“…[1] In the Cu(Ge) system, the hcp product phase is called f-phase. The precipitation of the f-phase from supersaturated Cu(Ge) solid solutions complies with the same crystallographic orientation relationship as holds for the fcc fi hcp phase transformations observed in Co, Ag-Al, etc., i.e.,ð11 1Þa==ð0001Þf; ½ 110 a==½11 20 f. [1,2,[4][5][6] In contrast with the allotropic fcc fi hcp phase transformation occurring in Co, [7] the transformation in the Cu-Ge system is associated with compositional changes [1,8] (cf. Figure 1) and therefore the transformation mechanism is an intermediate class of a diffusionless, displacive, martensitic fcc fi hcp phase transformation and a diffusional phase transformation: Ge must diffuse to the f-phase nucleation sites and/or the growing f-phase plates to establish the f-phase composition.…”