First-Principles Study Of Mechanical Behaviors And Toughening Mechanisms Of Boron Carbide

Boron carbide has been widely used in industry and military applications,due to its excellent properties,such as high strength and low density.However,because of the complex crystal structure of boron carbide,it is still a chanllenge to fully understand its mechanical behaviors,especially its micromechanical behaviors,which greatly limits its performance and wide applications.Thus,first-principles methods based on density functional theory were employed to explain the deformation behaviors and failure process of boron carbide from atomic scale,and then how to improve its toughness was further discussed.The main research contents and results are listed as follows:1.The elastic properties of B₄C and the mechanical behavios as well as microstructure evolution mechanisms of B₄C under different loads type were investigated in details.The results show that strong anisotropy of elastic properties of B₄C is observed.Under hydrostatic compression,the crystal structure of B₄C is almost unchanged.Under uniaxial compression,c-axis compressive strength of boron carbide is almost twice than that of a-axis compressive strength.Under complicated stress state,?=30°is the most likely to be activated biaxial compression system.Under uniaxial compression and complicated stress state,“the sudden bending of three-atom chains”is the“inherent”deformation behavior of B₄C.Under c-axis compression,the formation of new B^c-B^e bonds causes the sudden bending of three-atom chains in B₄C,leading to structural failure.Under others load conditions,the bending of three-atom chains is the elastic deformation of B₄C,and the fully destruction of icosahedra is its main failure mechanism.2.The effect of carbon content on the elastic properties and mechanical behaviors of boron carbide was investigated in details,revealing the microstructure evolution mechanisms of boron carbide.The results show that with higher carbon concentrations,the strength and hardness of boron carbides increase,while the toughness decreases.Under uniaxial compression,the destruction of icosahedra is the main deformation mechanism of B₁₂-CBC and B₁₀C-CC,where the deformation behaviors of B₁₂-CBC show strong anisotropy.The deformation of boron carbide in real applications can be suggested that for boron carbide with three-atom chains,such as B₁₁C^P-CBC and B₁₂-CBC,its deformations are led by the sudden bending of three-atom chains,and then the icosahedra are gradually destroyed due to the interation between three-atom chains and icosahedra,resulting in the destruction of boron carbide;for boron carbide with two-atom chains or one-atom chains,such as B₁₀C-CC,its deformations are mainly dominated by icosahedra if the strength and stability of atom chains are higher than that of icosahedra.3.The influence of different kinds of and different contents of atomic substitution on the toughness of B₄C and the underlying toughening mechanism were examined.The results show that Mg,N and Si atomic substitution may increase the toughness of B₄C,but the new structures are not stable after Mg and N atomic substitution.Thus,the different contents of Si atomic substitution on the toughness of B₄C and the underlying toughening mechanism were further studied.The results suggest that with the increase of Si content,the toughness of XSi-B₄C structure increases.Since Si atomic substitution can increase the stability of icosahedra,the substitution of Si atom may significantly affect the deformation behaviors of B₄C under a-axis compression and its failure strain increases 39.1%.