Study On The Formation And Strengthening Mechanisms Of Nanotwins Based On Nanoindentation Technique

Nanotwin structures can improve the mechanical properties of hardness, strength, ductility and fracture toughness of the material. Its manufacturing method and technology has become the multidisciplinary focus of advanced manufacturing, physics, materials science and mechanics. Nevertheless, the experimental and theoretical research of nanotwins is still in its infancy at present and most studies mainly focus on Cu and some non-metallic materials. At the same time, few present studies are about the preparation of nanotwinned surface of engineering alloy. Therefore, it's a very meaningful work to conduct the theoretical research of nanotwins and apply the manufacturing method and technology of nantwins into engineering alloy surface. In this article, on the basis of exploring the formation mechanism of nanotwins in cadmium zinc tellurium (CZT), the experimental and theoretical studies are extended to the low fault energy engineering nickel-based alloy C-2000.Deformation twinning evolution from a single crystal is conducted by molecular dynamics (MD) simulation of nanoindentation. Critical twinning stress of cadmium zinc telluride (CZT) calculated is 1.38 GPa. All the twin boundaries are along the [111] orientation, except the one with the [111] orientation, that supports the indentation, interpreting the unidirectional and continuous characteristics, confirmed in the experiment. Three twin thicknesses after unloading are 3.2,3.5, and 16 nm, which is consistent with the experimentally repeated pattern of a lamellar twin with thickness larger than 12.7 nm, followed by one or several twins with thicknesses smaller than 12.7 nm. In addition, the reasons why the hardness of nanotwinned CZT is higher than the single crystal and the nanotwinned CZT can maintain good plasticity in the the experiments are analyzed.Through the MD simulation of nanoindentation, the formation and strengthening mechanisms of nanotwins in nickel-based alloy C-2000 are studied. Deformation twinning evolution from a single crystal is conducted and the nucleation and extension mechanisms of nanotwins are deeply analyzed. Critical twinning stress of C-2000 calculated is 4.48 GPa. Twinning, detwinning, recovering and regenerating of the twin boundaries are closely related to the shear stress of the material. By comparing the models of single crystal alloy and nanotwinned alloy in nanoindentation simulation, the mechanical properties of hardness and strain-hardening of nanotwinned alloy are superior to the corresponding single crystal alloy. The reason why the nanowinned structures show strengthening characteristics is that partial dislocations are hampered by twin boundaries (TB) and accumulate around the TB. On the other hand, By comparing the models of nanotwinned alloy with twin thicknesses of 18 nm, 12 nm and 1.2 nm in the process of nanoindentation simulation, a conclusion is obtained that the density of twin boundaries has an important influence on the performance of nanotwinned materials.According to the formation mechanism of nanotwins, the nanoindentation experiments of nickel-based alloy C-2000 are conducted. At the peak load of 15 mN,100 mN,1.0 N and 5.0 N, the sample after chemical mechanical polishing (CMP) is selected to perform nanoindentation experiments. Through the selected area electron diffraction (SAED) and transmission electron microscopy (TEM) images of the indentation edge, the nanotwin structures are confirmed in the process of nanoindentation and the simulation results are verified. The unique structures can improve the mechanical properties of hardness and resistance to deformation of the alloy surface.