The Effect Of TiN Nano Grain Size On Its Mechanical Properties Studied By MD

In this paper, the molecular dynamics package lammps is used to simulate the single crystal and polycrystalline materials of TiN based on the2NN MEAM potential for exploring the influence of various factors on the TiN material hardness. In order to choose an effective potential which is suitable for this research, six different potentials of Si were compared respectively in this paper. The study found that the2NN MEAM potential is more applicable to describe the mechanical properties of materials and more suitable for the research of this topic. The software package lammps based on molecular dynamic with Modified Embedded-Atom Method(2NN MEAM) is used to simulate the tensile fracture behavior of single crystal TiN nanorod. The mechanical property of TiN with different cross sections is analyzed under different strain rates and different temperatures along [100],[111] directions and the stress-strain curve obtains. The tensile process is described in detail. In the tensile process of polycrystalline TiN, in order to make the simulation results closer to the natural environment, polycrystalline TiN model is constructed based on the Voronoi algorithm programmatically, the distribution of grain sizes is controlled. The tensile process and compression process of polycrystalline TiN are simulated by the software package lammps, the total process of the motion of grain, grain boundary and microscopic particles is described in detail, the stress-strain curve obtains by the calculation. The influence of different grain sizes, different strain rates and different temperatures on the mechanical property of polycrystalline TiN is analyzed.Through the above research, the following achievements have been obtained:1) The2NN MEAM potential is more applicable to describe the mechanical properties of materials, other types of potentials have an advantage in other ways, such as thermodynamic properties, etc. All kinds of the calculation results of TiN’s mechanical properties are consistent with actual, which shows that the molecular dynamics can be used in the simulation study of TiN materials very well. 2) For single crystal TiN materials, the tensile direction, cross section, strain rate and temperature all have different effects on the tensile process, yield stress and elastic modulus of TiN nanorod. During the tensile process along [100] direction, the larger the cross section, the lower the yield stress, but during the tensile process along [111] direction, the larger the cross section, the larger the yield stress. The yield stress and yield modulus increase with the strain rate, but decrease with the temperature. Under different tension condition, the tensile process of TiN nano-rod all contains three stages which are brittle failure, plastic failure and fracture stage..The elastic modulus along [100] direction is always lager than that along [111] direction.3) For polycrystalline TiN materials, different movement mechanism of grains and grain boundaries has an important influence on yield strength. During the tensile process, positive and inverse Hall-Petch relationship appears in the process of yield strength changing with the grain size. In the process of compression simulation, the deformation mechanism near grain boundary makes the yield stress increase with the grain size, then the yield strength value tends to be stable. Come to the conclusion eventually after the comparison of the simulation results of tensile process and compression process, when the value of average grain size is between3nm~5nm, the maximum value of the materials’ hardness achieves. In the tensile and compression process, the critical grain size, high strain rate and low temperature can contribute to the strengthening of polycrystalline materials, the strength of the grain boundary is one of the most important factors which affects the intensity of polycrystalline TiN. In conclusion, the strength of polycrystalline TiN material will improve effectively if the grain boundary doped with other elements which can strength the atomic bonding ability or increasing the resistance of the mutual movement between grains.