TRANSMISSION ELECTRON-MICROSCOPY STUDIES OF THE CRYSTALLOGRAPHY OF BCC 18R MARTENSITE IN FE-MN-AL-C/

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 tempera ture. The crystal structure is determined from the selected area diffr action (SAD) and high resolution imaging techniques. The 18R martensit e 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(4 ($) over b ar 2)(3) rather than an 18R(5 ($) over bar 1)(3) structure. [The expre ssion of 18R(4 ($) over bar 2)(3) and 18R(5 ($) over bar 1)(3) comes f rom the combined notation of Ramsdell and Zhdanov [Nishiyama, Martensi tic Transformation, p. 75 (1978)].] The stacking fault density relativ e to the 18R(4 ($) over bar 2)(3) structure is determined from high re solution imaging to be 0.096. Martensite crystallographic parameters s uch as orientation relationship, habit plane, shape strain direction a nd 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 we re confirmed using the two beam technique and computer image simulatio n. The observed interfacial dislocation structure and choice of]attice correspondence are discussed in terms of near-CSL concepts.