Microstructure and mechanical properties of Mn containing Mo2NiB2 boride base cermets

Ni-5 mass%B-53.25 mass%Mo-x mass%Mn model cermets with five levels of Mn content from 0 to 10 mass% were prepared to investigate the effects of Mn on the mechanical properties and microstructure of the Mo2NiB2 base cermets. The powder mixtures prepared form pure Mo and Mn, carbonyl Ni and MoB were ball-milled in acetone. After drying, the milled powders were pressed to green compacts and sintered in vacuum for 1.2 ks at temperatures between 1533 and 1593 K. The cermets exhibit a two phase structure with a tetragonal Mo2NiBi (M3B2) complex boride and a Ni base binder. Additions of 5 mass%Mn to the cermets resulted in refinement of the boride particles and remarkable improvement of mechanical proper ties such as transverse rupture strengh (TRS and hardness. The maximum TRS of 2.45 GPa and hardness of 85.6 HRA were obtained at 5 mass%Mn. Fracture toughness slightly increase with increasing Mn content up to 7.5 mass% and exhibited steep in crease between 7.5 and 10 mass%Mn, reaching 31 MPa(.)m(1/2). The high fracture toughness was attributed to enhanced uniformity of the hard particle distribtuion as well as spheroidization and coarsening of the particles.