Theoretical and experimental aspects of the “(225)” austenite-martensite transformation in iron alloys

Abstract

Measurements of the habit plane, p 1′, the direction of the shape strain, d 1; and the orientation relationship, R, were made for an Fe-Cr-C alloy. The classical “(225)” hahit plane is believed to be non-existent; allowing for experimental errors, all of the habit plane measurements obtained to date and all of the present p 1′ determinations do not lie on the boundary between (001) f and (111) f. The shape strain results indicate that a uniform dilatation (δ > 1 ) is more consistent with observations than the case where the interface is one of zero-average distortion (δ = 1). Particular states of interface strain such as that proposed by Frank and Suzuki, and an ad hoc δ-δ-δ∗ strain were studied for consistency. An anisotropic interface distortion is considered to be unlikely. The phenomenological theory does not permit the orientation relationship to be deduced from first principles. Within the framework of the theory, a dilatation is needed, but it is the required dilatation which produces the discrepancy between the observed and predicted orientation relationships. The theory in its present form, it is concluded, cannot account for all aspects of the “(225)” transformation: for example, the observed habit plane and orientation relation can be predicted, but the predicted direction of the shape strain differs from the observed one; alternatively the observed habit plane and direction of the shape strain can be successfully predicted, but the predicted orientation relationship is not consistent with that observed.