Microstructure And Mechanical Properties Of W/Ta Multilayer Composites

Tungsten materials are considered to be potential plasma-facing materials in the future nuclear fusion devices.However,its application is limited by the intrinsic brittleness,low recrystallization temperature and high ductile-brittle transition temperature of tungsten.Based on the composite toughening technology,the annealed tantalum(Ta)foils was used as the toughening layers,and W/Ta multilayer composites at1000~1400 ℃ were prepared by spark plasma diffusion bonding device in this paper.The microstructure and element interdiffusion behavior of W/Ta multilayer composites were studied.The interfacial formation process of composites was investigated.The toughening mechanism,ductile-brittle transition phenomenon and mechanism were analyzed,and the main conclusions obtained are as follows:(1)The W/Ta multilayer composites can effectively improve the toughness of tungsten materials,and show obvious "pseudo-plasticity" characteristics during the bending process.The W/Ta multilayer composites bonded at 1000 ℃ have the best comprehensive mechanical properties: flexural strength(Mode Ⅰ: 981 MPa,Mode Ⅱ: 715MPa)and fracture toughness(Mode III: 16.4 MPa?m0.5,Mode IV: 12.3 MPa?m0.5).Besides,its DBTT is the lowest,which is about 200 ℃.(2)The microstructures of W/Ta multilayer composites are different with different bonding temperatures.When the bonding temperature is 1000 ℃,the W layer is mainly slender fibrous grains.With the increase of the bonding temperature,the grains of the W layer begin to recrystallize.When the bonding temperature is raised to 1400 ℃,the grains of the W layer all grow into coarse equiaxed crystals.The grain size of the Ta layer increases with the increase of the bonding temperature.When the bonding temperature is1400 ℃,the grains of the Ta layer grow to a maximum size of 100 μm in the thickness direction.(3)During the bonding process,the interdiffusion of elements between the W layer and the Ta layer,the gradual decomposition of the oxide film on the surface of the W layer and the Ta layer,and the formation of fine recrystallized grains in the Ta layer lead to differences in the interface structure of the composites bonded at different temperatures.When the bonding temperature is 1000 ℃,there are a few micropores and oxide films on the interface.When the bonding temperature reaches 1200 ℃,a layer of fine recrystallized grains which is not complete is formed in the Ta layer near the interface.When the bonding temperature reaches 1400 °C,a layer of complete recrystallized grains is formed on the interface.Therefore,as the bonding temperature increases,the interfacial bond strength of the composites increases.(4)Due to the different interfacial bonding strength of W/Ta multilayer composites bonded at different temperatures,their fracture propagation behaviors are different.When the bonding temperature is lower than 1200 °C,the interface bonding strength of the composite material is weak,the interface debonding phenomenon occurs,and the crack expands in a single crack mode.The main way of energy dissipation is the plastic deformation of the Ta layer and the W/Ta interface debonding.When the bonding temperature is higher than 1200 ℃,the interfacial bonding of the composite material is enhanced,the interfacial debonding is limited,and the crack propagates in a multi-crack mode.The main way of energy dissipation is the propagation of cracks and the generation of tunnel cracks in the W layer.(5)The ductile-brittle transition temperatures of W/Ta multilayer composites prepared at different bonding temperatures are analyzed by uniaxial tensile and threepoint bending mechanical properties testing methods.The DBTT of the samples bonded at 1000 °C is the lowest,which is about 200 °C.While the DBTT of the samples bonded at 1200 °C and 1400 °C is about 200 °C to 300 °C.With the increase of the tested temperature,the strength and plasticity of the composite show a phenomenon of increase first and then decrease.Compared with the uniaxial tensile test,the stress state softness coefficient of the three-point bending test is higher.Therefore,the DBTT of the W/Ta multilayer composites is lower in the three-point bending test than in the tensile test.