Nitriding behavior and mechanical properties of AerMet100 steel and first-principles calculations of phase interfaces

In this paper, quenched AerMet100 steel specimens are plasma nitrided at different temperatures (from 440 °C to 500 °C), and their microstructures and mechanical properties are investigated. Surface layer hardness variation induced by plasma nitriding is also analyzed and correlated with microstructural characteristics and concentration distributions of carbon and nitrogen. Results show that wear resistance of AerMet100 steel after nitriding treatment exhibits various degrees of improvement compared with the un-treated specimen. Especially for the nitrided specimens from 460 °C to 500 °C, volume wear rate is reduced by more than 88%. Such great improvement is mainly attributed to the absence of brittle ε-Fe2-3N phase and the presence of a thick gradient nitrided layer. The interfacial properties of γ′-Fe4N(111)/α′N(011) are investigated by first-principles calculations. Six interface models with different γ′-Fe4N(111) termination and stacking sites are investigated to clarify their effects on the interfacial adhesion. The Fe-terminated Bridge site model exhibits a stronger adhesion tendency after relaxation, which can be regarded as the strongest interface. We found that the alloying additives benefit the γ′-(Fe,M)4N(111)Fe/α′N(011) interfacial adhesion in the order Si < Zr < Cu < Al < Ni < Mn < Co < Nb < Ti < V < Mo < Fe < Cr. Wherein, only Cr can be regarded as enhancers, and doping with Cr and Si are the most prominent representatives of strengthening and weakening effects, respectively. In addition, the bonding characteristics of two representative interface structures doped with Cr and Si are further studied. Density of states (DOS) and crystal orbital Hamilton population (COHP) analysis indicate that the significantly improved interfacial adhesion of Cr-doped is mainly due to the significant improvement of the enhanced Fe-Fe bonding. These results may provide some important inspirations for future experimental work of preparing high-performance nitrided layer.