Principle And Method For Grain Refinement And Microstructural Uniformity Of Tungsten

Tungsten(W)has high melting point,excellent high-temperature mechanical properties and thermophysical properties.W plate,crucible and target are key materials or components used in microelectronic,optoelectronic technology and high-temperature application.Due to its high melting point and brittleness,the processing of W involves complex thermo-mechanical environment,which causes microstructure difficult to be controlled.The high densification,grain size refinement and microstructural homogenization are critical challenges in the industry of refractory metal W.This thesis focuses on the microstructure and properties of W during sintering,plastic deformation and recrystallization,as well as the effects of impurity elements on them,aiming to explore the principle and method for grain refinement and microstructural uniformity of W.The main conclusions are as follows:The challenge of sintering ultrafine-grained W is hereby addressed by pressureless "two-step sintering",based on a new idea that the kinetics of densification and grain growth can be decoupled due to the variable activation energy for grain boundary(GB)migration.It successfully produced W samples with 99%theoretical density and 290 nm average grain size.The investigation of grain growth kinetics indicates a transition in the nominal GB mobility at 1400℃,above which the effective activation energy is～6.1 eV,below which GB motion is rapidly frozen with large activation energy of～12.9 eV.Such a transition in the GB mobility provides a theoretical basis for the successful practice of "two-step sintering".The activation energy for GB diffusion is calculated by using the Herring’s model,of which for uniform powders is about 10%lower than that for non-uniform powders.Optimized uniform nanopowders promote the densification and further refine grain size in the later stage of sintering.The transition in the GB mobility and uniformity of nanopowders are crucial for the grain refinement of the sintered W.The evolution of microstructure and properties of pure W during the whole rolling process(hot rolling and cold rolling)are studied.At the hot-rolling stage,the variation of microstructural parameters with the deformation strain is nonmonotonic.It undergoes three-stages evolution:original grain smashing,dynamic recrystallization and grain fibering.The intermediate recrystallization leads to a heterogeneous microstructure,which promotes the formation of fiber structure at a larger strain.At the cold-rolling stage,the variation of microstructural parameters with the deformation strain becomes monotonic.Compared to hotrolling,cold-rolling can refine the grain size down to sub-micron level and significantly change the grain structure.The recrystallization behavior and microstructure evolution of as-annealed W plate are studied.For W plate with low deformation strain,the softening behavior is dominantly governed by recovery process,while the softening behavior is dominantly governed by recrystallization process in W plate with high deformation strain.The evolution mechanism of recrystallized texture of rolled W plate is revealed.At low temperature,the nucleation of recrystallized grain is related to the formation of coincidence site lattice(CSL)boundary,and the formation of Goss texture at the plate surface is controlled by low angle grain boundaries(LAGBs).The microstructural inhomogeneity occurred in as-annealed W plate is contributed to two main effect:(i)"Through-thickness effect",namely,the recrystallized texture basically maintains the starting deformed texture in the bulk while the coarse Goss grains form at the surface,resulting in the through-thickness differentiation of microstructure;(ii)"Orientation-dependent effect",namely,{100}-oriented grains tend towards recovery and maintain the fiber shape while {111}-oriented grains tend towards nucleation/recrystallization and preferentially evolve into equiaxed grains,resulting in abnormal grain growth.The effects of impurity elements on the microstructure and properties of rolled and annealed W are studied.At the hot-rolling stage,impurities induce a milder dynamic recrystallization behavior,reduce the aspect ratio of fiber grains and weaken the texture.But impurities have relatively minor impact on the microstructure at the cold-rolling stage where impurities change the grain shape and reduce the grain aspect ratio.During annealing,impurities not only hinder the recrystallization,but also unexpectedly promote the microstructural uniformity by changing the dislocation structure and reducing the difference of stored energy between grains with different orientations.Furthermore,impurities significantly aggravate the embrittlement of W.The change of grain morphology induced by impurities is emphasized to be a critical factor affecting the mechanical properties of rolled W.