Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe-14Cr alloys with 1, 2, and 4 at. pct Hf addition at 1173 K (900 °C). Microstructural characterizations using aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy reveal high density of HfO₂ nanoparticles with sizes of ~4 nm dispersed throughout the ferritic matrix. This indicates that kinetic stabilization by HfO₂ nanoparticle pinning is primarily responsible for the observed high thermal stability. In addition, some Hf and Cr segregation on grain boundaries is observed in the Fe-14Cr-4Hf, suggesting the existence of thermodynamic stabilization at high Hf content. Second-phase precipitations such as hafnium carbide, M₂₃C₆, and Fe-Cr-Hf intermetallic phase are also found in the Fe-14Cr-4Hf, but their large sizes and inter-spacing suggest that their contribution to stabilization is minimal.