Research On The Microstructure Control And Oxidation Resistance Of Mo₂FeB₂-based Cermets At High Temperature

Mo₂FeB₂-based cermets by reaction boronizing sintering has received extensive attention for its excellent comprehensive mechanical properties and a lower cost.In this dissertation,effects of main sintering parameters and chemical composition on microstructure and mechanical properties of Mo₂FeB₂-based cermets were studied by using X-ray diffraction?XRD?,scanning electron microscopy?SEM?,energy dispersive X-ray analysis?EDS?and thermal analysis?DSC?.Based on these studies,the microstructure control of Mo₂FeB₂-based cermets would be realized.Besides,the influences of Cr addition on high temperature oxidation resistance of cermets were studied.Firstly,the phase transformation of Mo₂FeB₂-based cermets during sintering and the influnces of the different heating parameters on the microstructure of cermets were studied.Based on the above results,the effects of heating rate during solid reaction boronizing state sintering on the microstructure and mechanical properties of Mo₂FeB₂-based cermets were studied.Some conclusions were obtained as follows: Fe2B?Mo₂FeB₂?liquid phase L1 and liquid phase L2 appeared in order with the temperature rising.The morphology of the cermet hard phase did not change significally,when the holding time of 800??1010? and 1100? extended to 60 min,respectively.When the heating rate increased during solid reaction boronizing state,the hard phase with lattice parameter of a decreased gradually and lattice parameter of c kept almost invariant,the morphology of the cermet hard phase gradually changed from elongated morphology to quasi-equiaxed shape.When the heating rate was 5?/min during solid reaction boronizing state,the cermet had the better mechanical properties with TRS of 2367.5MPa,hardness of 88.2HRA and fracture toughness of 27.6MPa·m1/2.Then,the effects of Mo/B ratio and the addition of Cr on the microstructure and mechanical properties of cermets were studied.The results were drawn as follows: With the Mo/B ratio rising,the hard phase with lattice parameter of a firstly increased and then decreased gradually,and lattice parameter of c keeps almost unchanged,the morphology of the cermet hard phase changed from corners polyhedron morphology to elongated morphology and then turned to quasi-equiaxed shape gradually.The cermet had the better mechanical properties when the Mo/B ratio was 1.02.With the Cr content rising,the hard phase with lattice parameter of c increased gradually and lattice parameter of a remained almost constant,the morphology of the cermet hard phase gradually changed from elongated morphology to quasi-equiaxed shape.When the Cr content was 10 wt.%,the cermet had the better mechanical properties with TRS of 2124.1MPa,hardness of 89.0HRA and fracture toughness of 23.8MPa·m1/2.Finally,the influences of Cr content on the high temperature oxidation resistance of cermets were studied.The study obtained conclusions as follows: the oxidation kinetics of Mo₂FeB₂-based cermets performed at 750? for 100 h follows the parabolic law.With the Cr content increasing,the oxidation weight gain per unit area of Mo₂FeB₂-based cermets firstly increased and then decreased.When the Cr content is 10 wt.%,some oxides such as Fe2O3?Cr2O3?NiO and NiCr₂O₄?FeCr₂O₄ were produced in the cermet's surface and showed the lowest oxidation weight gain per unit area with the value of 19.24×10-4g·cm-2.