Establishment Of Dislocation Glide Model On Two-dimensional Barrier And Prediction Of Material Strength

Structural materials are defined as the materials on basis of mechanical properties and have a certain bearing capacity.With the rapid development of science and technology,the service environment of structural materials will become more and more complex.In order to ensure the safety of engineering components,it is necessary for the material to possess sufficient strength.Since the strength of a material is closely related to its components and microstructure,understanding and establishing the connection between the strength and microstructure has become a significant topic in materials science.For metallic materials,their strength models have undergone development from theoretical strength model,P-N model to Seeger model.In 1926,the theoretical strength model proposed by Frenkel was several orders of magnitude different from the experimental results due to using the atomic rigid slip model.In 1947,Nabarro revised the calculation of dislocation-activated stresses proposed by Peierls and developed the P-N model,but since it used a one-dimensional barrier for dislocation glide,it was valid only for metallic materials with low electron localization.In 1956,the Seeger model was developed,which takes into account the thermal activation effect,but it also has great limitations because the one-dimensional P-N barrier is still used.As a result,it is currently difficult to make quantitatively accurate predictions about the strength of metallic materials.For covalent materials,their hardness is generally studied by valence bond theory.In2003,Gao et al.argued that bond density and bond length are determinants of the hardness of covalent crystals.In 2006,?im?nek et al.introduced the concept of bond strength and proposed a new theoretical calculation model of hardness applicable to covalent and ionic crystals.In 2008,Li et al.introduced the concept of electronegativity and a Knoop hardness model for covalent crystals was developed.However,because the sp³ hybridized C-C bond in diamond is the strongest bond in 3D network.Therefore,it is natural to conclude that diamond is the hardest material in the world.However,the recently synthesized nano-twin diamond is more than twice as hard as single crystal diamond.Obviously,the valence bond theory alone cannot explain the origin of the super-hardness of nano-twin diamond.Therefore,a deeper theory is needed to study the hardness of covalent materials and its mechanism.It can be seen that for metallic materials,their strength is usually studied using dislocation theory,but the effect of chemical bond orientation and strength on dislocation glide is not considered.For covalent materials,it can be inferred from the plastic deformation that occurs during hardness measurements that the hardness of covalent materials is also dominated by dislocation motion.In fact,dislocations are bent and kinked during glide,and these bends and kinks are strongly correlated with the directionality and strength of the chemical bonds.Therefore,if the potential energy surface of dislocation glide is considered as a two-dimensional barrier,the influence of the directionality and strength of the chemical bonds on dislocation glide can be included in the kinetic equations of dislocation glide.In order to study the strength of materials in more depth,this paper combines the traditional dislocation theory and valence bonding theory to establish a dislocation glide model on two-dimensional barrier for both metallic and covalent materials,and establishes the corresponding dislocation kinetic equations.By solving the dislocation kinetic equations,we can calculate the critical shear stress of the dislocation glide and obtain the yield strength and hardness of the material through the polycrystalline Sachs model.In order to investigate the rationality,validity and applicability of this model,the strength of some typical covalent,ionic and metallic crystalline materials is investigated in this paper.The specific studies are as follows.(1)The dislocation glide model on two-dimensional barrier is proposed and developed.By studying the dislocation glide behavior of different types of materials,a two-dimensional dislocation glide barrier is proposed.Based on the influence of stress and temperature on the dislocation glide on the two-dimensional barrier,four modes of dislocation glide are proposed.The kinetic equations for dislocation glide under the four modes are established by quantitatively analyzing the system energy changes caused by the lattice distortion caused by dislocation,respectively.By solving the kinetic equations of dislocation glide in the four modes,the critical shear stresses of dislocation glide in different modes were obtained.Then the strength of the material is obtained by combining Sachs model and Tabor's law.(2)Hardness of covalent materials calculated by dislocation glide model on the two-dimensional barrier.The hardness of typical zinc-blende structured covalent materials is calculated based on the dislocation glide model on two-dimensional barrier and the factors influencing the hardness of zinc-blende structured covalent materials are investigated.The hardness of wurtzite structured diamond and BN were then predicted using the dislocation glide model on two-dimensional barrier.(3)Hardness of typical ionic crystals calculated by dislocation glide model on the two-dimensional barrier.The dislocation glide model on two-dimensional barrier was applied to the ionic crystals Na Cl and the mantle mineral?-Mg₂Si O₄,and the hardness of Na Cl and?-Mg₂Si O₄ were calculated to be in agreement with the experimental values.And the types of slip systems activated in Na Cl and?-Mg₂Si O₄ at different temperatures were analyzed according to the Sachs model and compared with the experimental observations.(4)Strength of metallic materials calculated by dislocation glide model on the two-dimensional barrier.The dislocation glide model on the two-dimensional barrier is applied to face-centered cubic metals(Cu and Al)and body-centered cubic metals(Fe and Mo)to investigate the temperature-dependent trends of their yield strengths and compare them with the corresponding experimental results.(5)The correlation between hardness and conductivity based on the dislocation glide model on two-dimensional barrier.Taking Ti B₂ as an example,the high-temperature hardness of Ti B₂ was calculated using the dislocation glide model on two-dimensional barrier with Sachs model and Tabor's law.The electronic structure of Ti B₂ was analyzed by energy band.The relationship between material hardness and material electrical conductivity is investigated by comparing and analyzing the difference between the potential energy surface of dislocation glide and the potential energy surface of electron transport.In this paper,the dislocation glide model on the two-dimensional barrier and a material strength prediction method are proposed,and a dislocation theory model is developed for the high-temperature hardness of covalent,ionic and metallic materials.These findings help to reveal the physical mechanism of hardness and provide direct guidance for the design of new structural materials,especially those with high temperature resistance.