C alloy, after cooling from high temperature. The crystal structure is determined from the selected area diffraction (SAD) and high resolution imaging techniques. The 18R martensite has an orthorhombic lattice with lattice constants a = 0.448 nm, b = 0.259 nm and c = 3.865 nm and is best described as an 18R(42)3 rather than an 18R(51)3 structure. [The expression of 18R(42)3 and 18R(51)3 comes from the combined notation of Ramsdell and Zhdanov [Nishiyama, Martensitic Transformation, p. 75 (1978)].] The stacking fault density relative to the 18R(42)3 structure is determined from high resolution imaging to be 0.096. Martensite crystallographic parameters such as orientation relationship, habit plane, shape strain direction and magnitude of lattice invariant shear are calculated using the CRAB theory, in good agreement with experimental observations. The Burgers vector of anticoherency dislocations and the shape strain direction were confirmed using the two beam technique and computer image simulation. The observed interfacial dislocation structure and choice of lattice correspondence are discussed in terms of near-CSL concepts.
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