Phason defects in Al-Pd-Mn approximant phases

Structural defects, observed by TEM, in Al-Pd-Mn approximant phases are identified as phason defects. Assuming their motion under an applied shear strain, it is shown that two approximant phases can be related between them by a phase transformation occurring through plastic deformations. Then, other approximant phases, experimentally observed, can be interpreted as intermediate states of such a phase transformation. Atomic motions corresponding to these phason shifts are partly examined on the basis of HREM observations and crystallographic data, obtained from a single-crystal X-ray study on one of these approximants. The description of this transformation in six-dimensional hyperspace is proposed in order to establish a relationship with the icosahedral quasicrystalline state. The possibility that a plastic deformation of the icosahedral phase, occurring without the strain-hardening effect, could only result in a motion of phason defects is briefly discussed.