| In recent years， the mechanical properties and materials deformationmechanisms under micro-nanoscale have been urgently attention to study andunderstand. But the macro-theories, simulation and experimental study methods ofmaterials deformation which based upon classic continuum medium mechanismscan not be applied to the research of materials deformation under micro-nanoscaledirectly. The nanoindentation test is an excellent method for measuring the localstrength of a material on the micro/nanometer scale, which has been widely used inevery field of material science. Specifically, nanoindentation deformation behaviorcan be used for interpreting the initial yield characteristics of materials and themechanisms for the emission of the first dislocation. However, at present, the studyof nanoindentation deformation behavior is still very lack, and most of the currentnanoindentation studies are focused on materials that do not containdefects.Compared to single crystal metal materials, the plastic deformation ofpolycrystalline metal materials is more complicated. The grain boundarys(GBs) areconsidered to be the dominant and most effective sources of dislocations at theinitial stage of plastic deformation. Therefore, dislocation and grain boundaryrelationship of polycrystalline metal materials has became the material science,mechanics and physics research object.In this paper, based on the more novel multiscale simulation method–quasicontinuum(QC) method, combined with nanoindentation experiment toresearch plastic deformation behavior under micro/nano scale. The nanoindentationdeformation behavior and plastic deformation mechanism of crystal material withdifferent crystal structure (body centered cubic crystal and centered cubic crystal)was analysed. The dislocation emission mechanism of initial plastic occurs andgrain boundary、indenter position、indenter size and load on the influence of theinitial plastic deformation was explored.A series of indents were indented on the bicrystal copper after ion etching fornanoindentation experiment. The distance between indenter and GB was changedwith different preinstall load. The surface morphology before and after theindentation were observed. Through the load-displacement curve, contact depthand contact area of indentation point were calculated, so as to analyse GB effect onnanoindention deformation behavior of the bicrystal copper. The experimentalresults show that the load-displacement curves of the experiment on grainboundary and grain internal have discontinuous phenomenon, which marks thebeginning of the initial plastic deformation. The nanohardness of the GB is higherthan other indents inside the grains. With the increasing of the distance between the indenter and GB, the nanohardness descend sharply when the indenter near the GB1μm. And then it increase slightly and decrease lastly as the indenter far away fromthe GB. The load dependence was revealed on the GB: with the increased load, theheight of the contact depth increase continually, the nanohardness increase. Whenload reached200μN (critical application load), the nanohardness is maximum, andwhen the load is more than critical application load, load is bigger, nanohardnessdrop instead.The key technology and important parameters were studied and setted, makingit can be applied to nanoindentation in the numerical simulation. And using thismethod can build several typical grain boundary model of bcc and fcc crystalprecisely. For fcc structure of the material, the nanoindentation deformationbehavior of twin aluminum、copper∑5(310) θ=53.1°symmetric tilt grain boundaryand single crystal aluminum with void were simulated, get the following conclusion:The elastic-to-plastic transition during nanoindentation of fcc materials is caused byfirst dislocation emission. For Al twin crystal, the Shockley partial dislocations arenucleated and slide occurs along112direction. The initial plastic critical load oftwin boundary and both two sides grain internal change regularly with the indenterposition. Maximum critical load appeared in the vicinity of GB. The initial plasticcritical load is remained stable when the indenter is farther from the grain boundary.The initial dislocations of bicrystal copper with∑5(310) θ=53.1°are perfectdislocation along the [110] direction. The maximum initial plastic critical loadappears in the middle of the symmetrical tilt grain boundary. The critical load anddepth of indentation exhibit a tendency to increase and the nanohardness exhibit atendency to decrease with enhancive indenter size.Comparing the simulation resultsand experimental results, the change rule of nanohardness are consistent. Voidseffect the initial plastic deformation of single crystal aluminum, when the voidsdistance indenter is close, the initial plastic critical load reduce, when voids distancemodel surface is farther, the initial plastic critical load almost no influence. Voidsize is much larger, the initial plastic critical load of single crystal aluminum islower.The nanoindentation initial plastic deformation behavior of bcc bicrystalmolybdenum was studied. A symmetric tilt structure∑5(310)[001] θ=36.9°ofbicrystal molybdenum was structured using QC method. A series of multiscalesimulations and nanoindentation experiment have been performed to comparativestudy the nanoindention deformation behavior of bicrystal Mo. The experimentalresults and simulation results were qualitative contrasted, both results are consistent.The elastic-to-plastic transition during nanoindentation of bicrystal Mo is caused byfirst dislocation emission. The partial dislocations are nucleated and slide along [110] direction. The critical load and depth of indentation exhibit a tendency toincrease and the nanohardness exhibit a tendency to decrease with enhanciveindenter size of bicrystal Mo. The maximum initial plastic critical load appears inthe middle of the symmetrical tilt grain boundary, the load may dropped first androse later with the indenter away from the GB. |
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