
2510—VOLUME 29A, OCTOBER 1998 METALLURGICAL AND MATERIALS TRANSACTIONS A
In order to understand the influence of Mg additions to
aluminum, it is first necessary to examine the precise effect
of adding magnesium in solid solution. It is well established
that the presence of Mg atoms in an Al matrix leads to
solid-solution strengthening by reducing the dislocation
mobility. In the ECA pressing of pure Al and Al-Mg alloys,
very large numbers of dislocations are introduced on the
first passage through the die because of the intense plastic
shearing. However, dislocation mobility is reduced in the
Al-Mg alloys and the rate of recovery is, therefore, slower
than in pure Al, so that additional straining and, therefore,
further pressings through the die are required in order to
attain a reasonably homogeneous microstructure. It is rea-
sonable to anticipate also that the higher density of dislo-
cations remaining in the Al-Mg alloys, as compared to pure
Al, will lead to a more complex slip pattern and, therefore,
as observed experimentally, to a refinement of the equilib-
rium microstructure.
Two experimental observations are available from this
study to confirm the easier recovery in pure Al as compared
to the Al-Mg alloys. First, the observations reported earlier
on pure Al after ECA pressing showed that the grain
boundaries were reasonably straight and smooth,
[3,12]
whereas, as illustrated in Figure 5(b), the boundaries are
poorly defined and generally curved after ECA pressing of
the Al-Mg solid-solution alloys. Second, the increase in the
elongation to failure in pure Al at and above four pressings,
as shown in Figure 10, is consistent with the more rapid
attainment in this material of a stable equilibrated micro-
structure.
From these experiments, it is reasonable to conclude that
alloys exhibiting low rates of recovery should be especially
attractive materials for attaining exceptionally small grain
sizes through the use of the ECA pressing technique.
V. SUMMARY AND CONCLUSIONS
1. The ECA pressing was conducted at room temperature
on samples of Al-1 pct Mg and Al-3 pct Mg solid-so-
lution alloys and comparisons were made to pure Al.
The results show that all three materials exhibit the for-
mation of bands of subgrains on a single passage
through the die and the subsequent evolution, with ad-
ditional pressings, into an array of equiaxed grains hav-
ing high-angle grain boundaries.
2. The experiments demonstrate that the addition to an Al
matrix of Mg atoms in solid solution has two important
consequences: (a) there is an increase in the number of
pressings necessary to establish a homogeneous and
equiaxed microstructure, and (b) there is a significant
decrease in the grain size attained ultimately in the stable
equiaxed condition.
3. The effects of Mg additions are attributed to the reduc-
tion in dislocation mobility and to the consequent lower
rates of recovery in the solid solution alloys. It is con-
cluded that materials having low rates of recovery
should be especially attractive for the production of ul-
trafine microstructures through grain refinement using
the procedure of ECA pressing.
ACKNOWLEDGMENTS
We are grateful to Mr. Moritaka Hiroshige (Nishiki
Tekko Co., Kurume, Fukuoka, Japan) for fabricating the
ECA pressing die with a single circular channel. This work
was supported in part by the Light Metals Educational
Foundation of Japan, in part by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Science,
Sports and Culture of Japan, in part by the Japan Society
for the Promotion of Science, and in part by the National
Science Foundation of the United States under Grant Nos.
DMR-9625969 and INT-9602919.
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