Effects of α-γ-α Phase Transformation on the Σ3 Boundaries in High-Purity Iron

Phase transformation is a basic issue in physics, and it has been extensively used to manipulate the microstructure and thus to manipulate the properties of the materials. In present work, the alpha-gamma-alpha phase transformation was applied to the treatment of a high-purity iron sample. Then an integrated method, which involves electron backscatter diffraction, grain boundary filtration and stereology based five-parameter analysis, was used to characterize the Sigma(3) grain boundaries in the sample. The results show that both the frequency and the grain boundary inter-connection of Sigma(3) boundaries were changed after the treatment of the alpha-gamma-alpha phase transformation. That is the frequency was doubled and the grain boundary inter-connection was changed from its initial multi-components to a single component of {0 1 1}/{0 1 1}. Theoretical analysis based on the crystallography of phase transition suggests that such effects come from the Kurdjumov-Sachs mechanism during the phase transformation from gamma to alpha. Further analysis of crystallography indicates that the Sigma(3) boundaries with {0 1 1}/{0 1 1} inter-connection have much higher degree of structural ordering compared to the random boundaries. In the view of grain boundary engineering, such Sigma(3) boundaries are more resistant to the intergranular attacks in the service if the geometrical factors are the most important. Therefore, it is suggested that to further enhance the frequency of the Sigma(3) boundaries with {0 1 1}/{0 1 1} inter-connection through an optimized treatment of alpha-gamma-alpha phase transformation would be pertinent to grain boundary engineering of body-centered cubic materials featured with the alpha-gamma-alpha phase transformation.