Research On The Manufacturing Technique And Alloying Of Mo 2 FeB 2 -based Cermets With High Mechanical Properities

In this thesis, the relationship among composition, manufacturing process, microstructure and mechanical properties of Mo₂FeB₂-based cermets had been systematically investigated by different scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrodmeter (EDS) and transmission electron microscope (TEM).In the first part of the thesis, the development and present research situation of Mo₂FeB₂-based cermets have been overviewed, which includes the mechanisms of liquid phase sintering of Mo₂FeB₂-based cermets, the influence of manufacturing process, and composition on the microstructures and properties of Mo₂FeB₂-based cermets, The progress on the behaviours of grain growth inhibitors in fine-grain cermets and the effect of SiC wisker on the properties of composite material was also summarized. Based on the above work, the purpose and significance of the thesis have been pointed out.The influence of the sintering temperture, soaking time, heating rate of liquid phase sintering on the microstructure and properties of the Mo₂FeB₂-based cermets has been studied. The conclusions were obtained as follows: the cermets sintered at 1250℃for 40 minutes with a heating rate of 1℃/min at L1 stage and 5℃/min at L2 stage during liquid phase sintering had fine grains and homogeneous microstructure, which exhibited the highest mechanical properties in the present study.The influence of cold isostatic press (CIP) process on the microstructure and properties of the Mo₂FeB₂-based cermets has been studied. It was found that the cermets pressed by CIP were characterized with more homogeneous microstructure, smaller average grain size and narrower distribution of grain size compared with that by ordinary die compaction.The influence of the chemical composition on the microstructure and properties of the Mo₂FeB₂-based cermets had been studied. The conclusions were obtained as follows: 0.5 wt.% C addition could eliminate the adsorbed oxygen completely, and no free carbon and brittle phase occurred, which resulted in the best mechanical properties. The wettability of hard phase by binder phase was improved by Mn addition, which decreased the grain size of Mo₂FeB₂ phase. The highest TRS of about 2250 MPa was obtained at a 10 wt.% Mn addition, whereas 5 wt.% Mn addition resulted in the maximum hardness. VFe addition decreased the wettability of hard phase by binder phase, and accordingly increased the porosity of the cermets. However, the solubility of ceramic phase in the binder decreased with increasing VFe addition, which led to a decrease of Mo₂FeB₂ grain size. The cermets with 4.86 wt.% VFe addition showed the highest TRS of 2350 MPa and hardness of 90.6HRA.The influence of three kinds of grain growth inhibitors and SiC wisker on the microstructure and properties of the Mo₂FeB₂-based cermets has been studied. The conclusions were obtained as follows: No remarkable decrease of grains size occurred with Cr3C2 or VC addition. TiC addition resulted in the aggregation of ceramic grains and an increase of porosity, so the mechanical properties decreased greatly. The cermets with 0.5 wt.% SiC whisker addition were characterized with homogeneous microstructure and fine grains, which resulted in higher mechanical properties. At higher SiC whisker additions, the wettability of hard phase by binder phase decreased and the porosity of the cermets increased, which resulted in the decrease of mechanical properties.FeMo alloy was used to replace some Mo and Fe in the preparation of Mo₂FeB₂-based cermets. The microstructure and properties of the cermets have been studied. The conclusions were obtained as follows: the cermets fabricated by FeMo alloy exhibited a homogeneous microstructure and the average grain size was also small. As a result, the mechanical properties of the cermets were comparable to that without FeMo alloy. Thereby, it was feasible that cermets were fabricated by FeMo alloy.