INTERFACE BETWEEN SIMPLE CRYSTAL AND ICOSAHEDRAL-SYMMETRY RELATED CRYSTAL

The phase diagram of transition metal intermetallics exhibits a system atic sequence of phases involving simple crystals and complex crystal structures with icosahedral motifs (beta-W, sigma, chi phases). Such i ntermetallic alloys provide then convenient examples to establish basi s for the analysis of simple crystal/ icosahedral phase interfaces. Th e sigma and chi intermetallic phases in a Fe-Cr-Mo steel have been stu died by Transmission Electron Microscopy: the interface as well as the orientation relationships has been identified. A simple geometrical a nalysis is proposed to account for the ferrite/sigma-phase interface, sigma/chi orientation relationship and chi-phase antiphase boundaries. This approach emphasizes on the necessity to consider corrugated comp act layers instead of lattice planes in icosahedral phases and the con servation of the site density at the interface plane.