An Electron Microscopy Investigation On The Crystallography Of Widmannst(?)tten Cementite In A High Manganese Steel

Most structural metallic materials contain multi-phase microstructures,and their mechanical properties are affected by the number,morphology,distribution,orientation,and size of various phases.With the advancement of techniques and instruments for characterization,it becomes possible to investigate the crystallographic morphology of fine precipitates in the microstructures.However,so far the relevant knowledge is rather limited for most materials.In this work,the morphology,crystallography,interface migration mode,and the surface relief effect associated with precipitation of proeutectoid Widmannst?tten cementite in a high manganese steel were investigated systematically,aiming to improve understanding of the morphology and crystallography of the compound precipitates.The morphology and interfacial structure of Widmannst?tten cementite were characterized by transmission electron microscopy(TEM)and scanning electron microscopy(SEM).It was found that the Widmannst?tten cementite in the experimental steel mainly possesses the plate and lath morphology.The cementite plates hold an exact Pitsch orientation relationship with austenite,but the orientation relationship associated with the cementite laths disperses near the F-E orientation relationship.The broad interface of cementite plate mainly consists of habit plane and growth ledges with a specific height,and the habit plane itself contains structural ledges associated with secondary dislocations distributed periodically.The geometry structure of these two types of ledges and the Burgers vector of the associated dislocations were quantitatively characterized.The study using in situ TEM revealed that the broad interface of cementite plate migrates by lateral movement of numerous growth ledges,and the extension of the broad interface at the cementite edge is accompanied with emitting dislocations to the austenite matrix.The broad interface of cementite lath contains irregularly distributed ledges and near parallel dislocations.The line direction and Burgers vector of the dislocations have been characterized.After a long-time isothermal heat treatment,the facet on the broad interface tends to lie parallel(0 0 1)_C.In addition,a new and unreported orientation relationship between cementite and austenite has been found.The related habit plate contains three sets of dislocations,with the geometry and Burgers vectors of the two sets been quantitatively characterized.According to the rule of singular structure in a preferred interface,an integrated geometric analysis method has been established for rationalizing the preferred orientation relationship and the preferred interface.This method includes examining the potential preferred orientation relationship in the reciprocal space according to the distribution of g andΔg rows,graphic analysis of the GMS cluster distribution,and the application of CSL/DSCL model and O-lattice theory to calculate the misfit strain field,invariant line direction,and secondary dislocation structure.Using this method,the experimental results on the orientation relationships between cementite and austenite,the orientation of the preferred interfaces,the ledges on the broad interface,and the configuration of dislocations have been quantitatively interpreted.Especially,the reason for the puzzling characteristic of non-unique habit plane of the lath-shaped cementite associated with the F-E OR has been clarified.The surface relief effect accompanying the cementite transformation were studied by focusing ion beam technique,scanning electron microscopy and atomic force microscopy.The tilt angle of surface relief accompanying the precipitation of cementite plate was measured.According to the model of the interfacial structure of cementite plate,the long-range strain field accompanying the interfacial migration has been estimated,and it was used to interpret the experimental results.Based on the long-range strain field accompanying the interfacial migration of cementite plate and a calculation of the Schmid factor,the Burgers vector and slip system of the emitted dislocations from the cementite edge to the austenite matrix have been interpreted.