Strengthening mechanism of Cr doping in Mo2FeB2-based cermets and effects on biphase interface

Abstract

EDS, SEM, and TEM were utilized to investigate the effects of various Cr doping amounts on the microstructure and properties of Mo2FeB2-based cermets. The mechanism behind Cr doping and its influence on the biphase interfaces were elucidated by first-principle and EET theories, respectively. The results show that a moderate amount of Cr doped into Mo2FeB2-based cermets is preferentially solid-solved in the Mo2FeB2 hard phase, which leads to an enhancement in the wettability of the system, as well as an increase in the fracture toughness and transverse rupture strength (TRS) of the cermets. Excessive Cr addition leads to a decrease in mechanical properties due to coarser aggregation of hard phase particles and a decrease in the volume fraction of the binder phase. With increased Cr addition, a significant amount of Cr starts to solid-solve in the Fe-based binder phase when the saturation point for solid-solved Cr in the hard phase is approached. However, Cr solid-solved in the hard phase has the most pronounced effect on the mechanical properties of cermets. Combined with the first principle calculation, it can be seen that Cr tends to occupy the position of iron atoms when solid-solved in the hard phase, thus enhancing the covalent bonding in the system. Analysis of the valence electron structure at the interface reveals that adding a moderate amount of Cr increases the electron density and the number of atomic state groups at the interface. At this point, the interfacial stability and bond strength between the two phases are increased, which positively affects the mechanical properties of the cermets. Nevertheless, an excess of Cr is detrimental as it leads to a decrease in the ρ' and σN, thus affecting the bond strength at the interface.