Molecular Dynamics Simulation Study Of Size Effect In Nanocrystalline Nickel Microforming

Nanocrystalline materials have different mechanical properties from the coarse-grain materials due to small grain size. With grain size change, the stress-strain relationship, plastic deformation properties and forming parameters will change with different trend from the coarse-grain materials, and it is called size effect. So the plastic deformation of nano polycrystalline and monocrystalline nickel pillar was simulated, and the size effect was observed and analyzed based on the molecular dynamics simulation in the paper.To study the grain size effect, the deformation of nanocrystalline nickel pillar with grain size of 5.818 nm, 7.33 nm, 9.236 nm, 11.636 nm, 14.662 nm, 18.471 nm and 22.63 nm under uniaxial compression at 0K and 300 K was simulated by MD. The results show that the size effect is mainly affected by plastic deformation mechanism. The plastic deformation is dominated by twin deformation at 0K, and movement of grain boundary at 300 K. The grain size effect was obviously indicated. The decrease of flow stress with the decrease of grain size is consistent with the inverse Hall-Petch relationship. This is mainly caused by the size effect of twin. In large grains twin is hardly observed and formed by the extend dislocations which is difficult to nuclear, and in small grain more twin is observed which is formed by the Shockley partial dislocation emitted from grain boundary. The movement of grain boundary can weaken the size effect caused by twin in the simulation at 300 K. The grain shape may change under the effect of grain boundary, even grain tend to grow up by the grain boundary splitting. We also simulated the deformation of nanocrystalline nickel pillar with different diameter under uniaxial compression in order to study the specimen size effect. The yield stress will decrease with the increase of diameter and this phenomenon was explained by the surface layer model.Single-crystal nickel pillar with different orientation of [100], [111], [110], [112?] and [2?6?5] was simulated under the uniaxial compression at 0K in order to study the orientation effect. The results show different plastic deformation mechanism of different orientation. Meanwhile simulation of different single-crystal grain size was performed to study the size effect in single-crystal nickel. The yield stress varies with the change of the grain size in a trend of "the smaller the higher".To study the specimen size effect of nanocrystalline materials, we prepared the nanocrystalline nickel foil with the grain size of about 50 nm by electrodeposition. And tensile test of different thickness foil was performed at room temperature. We also established the model of different thickness of nanocrystalline nickel and simulated the deformation progress under uniaxial tensile. From the simulation and experiment, it is found that with increasing thickness of foil, the mechanical properties such as elastic modulus and yield stress will increase as well.