Preparation,Thermal Stability And Property Analysis Of Nanostructured W-Cu Based Composites

W-Cu based composites are widely used in national defense,aerospace,microelectronics and other high-tech fields due to the combined advantages of both W and Cu.With the rapid development of modern science and technology,traditional WCu based composites can hardly meet the high demand on excellent performance in extreme conditions.For the typical immiscible W-Cu based composites,researchers have been pursuing refining their microstructure by adding dispersive refractory metal carbides and metal oxides nanoparticles.However,it is difficult to achieve high density and nanostructure concurrently,so there lacks systematic research on mechanical and physical properties of W-Cu based composites.Based on this background,the main target of this research is to prepare nanostructured W-Cu based bulk composites with high relative density,high thermal stability and high-performance.For this end,a series of experiments were carried out on the preparation of nanocomposite powders,densification process,regulation strategies of nanostructure,and characterization of room-temperature and hightemperature performance of W-Cu based composites.The microstructure and preparation process were optimized to obtain nanostructured composites with high thermal stability and high performance.The design and preparation method for the nanostructured W-Cu based composite with high thermal stability was established.Furthermore,the influence of composites’ composition and structural stability on hightemperature mechanical properties and wear resistance was disclosed.Eventually,the regulation strategies and mechanism to obtain the nanostructure with high thermal stability,superior room-temperature and high-temperature comprehensive properties for W-Cu based composites were proposed.The main contents and results of this dissertation are as follows:The initial powder was nanocrystallized through two different design strategies.In one strategy,W and Cu particles were simultaneously nanocrystallized by cationic surfactant,and the W-Cu nanocomposite powder with a mean particle size of 30 nm was prepared.The composite powder has advantages of pure phase,uniform distribution and good dispersion.The other strategy is to realize nanostructure inside the W phase using two-step ball milling.W-Cu based composite powders with phase separation element and carbides dispersed in the W phase were prepared,and the mean grain size was reduced to 10 nm.The sintering process of nanostructured W-Cu based bulk composites with high relative density was established.The stabilization mechanism of nanostructure was revealed through characterization and analysis of the microstructure of sintered bulk.A W-Cu-Cr composite with a mean W grain size of 46 nm was prepared using the segregation effect of the phase separation element Cr during the sintering process.Furthermore,taking advantage of providing more nucleation sites for the phase separation element by dispersed nanoparticles introduced at the W grain boundary,the distribution uniformity of the segregated element was improved.Under the synergic effect of the segregated element at W grain boundary and in-situ generated stable ternary carbides,the mean W grain size is further reduced to 39 nm.Based on the above research,the hierarchically structured W-Cu based composites with nanograins in the micron-scale W phase were prepared.The hardness,wear resistance and arc ablation resistance of the nanostructured WCu based composites were tested to reveal the effects and working mechanism of the nanostructure and component on the mechanical properties,room-temperature wear resistance and arc ablation resistance.The results show that the hardness of the nanostructured W-Cu based composites can reach 2 to 3 times of that of the coarsegrained W-Cu composite due to the combined effects of fine grain size,solid solution and highly dispersive hard nanoparticles.The high hardness endows the nanostructured W-Cu based composites with good resistance to plastic deformation.During abrasion process,a mechanically mixed layer with higher hardness is formed on the surface of wear scar,which has a good protective effect on the matrix.The main wear mechanism of nanostructured W-Cu based composite is delamination and detachment.In addition,the highly dispersed refractory Zr C and Cr components with relatively low work function in the composite play an effective role in dispersing the arc.Therefore,the nanostructured W-Cu based composite exhibits excellent arc ablation resistance.The evolution of nanostructure in W phase after heat-treatment at different temperatures was investigated.The thermal stabilization mechanism of W nanostructure and its influence mechanism on high-temperature mechanical properties and wear resistance of the W-Cu-Cr-Zr C composite were revealed.The W phase in the W-Cu-Cr-Zr C composite still maintains a nanostructure after heat-treatment at 1000℃for 2 h.As a result,the compressive strength at 900℃ is 1150 MPa,which is approximately four times as high as that of coarse-grained composite.This indicates the importance of high thermal stability of W nanostructure for obtaining excellent high-temperature strength.The critical conditions for the coarsening of W grain were determined,and the thermal instability mechanism of the W nanostructure caused by element redistribution during phase transformation was revealed.These research results provide theoretical guidance and innovative technology for the development of W-Cu based composites with excellent high-temperature mechanical and physical properties.