Mechanical Properties Of Single Crystal And Twin Copper Under Cyclic Deformation

Nanostructured metals often have unique physical,mechanical and chemical properties.The mechanical properties of metals are affected not only by the internal structure,such as crystal orientation,grain size and twin layer thickness,but also by large deformation treatment.For macroscopic bulk materials,the mechanical properties of metals can be improved by severe plastic deformation.However,severe plastic deformation is not suitable for micro-and nano-scale metallic materials.It has been reported that the mechanical properties of nanometallic materials can be further improved by cyclic deformation treatment.However,there is still no systematic study on the mechanical properties of nanometallic materials with different internal structures under cyclic deformation,and there is still no consensus on the deformation mechanism.In this paper,single crystal,ultrafine crystalline and nanocrystalline twin copper were studied.Ultrafine crystalline and nanocrystalline twin copper were prepared by magnetron sputtering.Cyclic deformation of the samples was carried out by nanoindentation and hardness was mearsured.The microstructure of the samples was characterized by X-ray diffraction,scanning electron microscopy and transmission electron microscope.The mechanical behavior of single crystal,ultrafine crystalline and nanocrystalline twin copper under cyclic deformation was studied,and the mechanism of microplastic deformation was discussed.The main conclusions are as follows:1.For single crystal copper,the hardness of different orientations under monotonic loading conditions is(111)>(110)>(100).Under the condition of cyclic deformation,the hardness remains constant as the number of cycles increased,and the hardness increases only after the step length is less than 5 nm.The critical step size is still 5 nm even when the three orientated single crystal copper is pressed at different depths.When the step size is less than 5 nm,the image force of the free surface can attract the dislocation produced by nucleation and attract it to the free surface,which reduces the dislocation density inside the crystal,and the further plastic deformation mainly depends on the dislocation nucleation.Dislocation nucleation requires higher stress.Therefore,the cyclic strengthening mechanism of single crystal copper is dislocation starvation.This is in good agreement with our theoretical result(when the dislocation is less than 6.4 nm away from the free surface,it is absorbed by the surface).2.For ultrafine crystalline twin copper thin films,the hardness increases with the increase of cycle times when the indentation depth is different under cyclic deformation.This is due to the fact that the grain size is 212 nm and the twin layer thickness is 15.1 nm,which has high interface density and high dislocation storage capacity.During cyclic deformation,dislocations can be accumulated at grain boundaries and twin boundaries,forming forest dislocations,increasing the resistance of dislocation motion,and thus improving the strain-hardening behavior of thin films.Therefore,the strengthening mechanism of ultrafine crystalline twin copper during cyclic deformation is dislocation pile-up.3.The hardness of nanocrystalline twin copper film varies with the cycle times,which is different from that of single crystal and ultrafine copper.The hardness of nanocrystalline twin copper increases first and then decreases with the increase of cycle times at different indentation depth,and there is a maximum value in the hardness of nanocrystalline copper.This is due to the fact that the grain size is 22.4 nm and the twin layer thickness is 2.3 nm.During cyclic deformation,the dislocations accumulated at the grain boundaries and twin boundaries are easily saturated.When the dislocations reach saturation,the incoherent twin boundaries will first be formed in the middle of the twin boundaries.Detwinning occurs and then induces grain boundary slippage,which reduces the storage capacity of dislocations and softens the sample.Therefore,the cyclic hardening mechanism of nanocrystalline copper is the reaction of dislocation and interface,and cyclic softening is the result of detwinning and grain boundary sliding.