Molecular Dynamics Simulations Of Nanoindentation Of Cuni Alloy

Copper-nickel(CuNi)alloy has excellent mechanical properties,corrosion resistance and oxidation resistance,etc.,and is mostly used to make corrosion-resistant,hightemperature fatigue-resistant engineering structural parts,and is widely used in aviation engines,ship instrumentation devices,chemical machinery parts,medical devices and other fields,and its beautiful color can also be used for decorative crafts industry.The macroscopic mechanical properties of CuNi are closely related to its microstructure and deformation mechanism.In this paper,a series of numerical simulations and analyses of single-crystal CuNi alloys using molecular dynamics methods are carried out to investigate the effects of indentation velocity,test temperature,crystal orientation and twin boundaries on the mechanical properties of the material and the corresponding intrinsic microstructural deformation mechanisms,and the following conclusions are obtained:(1)For different indentation velocities,the hardness of CuNi alloy increases with the increase of the indentation velocity,and the elastic modulus obtained at different indentation velocities is approximately the same.For different temperatures,an increase in temperature leads to a decrease in hardness and modulus of elasticity.For CuNi alloys with different crystal orientations,the hardness and modulus of elasticity obtained from the[111] crystallographic simulation are the largest.For CuNi alloys with preexisting twin boundaries at different inclined angles,the material has the highest modulus of elasticity and hardness at 90° inclined angle,while the hardness of CuNi alloy is lowest at 45°inclined angle.The hardness tends to decrease and then increase with the increase of the twin boundary inclined angle.For CuNi alloys with preexisting twin boundaries of different spacing,there is almost no difference in the modulus of elasticity for different twin boundary spacing;with the increase of twin boundary spacing,the hardness tends to increase and then decrease,and reaches the maximum at the twin boundary spacing of 1.25 nm.Based on the simulation results,an empirical model was fitted to predict the hardness of the material at a twin boundary spacing greater than 1.25 nm.(2)During the plastic deformation stage,the overall dislocation length increases with increasing indentation depth,and the dislocation density tends to be a constant with increasing indentation depth.The relationship between dislocation density and hardness is in accordance with the Taylor hardening model,and the trend of Taylor hardening parameters under different working conditions(indentation speed,ambient temperature,crystal orientation)is the same as that of hardness.When the ratio of indentation depth to contact radius is about 0.6,the plastic zone factor f tends to a constant.(3)During nanoindentation of CuNi alloys without preexisting twin boundaries,plastic deformation is mainly dominated by dislocation nucleation and slip,growth of stacked lamellar dislocations,and slip of extended dislocations,either individually or jointly.When indenting along the [110] direction,deformation twins appear in the matrix to promote dislocation slip,making the matrix hardness decrease;when indenting along the [111] direction,deformation twins appear to hinder dislocation slip,making the matrix hardness increase.(4)During the nanoindentation of CuNi alloy with preexisting twin boundaries,the plastic deformation is mainly dominated by the nucleation and slip of dislocations,the migration of twin boundaries,and the mutual reaction between twin boundaries and dislocations.With the increase of the inclined angle of the twin boundary(i.e.,the angle between the indentation direction and the twin boundary),the obstructing effect of the twin boundary on the dislocation is enhanced.When the twin boundary spacing is greater than1.25 nm,the plastic deformation is mainly dominated by the nucleation and slip of dislocations and the interaction between dislocations and twin boundaries;when the twin boundary spacing is less than 1.25 nm,the plastic deformation is mainly dominated by the slip of twin boundaries.