Mh. Wu et al., TRANSFORMATION CHARACTERISTICS OF ALPHA-1 PLATES IN CU-ZN-AL ALLOYS, Metallurgical and materials transactions. A, Physical metallurgy andmaterials science, 25(12), 1994, pp. 2581-2599
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.