Bainite self accomodating shear strains

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Sandvik resolved this issue by looking at the displacement of austenite twin boundaries by plates of bainitic ferrite using transmission electron microscopy and reported a shear strain associated with an individual plate to be 0.22 [14].Swallow and Bhadeshia [15] used atomic force microscopy to determine the shear strain to be about 0.26; theory predicts values in the range 0.22–0.28.Figs 2 and 3 show the three–dimensional (3D) representations of the areas scanned.As will be seen in the other images the total width of the transformation upheaval of each sub-unit is around 200 nm wide.

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The plates have a lenticular morphology [20, 21] so if only the periphery of a deeply located plate intersects the free surface then the apparent width will be smaller.

These are conical in the three dimensions and are thought to occur due to oxidation or some other contamination of the surface. The plates, however, will in general be inclined to the surface so that the measurement represent an apparent shear component = height/width.

The samples tended to degrade with time, becoming more oxidised or contaminated, until the sub-units could no longer be seen. There is a further complication that the austenite adjacent to the bainite plate may relax by plastic deformation [15].

This, together with the fact that the orientation of the displacement vector of the shape deformation can vary relative to the free surface, could explain why different values of apparent shear are observed for the same apparent width in Fig. Neither of these observations detract from the fact that the maximum observed value of the shear will be closer to the actual shear.

The results are interesting in that the true shear estimated for the nanostructured bainite is likely to be about 0.46, which is much larger than has been observed for ordinary bainite at about 0.26 [15].

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