Research Progress on Microalloying of Refractory Metals Molybdenum and Tungsten
Refractory metals molybdenum and tungsten have the advantages of excellent electrical and thermal conductivity,high temperature strength and creep resistance,and are indispensable strategic metals in the field of military equipment and national eco-nomic development.However,as transition metal elements with body-centered cubic structure,pure molybdenum and pure tungsten have some problems such as brittleness at room temperature,easy oxidation at high temperature and high deformation resistance,which limit their application in more extreme environments.As an advanced material design and control concept,microalloying tech-nology has been widely studied and applied in the field of steel and non-ferrous light metals,and has broad development prospects.Mi-croalloying is one method to improve the brittleness,recrystallization temperature and mechanical properties of pure molybdenum and tungsten substrates by doping trace amounts of solute elements on the basis of maintaining their original characteristics.There are many kinds of microalloying elements and their action mechanisms are different.According to the classification of microalloying ele-ments into non-metallic elements and metallic elements,the research status of the microalloying of refractory metals molybdenum and tungsten was summarized.The results showed that trace amounts of Si,C and B were beneficial to enhance interfacial bonding.The in-fluence of different Si content on molybdenum was discussed:adding 0.1％Si to molybdenum could reduce the yield strength and im-prove its ductility at room temperature.Molybdenum could be strengthened by solid solution with the increase of Si content.The addi-tion of trace C could improve the low-temperature toughness of molybdenum,inhibit the intergranular brittleness caused by the segre-gation of O at the grain boundary,and directly strengthen the grain boundary,which significantly improved the fracture toughness of pure molybdenum.B had excellent strengthening effect on both molybdenum and tungsten,and it could strengthen grain boundaries and improve ductility by increasing grain boundary cohesion and replacing O at grain boundaries.The effects of microdoping of metal-lic elements Ti,Zr,Re,Hf,K and Group Ⅷ elements on the macroscopic properties and microstructure of molybdenum and tungsten materials were studied.The addition of trace elements of Ti and Zr could improve the high temperature strength and creep resistance of the alloy,which existed in the form of solid solution and the second phase such as TiC,(Ti,Zr)C.It was enumerated that the effect of adding trace Re and Hf to molybdenum was different from that of adding high content:the strength of molybdenum alloy with 0.2％～0.5％Re was not lower than that of alloy with 4％～36％Re at 1800 ℃,and excellent high temperature resistance was also obtained.When the doping amount of Hf was 0.15％～0.28％(atom fraction),the hardness of molybdenum decreased at room temperature and low temperature(77 and 183 K),and had significant solution softening,which was different from the solution strengthening and dis-persion strengthening molybdenum with high Hf content.The effect of the addition of trace K element in molybdenum and tungsten was summarized.K bubbles were formed to hinder the movement of grain boundaries and dislocations,improve the strength of matrix,and inhibit recrystallization.Due to the difference in the number of bonding(s+d)electrons with molybdenum,Group Ⅷ elements have the effect of solid solution softening,which can reduce the ductile-brittle transition temperature and improve the toughness of molybde-num.In particular,a small amount of Ni can improve the sintering property of tungsten and improve the compression toughness and room temperature fracture toughness.The influence of Mg,Sn,Y and W elements on improving the toughness and plasticity of molyb-denum wire and foil was briefly reviewed.The strengthening and toughening mechanisms of microalloyed molybdenum and tungsten were summarized,including solid solution softening,solid solution strengthening and second phase strengthening.The hardness of the alloy was related to the electron concentration of solute elements.Elements with more s+d electrons would soften molybdenum and tung-sten.The reason was that solute atoms with more s+d electrons increase the nucleation of double kink and provide a more favorable nu-cleation position,which improved the mobility of screw dislocation and made the plastic deformation of metal easier.Solute atoms changed the core structure of the dislocation,and the screw dislocation structure changes from symmetric core to asymmetric core,re-ducing Peierls stress and dislocation barrier,reducing dislocation movement resistance,and making sliding easier.The solid solution strengthening of Zr,Hf,Ti was that the solute atoms would be located in the maximum dislocation accumulation area and cause lattice distortion after they were incorporated into the solid solution,which increased the Peierls stress of screw dislocation,inhibited the twisting nucleation and migration of screw dislocation at high temperature,making it difficult to slip.The second phase strengthening was to improve the strength of the matrix metal through the pinning effect of the second relative dislocation of fine particles or even nanoparticles dispersed in the matrix,which hindered the dislocation movement.In the refractory metal of molybdenum and tungsten,there were mainly three ways:precipitation strengthening,dispersion strengthening and K bubble strengthening.Finally,the prospect of the research and development of the microalloying of molybdenum and tungsten was highlighted.
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