Creep-like Stress Response Of TMB₁₂(TM=Sc,Y,Zr,Hf) And Nanotwinned Structure Design Of ZrB₁₂

From the ancient stone,bronze and iron age,the hardness of materials determines the level of productivity.The international society of production engineering pointed out in its report:"the development of new high hardness materials has doubled the material processing efficiency every 10 years."Transition metal light element(B,C,N)compound material is one of the potential research systems of high hardness materials.Its theoretical basis is that transition metal atoms have high electron density and compression resistance.Light elements can form long and short bonds and directional covalent bonds to form a three-dimensional covalent bond network to enhance the shear resistance.The combination of high compression resistance and high shear resistance makes transition metal light element compounds exhibit extremely high hardness.In practical application,the hardness of materials is often limited by the shear resistance.Therefore,the design theory of high hardness materials predicts that"transition metal light element rich materials have potential high hardness and high strength."In light elements,B is an electron deficient atom,which is easier to combine with metal atoms to form compounds with various ratios.Transition metal dodecaboron compounds(TMB₁₂,TM=SC,Y,Zr,Hf)are a kind of cemented carbide materials with high hardness and low density.They also have high conductivity and superconductivity.They are truly multifunctional high hardness materials.Their mechanical properties have been studied experimentally and theoretically for many years,but their mechanical response behavior under large strain and the stress failure mode of crystal structure under strain limit have not been clearly analyzed.Industrial materials mainly exist in the form of polycrystals.Reducing the grain size is a common method to strengthen the mechanical properties of materials.With the decrease of grain size,the rapidly increasing grain boundary area will hinder the dislocation movement of the material and improve the hardness of the material.This phenomenon is called Hall-Petch effect.Unfortunately,in traditional processing,when the grain size is small to a certain limit,the hardness of the material will not increase,but will decrease.This phenomenon is widely recognized as the anti-Hall-Petch effect.In recent years,with the continuous improvement of material processing methods,nano twin materials with grain size of tens or even several nanometers can be synthesized experimentally.It is found that the twin materials in nano scale are no longer limited by the anti-Hall-Petch effect.For instance,the hardness and toughness of nano twin diamond and nano twin cubic boron nitride are better than that of single crystal phase:the low load hardness of twin diamond with grain size of L=3-6nm is about 200 GPA,and the hardness of twin cubic boron nitride with grain size of L=3.8nm is about 103 GPa.Their hardness is nearly 100%higher than that of single crystal,and their toughness is also greatly improved.Nano twin materials may be a powerful means to further obtain high hardness materials.As a new research discovery in the past 10 years,the properties of nano twin materials have broken through people's understanding of the hardness mechanism of materials.Scientists are making continuous efforts to explain the mechanical properties and underlying atomic mechanism of nano twin materials.At present,the physical mechanism of nano twin strengthening is studied theoretically by constructing atomic scale high-density twin boundary structure model and first principle calculation method.It is an important subject to promote the progress of relevant scientific research to develop the method of designing reasonable nano twin crystal structure for the study of mechanical response behavior and structural fracture mode.In this paper,the mechanical response behavior and atomic-scale structural fracture modes of transition metal boron-rich compounds(TMB₁₂,TM=Sc,Y,Zr,Hf)are investigated by the first-principles stress-strain method and twinned Zr B₁₂ideal strength has been investigated by developing twinned structure design software.The main contributions are summarized as follows:1)The ideal strength and fracture mode of TMB₁₂compounds were systematically studied.It was found that the bonding between the cages of TMB₁₂was weak,and the boron cages were prone to slip under large strain,but could not effectively carry the stress.This changed the previous observation and understanding that the higher the content of light elements,the greater the hardness and strength of the compounds.2)By introducing operations such as mirror image,translation and rotation,the design method and software of nano twin covalent structure are developed,and a series of nano twin structures of TMB₁₂with significantly improved toughness compared with single crystal structure are designed,which provides a powerful tool for developing twin covalent structure superhard materials.