Study On Microstructure Of Tungsten-based Ultra-high Temperature Refractory Metal Composites

Refractory metals generally refer to five transition metals:tungsten(W),molybdenum(Mo),tantalum(Ta),niobium(Nb),and rhenium(Re).With many advantages including high melting point,high-temperature strength,strong resistance to liquid metal corrosion,good processing plasticity and low cost,W and its alloys have better application in high-temperature service environments.However,in recent years,with the development of advanced scientific and technological fields,such as defense military industry,aerospace and nuclear industry,the strength and corrosion resistance of relevant mechanical components under ultra-high temperature conditions are unable to meet requirements.Therefore,it is necessary to improve the overall performance of W and its alloys through solid solution,dispersion and deformation strengthening mechanisms,as well as multiple composite strengthening mechanism.Since hafnium carbide(HfC)is an ultra-high temperature and oxidation resistant ceramic material and has the advantages of good electrical and thermal conductivity and small thermal expansion,it has been an efficient second-phase strengthening particle in W and its alloys.Nevertheless,the strengthening effect of HfC is related closely to its thermal stability,size and distribution in W matrix.The W-HfC binary system and W-Re-HfC ternary system are considered as the research objects in this thesis.The solid solution strengthening of Re and the dispersion strengthening of HfC are used to meet the requirements of performance under ultra-high temperature conditions.The phase composition and microstructure of the two systems were characterized and investigated in detail.The morphology and distribution of various second-phase particles,as well as the effects of the original composition and preparation process on the microstructure of the alloy,were discussed and analyzed particularly in this thesis.It is found that that the W-HfC binary system is composed of W matrix and a small number of second-phase particles involving mainly HfO2,W6C2.54,and W2C-like particles.The initial carbon source reacts with W,leading to the formations of W6C2.54 and W2C-like particles.Meanwhile,the Hf atoms in HfH2 preferentially react with 0 to form HfO2,and only a small amount of Hf atoms is incorporated into the W2C lattice to form a secondary solid solution W2-xHfxC.In the two types of W-ReHfC systems containing 0.3 wt.%Hf and 3.0 wt.%Hf respectively,the HfC-like second-phase particle with a face-centered cubic(FCC)structure can be captured frequently.Unfortunately,the sizes of particle were large.Here,Re solid solution into W matrix plays a role in solid solution strengthening.The addition of Re into W matrix causes a decrease in solid solubility of Hf in W matrix,resulting in the formation of some HfC particles on the grain boundaries of W matrix.When HfH2 and C react to form HfC in the ternary system containing 3.0 wt.%Hf,some W atoms are also merged into HfC-like FCC structure and occupy the Hf-sites,bringing about an appearance of the secondary solid solution Hf1-xWxC.Additionally,the excess Hf atoms and insufficient O atoms form the HfO2-x phase.The research in this thesis provides a certain theoretical basis for obtaining fine HfC second-phase particles to meet the requirements of ultra-high temperature mechanical properties.