Plastic deformation and hardness in Mg–Zn–(Y,Ho) icosahedral quasicrystals

High-quality icosahedral poly-quasicrystalline samples have been produced for Mg30Zn60Y10 and Mg30Zn60Ho10 alloys by melting and annealing the constituent elements in a sealed tantalum ampoule. Compression tests with a constant crosshead speed and stress-relaxation tests at about 10% compressive strain have been performed at high temperatures. The Vickers hardness has also been measured at high temperatures for undeformed and compressed samples. The thermal activation analysis of plastic deformation has been successfully performed by estimating the dragging stresses at various temperatures from the stress dependence of the activation volume; the total activation enthalpy is about 2.5 eV in both alloys. The higher the plastic strain, the lower is the Vickers hardness at high temperature. This work softening phenomenon is interpreted to be caused by the effects of the deformation-induced phason strain on the dislocation motion.