The transition from elastic to plastic behaviour in an Al-Cu-Fe quasicrystal studied by cyclic nanoindentation

Abstract

Abstract Comparative mechanical tests of an Al-Cu-Fe quasicrystal and a tungsten (001) single crystal have been carried out using cyclic nanoindentation by a Berkovich indenter. The first loading-unloading cycle up to 30 mN formed the initial indent. On reloading of the indenter up to 50 mN, a transition from elastic to plastic deformation of the material was observed. To reveal the region of the transition better, the reloading curve was differentiated with respect to time. It was found that, for tungsten, the transition from elastic to plastic behaviour was smooth. The transition began at 25 mN and terminated at 34mN only. During the transition the strain rate in the indent trebled. In the Al-Cu-Fe quasicrystal, the transition from elastic to plastic deformation was very abrupt. No evidence of plastic deformation in the indent was observed up to 32 mN. Only a further increase in the load by 0.35mN caused the onset of plastic deformation. A pop-in was observed in the reloading curve at this point. The displacement increased by about 10 nm and the pressure in the indent dropped by 360 MPa. This was probably due to the destruction of the quasicrystalline structure and the formation of the crystalline structure. A plastic crystalline phase appeared to be pressed between the indenter and the hard quasicrystal substrate and then to be extruded from the indent. Because of this, the pressure in the quasicrystal drops during the transition from elastic to plastic deformation.