Study On Fracture Behaviors Of Nickel Nano-crystals With Through-thickness Straight Crack Under Different Constraints

As a crucial role of structural material in aero engines and gas turbines,there inevitably exist some micro defects such as voids and micro-cracks during manufacturing,transporting and assembling of nickel based superalloys,which ultimately cause deformation,expansion and even failure.Therefore,it is of great significance to analyze the dislocation initiation,movement and three-dimensional stress state from atomic scale.In this paper,the mechanical properties of nickel nano-crystals with through-the-thickness straight crack under different constraints are simulated by using molecular dynamics method based on the embedded atom potential function.The main contents are listed below:(1)Effects of thickness constraint on fracture behavior of nickel nano-single-crystal with through-thickness straight crack.The mechanical properties and fracture behavior of samples with different thicknesses are simulated by molecular dynamics method.The elastic limit of different samples increases first and then decreases and tends to a steady value with the increase of thickness.The sample with 4 nm thickness shows the maximum elastic limit.Out-of-plane constraint exists in most areas of samples with less than 4 nm thickness so they have smaller elastic limits,while the elastic limit of samples with larger than 4 nm thickness gradually reduces due to the stress concentration induced by the pile-up of dislocations.The samples with larger thickness have larger plasticity.In addition,as the thickness increases from the finite to the infinite,it can be observed that the samples transform from flexible specimen to brittle specimen.(2)Effects of displacement constraint on fracture behavior of nickel nano-single-crystal with through-thickness straight crack.The mechanical properties and fracture behavior of samples with displacement constraint are simulated by molecular dynamics method.It is found that the boundary atoms in the thickness direction are subjected to stable surface constraint and there is no dislocation occurring during tensile process,which is different from that of samples with free boundaries.The out-plane constraints in the sample are maintained at a stable level,and the stress of crack-tip is evenly distributed along the thickness direction.As the strain increases,the crack expands steadily to break,which shows that the sample with displacement constraint is brittle.(3)Effects of grain boundary constraint on mechanical properties of nickel nano-crystal with through-thickness straight crack.The mechanical properties and fracture behavior of nickel polycrystal are simulated by molecular dynamics method.It is found that unlike the single crystal with the same size,the elastic phase of the polycrystalline sample is not completely linear due to the existence of the grain boundary constraints.After reaching the elastic limit,it enters the yield phase and strengthening phase,similar to the uniaxial tensile experiment of low carbon steel.During the whole tensile process,crack in the polycrystal sample did not expand significantly and a large number of dislocations occur in the plastic phase which causes a very long state of plastic deformation.The nano-polycrystal shows a property of ?crack insensitivity‘.