The effect of stress on the Widmanstنtten ferrite transformation

Transformation of Widmanstنtten ferrite has been considered to be displacive with a large shear component (larger than martensite and bainite). It is expected that the external stress exerts a profound effect on Widmanstنtten ferrite formation. Optical metallographs show that under the influence of an applied compressive stress (below the yield strength of austenite) the number of crystallographic variants of Widmanstنtten ferrite is reduced drastically and there is a strong tendency for the plates that do grow to align. Transformation strains (including longitudinal and radial strains) during Widmanstنtten ferrite formation are shown to be consistent with the microstructural changes. Furthermore, transmission electron micrographs clearly reveal that the applied stress leads to the decoupling of the adjacent accommodating plates of Widmanstنtten ferrite. All the experimental results are explained in terms of the invariant plane-strain displacive mechanism for Widmanstنtten ferrite.