Diffuse X-ray scattering from doublet defects in martensite and ferrite

Abstract

Using recent calculations of the doublet-force displacement field about a carbon interstitial in α-iron, we have calculated the X-ray diffuse scattering in the vicinity of the reciprocal lattice points in iron-carbon martensite and in ferrite. Contours of the diffuse scattering in electron units per iron atom divided by p(l − p), where p is the fraction of carbon atoms are given for both martensite and ferrite. Martensite is characterized by the alignment of the doublet displacements along [001] due to the preferred occupation of one of the three interstitial octahedral sites by carbon. In martensite the contours are of a complicated shape, often quite different from those for defects with spherical symmetry. Reflections of the type (00L) and (HK0) always show nodal planes perpendicular to the vector from (000) to (HKL). The diffuse scattering at (000), the small angle scattering shows the cylindrical doublet symmetry. In ferrite the three octahedral sites are equally populated, and the doublet displacements are equally distributed amongst [100], [010] and [001]. The contours are even more complicated, some displaying no nodal surfaces. Reflections of the type (H00), (0K0), or (00L) show nodal planes perpendicular to the vector from (000) to (HKL). The results for martensite and ferrite reveal that the origin scattering exhibits the orientation and symmetry of a defect in a more direct manner than the diffuse scattering about any other point in reciprocal space.