Research On Microstructure Formation And Ceramic Growth Kinetics Of Mo₂FeB₂-based Cermets

In this dissertation,the microstructure formation processes and mechanism,the influence of main metallurgical factors on the growth kinetics of ceramic grains,the formation mechanism of ceramic grains and its influence on the mechanical properties of Mo₂FeB₂-based cermets were studied by Monte Carlo（MC）simulation,valence electron theory,thermal analysis（DSC）,scanning electron microscopy（SEM）,and transmission electron microscopy（TEM）.In addition,the results of simulation and calculation were verified by relevant experiments.Firstly,the microstructure formation processes of Mo₂FeB₂-based cermets with different Mo/B atomic ratios were studied by Monte Carlo（MC）simulation and experiments.The conclusions were obtained as follows:the modified simulation results were basically consistent with the experimental results.The ceramic grains grew by dissolution and precipitation process.The morphologies of the ceramic grains were relied on the solid-liquid interface energies between different crystal faces of ceramic grains and liquid phase.The growth kinetics of the ceramic grains of Mo₂FeB₂-based cermets with different Mo/B atomic ratios during liquid phase sintering were studied.When the Mo/B atomic ratio was 0.9,the grain growth of the ceramic phase along the direction of the long-axis direction satisfied the equation（r³-r₀³=kt）,and along the direction of the short-axis direction satisfied the equation（r⁵-r₀⁵=kt）during the L1 and L2 liquid phase sintering stages.The growth activation energy along the long axis direction were 242.5±9.5 kJ/mol and 54±5 kJ/mol during the L1 and L2 liquid phase sintering stages,respectively.The growth activation energy along the short axis direction were 321.5±43.5kJ/mol and 227±39 kJ/mol during the L1 and L2 liquid phase sintering stages,respectively.The grain growth of the ceramic phase along the long axis direction was controlled by diffusion,and along the short axis direction was controlled by the interface reaction.The grain grew preferentially along the[001]crystal orientation.When the Mo/B atomic ratio was 1.1,the grain growth of the ceramic phase satisfied the equation（r³-r₀³=kt）during the L1 and L2 liquid phase sintering stages.The growth activation energy of the ceramic grains were 103.5±6.5 kJ/mol and 153.5±30.5 kJ/mol during the L1and L2 liquid phase sintering stages,respectively.The ceramic grain growth was controlled by diffusion.The"phase structure formation factor"of（001）crystal plane and（210）crystal plane of the ceramic phase in Mo₂FeB₂-based cermets were calculated.The calculation results were basically consistent with the experimental results.The statistical average S_F’of the"phase structure formation factors"could be used to investigate the relative rates of growth in different crystal faces of ceramic grain.The rising absences of Mo increased the difference between the S’_F（001）of（001）crystal plane and the S’_F（210）of（210）crystal plane,resulting in the grain preferential growth along the[001]crystal orientation.Hence,the elongated ceramic grain developed.The rising absences of B made the difference between the S’_F（001）of（001）crystal plane and the S’_F（210）of（210）crystal plane tend to zero gradually,resulting in the substantially same growth rate of the（001）crystal plane and the（210）crystal plane.So the nearly equiaxed ceramic grain developed.Finally,the effects of ceramic grains with different morphologies on the mechanical properties of Mo₂FeB₂-based cermets were investigated by valence electron theory.Some conclusions were drawn from the calculated results:compared with the Mo₂FeB₂-based cermets with nearly equiaxed ceramic grains,the Mo₂FeB₂-based cermets with elongated ceramic grains had higher interfacial bonding strength and larger total contact area of the（210）_Mo2FeB2/（100）_Fe interface,which enhanced the ability of the materials to resist crack propagation.Therefore,the Mo₂FeB₂-based cermets with elongated ceramic grains showed a higher fracture toughness.