Study On Mechanical Properties And Stability Of Y / Nb Carbide Hard Phase In High Speed ​​Steel By First Principle

With the development of the modern steel material industry, steel materials are widely applied in all fields. Every year, more than1billion tons of steel are required in daily life. However, there are less than1,000tons of high speed steels. Thus, it is an urgency to solve the problems in how to improve the properties of steel, how to forecast and design the properties of steel from the microscopic mechanism aspect and how to acquire high-qualified but cheap-price steel. As the wide use of high speed steels, it is further required to study the functions of alloying element in steel materials. Carbides play an important role in the high speed steels, yet the basic natures of carbides have been seldom studied. Thus, this article fixes focus on the yttrium (Y) and niobium (Nb) carbides of high speed steels based on the first principles which are density functional theory calculations. It compares the crystal structure of these two types of carbides and analyses the thermodynamic stability and mechanical properties of carbide hard phase with different stoichiometric ratio.In this way, combined with the calculation method of quantum mechanics, the relationship between the macro properties and the microstructure in carbide hard phase is fundamentally established.This work studies thermodynamics and mechanical properties of seven different stoichiometric ratios of YxCy compounds and NbxCy compounds through the usage of C ASTEP module of the Material Studio. The study goes through the establishment of crystal compound model to optimize the geometrical, and finally leads to physical quantities of formation enthalpy, the cohesive energy, elastic modulus, Poisson’s ratio, Debye temperature, elastic anisotropy, hardness etc. Then, it studies stability of the two compounds which is determined through the application of the formed enthalpy, the Debye temperature and elastic constant stable discriminator formula. In this way, it can be found that each carbide compound is stable but the pmc21-Nb2C structure.Due to the analysis of physical parameters of elastic constants, elastic anisotropic and Poisson’s ratio, we have found these results:the ability of the carbides to resist the normal stress is stronger than to the shear stress; carbide compounds reveal the feature of elastic anisotropy; the result of the carbide toughness and brittle are consistent with the predications of Poisson’s ratio and B/G ratio(YxCy compounds and two kinds of NbxCy compounds (Nb6C5and NbC) show the feature of brittle, while P-3ml-Nb2C, Plml-Nb2C and Nb4C3of toughness). Although the calculation of compound hardness is conducted by two different methods, we find that the results do not differ much. Among which, one is that the size of the material hardness showing by the shear modulus and Young’s modulus is more markedly accurate than by the bulk module with. The hardness of YxCy compounds own less than17GPa and NbxCy compounds less than20GPa.