TRANSFORMATION CHARACTERISTICS OF ALPHA-1 PLATES IN CU-ZN-AL ALLOYS

The formation of a, plates during isothermal aging of a Cu-26.7 wt pct Zn-4.0 wt pct Al alloy at 150 degrees C to 350 degrees C follows ther mally activated incubation kinetics. Early stage alpha(1) plates posse ss an ordered 18R or 9R long-period stacking order (LPSO) crystal stru cture, with antiphase domain boundaries running continuously across th e interface. The plates also exhibit invariant plane strain (IFS) crys tallography consistent with calculations of the phenomenological theor y of martensite crystallography (PTMC). The ordered LPSO structure and IPS crystallography are gradually annealed out only after extended ag ing as the structure changes to the equilibrium disordered face-center ed cubic (fee) one. High-resolution transmission electron microscopy ( TEM) analyses reveal that early stage alpha(1) plates have straight co herent interfaces, Prolonged aging induces misfit dislocations at the interface and causes the interface to protrude into the parent phase. Although microanalytical analyses indicate that a composition differen ce exists between the alpha(1) plates and the parent matrix, solute de pletion was observed at neighboring defects. These observations suppor t the proposed transformation mechanism that the alpha(1) plates nucle ate at solute depleted defects through a shear mechanism and that subs equent plate growth is then controlled by a diffusional process.