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Vol. 34, Nos. 14/15
existence of a peritectic reaction. In 1938, the high-temperature phase disappeared from the
phase diagrams, leaving a monotectoid reaction. Observations by Presnyakov et al. [4] and
Goldak and Parr [5] of anomalies in high-temperature lattice parameters led to the reinstate-
ment of a very narrow two-phase region (␣ ϩ ) at 51.48 at% Zn (72 wt% Zn). Controversies
still exist, however, regarding the peritectic transformation, the presence of a high-temper-
ature intermetallic phase, the presence of the two-phase region, and the maximum percentage
of zinc up to which the eutectic halt is extended.
The present work is an attempt to resolve the high-temperature intermetallic phase
question, using high-temperature electron diffraction and high-temperature X-ray techniques
applied to flat-surfaced block specimens.
EXPERIMENTAL
Alloy Preparation. The alloy was prepared from 99.995% Zn and 99.98% Al. Melts were
cast and homogenized at 380°C for 24 h, and then furnace-cooled to room temperature to
obtain stable pearlite structure. Analysis of the final product showed the composition to be
Zn–40.32at%Al (21.8 wt% Al).
Electron Microscopy and Diffraction. Thin foils were prepared by ion beam thinning.
They were examined in the heating holder of a JEOL 1200 electron microscope working at
1
20 kV. It was found that the initial microstructure of the alloy was composed of bands
(pearlite structure) of ␣ phase (aluminum rich) and phase (zinc rich). When the transfor-
mation temperature (277°C) was reached, the banded structure transformed into a one-phase
structure, indicating that the high-temperature phase had been obtained. The working tem-
perature was about 350°C. A typical transmission electron micrograph of the structure, taken
after a stabilization of 2 h, is shown in Figure 1. The observed average grain size of the
transformed structure was 2 m.
Zones of the  phase were selected at random and the electron diffraction patterns
recorded. A frequently observed pattern is shown in the inset of Figure 1. Careful measure-
ments of the angle between reflections, made on this scanned pattern, show that this zone axis
cannot be the [110] zone of aluminum. The measurements were done using the Adobe
PhotoShop application program [6] for measuring angles and distances, with an accuracy of
0
2
.1 mm in distance and 0.2° in angular measurements. The angle between reflections 1 and
is 54.8° and between 3 and 4 is 54.2°, which is not in accord with cubic symmetry.
X-ray Diffraction. High-temperature X-ray experiments were performed using Cu K␣
radiation in a Rigaku Dmax 2200 diffractometer equipped with a high-temperature chamber
and calibrated with a certified standard of silicon at the working temperature (370°C). The
Zn–Al specimen was heated up to 370°C, at a heating rate of 5°C/min. The specimen was
maintained at 370°C for 9 h to assure complete transformation before the diffraction
experiments were begun.
Figure 2 shows the diffraction patterns after a stabilization of 9 h. The observed peaks
almost match those of an aluminum pattern. A detailed examination of the diffraction peaks
revealed the presence of extra reflections. In order to obtain a relative separation of the
diffraction peaks, we explored selected Bragg peaks, using a peak identification program [7].
We observe multiplets at each reflection in Figure 2, where the (111) and (200) reflections
of the “aluminum” pattern (Fig. 2a) split up into four and three peaks, respectively, and at