Molecular Dynamics Study Of The Dislocation Evolution During Nanoindentation On Single Crystal Copper

With the rapid advancements of the precision and ultra-precision processing of metal materials,the discreteness of materials’ mechanical behaviors attracts attention.Molecular Dynamics(MD),which finds a wide application in micro-and nano-mechanics in recent years,could successfully cast light on this characteristic during metal materials processing.Based on MD theory,here simulations of nanoindentaion on face-centered cubic(FCC)copper are carried out for the analysis of plastic deformation and dislocation behaviors,the results obtained are as follows:(1)Nanoindentation on single-crystal copper(100)via large-scale MD simulations is presented to provide clarification on load drop and the correlated hardness drop during plastic deformation of a workpiece placed beneath an indenter.Via an in-house code,we calculate the contact area between the indenter and workpiece,the general hardness and spatiotemporal distribution of the dislocations around the indenter during the process of nanoindentation.The length of total dislocations includes a substantial proportion of the one of Shockley partials.The hardness-drop event,which marks the abrupt and massive growth of the Shockley partials occurs prior to the load drop,and is examined from a comparison of the variation of the load and hardness with the dislocation distribution.The process of dislocation nucleation and growth of the Shockley partials occurs repeatedly,which results in a sawtooth pattern of the load-depth and hardness-depth curves over the entire indentation process.(2)MD simulation of the nanoindentation on single crystal copper(111)with coherent twin boundaries(CTBs),whose spacings are equal to 17.5 nm,has been presented.At the initial stage of nanoindentation,indenter moves downward and prismatic dislocation loops(PDLs)form.Thompson’s tetrahedron notation has been adopted to scrutinise the process of PDL’s formation without cross-slip and release from the indented place.Dislocations at PDLs’ side edge are stair-rod partial dislocations,including two Lomer-Cottrell locks and two Hirth Locks.(3)The interaction between PDL and CTB is also analyzed,which drives the migration of CTB.As a result,a parallelogram step which is bounded by Frank partial dislocation has been left,implying the formation of twinning dislocations.When the indenter moves further,the interaction repeats and twinning dislocations multiply.Our simulation results shed light on the interaction between PDL and CTB and would provide a new insight into understanding the role of CTB during mechanical deformation.(4)A novel mechanism without involvement of vacancy or Frank loop is put forward to cast light on the formation of stacking fault tetrahedron(SFT)during plastic deformation of bulk single-crystal Cu with pre-existing parallel CTBs,whose spacings are equal to 5 nm,via large scale MD simulation.The fresh mechanism is totally different with Silcox-Hirsch mechanism and its sequelae,which confirms that to produce an SFT during plastic deformation,formation of Frank loop is not essential in metals with low stacking fault energy(SFE).