The rare earth/transition element intermetallics R2T17 are essentially topologically close packed phases for which layer structure models have already been presented. Many of these compounds are known to undergo allotropic transformation of the type at elevated temperatures. It is not unexpected that shear transformation mechanisms are involved in view of the layering character of the structures. The transformations are evidently quite sluggish, illustrated in furnace cooled Dy2Co17 by the fact that only rarely has the low temperature rhombohedral form been seen. The more usual structures observed so far in furnace cooled alloys include 4H and 6H in Dy2Co17 (Figs. 1 and 2) . In any event it is quite clear that the general microstructure is very complicated as a consequence of the allotropy, illustrated in Fig. 3. Numerous planar defects in the layer plane orientation are evident as are non-layer plane defects inherited from a high temperature structure.
