Strengthening Mechanism Of Grain Boundary Affected Zone Of Nano-polycrystalline Tungsten Based On Rhenium Substitution

The strength or hardness of metallic materials follows the well-known Hall-Petch effect within a limited grain size,that is,the increase in strength is inversely proportional to the square root of the grain size,but when the grain size decreases to a certain value on the nanometer scale,the material softens and the strength decreases as the grain size decreases.Most studies attribute it to a large amount of grain boundary migration.The movement behavior of the grain boundary largely determines the mechanical properties of the matrix.In order to overcome the defects such as softening and poor thermal stability of small-sized nanocrystals,researchers have taken a series of measures from the perspective of improving the stability of grain boundaries(GBs)and achieved good results,such as the use of alloy elements for segregation and solid solution,modifying the distribution of GBs in space,etc.In fact,the stability of the GB itself is closely related to the state of the surrounding area.In the study of nanocrystalline materials,the GB and its surrounding area should be regarded as a whole region,that is,grain boundary affected zone(GBAZ),the deformation behavior of such region is different from the pure grain boundary and the grain interior,but there is very little research around GBAZ.In a high-temperature service environment,compared with pure tungsten(W),tungsten-rhenium alloys have higher strength and stronger plasticity due to the addition of rhenium(Re),and have good thermal stability even at the nanometer scale.In previous study,we found that the doping of Re in the GBAZ will obviously strengthen its high-temperature mechanical properties,but the inherent mechanism is still unclear.Therefore,in order to study the effective strengthening position of Re atoms in the nanocrystalline W matrix and the special mechanism,in this paper,the Voronoi algorithm is used to construct a pure nano-polycrystalline W film with a grain size of 16 nm as the initial model,so that the Re atoms are randomly dissolved in the GB,GBAZ10(10? near the GB),GBAZ20(20? near the GB)area,and establish a model of random solid solution of the whole grain to compare.Subsequently,EAM potential and molecular dynamics simulation method were used to study the influence mechanism of the Re atoms solid solution position on tensile,friction,impact,and fatigue properties of each model at high temperature.The simulation results show that when at high temperature from the GB to the GBAZ to the inside of the grain,the strain of the atoms shows a decreasing trend,Re atoms can improve the high-temperature strength and plasticity of tungsten in the GB,grain interior,and GBAZ,the concentration and distribution of Re atoms are the key factors affecting the tensile and fatigue performance.The higher the degree of strain that the atoms in this part can withstand,the stronger the ability to resist cracks,making the GB more resistant to slippage and more difficult to crack,making it more resistant to crack initiation and expansion.The first-principles calculation method was then used to investigate the effects of Re on the electronic structure at the W matrix and W GB.It was found that the solid solution Re atoms can not only strengthen the GB,but also strengthen its connection with the area near the GB.The further solid solution of Re atoms in the area near the GB not only strengthens the GB,but also plays a role in increasing the charge density,reducing the population of bonds,reducing covalentity,and improving the brittleness of the W matrix.So that the strength and plasticity can be improved.According to the traditional concept,the nano-grain is only regarded as a compose of a GB with 2-3 interatomic distance and the interior of the grain.While in this paper,the importance of GBAZ was proposed.The research results show that the solid solution atoms in such area can also play a role in the mechanical properties of the matrix,this research will help further to understand the solid solution strengthening mechanism of nanocrystals,and will further inspire the development of new solid solution alloys and GB modification processes.