Microstructure characterization and mechanical properties of gradient nanostructured copper generated by ultra-precision machining

Abstract

For a long time, human has been trying to strengthen the materials. Composite material and alloying have largely improved the mechanical properties of materials, especially to reduce the weight-strength ratio which is most meaningful to aviation industry. Another trade-off relation is between strength and ductility which is difficult to be solved by conventional metallurgy means. Impressed by the nature, researchers found that gradient structure is an effective method to strengthen metal while retail the ductility. Moreover, it is believed if the gradient structure covers a range gradually form nanometer grains to coarse grains, additionally with nanotwins. The generation of dense nanotwins and extreme grain refinement via plastic deformation requires very high strain rate. To achieve these, a much concentrated and large stress is essential. Thus, single point diamond turning (SPDT) technique was introduced in this program to produce gradient nanostructure (GNS). Besides, SPDT can also create high quality surface which contains much less cracks and impedes the stress concentration under loading leading to extra improvement of endurance and mechanical properties. However, there are issues remained such as how the microstructure evolutes during the SPDT machining to achieve the extreme grain refinement. Why and how the GNS with nanotwins strengthen the metals while sacrifice only a little ductility. Finally, mechanical tests are essential to quantitively evaluate the improvements on the mechanical properties of the SPDT produced GNS bulk samples. Apart from providing synthesis approach of the metallic GNS bulk materials by SPDT, this thesis also discovers and proposes the mechanisms in which nanotwins facilitate the grain refinement, stabilize the microstructure and increase the strain hardening. In terms of the works from which this thesis is extracted, the thesis can be divided into three parts. In the first part, the GNS copper have been successfully fabricated by SPDT. Based on the transmission electron microscopy (TEM) inspection as well as the further characterization and molecular dynamics simulation, a grain refinement mechanism via firstly twinning and then detwinning for nanograins with tens of nanometers under high shear strain rate is discussed. Besides, there is a critical twin thickness for specific grain size which determines the strengthening-softening transition of role that nanotwin plays. Consequently, the microstructure evolution progress during SPDT machining has been uncovered. In the second part of the thesis, the tribological performances of the GNS samples fabricated by SPDT is reported. To further study the role the nanotwin plays and the contribution from the nanotwins against wearing, two GNS sample with drastically different fractions of nanotwins have been produced and tested. Gradient structure can improve tribological properties for the better accommodation of stress/strain. The SPDT created nanometer level refinement also further strengthen the gradient structure layer. Thus, the key to maintain the elevated tribological properties is to stabilize the GNS structure, in other words, constrain the strain induced grain coarsening. Compared with the GNS sample without many nanotwins, the one with dense nanotwins suffer less grain coarsening verified by its best tribological performance among all sample tested. According to the TEM observation on the worn samples, the nanotwins can resolve lots of strain induced dislocations by twin thickening and detwinning process. Thus, the grain rotation and grain boundary migration, which lead to grain coarsening are effectively compressed. The third part mainly focuses on the strain hardening improvement resulted from nanotwins on GNS materials based on various macro mechanical property tests. By measuring the variation of activation volume against time and dislocation density changing at specific strains, the strengthening mechanisms by the interaction between nanotwins and dislocations are proposed and discussed regarding the TEM observation on the deformed samples as well. Apart from the strain hardening attributed to the nanotwins, the back stress effect due to the heterogeneous intrinsic characteristics of GNS, particularly with twins is also calculated and discussed in this part of the thesis. The originality and significance of this research lies in the provision of a novel means to fabricate GNS metallic materials with dense nanotwins by SPDT machining and more importantly, revealing the microstructural evolution of the material with such microstructure no matter during the fabrication process or deformation progresses when testing. Particularly, the roles that the nanotwins play in the grain refinement, grain boundary stabilization and strain hardening. The study contributes to the body of knowledge by discovering: (1) a grain refinement mechanism for nanograins via twinning and detwinning; (2) inverse Hall-petch relation appearing under critical twin thickness due to the softening mechanism of nanotwins; (3) the ability of nanotwins to stabilize grain boundaries and impede the grain coarsening by twin thickening and detwinning which finally leads to a superior tribological performance of GNS metal armed with dense nanotwins; (4) the ability of the nanotwin boundaries to accumulating much more dislocations providing high extra strain hardening and retained ductility to GNS metal as its coarse grain counterpart.