Nanoindentation studies on single crystals of Zn–Mg–Er and Zn–Mg–Ho icosahedral phases

Single crystals of Zn–Mg–Er and Zn–Mg–Ho icosahedral phases were grown from the melt using the liquid encapsulated top seeded solution growth technique. The fivefold planes of these as-grown single crystals were polished and used for the present nanoindentation studies using Hysitron triboscope attached to an atomic force microscope with a maximum load up to 16 mN. The reduced Young's modulus (Er) and nanohardness (H) were found to be 135 ± 10 GPa and 8.5 ± 0.2 GPa for the F-type Zn65Mg25Er10 quasicrystal whereas for the P-type Zn74Mg15Ho11 quasicrystal (QC) these were 135 ± 10 GPa and 7.8 ± 0.2 GPa, respectively. Using the appropriate constants, the average E in both the quasicrystalline alloys was estimated as 140 ± 10 GPa. The elastic and plastic deformation characteristics in load–displacement, F–h, curve appears to be identical in both the QC phases. The peculiarity in the load–displacement curves, in terms of pop-ins were observed frequently. AFM images showed the steps in the pile-up material which can be identified as shear bands and correlated with the pop-ins. The first pop-in was observed in the load range of 70 μN in case Zn–Mg–Ho and 84 μN in case of Zn–Mg–Er QC. The contact pressure, p, turns out to be 13 GPa for Zn–Mg–Ho and 18 GPa for Zn–Mg–Er QC. The maximum shear stress under the indent, τTresca is found to vary from 6 to 8 GPa, which is close to the theoretical shear strength of this material (∼E/20). The energy required for the indentation was determined from the first pop-ins and it turned out to be 0.88 eV/atom, close to the thermal energy required for deformation of quasicrystals. Thus the ‘pop-ins’ are attributed to the elastic-plastic transition during indentation.