Thermoelastic strain of the lattice plane spacings in white cast iron

White cast iron consists of the two phases ferrite and cementite. On cooling, strong thermoelastic stresses Δσij occur which-because of equilibrium conditions-have different signs in the two phases. According to the sin2 ψ-law of conventional X-ray stress analysis (ψ is the angle between the reflecting lattice plane and the irradiated surface of the specimen), the corresponding thermoelastic strains Δd of the lattice plane spacings, if plotted versus sin2 ψ, should yield straight lines with slopes proportional to Δσij components. The slopes observed for the two phases should, therefore, have different signs. Experimentally, the opposite was observed. It is shown that this unexpected behavior is related to the anisotropic microscopic thermal expansion coefficients of the cementite combined with anisotropic macroscopic thermal expansion coefficients of the specimen. The latter is a result of the texture of the cementite phase. This changes more or less the conventionally expected Δd(sin2 ψ) curves of the two phases.