2509.     ÖHMAN, E.  Crystal Structure of Martensite.  Nature, vol. 127, 1931, pp. 270-272; Chem. Abs., vol. 25, 1931, p. 2400.

        Present knowledge of tetragonal martensite is summarized, and results of earlier investigators are reviewed with reference to C content and lattice dimensions.  The author employed Cr-K-radiation and obtained photograms of quenched steel specimens in which the line (101) of the tetragonal phase is separated from the γ-Fe line (111).  Thus, it was possible to determine the lattice dimensions of the tetragonal structure.  Results agree with those of Kurdumoff and Kaminsky that the tetragonal martensite is a supersaturated solution of C in a-Fe.  Ferrite and tetragonal martensite are thus one and the same phase, but, as they are often present in one and the same specimen as separate constituents, it seems convenient to denote the tetragonal phase as a’.  Westgren and Phragmen suggested that the C atoms dissolved in γ-Fe do not occupy any points of the face-centered lattice but are statistically distributed in the interstices between the Fe atoms.  Seljakow, Kurdumoff, and Goodtzow suggested that the same might be the case in the a’-phase but the author thinks this improbable as the space available for the C atoms would be extremely small.  Another probable explanation is that there is a complex substitution in such a way that a group of 2 C atoms is substituted for 1 Fe atom in the lattice.  These compounds have a tetragonal structure in which the C₂ group is oriented parallel to the tetragonal axis, an explanation that holds equally well for tetragonal martensite.  By density measurements, it is shown that the assumption of a complex substitution of C atoms is in good agreement with X-ray intensities.  The only suggested structure of tetragonal martensite that explains the observed density, the increase of volume with the C content, and the elongation of 1 of the crystallographic axes may be described as follows:  In the body-centered lattice groups of 2 C atoms statically distributed replace some of the Fe atoms.  The C atoms are most probably oriented in such a way that the axes of the C₂ groups parallel the tetragonal axis of the lattice.