The plasticity of icosahedral quasicrystals

A quantitative model of plastic deformation of icosahedral quasicrystals is established. It is based on shears occurring in planes of high atomic density via a dislocational activity. A single-slip dislocation friction stress is first derived on a microscopic basis and introduced into a viscoplastic Kocks-type constitutive law where the dislocation density is the single microstructural internal variable. Dislocations are assumed to be stored and annihilated dynamically during deformation, as in simple crystals. The specificity of quasicrystals with long-range but non-periodic atomic order is the introduction of a friction stress limiting the dislocation mobility which decreases with increasing strain, as evidenced for instance by molecular dynamic simulations. Such a constitutive plastic law leads to shear localization and strain softening. The mean-field single-slip behaviour is then introduced in a multiple-slip state, where quasilattice rotation and activation of the various slip systems are calculated. The role of the quasicrystal symmetry relating the available slip systems is examined as a function of the quasilattice orientation. The results and consequences of the model are compared with experimental data on single icosahedral Al-Mn-Pd quasicrystals deformed at high temperatures at a constant strain rate or constant stress in uniaxial compression and discussed.