||In this research, the effect of residual stresses on the time-dependent deformation of a bulk metallic glass is investigated by the nanoindentation technique. In order to induce residual stresses, a beam sample was elastically bent and constrained in a steel ring. The upper side of the beam experiences the tensile residual stress, the lower side the compressive residual stress, and the central line nearly nil stress. Afterward, nanoindentation creep tests are performed on this stressed sample at room temperature. The creep rate is apparently higher on the tensile side, and remains lower and nearly fixed on the compressive side. The behavior can be explained by the joint influence of the residual stress and indention loading. |
We also attempt to investigate the nanoindentaion time-dependent relaxation tests were performed on the amorphous ZrCu, nanocrystalline Zr and multilayer ZrCu/Zr thin films aiming to explore the different time-dependent behaviors of these materials under the similar load level at room temperature. There appears an interesting crossing phenomenon of the creep rate as a function of applied stress. In comparison with the ZrCu thin films, the Zr film shows higher load/stress sensitivity for the creep response, suggesting the operating of dislocation creep along various slip systems and some minor grain-boundary-sliding creep mechanism. Multilayered ZrCu/Zr thin films also exhibit higher creep response due to the presence of numerous interfaces.
Eventually, an amorphous-ZrCu/crystalline-Zr nanolaminate (500 nm each layer) was initially synthesized using sputter deposition and, then, fabricated into micropillar samples using focus ion beam machining with the amorphous-crystalline (a-c) interfaces inclined to the pillar axis. These pillars were, subsequently, tested in compression in order to study the response of a-c interfaces to the applied shear stress, and further compared with the one that tested with their a-c interfaces normal to the compressive direction. Comparison was made on the experimental, MD and Tsai-Hill data on modulus and yield strength as a function of the angle φ with respect to the loading axis. The extracted interface shear modulus and interface strength for the current ZrCu/Zr is about 25 GPa and 1.0 GPa, respectively. Molecular dynamic simulations are also carried out to reveal the elastic-plastic behavior and, in particular, the deformation mode, of the pillars. The computed results are in excellent agreement with the experimental observations.
Keywords: Metallic glasses, Multilayer, Mechanical properties, Creep, Interface