APPLICATION OF THE THEORY OF MARTENSITE CRYSTALLOGRAPHY TO DISPLACIVEPHASE-TRANSFORMATIONS IN SUBSTITUTIONAL NONFERROUS ALLOYS

It has been demonstrated that the theory of martensite crystallography is capable of accounting successfully for the form and crystallograph y of a range of plate- or lath-shaped transformation products, even wh en the formation of the product phase involves significant substitutio nal diffusion. These transformations include the precipitation of meta stable hexagonal gamma' (Ag2Al) plates in disordered face-centered cub ic (fcc) solid-solution Al-Ag alloys, the formation of ordered AuCu II plates from disordered fcc solid solution in equiatomic Au-Cu alloys, and the formation of metastable 9R alpha1 plates in ordered (B2) Cu-Z n and Ag-Cd alloys. The application of the theory to these transformat ions is reviewed critically and the features common to them identified . It is confirmed that, in all three transformations, the product phas e produces relief at a free surface consistent with an invariant plane -strain shape change and that the transformations are thus properly de scribed as displacive. The agreement between experimental observations and theoretical predictions of the transformation crystallography is in all cases excellent. It is proposed that successful application of the theory implies a growth mechanism in which the coherent or semicoh erent, planar interface between parent and product phases maintains it s structural identity during migration and that growth proceeds atom b y atom in a manner consistent with the maintenance of a correspondence of lattice sites. In the case of the coherent, planar interfaces asso ciated with gamma' precipitate plates in Al-Ag alloys, there is direct experimental evidence that this is accomplished by the motion of tran sformation dislocations across the coherent broad faces of the precipi tate plates; the transformation dislocations define steps that are two atom layers in height normal to the habit plane and have a Burgers ve ctor at least approximately equivalent to an (a/6)[112] Shockley parti al dislocation in the parent fcc structure. However, for AuCu II plate s, where the product phase is twinned on a fine scale, and for alpha1 plates, for which the lattice invariant strain leads to a substructure of finely spaced stacking faults, the structures of the semicoherent interphase boundaries and thus the details of the transformation mecha nism remain less clearly defined.