Simulation And Strengthening Mechanism Of Tungsten During High Pressure Torsion

Uniting the superiorities of high melting point,high strength,high sputtering resistance,low deuterium/tritium retention and good thermal-mechanical properties,tungsten has been applied in many fields such as aerospace,national defense,military,and nuclear industry.However,the service environment under extreme conditions requires higher performance of tungsten.Therefore,it is urgently needed to develop high performance tungsten.At present,powder metallurgy and subsequent plastic processing are used to fabricate high performance tungsten,but the stress state of traditional plastic processing technologies,such as rolling and extrusion,is not suitable for the deformation of low-plasticity tungsten,so the effect of microstructure refinement and property improvement is limited.High hydrostatic pressure and large shear strain introduced by HPT can effectively refine the microstructure and significantly improve the comprehensive properties of the materials.And this technique has been widely used to prepare block UFG materials.In this paper,the HPT technology was successfully applied to prepare UFG tungsten with excellent performance.Based on the finite element theories and finite element method(FEM),the behavior ofthe tungsten sample under pressure and torsion in a quasi-constrained configuration was studied by the large commercial software of ABAQUS FEM code.And the evolution of contact state between anvils and sample,as well as the distribution of stress field and strain field were obtained.The results show that the contact stress on the surface of the specimen is always at a high level,which indicates that no slippage occurs.At the same time,the sample is always under high hydrostatic pressure,and the three normal stress components remain stable.However,the the three shear components experience significant change:at the compression stage,there are two significant shear components,?_zr and??r,because of the radial material flow;at the torsion stage,due to the rotation of the anvil,the circumferential component of shear stress??z increases as the radial components of the shear stress?_zr and??r decrease to zero.In addition,the equivalent strain increases gradually along the radial direction,viz.,the strain at the center is approximately zero,and that at the edge is maximum.Along the height direction,the strain is always heterogeneous,which decreases gradually from the upper and lower contact surfaces of the sample to the heart.With torsion angle increasing,the strain gradient gradually weakens.When the torsion angle is large enough(1/4 turn),the strain at the heart of sample is slightly higher than that at contact surface due to the occurrence of dead zones.Pure tungsten was processed by high pressure torsion with 1 turn,2 turns and 5 turnsunder the applied pressure of 1.5 GPa,with the temperature of 823 K for sample and 623K for anvils,and the microstructure HPT processed samples were observed by means of XRD,EBSD and TEM.It was found that HPT processing results in significant grain refinement and with torsional strain increasing,this phenomenon becomes more obvious.The dislocation density increased significantly from initial 1.18×10¹⁴ m^-2 to 3.89×10¹⁴m^-2 for the 5 turns HPT processed samples.When the strain increased to a certain extent,the grain size and orientation tend to be homogeneous,accompanied by dynamic recrystallization.During HPT processing,the LAGB gradually transformed to HAGB,and after 5 turns HPT processing,the fraction of HAGB increased to 62.1%.Moreover,at the beginning of the deformation,the main deformation mode is dislocation sliding,but the grain boundary sliding becomes essential deformation mechanism besides dislocation sliding when grains size is refined to ultrafine scale,equal to the mean free dislocation path.The HPT processed tungsten were tested by microhardness and nano-indentation to evaluate the strengthening effect of HPT.It can be concluded that HPT results in significant enhancement of microhardness and as the strain increasing,the Vickers microhardness increases gradually and becomes more homogeneous.With the comprehensive influences of relative density,residual stress and dislocation density,the elastic modulus increases evidently after HPT and decreases gradually as the strain further increasing.The rapid rise and subsequent stability of micro-strength and nano-hardness are regarded as the comprehensive influences of pores closure,grain refinement,lattice distortion,dislocation multiplication and HAGB formation.Based on the grain boundary strengthening and dislocation strengthening theories,the strengthening model was established to gain an in-depth understanding of the enhanced strength or hardness during high pressure torsion.Compared with dislocation strengthening,grain boundary strengthening was of the greater significance.And the deviation from classic Hall-Petch relationship was revealed and it can be attributed to grain boundary and dislocation density.