| Tungsten(W)materials are considered to be excellent plasma-facing materials in nuclear fusion generating devices.However,due to the brittleness and low toughness,W often fractures and fails quickly during use,which greatly limits its application in practical engineering.In order to solve this problem,titanium(Ti)foils and tantalum(Ta)foils are used as toughening layers,so a series of tungsten-based multilayer composites are designed and prepared by discharge plasma connection technology.By studying the interdiffusion behavior of elements,the characteristics of microstructure and interfacial structure in the composites,the microstructure formation mechanism of the composites and its correspondence with the mechanical properties are analyzed.On this basis,the strengthening and toughening mechanism of the composites are clarified,and the main conclusions obtained are as follows:(1)By comparing and analyzing the microstructure and mechanical properties of W/Ta and W/Ti multilayer composites,W/(Ti/Ta/Ti)multilayer composites are designed.The composite has a multi-layered organization and shows a gradient of strength and ductility change from W layer to Ti layer to Ta layer,thus realizing the simultaneous increase of strength and elongation of the tungsten material.The W/(Ti/Ta/Ti)multilayer composites bonded at 1200 °C have the best comprehensive tensile mechanical properties at room temperature.The tensile strength is 631 MPa and the elongation is 2.9 %.(2)During the preparation of W/(Ti/Ta/Ti)multilayer composites,the isotropic transformation in the Ti layer at high temperature,which results in the formation of a higher vacancy concentration in the Ti layer.Therefore,the diffusion rate of W and Ta into Ti is higher than that of Ti into W and Ta.The calculations show that during the interdiffusion of W and Ti,the diffusion activation energy of W element is 90.9 KJ/mol,while that of Ti element is 161.4 KJ/mol.During the interdiffusion of Ti and Ta,the diffusion activation energy of Ta element is 70.7 KJ/mol,while that of Ti element is 122.6KJ/mol.It can be seen that the diffusion rate of Ta into Ti is the fastest.(3)The microstructures formed in W/(Ti/Ta/Ti)multilayer composites are mainly affected by grain growth and element diffusion.For the W layer,when the bonding temperature is lower than 1200°C,elongated fibrous structures are mainly formed;when the bonding temperature is higher than 1200 °C,the recrystallization of grains occurs.For the Ta layer,the grain size gradually increases with the increase of the bonding temperature.For the Ti layer,since the W and Ta elements diffused into the Ti layer have the effect of stabilizing the ?-Ti phase,when the temperature is lower than 1300°C,the structure in the Ti layer is divided into three layers,?-Ti(W)solid solution region,?/?dual-phase region and ?-Ti(Ta)solid solution region.When the temperature reaches1400 ?,the Ti layer is only composed of pure ?-Ti single-phase region.(4)In W/(Ti/Ta/Ti)multilayer composites,the microhardness of W layer is the highest,that of Ti layer is the second,and that of Ta layer is the lowest.With the increase of bonding temperature,the microhardness of W layer decreases gradually due to recovery and recrystallization,while the microhardness of Ta layer changes little.The microhardness distribution of the Ti layer is closely related to its organization and to the W and Ta contents.The strengthening mechanism of the ?-Ti single-phase region in the Ti layer is mainly solid solution strengthening.Due to the precipitation of nanoscale acicular martensite structure in the ?/? dual-phase region,the strengthening mechanism is solid solution strengthening and second phase strengthening(5)The W/(Ti/Ta/Ti)multilayer composites performs good ductility in tensile and bending tests at room temperature,which is mainly due to the significant plastic deformation of the Ti/Ta/Ti layers.The fracture analysis shows that wavy slip bands appear in the Ta layer,while planar slip bands are presented in the Ti layer,which is closely related to their plastic deformation modes.The formation of planar slip bands in Ti layers is mainly due to the Burgers orientation relationship between ?-Ti and ?-Ti,which makes their glide planes and glide directions parallel,resulting in a consistent slip band in ?-Ti and ?-Ti.(6)The results of uniaxial tensile tests at different temperatures show that the tensile strength of W/(Ti/Ta/Ti)multilayer composites is the highest at 100 °C,and the elongation can reach more than 10 %,which shows that the DBTT of the composites is around 100 °C.Meanwhile,the tensile strength model of W/(Ti/Ta/Ti)multilayer composites is constructed according to the stress concentration coefficient of W/Ti interface after nucleation of microcracks.The calculated results can be in good agreement with the experimental results.(7)With the increase of bonding temperature,the bending strength of the composite increases first and then decreases,reaching the maximum value of 1700 MPa at 1200 ?.The toughening behavior of W/(Ti/Ta/Ti)multilayer composites is closely related to the weak interface between W layer and Ti layer.The W/Ti interfaces are prone to form long interfacial cracks during three-point bending,so the bending stress-strain curves show typical "pseudo-toughness" characteristics.At the same time,the toughening mechanism of the composites includes the formation of tunnel cracks in W layer,the deflection of main cracks and the plastic deformation of Ti/Ta/Ti layer. |
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