Quantitative evaluation of dislocation density in as-quenched martensite with tetragonality by X-ray line profile analysis in a medium-carbon steel

The crystal structure of as-quenched martensite in steel changes from body-centered cubic to body-centered tetragonal because of the presence of interstitial carbon, and the c/a ratio increases with increasing carbon content. Because the X-ray diffraction peak profiles associated with the c-axis overlap those of the a-axis, the diffraction peak observed in martensite gradually broadens as the solute carbon increases. This may cause an overestimation of the dislocation density in the X-ray line profile analysis (for example, modified Williamson-Hall/Warren-Averbach method). In this study, the effect of the tetragonality of martensite in the X-ray line profile analysis was investigated to evaluate the dislocation density in a medium-carbon martensitic steel. The apparent dislocation density estimated without considering peak broadening due to tetragonality was calculated to be 1.7 × 10¹⁶ m⁻² in the as-quenched 0.55%C steel. This value is approximately six times higher than that estimated using transmission electron microscopy (TEM), indicating that the dislocation density was overestimated. To eliminate the effect of tetragonality, the specimens were tempered at temperatures below 623 K. The estimation based on the TEM method revealed that the dislocation density only slightly decreased by tempering at 573 K, indicating negligible recovery at this temperature. The dislocation density obtained in the tempered specimen was calculated to be approximately 4.5 × 10¹⁵ m⁻² using the mWH/WA method. This value was regarded as the true dislocation density of the as-quenched martensite in the 0.55%C steel.