Surface-Stress-Induced Lattice Distortion in Nanolamellar Cementite

Cementite morphology is known to govern the mechanical performance of steels, yet its influence on actual crystal structure, like unit cell parameters, is not well quantified. Here, we show that cementite morphology-dependent surface stresses generate measurable lattice distortion in nanocrystalline cementite. Lamellar and spheroidized cementite powders were extracted from a Cr-alloyed hypereutectoid steel and characterized by high-energy synchrotron X-ray diffraction combined with double-Voigt profile modeling. Despite a higher Cr content in spheroidized cementite, lamellar cementite exhibits systematically larger lattice parameters, corresponding to an average volumetric lattice strain of 6.3 ± 2.2 × 10^-4. This counter-intuitive expansion is rationalized by a continuum mechanics framework that incorporates the orthorhombic elastic anisotropy of cementite and particle morphology. The model predicts the sign and magnitude of morphology-induced lattice strain, demonstrating that surface-stress effects constitute a systematic bias in diffraction-derived lattice parameters of nanostructured carbides.