Abstract
Recent experimental results on the axial ratios of bct and fct martensitic phases in both carbon steels and carbon-free ferrous alloys are reviewed, and a number of recent theories are examined. The topics discussed include the recent work of Kajiwara and Kikuchi on the influence of morphology and substructure on the axial ratio, the factors contributing to non-cubic structures in interstitial-free alloys and the changes in c⁄a produced by heating of virgin martensites from 90 K to room temperature. The Roitburd-Khachaturyan-Kurdjumov description of low axial ratio bct or orthorhombic martensites in terms of {011}⟨0\bar11⟩ twinning is re-examined, and it is pointed out that it involves the concept of an unusual, high shear twinning mode. It is concluded that this model only applies to untwinned rather than twinned martensite.
