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The propagation process of crack in the nanocrystalline Ni is simulated via the quasicontinuum method. The results show that the stress near the crack tip could prompt the disassociation of grain boundaries, and the formation of stacking faults and deformation twins. Farther from the crack tip, fewer deformation twins can be found. There are more stacking faults than deformation twins in the grains, which approximately have the same distance to the crack tip. The effect on deformation twins from the variation of local stress and generalized planar fault energies is manifested by these results. The distribution of hydrostatic stress on atomic-level around the crack tip is also calculated. It is shown that nanovoids can be easily created in grain boundaries in front of the crack tip. There exists an intense tensile stress state in the grain boundary regions around these nanovoids. As a result of the stress accumulation, the crack propagates along the grain boundaries. Our simulated results qualitatively uncover the propagation process of crack in nanocrystalline Ni, which agrees well with the relevant experimental results.

The dynamical shear deformation induced hexagonal close-packed （HCP）face-centred cubic （FCC） processes in TiAl/Ti3Al system are investigated by the molecular dynamics method. The details of the dislocation initiation， the microstructure evolution and the force field effect are presented and discussed. The analyses of the potential variation and the structural snapshots show that the shear deformation is related to the “stick-slip” behaviour. The interface can transit the energy and counterpoise the deformation between the hetero-phases. The HCP-Ti3Al （FCC-TiAl） shows obvious （little） covariant deformation stage before the initiation of the fault transition. For HCP-Ti3Al， the final structure is the continued FCC stacking in a region with the large force field， and the FCC and HCP plates alternatively occur near the interface. For FCC-TiAl， twin and SISF are the main remnants， and the meta-stable HCP phase may occur with the increment of the force field.

Molecular dynamics (MD) simulation is performed to study the vacancy-interstitial annihilation in crystalline silicon. We choose the Stillinger-Weber potential , which is commonly used for silicon, to describe the interaction between atoms. The system is relaxed under 300K and 1400K respectively. We have found that is the preferred recombination direction and propose the presence of an energy barrier in the direction. From the calculated value of energy barrier alon g we give a reasonable explanation for the difference between Tang's and Z awadzki's data.

The structure changes of a Cu Σ=5（310）/［001］ symmetrical tilt grain boundary are investigated using the molecular dynamics method by heating and quenching. The simulated results showed that with increasing temperature,there are structure changes of the grain boundary. At a temperature significantly below the bulk melting point of copper,the grain boundary undergoes a structure change,and there exists the crystal-melt coexistence in the grain boundary zone. The grain boundary structures can be changed by quenching,which are compared to those at the same temperatures by heating.