Three dimensional morphology of cementite precipitates

Deep etching experiments on an isothermally transformed Fe-1.34 wt. % C-13.1 % Mn alloy clearly reveal the three dimensional morphology of cementite precipitates. Grain boundary films initially consist of discrete precipitates exhibiting a "dendritic" or "fernlike" morphology and, contrary to previous reports, do not exhibit the classical allotriomorphic morphology [12,13] often approximated as ellipsoidal or hemispherical [4,22]. Random two dimensional cross sections in typical metallographic planes of polish may belie this fact. The cementite precipitates exhibiting a dendritic morphology lie essentially within the parent austenite grain boundary plane; grain boundary coverage is accomplished by impingement and overgrowth of these fernlike precipitates. There is no evidence that cementite "sideplates" form by continuous growth instabilities from the dendrite arms of the grain boundary precipitates. This investigation illustrates the importance of deep etching and/or serial sectioning techniques to understand the true three-dimensional morphology of microstructural features.