Effect of Mo/B atomic ratio on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo2FeB2 based cermets with the Mo/B atomic ratio in the range from 0.8 to 1.1 were prepared by reaction sintering process. The microstructure and crystalline phases were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The results indicate that the transverse rupture strength (TRS) of the cermets increases with an increase of Mo/B atomic ratio and shows a maximum value of 1.9 GPa at a Mo/B atomic ratio of 0.9. At a higher atomic ratio the TRS decreases. The hardness of the cermets decreases monotonically from 90.8 HRA to 88.8 HRA with an increase of Mo/B atomic ratio. In Mo-rich cermets with an atomic ratio of Mo/B above 1.0, a new M23B6 type phase (M23B6, where M represents a metal) is found. This phase has a lattice parameter a = 1.09 nm containing Mo, Fe, Cr and B with an atomic ratio close to 16:6:1:6 and is precipitated at the interface of Mo2FeB2 grains or at the Mo2FeB2 grain boundaries.