Research On Plastic Deformation And Elastic Properties Of Nanometals

Nanomaterials exhibit novel plastic and elastic behaviors comparing with their bulk counterparts due to their small grain size and high density of grain boundaries.Previous studies indicated that with a decreasing grain size,grain boundary mediated processes such as grain boundary sliding,grain rotation and grain boundary diffusion,substitute for conventional dislocation nucleation and motion as the dominant deformation mechanisms in nano metals.It is believed that this plastic deformation mechanism change leads to the softening of nanometals at a critical grain size of 10-15 nm,i.e.inverse Hall-Petch effect.In addition,the compressibilities of some nanomaterials show increases with decreasing grain sizes,however,some contraries were observed in other nanomaterials.In this dissertation,I reported my study on the grain rotation and dislocation activity in nickel of various particle sizes,plastic and elastic strength of nano nickel and nano gold by using in-situ high pressure X-ray Laue Micro-diffraction method and high pressure radial X-ray diffraction techniques.In the high pressure X-ray Laue Micro-diffraction experiments,we successfully observed the difference of grain rotation of the same tungsten carbide(WC)marker crystal embedded in nano nickel media of different sizes.It is found that WC markers embedded in 70 nm nickel nanocrystals rotate the most at the same stress level,indicating that 70 nm nickel has the largest grain rotation.This reversal in the size dependence of grain rotation is different from traditional belief that samller nano grains rotate more under stress based on the grain boundary dislocation model.By performing high pressure radial DAC X-ray diffraction experiments,we found that the deformation texture of nano nickel decrease with a decreasing grain size,indicating that grain interior dislocation activities become weak in finer nano grains but are still active in untrafine naocrystals.Combining with previous studies about the enhanced grain boundary dislocations in finer nanocrystals,we infer that the competition of grain interior and grain boundary dislocations results in the maximal grain rotation of a certain sized nanocrystals.The reversal in the size dependence of grain rotation shows the mechanism crossover from grain interior dislocation-mediated to grain boundary dislocation-mediated deformation mechanisms in nanomaterials.We studied the yield strength of nano nickel and nano gold of different sizes by using radial DAC X-ray diffraction techniques.Our results show that instead of softening,the strength of nano nickel and nano gold are significantly enhanced with decreasing grain sizes down to 3 nm.It indicates that Hall-Petch effect can be extended down to 3 nm.Moreover,the strength of 3 nm nickel is increased more than three times comparing with its bulk counterparts.Our analysis on the dislocation activity of nano nickel and TEM images of quenched samples from high pressure show that two possible reasons,including the fewer sliding in smaller-sized nanocrystals and the enhanced interface networks consisting of grain boundaries,disorders and dislocations in the nano-metals,may account for the strength enhancement in smaller-sized nanometals.The compressibility of nano nickel of different sizes was studied by using the lattice strain theory and fitting the three-order Birch-Murnaghan equation.The bulk modulus of nano nickel increases up to a maximum value at 20 nm and then decrease with decreasing grain size,which can be well explained by the dislocation pile-up effect.With a grain size decreases from 200 nm to 20 nm,the proportion of grain boundary is increased,resulting in an enhanced dislocation pile-up at grain boundaries and thus an increased bulk modulus.When the grain size is below to 20 nm,the dislocation activity becomes very weak,leading to the weakening of dislocation pile-up effect and a lower bulk modulus,although the proportion of grain boundary is continuously increased.My work revealed the plastic deformation mechanisms,strengthening mechanisms and the influence of dislocation activity on the elastic properties of nano meatals,providing a guidance for producing strong metals and optimizing properties of nanomaterials for industrial applications.