Microstructure And Oxidation Protection Of Spark Plasma Sintered MoNbTaW Refractory High-Entropy Alloys

Refractory high-entropy alloys（RHEAs）are considered as one of the potential candidate materials for the next generation of advanced high-temperature structural materials,due to their higher strength,higher plastic toughness and good structural stability.They have potential applications in fields such as aerospace and thermomechanical engineerings.However,the high brittleness,difficult processability,and poor oxidation resistance of RHEAs at high temperatures are the main technical obstacles to their engineering applications.In this thesis,high-entropy phase formation mechanism,preparation method,complex shape forming technology,and anti-oxidation coating of MoNbTaTiW RHEAs are investigated.The solid-phase sintering behavior of MoNbTaW RHEAs and the formation mechanism of high-entropy phases are explained.The relationship between the preparation process of spark plasma sintering（SPS）and its microstructure and properties are studied.The effects of alloying element Ti on the properties and structure of MoNbTaTiW RHEA are analyzed.The hot extrusion molding method of complex-shaped MoNbTaTiWAl_0.2RHEA parts is explored.Silicide anti-oxidation coating is prepared on the surface of MoNbTaW RHEA and the antioxidant mechanism is discussed.The main research contents and results are as follows:（1）Since porous materials prepared by pressureless sintering can retain the microstructure and morphological characteristics of particle surfaces and sintered necks during the sintering process,the porous MoNbTaW RHEA is prepared by pressureless spark plasma sintering（PSPS）.The sintering behavior of MoNbTaW RHEA during solid-phase sintering is studied,and the formation mechanism of MoNbTaW high-entropy phase and the evolution of chemical compositions and microstructure are revealed.The formation of MoNbTaW RHEA is a dynamic process through which a continuous solid solution forms as result of atomic diffusion.The elemental atoms migrate and dissolve in a sequence of Mo→Nb→Ta→W.The order of migration and solid solution is related to the binding energies and evaporation rates of the elements.The lower the binding energy,the easier the elements diffuse and form solid solutions.From 1300℃ to 1700℃,evaporation-agglomeration is the main sintering mechanism controlling atomic migration,while at 1900℃,diffusion is the main sintering mechanism.A TFDC model is proposed to calculate the lattice constants of MoNbTaW RHEA,which are more accurate than those obtained by Vegard’s law.（2）MoNbTaW RHEAs with a single BCC phase are prepared by SPS using mixed powders of Mo,Nb,Ta,and W.It is found that the sintering temperature has a significant effect on the properties of MoNbTaW RHEA.With the increase of the sintering temperature,the grain size,densification,hardness,and yield strength of the RHEA increase.The densifictation of 99.8%,grain size of 32.4μm,Vickers hardness of 4.76 GPa,and yield strength of 1314 MPa are achieved at the sintering temperature of 2000 ℃.（3）Since Ti can improve the room-temperature yield strength and sintering behavior of RHEAs,it is added to MoNbTaW RHEAs.The results show that the addition of Ti can reduce the sintering temperature of MoNbTaW RHEA.Fully dense MoNbTaTiW RHEAs are obtained after sintering at 1580 ℃.The compressive yield strength of MoNbTaTiW RHEA at room temperature and 1200℃ are 1547 MPa and 700 MPa,respectively.（4）For the problem that RHEAs are difficult to process and form,the sintered MoNbTaTiWAl_0.2RHEA is prepared by SPS and then hot-extruded with a graphite die,finally a complex shape is obtained through plastic deformation.The hot-extruded MoNbTaTiWAl_0.2RHEA has a dual phase structure:a RHEA matrix（BCC）and a complex of carbide（FCC）.The crystalline orientation relationship between the matrix with the<110>fibre texture and the carbide with the<001>fibre texture is{001}_BCC//{110}_FCC.Interlocking grain boundaries are formed between the matrix and the carbides.（5）To improve the anti-oxidation of MoNbTaTiW RHEAs,Si-20Cr-20Fe protective coating is prepared on the surface of MoNbTaTiW RHEA by a fused slurry method.The silicide coating could effectively prevent the oxidation of the MoNbTaTiW RHEA.Under high-temperature oxidation conditions,the silicon element diffuses from the silicide coating to the RHEA substrate.During the oxidation process,Si,Fe and Cr continually diffuse into the MoNbTaTiW RHEA substrate at different diffusion rates.The structure and compositions of the outer surface of the oxide layer strongly depend on the oxidation temperature.