The effect of Cr content on intragranular κ-carbide precipitation in Fe-Mn-Al-(Cr)-C low-density steels: A multiscale investigation

The effect of Cr content on intragranular kappa-carbide precipitation behaviors in Fe-Mn-Al-Cr-C low-density steels was investigated by X-ray diffraction (XRD), scanning (SEM) and (high-resolution) scanning transmission electron microscopy ((HR)STEM), atom probe tomography (APT), thermodynamic and first-principles calculations. We found that the volume fraction of kappa-carbides in near-rapidly solidified samples was decreased with increasing Cr content. The increased Cr content also significantly slowed down the growth rate of kappa-carbides during isothermal aging treatment (600 degrees C/9 h). Thermodynamic calculations showed that the increased Cr content decreased the precipitation temperature of kappa-carbides and narrowed the temperature range of kappa-carbide formation. The impurity of Cr atoms in kappa-carbide structure increased its interfacial energy and elastic strain energy for nucleation, thereby significantly increasing the critical energy for the kappa-carbide formation in gamma-austenite. Simultaneously, the results of APT and first-principles calculation indicated that the impurity of Cr atoms in the kappa-carbide structure preferred to occupy the Al sites, which increased the formation energy of kappa-carbides making the kappa-carbide formation difficult. In addition, the increased interfacial energy between the kappa-carbides and gamma-austenite, the improved solubility of Al and C in the gamma-austenite and the decreased diffusion coefficient of C in gamma-austenite caused by increasing Cr content resulted in a slower growth rate of the kappa-carbides.

Automated summary

The effect of Cr content on intragranular kappa-carbide precipitation behaviors in Fe-Mn-Al-Cr-C low-density steels was investigated. It was found that the volume fraction of kappa-carbides decreased and the growth rate slowed down with increasing Cr content. The precipitation temperature and temperature range of kappa-carbides were also affected by the increased Cr content. The presence of Cr atoms in the kappa-carbide structure increased interfacial energy and nucleation energy, making kappa-carbide formation more difficult. The increased interfacial energy, improved solubility of Al and C, and decreased diffusion coefficient of C in gamma-austenite caused by increasing Cr content resulted in a slower growth rate of the kappa-carbides.