Combined effect of stress and strain on crystallographic orientation of bainite

The combined influence of prior plastic strain in the austenite, and applied stress in the course of the transformation, on the crystallographic texture of a bainitic steel was studied in this work. The experimental data were obtained using Electron Back Scatter Diffraction (EBSD) method and analyzed using two methodologically different approaches. It was concluded that the plastic deformation of the parent austenite grains could suppress variant selection in the bainite despite maintaining an externally applied load throughout the bainitic transformation. 1Introduction Bainite forms in steels by transformation of the parent austenite, induced by a homogeneous plastic deformation which does not require diffusion [1–4]. Hence, bainitic transformation and ordinary plastic deformation are somehow similar. For instance, in both cases certain crystallographic planes and directions (i.e. variants) are favoured. The favouring of certain variants over the others is known as “variant selection”. The crystallography of each plate of bainite can be described in terms of a mathematically connected set of habit planes, orientation relationships with the parent austenite, and the shape deformation [5]. The interaction between the applied stress and the transformation can be treated by adding a mechanical driving force (GMECH) to the chemical term (GCHEM) which would be the driving force for the transformation even in the absence of applied stress. Hase et al. studied the effect of applied compressive stresses on the bainite start-temperature and variant selection in a carbide-free bainitic steel [6]. The work showed that the applied compressive stress of about 200 MPa raised the transformation start-temperature and led to micro-structural alignment of the bainite sheaves. The optical micrographs clearly confirmed that the majority of bainite sheaves were formed on the planes of maximum shear stress (i.e. 45° angle to the applied compressive load), and the higher the applied load the higher the abundance of aligned sheaves. Visual examination of the electron back scatter diffraction (EBSD) maps revealed that certain crystallographic orientations were favoured within some large regions containing parallel bainite sheaves; indicating the presence of a strong variant selection. No quantitative assessment of the variant selection was carried out in Hase et al. work. In more recent work, Shirzadi et al. investigated the influence of plastic strain alone (i.e. without any applied stress during the transformation) on the crystallographic orientation of bainitic [7]. They found no strong variant selection in the bainitic steel which transformed from a plastically deformed austenite to bainite under zero applied load. This was in contrast with Bokros and Parker’s work on a single crystal austenite in which a strong variant selection in martensite was observed due to prior plastic deformation of the parent austenite [8]. It is possible that the complexity of slip systems in the polycrystalline steel used by Shirzadi et al., where multiple slip systems must operate in order to maintain continuity, led to the lack of strong variant selection. Whereas, in Bokros and Parkers’ work, the observed variant selection occurred because of the anisotropic nature of the dislocation debris in the austenite single crystal. The purpose of the present work was to study the combined influence of (1) prior plastic strain in the parent austenite, and (2) applied stress in the course of the transformation, on the crystallographic texture of bainite. 2Crystallographic orientation in steels The room temperature microstructure and texture of a steel mostly depend on its composition, thermo-mechanical history and initial crystallographic orientation of the parent austenite at the elevated temperature prior to its transformation to a low temperature phase, e.g. ferrite or martensite. When austenite transforms to ferrite by a reconstructive mechanism, the product phase nucleates heterogeneously at austenite grain boundaries. Although a reproducible, low energy orientation relationship is expected to exist between the ferrite and that of the parent austenite grains, it is possible that the ferrite simultaneously adopts this orientation with more than one austenite grain. On the other hand, in displacive transformations the crystal structure of the parent austenite is deformed into that of the product without the need for any diffusion. In displacive transformations, the coordinated movements of atoms cannot be sustained across the grain boundaries. Hence, it is reasonable to assume that the product is confined to the parent grain with which it has an orientation relationship [9]. In the case of displacive transformations such as martensite and bainite certain sets of the orientation relationship, shape deformation and habit planes can be defined using the theory of martensitic transformation. Such transformation are dominated by strain energy due to the shape deformation, therefore many of the crystallographic variables cannot be varied independently [10]. Hence, having assumed that the product phase grows in the same parent 1 Some crystallographers argue that formation of bainite is also a reconstructive transformation since it requires breaking up the atomic bonds and limited amount of diffusion. crystal in which it nucleates, the texture of final phase can be predicted once the orientations of the parent crystals are known [11]. In order to study and predict the evolving textures in polycrystalline steels, the original crystallographic orientation of the austenite has to be known. Various simulation techniques and experimental methods are used to determine the texture of retained austenite [12-15]. For instance, advanced X-ray and neutron diffraction methods can be used to determine the texture of the parent austenite at 950C prior to its transformation to low-temperature phases [16]. 3Determining variant selection In this work, two methodologically different approaches were used to investigate the presence (or lack) of the variant selection in a bainitic sample, which was formed under a compressive stress from a plastically deformed parent austenite. In the first approach ARPGE software were used to reconstruct the orientation of the parent austenite grains followed by a statistical analysis of the bainite texture [17,18]. The second approach relies on the micro-texture development within individual austenite grains as a function of the direction of the applied compressive stress. The micro-texture of the grains could be illustrated and compared using the corresponding pole figures. The details of the second approach were given elsewhere [7]. Statistical approach The ARPGE software is capable of determining the crystallographic orientation of austenite grains using the EBSD data acquired from the room temperature phase (i.e. bainite) and without requiring any experimental data on the parent austenite grains [17,18]. The reconstruction of parent austenite is performed by (a) checking if the experimental misorientations between the BCC grains are close to theoretical misorientations (i.e. operators), (b) checking if the composition of the operators corresponds the theory (groupoid composition), (c) deducing the indexes of the variants and (e) calculating the orientation of the parent austenite grainssee Ref [18] for more details. Finally, the global pole figures corresponding to the texture of the parent grains are drawn, and the frequencies of the detected variants and used operators are represented on columnar charts. If the frequencies of variant indexes and/or operators between the neighboring grains are high, then one can conclude that the variant selection has most likely occurred. The flow chart in Fig. 1 shows the procedure used to determine whether variant selection occurred in the sample studied in this work. Fig. 1: Procedure of analyzing variant selection in a fully bainitic steel using EBSD data. Experimental EBSD map of bainite