Molecular Dynamics Simulation Of Intragranular And Intergranular Crack Propagation In Nanocrystalline Ni Under Single Cycle Loading

Numerical simulation is an important means to study fatigue crack growth.With the rapid development of computer technology,the research on fatigue crack growth has transformed from macro-scale to micro/nano-scale.With further research,the micro-mechanisms of fatigue crack initiation and propagation at nano-scale have attracted more attention.In this paper,the fatigue crack growth behaviors of a nanocrystalline Ni symmetric model with intragranular and intergranular cracks under a single impulsive cyclic load are simulated and studied by molecular dynamics method.The effects of temperature,strain rate and crack opening size on crack growth behavior are compared and analyzed.In the initial stage of loading,both the intragranular and intergranular crack models lie in the elastic stage,and their cracks gradually open.Subsequently,under the guidance of Schockley dislocation slipping along{111}?27?112?29?direction,the intragranular crack begins to propagate along the slip direction of the dislocation and its tip is gradually passivated.The intergranular crack propagates along the grain boundary and is blocked at the triple junction.Whether in the intragranular crack model or in the intergranular crack model,a new crack will initiate at the grain boundary located in front of the crack and perpendicular to the loading direction during the loading stage.With increasing load,the new crack will propagate in the opposite direction to the original crack.They are close to each other.In the unloading stage,the cracks in the two models will be sharpened.In the final stage of unloading,the intragranular crack will be locally closed and divided into two parts,namely a big part and a small part.The small part further evolves into a void in front of the crack.However,the intergranular crack is not locally closed completely.Elevated temperature is beneficial to crack passivation,so it will hinder crack propagation.A low strain rate can not only promote intragranular crack propagation along the crack extension line,but also promote the transformation from an intergranular crack to an intragranular crack.When the strain rate is 5×10⁷s^-1,there will be obvious"steps"in the nominal stress-strain curve in the loading direction,and the crack will grow at an accelerated speed.A Large crack opening size is conducive to intragranular crack propagation along the dislocation slip direction of?27?112?29?and may promote twins to occur in front of the intergranular crack and hinder crack propagation.