The Mechanism Of Microstructure Evolution And Performance Improvement Of Pure Tungsten Processed By High Pressure Torsion

Tungsten is widely used in aerospace,military industry and nuclear reactor structural materials due to its excellent mechanical properties and stable chemical properties.However,tungsten that prepared by traditional sintering process has a low plasticity because of the coarse grain and inhomogeneous microstructure,which limits its engineering application.The severe plastic deformation such as high pressure torsion can improve the performance of tungsten by refine the grains.However,the mechanism stays still unclear.This paper adopts the method of high-pressure torsion to prepare ultrafine/nanocrystalline tungsten to improve the plasticity of pure tungsten and the mechanism of microstructure evolution and performance improvement have been deeply discussed.In this paper,high-pressure torsion experiments of pure tungsten were carried out under the condition of 550?while the applied pressure is 1.5GPa,and the turns were 1,2,5,and 10.The microstructures of sintered tungsten and HPT-processed tungsten samples were characterized by X-ray diffraction(XRD),electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM).The results show that after 10 turns of high-pressure torsion deformation,the texture of the pure sintered tungsten sample changed from the initial random orientation to(001)<110>and(110)<1-10>plate textures,and the texture types increased first and then decreased.The average grain size was refined from the initial 65.45?m to 0.69?m;the proportion of the large-angle grain boundary rises from the initial 9.6%to 70.5%.In early stage of deformation,the microscopic deformation of pure tungsten samples is determined by intragranular dislocation slip,and the refinement mechanism during HPT is dislocation cell nucleation and dislocation wall division grain refinement.As the turns of HPT were higher,the grains are refined.When the size of the grains is less than the intragranular mean free path of dislocations,the microscopic deformation mode of the pure tungsten sample was transformed into twining deformation at the grain boundary and the the reaction between grain boundary and dislocations.The refinement mechanism in the pure tungsten sample is twinning refinement at the grain boundary.In the whole process of HPT,the proportion of large-angle grain boundaries in the sample kept increasing,and as the degree of deformation increases,the additional dislocation energy stored at the grain boundaries becomes higher and higher.The same phenomena happened for intrinsic dislocation energy,elastic strain energy and crystal due to the change of deformation mode.The total grain boundary energy of the sample has a trend of increasing.When the size of the grains is smaller than the mean free path of dislocations,no intragranular dislocation slip can occur inside the sample.The local twinning deformation and the dislocation reactions between at the grain boundaries lead to an increment of grain boundary thickness.Also,observable moiréfringes occurred at the grain boundary.Thickened grain boundary and observable moiréfringes can be the criterion to define non-equilibrium grain boundary.At the same time,the deformation-induced amorphization took place during HPT process of tungsten.due to the large accumulation of dislocations at the grain boundaries,when the local dislocation density at the grain boundaries exceeds the tolerance range of the crystal lattice,deformation-induced amorphization will occur.The Apearance of deformation-induced amorphization is related to experimental temperature,local dislocation density and grain size.Under the experimental conditions in this article,the local critical dislocation density and grain size of deformation-induced amorphization is6×10¹⁶ m^-2 and 62.4nm.The hardness test and nanoindentation curve results of the samples show that the strength and plasticity increased with higher turns of HPT.the ultimate elongation of pure tungsten at room temperature increases from 2.13%to 6.12%,which effectively improves the plasticity of pure tungsten.At the same time,molecular dynamics simulation results also show that the non-equilibrium grain boundaries with high-density dislocation structure after high-pressure torsion of pure tungsten can coordinate the macroscopic deformation more effectively compared to normal grain boundary,which is able to inhibit the generation and propagation of cracks.Pure tungsten with normal grain boundaries will have intergranular fracture and poor plasticity during tension.When nonequilibrium grain boundaries with high-density dislocation structure are stretched at low strain rates,twin-coordinated macroscopic deformation occurs at the grain boundaries,which inhibits the generation of cracks.Twinning deformation occurring at the crack tip under high strain rate of tension can decrease the stress near the crack,which inhibits crack propagation to a certain extent and improves plasticity of pure tungsten.