The mechanical deformation of cellular structures in the selective laser melting (SLM) of aluminum was investigated by performing a series of molecular dynamics (MD) simulations of uniaxial tension tests. The effects of crystalline form, temperature, and grain orientation of columnar grains on the mechanical properties of SLM aluminum were examined. The MD results showed that the tensile strength of SLM aluminum with columnar grains at different temperatures was lower than that of single-crystal aluminum, but greater than that of aluminum with equiaxed grains. The tensile strength and Young’s modulus both decreased approximately linearly upon increasing the temperature. The deformation mechanisms of equiaxed and columnar grains included dislocation slip, grain boundary migration, and torsion, while the deformation mechanisms of single crystals included stacking fault formation and amorphization. Finally, the influence of the columnar grain orientation on the mechanical properties was studied, and it was found that the Young’s modulus was almost independent of the grain orientation. The tensile strength was greatly affected by the columnar grain orientation. Reasonable control of the grain orientation can improve the tensile strength of SLM aluminum.
Buckling behavior of boron nitride nanotubes under combined axial compression and torsion is presented by using molecular dynamics simulation. In order to study the effect of helicity and nanotube size, three groups of nanotubes are considered. The first group is a pair of boron nitride nanotubes with a similar geometry but different helicities, the second group includes three armchair naotubes having equal length but different radii, and three armchair (8, 8)-nanotubes with different lengths form the third group. The simulation is conducted by applying Nose-Hoover thermostat in a temperature range from 50 K to 1200 K. Based on the interatomic interactions given by Tersoff-type potentials, the molecular dynamics method is used to study variations of atomic interaction in initial linear deformation and postbuckling stages with various load-proportional parameters, and to determine the interactive buckling loads relationship. By comparing typical buckling modes under different loads, it is found that the boron nitride nanotube experiences complex micro-deformation processes, resulting in different variations of atomic interaction and strain energies. When the axial compressive load is relatively large, the change of atomic interaction for boron nitride nanotubes under combined loads is similar to that found under the pure axial compression. The onset of buckling leads to the abrupt releasing of strain energy. As the torsional load is relatively large, the nanotube shows torsion-like buckling behavior, no obvious reduction of strain energy is observed after the critical point. The present simulation results show that both the armchair and zigzag nanotubes exhibit nonlinear interactive buckling load relationships. Rise in temperature results in the decrease of interactive buckling load, and the effect of temperature varies with the value of load-proportional parameter. That is, the axial compressive load is relatively large, and the effect of temperature is more significant. It is found that the buckling behavior in the case of combined loading is strongly size dependent. The interactive critical axial and shear stress decrease as nanotube radius or length increases. The studies also reveal that under both simple loading and combined load condition, carbon nanotubes possess higher buckling loads than those of boron nitride nanotubes with a similar geometry, which provides valuable guidance for forming carbon and boron nitride hybrid nanotubes as well as coaxial nanotubes with superior mechanical properties.
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