Pengbo Zhang

Solution strengthening is an effective strategy to improve the properties of martensitic steels (MSs). However, the microscopic mechanism of solution strengthening between Cr and interstitial atoms (IAs; C/N/O) remains elusive in dilute Fe-Cr system. Herein, this research aimed to ascertain the stability of Cr and IAs, and the interaction mechanisms between Cr and IAs by energy analysis and four-level electronic structure analysis (projected density of states, Electron Localization Function, crystal orbital Hamiltonian population, and charge density difference) in Fe-Cr alloys based on the first-principles calculations. First, studies on the thermostability of Cr and IAs show that Cr tends to occupy a central substitution site, IAs prefer to occupy octahedral interstitial sites (O-sites), whereas Fe₅₃Cr₁N/O is the most stable structure in the Cr-rich region. Therefore, the Fe₅₃Cr₁X (X=C/N/O) is selected to investigate the interaction between Cr and IAs. Moreover, the formation energy of IAs in the Fe-Cr system is significantly lower than the solid solution of IAs in the pure Fe system, indicating that there is a cooperative effect between Cr and IAs. Then, the four-level electronic structure analyses of Fe₅₃Cr₁X reveal the strong bonding between IAs and Cr, implying that the system has high stability. Furthermore, compared to the pure iron system, the increase in the dissociation temperature of IAs in the Fe-Cr system again verifies the enhancement of stability by cooperative strengthening. The results provide a theoretical basis for understanding the solid solution strengthening of IAs and Cr in martensitic steels.

Laser cladding rapid solidification technique is an effective strategy for manufacturing ultra-high-strength martensitic stainless steels (UHS-MSS). Due to super-saturation solution strengthening of interstitial atoms (IAs), martensitic stainless steels containing IAs exhibit excellent ultra-high strength and toughness and have high tolerance for oxygen impurities. Hence, studying the specific speciation and structural characteristics of IAs is of great significance for guiding laser cladding of ultra-high-strength steels. Herein, we use density functional theory (DFT) computations to analyze the stable occupancies of IAs and their interactions in body-centered cubic iron (BCC Fe). The findings show that single IAs prefer to occupy octahedral sites over tetrahedral sites. Therefore, octahedral sites are selected as the optimal sites for the following double IAs study. For homo IAs, C-C and N-N configurations exhibit greater stability at long-range distances, whereas O-O demonstrate optimal stability at intermediate distances. Crucially, hetero IAs configurations are more stable compared to single IAs and homo IAs, exhibiting a synergistic effect. Especially, the C-O combination shows the highest stability and strongest bonding character. Meanwhile, the dissociation behavior of O indicates that C-O and N-O have higher dissociation temperatures than single O, further verifying the synergistic effect of hetero IAs. This provides a theoretical basis for understanding the interstitial solution strengthening of laser cladding UHS-MSS.