Dilatometric and microstructural response of variant selection during α′ transformation

Abstract

Variant selection during displacive transformation is popularly monitored by assessing the pole figure or orientation density function of the bulk texture or pole figures from single prior austenite grains. It can also be done by measuring the physical orientation of α′ (bainite or marteniste) plates in the microstructure. It is known that when variants of α′ form randomly, i.e. there is no variant selection, the shear strain associated with each α′ plate gets canceled and transformation strain becomes equal to the volume strain. Under this condition, orientation of sheaves of α′ also shows random distribution. In this work, it has been shown how transformation strain and orientation of sheaves change when variant selection occurs. Mathematical models have already been developed to calculate the transformation strain under various loading conditions; extensive experiments have been conducted during the course of this work to show how the nature of externally applied stress influences the transformation strain. In addition to this, it has also been shown that the variant selection might leave its trace in the microstructure by affecting the alignment of the sheaves under externally applied stress and strain. Mathematical models have been developed to show that variant selection has negligible influence on physical orientation of α′ plates when transformation occurs under external stress, but prior plastic deformation can dictate the orientation to a great extent.