HRTEM study of small hexagonal precipitates produced during the dezincification of -CuZn.

H. TROIANI; A. TOLLEY; M. AHLERS

PHILOSOPHICAL MAGAZINE A

Taylor and Francis

Año: 2000 vol. 80 p. 1379 - 1391

0141-8610

The evaporation of Zn from β-phase Cu-Zn alloys produces a phase transformation in the material. For low evaporation temperatures (450°C) and short dezincification times (around 14 min) the phase transformation is observed solely close to the sample surface. Two phases are formed during this process: the fcc phase, expected at lower Zn concentration, and a metastable phase with an approximately hexagonal structure, which is not expected in principle. Previous transmission electron microscopy studies have reported that this hexagonal structure has a 4H stacking sequence and that the precipitates contain a high density of stacking faults. In this paper the formation of hexagonal precipitates is studied with high-resolution transmission electron microscopy. It is shown that the precipitates do not have a homogeneous structure. Different regions present different stacking sequences, which on the average result in a diffraction pattern corresponding to a 4H structure. The interface with the matrix appears as a relatively extended region. The results indicate that the β phase transforms to a hexagonal structure by a simple shuffle mechanism, a result not previously observed. All results can be understood as a consequence of the collapse of the high vibrational entropy due to the large amplitude soft mode {110}<110> vibrations of the β phase.