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Interaction between solute magnesium atoms and vacancies in aluminium
INTERACTION
BETWEEN SOLUTE MAGNESIUM VACANCIES IN ALUMINIUM*
C. PANSERI,t A study by resistivity
F. GATTOt
measurements
and
T.
ATOMS
AND
FEDERIGHIt
at 20°C has been carried out on the effects of small magnesium
concentrations (to a magnesium of 1.08 per cent at. fr. Mg) on the annealing out of vacancies frozen-in by quenching sluminium wires (quenching medium: brine at 2°C). The following results are observed: (1) a concentration as small as about 0.13 per cent at. fr. of Mg atoms is sufficient nearly to suppress the annealing out of vacancies which is observed in pure aluminium at room temperature; (2) in the meantime we have an increase of resistivity of the sample which is sensitive to the quenching speed; (3) this increase is annealed out in isochronal annealing in the range These facts are easily interpreted if we assume, in agreement with a point of view advanced SO-120°C. recently by Perryman in the case of the examination of other properties of Al-Mg alloys, that magnesium atoms in aluminium are able to trap vacancies at room temperature and that only at about 80-120°C vacancies can free themselves from traps. A preliminary study of the isothermal annealing out of vacancies in the presence of magnesium has shown that the phenomenon is complex and not well understood. Finally, some additional experiments on the isochronal recovery of resistivity of cold-worked Al-Mg alloys have shown that in the same range in which it is possible to observe the recovery after quenching, a This result is in agreement with the larger recovery, the greater the magnesium content, is observed. view of generation of vacancies by cold-working.
INTERACTION
ENTRE
LACUNES
ET ATOMES L’ALUMINIUM
DE
MAGNESIUM
DISSOUS
DANS
Des mesures de resistivite ont Bte effect&es it 20°C en vue d’etudier les effets de concentrations faibles de magnesium (jusqu’a 1,68%) sur la restauration des laounes gelees par trempe dans des fils d’aluminium (trempe dans la saumure It 2%). Les resultats suivants ont Bte obtenus; (1) une concentration aussi basse que 0,13% de magnesium est suf%lsante pour supprimer presque completement la restauration des lacunes qui est observee dans l’aluminium pur aux temperatures ordinaires; (2) on observe une augmentation de resistivite dependant de la vitesse de trempe; (3) cet accroissement se restaure par un traitement isochrone entre 80 et 120°C. Ces observations peuvent s’interpreter facilement si l’on suppose, en accord avec le point de vue de Perryman It propos d’autres proprietes des alliages Al-Mg, que les atomes de magnesium sont capables de bloquer des lacunes it la temperature ordinaire et qu’entre 80 et 12O”C, ces lacunes se lib&rent de leurs pieges. Une etude preliminaire de la restauration des lacunes en presence de magnesium a montre que le phenomene est complexe et encore ma1 compris. Enfln des experiences supplementaires sur la restauration de la resistivite d’alliages AI-Mg Bcrouis ont montre que dans certaines conditions la restauration est d’autant plus grande que la teneur en magnesium est forte. Ce resultat est en accord avec l’idee de la formation de lacunes par l’ecrouissage.
WECHSELWIRKUNG
ZWISCHEN
GEL&TEN MAGNESIUM-ATOMEN IN ALUMINIUM
UND
LEERSTELLEN
Die Erholung eingefrorener Leerstellen in Aluminium in Gegenwart kleiner Magnesiumgehalte (bis zu 1,68 at.% Mg) wurde durch Widerstandsmessungen bei 20°C untersucht, mit folgenden Ergebnissen: (1) Bereits 0,13 at. o/0 Mg reichen aus, um das Ausheilen der Leerstellen, welches in reinem Aluminium bei Raumtemperatur auftritt, fast vollstandig zu unterdrticken. (2) An Stelle dessen bekommt man einen Widerstandsanstieg, der von der Abschreck-Geschwindigkeit abhangt. (3) Dieser Anstieg heilt im isochronen Aufheizverfahren zwischen 80”-120°C aus. Diese Tatsachen lessen sich durch die Annahme deuten, die such Perryman fiir andere Eigenschaften von Al-Mg-Legierungen gemacht hat, dass namlich Mg-Atome in Aluminium Leerstellen bei Raumtemperatur binden kiinnen, und dass sich diese erst bei 80” bis 12O’C losreissen k&men. Das isotherme Erholungsverhalten ist in Gegenwart von Magnesium sehr komplex und noch schlecht verstanden. Schliesslioh wurde noch die isochrone Erholung des Widerstandes kalt bearbeiteter Al-Mg-Legierungen untersucht, mit dem Ergebnis, dass im gleichen Bereich wie beim Abschrecken eine starke Erholung zu beobachten ist, welche mit wachsendem Magnesium-Gehalt zunimmt. Das ist mit der Vorstellung einer Leerstellenerseugung bei Verformung im Einklang.
* Received July 17, 1957. t Istituto Sperimentale dei Metalli ACTA
METALLURGICA,
VOL.
6,
Leggeri, MARCH
Divisione 1958
Ricerche, 198
Via Dell8
Post8
8-10,
Milano,
Italy.
PANSERI,
GATT0
AND
FEDERIGHI:
It is currently assumed that vacancies can be introduced in metals in several ways, as quenching from high ~mperat~e, cold-working and irradiation(r) and that eventually they can be trapped by impurity atoms.c1-3) In the present paper, results obtained in a research carried out to confirm the existence of an interaction between vacancies and solute magnesium atoms in aluminum are reported. The existence of such interaction was recently advanced by Perryman,(4y5) who reported that several properties of cold-worked Al-Mg alloys (on which we will come later into discussion) are easily explained by such assumption. In this research, excess of vacancies has been introduced principally by quenching and in some additional experiments by cold-working; the excess of vacancies has been evaluated by accurate resistivity measurements at room temperature. The possibility of quenching-in defects in aluminium was proved recently;t6-‘l~ since these quenched defects appertr to be thermally generated and activated, the interpretation in terms of vacancies (or groups of vacancies) appears to be the most straightforward; in this paper such interpretation is accepted a,s a working hypothesis. Vacancies appear to conserve a great mobility in pure aluminium at room temperature, so to rapidly cluster or precipitate on grain boundaries or on dislocations, when an excess is produced.(6-i2) In preliminary experiments, however, it was found that the presence of some kind of solute atoms can slow down the rate of room-temperature recovery of resistivity in quenohed aluminium, which is interpreted as due to the precipitation or clustering of vacancies. For this purpose, solute magnesium atoms seemed to be very efficient, and this fact might again be ascribed to the existence of an interaction between vacancies and solute magnesium atoms, in agreement with Perryman’s view: due to this interaction, vacancies could be trapped by solute magnesium atoms remaining in excess in respect to the equilibrium conoentration. To support this view, however, it was considered necessary to acquire some more experimental information. 1. SECTIONS
OF THE
RESEARCH
Measurements have been carried out on four Al-Mg alloys, marked A, B, C, D, prepared with aluminium 99.995 per cent and pure magnesium; the composition (in atomic fraction) were: B = pure aluminium, B = 0.13 per cent Mg, G = 0.42 per cent Mg and D = 1.68 per cent Mg; solubility of Mg in Al, at room temperature, was not surpassed.
Mg ATOMS
AND
VACANCIES
IN
Al
199
To understand the various stages of the research, it must be noted that the preliminary experiments had shown that by quench~g from high temperature it is possible to observe: (a) a recovery of resistivity after quenching, at room temperature; (b) a “permanent” i.ncrease of resistivity, that is, an increase of resistivity with respect to the value before quenching (this increase must be considered permanent only at room temperature). The research was divided, therefore, in several sections: (I) Study of room-temperature recovery and of the permanent increase of resistivity after quenching from 550°C; these experiments show clearly the trapping of vacancies by solute ma~e~um atoms; (II) Study of recovery of the permanent increase of resistivity by annealing at increasing temperatures, after quenching from 550°C; these experiments show in what range of temperature (NW-120°C) vacancies can be set free from magnesium atoms and the equilibrium concentration re-establish~; (III) Study of the effect of quenching speed from 550% on permanent increase of resistivity (this study was carried out on alloy C only); (IV) Study of isothermal recovery of the permanent increase of resistivity at temperatures in the range of 75 to lZO”C, to know the rate of precipitation of vacancies (this study was carried out on alloy B only) ; (V) Study of recovery of resistivity after coldworking, by annealing, as in (II) (this study was carried out to make clear the interaction between magnesium and vacancies generated in cold-working). Since in Sections (I)-(IV) of the research samples were slways annealed 1 hr at 570% before the performance of the experiments, it has been assumed that density of dislooation should remain practically unchanged subsequently; the variation of resistivity has been interpreted therefore as a varia,tion of concentration of puntiform defects. This may be considered correct for pure aluminium and for alloy B; in this last case concentration of ma’gnesium can be estimated to be small enough to prevent appreciable clustering, so that the contribution of magnesium atoms to resistivity may be considered constant. For alloys C and D, though precipitation phenomena are to be excluded, the presence of clustering cannot be, a priori, rejected. 2. EXPERIMENTAL
PART
Owing to the very small resistivity variations, these were always computed from resistance variations of wire samples ($ 1.56 mm, length 120 cm), with soldered contacts.
ACTA
200
To
simplify
manual
operations,
METALLURGICA,
samples
wound as lamp spirals and supported
VOL.
1958
The relatively
were
by an insulating
6,
observed
large resistivity
in pure aluminium
variation
appears
which is
to be notably
Quenching operations were carried out by rod. manual extraction of samples from furnace and by
reduced by the presence of about 0.13 per cent at. Mg
rapid
of Mg.
immersion
about 2°C). In Sections
in the
cooling
medium
was carried out in an air furnace;
20°C by a potentiometric
were taken;
were carried
out at
system, using an oil-stirred
to eliminate only,
to
a Wenner-type
use a different
out
current direction resistance-time;
rapidly,
thermal
recovery
(Section
of resistance,
procedure:
parasitic
study
at room temperature
the rapid variation were carried
and practically
suppressed
by a higher concentration
due to a trapping of vacancies operated by magnesium, we must expect by
was used.
Normal precautions quenching
of samples
in Section (IV) an
bath to assure a constant temperature;
e.m.f.
at
If we assume that the absence of variation is really (II) and (V) the annealing
oil-bath was employed. All resistance measurements
potentiometer
(brine
after
(I)) due to
quenching,
inverting
and hence plotting
As a matter of fact, experience has pointed out that
the first alloys, and less for the others (this fact is connected probably with a small variation of Mg content due to its preferential
oxidation).
TABLE 1. Permanent increase of resistivity at room temperature, by quenching from 550°C
the from
the results were
0.103
A
2.8 -10.0 -10.5 -11.0
R 0.42 1.68
3. RECOVERY AT ROOM AFTER QUENCHING
Typical quenching
results from
for
the
TEMPERATURE (SECTION I)
recovery
550°C are reported
at
20°C
in Fig.
1.
after The
differences Ap, expressed for its small value in rnp fi cm, has been
computed
by taking
in
two diagrams of
true results have been computed
the mean value of the two diagrams; satisfactorily reproducible.
of resistivity
such increase exists as reported in Table 1. Such data have been reproducible well enough for aluminium and
measurements each time
increase of resistivity
respect to the value before quenching.
it was necessary to
resistance
a permanent
that is an increase
the values
of
each
sample 1 hr after quenching as reference value.
The reported permanent increases appear to be very interesting from two standpoints: (1) The permanent increase of Mg alloys, which is relatively large, is nearly independent from Mg concentration;
this fact shows that about 0.1 per cent at.
fr. Mg is sufficient to trap nearly all the vacancies,
in
agreement with the very small variation of resistivity observed after quenching (Fig. l);* (2) A small permanent pure aluminium the following
increase appears to exist for
also;
we shall return to this fact paragraph.
in
4. ANNEALING AFTER QUENCHING (SECTION II)
Keeping
in mind the preceding
considerations,
we
may expect that by annealing the quenched samples of Al-Mg alloys at increasing temperature, the permanent increase of resistivity
due to the quenching
action can be reduced.
min FIG. 1. Time variation of resistivity at 20°C after quenching from 550°C for Al and Al-Mg alloys. The reference value in ordinate (zero value) is the value of each sample 1 hr after quenching.
* It is easy to note that the observed permanent increase in presence of Mg (-10-11 rnp R cm) is much higher than the observable recovery reported for pure Al (Fig. 1) at 20°C; this is not a contradiction, because by the adopted technique the first measurement has been carried out only 40 set aftell quenching, and so we have had a noticeable recovery. Our subsequent air-liquid measurements on quenched alumiuium (to be published) have shown that the increase of resistivity by quenching from 55O”C, is about 10-l 1 rnp fi cm, hence of the expected value, in agreement with the given interpretation.
PANSERI,
GATT0
AND
FEDERIGHI:
Mg ATOMS
AND
0
VACANCIES
IN
Al
201
0
Fig. 2. (a): resistivity variation at 20°C of quenched samples, after annealing + br at increasing temperatures. The reference value in ordinate is the value of each sample 1 hr after quenching. (b): as above, for cold-worked samples (70 per cent R.A.). The reference value in ordinate is the value of each sample two months after cold-working.
Typical results are reported in Fig. 2(a). As a matter of fact in presence of magnesium, a sensible recovery appears to exist in a range of temperatures between 80 and 120°C; the magnitude of the recovery is of the same order as the permanent increase observed at room temperature by quenching, in full agreement with the proposed interpretation of trapping of vaca,neies: in pure aluminium, vacancies can rapidly precipitate at room temperature; ‘in the presence of magnesium, vacancies are trapped at room temperature, so we observe no recovery of resistivity; such a recovery is, however, observed at higher temperatures, when vacancies are set free from magnesium atoms, or when solute-vacancy couples have acquired a greater mobility to precipitate on dislocations or on grain- boundaries. The sma,Upermanent increase of resistivity of pure aluminium (Table 1) is partially reduced by annealing in a range from 150-170°C; no recovery is observed in t,his range in presence of magnesium; therefore this phenomenon must be considered peculiar of the recovery of vacancies in pure aluminium and its analysis will be done in an appropriate paper. * It is interesting to note in Fig. 2 that annealing of 4 hr at about 200°C is sufficient for recovery of all
the quenched vacancies. This fact suggests evnluating the magnitude of Ap due to the quenched vacancies, as the difference between the value of resistivity after quenching and the value of the same sample after annealing at a convenient temperature, i.e. Q hr at 240°C (instead of taking Ap as the difference between the value before and after quenching which can be altered by the preferential oxidation of magnesium). Such procedure has been always adopted in the following. 5. EFFECT
OF QUENCHING (SECTION III)
SPEED
We must expect the perma.nent increase of resistivity by quenching in presence of magnesium to be sensitive to quenching speed, due to the variation in the number of trapped va,cancies. Experiments have been carried out on alloy C only, a.nd results are reported in Table 2. * We anticipate, however, that our air-liquid measurements on pure aluminium will con&m that the recovery of A,, due to the quenching is annealed out in two stages, the first of which takes place at about room temperature (see Fig. 1) and the second at about 160°C (see Fig. 2). The last stage, which is not observed in presence of Mg, is probabIy due to the annealing out of clustered vacancies.
ACTA
202
METALLURGICA,
VOL.
6,
1958
min
FIG. 3. Isothermal annealing of alloy B after quenching from 550°C the resistivity
The
effect
of
(pi, pt and pa we, respectively, of the sample after quenching, after time t at the annealing temperature, and after & final annealing of 4 hr at 240°C).
quenching
speed
air-cooling
or furnace-cooling
vacancies;
on the contrary,
is very
giving
evident,
no trapping
brine-quenching
of
appears
in about the same time are nearly equally effective in trapping vacancies. The possibility of
trapping
all vacancies
sufficient concentration
of magnesium
obtained in respect to the quenching in water at 17°C.
method
the
Intermediate
vacancies
to trap probably
all the vacancies,
results
are
as no increase is
obtained
by
less
drastic
for
studying
in aluminium
energy
by
a
suggests a new
of
formation
by quenching
of
from various
quenching. It is interesting to note, however, that quenching in
temperatures, viz. it should be possible to substitute the freezing action of low temperatures (to immobilize
water at 50°C appears to be as effective as quenching in brine, in trapping nearly all vacancies. This fact
vacancies) by the trapping action of magnesium. In this way it should be possible t’o conduct measure-
can be related to the reduced mobility
ments at room temperature.
Al-Mg
alloys due to interacting
of vacancies in
action of magnesium
atoms; it is probable that when the temperature of the sample is reduced to about 100°C during the quenching action,
all vacancies
are trapped
by magnesium
as
shown in the preceding sections. The meaning of this fact is that all quenching media that are able to reduce the temperature
of the sample to about 100°C
TABLE 2. Influence of cooling media from 550°C on the increase of resistivity at 20°C of alloy C Cooling medium Brine at 3°C Water at 17OC Water at 50°C Oil at 28°C Air-jet Still air Furnace
rn/_8Klccm 10.5 10.5 9.0 2.9 0.2 0 0
6. ISOTHERMAL
Results peratures
(SECTION
of isothermal
annealing
of alloys
are reported curves,
ANNEALING
QUENCHING
at several
tem-
B, after quenching from 55O”C,
in Fig.
results have
AFTER IV)
3.
To normalize
been expressed
by
the various (pt - pR)/
(PQ - PR) where PD PQ and pR are respectively the resistivity of the sample at time t, after quenching, and after a final annealing at 240°C. The curves appear to be very complex; they are not exponential, and an attempt to prove that they were curves of some order n has given negative results; they are not therefore like the recovery curves observed in quenched pure aluminium, which we have found to be approximately of order two. Similarly, they are not curves of strain-ageing type.
PANSERI,
GATT0
Besides, it was impossible energy computing,
AND
FEDERIGHI:
Mg
Typical
to deduce an activation
as usual, the time necessary
for a
given variation, because in the usual diagram vs. l/T a straight line is not obtained.*
log t
The conclusion the
equilibrium
presence of magnesium
of vacancies
ANNEALING
other experimental
AFTER
recovery
therefore by
of resistivity
in
Fig. 2(b).
270°C due to recrystallization,
It
is
a recovery
content.
In Fig. 4 the decrement together
in
80 to 13O”C, whose magnitude
increases with magnesium
lization;
COLD-WORKING
Ap evaluated
at 240°C
with the total variation
of Mg
evaluated
if it was possible after
cold-working,
to a
in the same range as observed
after quenching, which t,akes place at temperatures low enough to be distinguished from resistivity due to recrystallization.
the
while the last is scarcely dependent
content, the Ap observed in recovery
V)
to know
annealing
are reported
203
Al
to observe, as distinct from the decrease at
content,
It is currently assumed that vacancies can be introduced in metals by cold-working; it has seemed demonstrate,
results
IN
at 360°C and with the variation due only to recrystal-
(SECTION
interesting
VACANCIES
(by the data of Fig. 2) is reported in function
results are perhaps necessary. 7.
about
in the
is not a simple phenomenon,
and for it& correct interpretation
possible
AND
the range from about
therefore is that the return towards concentration
ATOMS
decrea,se of
linearly dependent
on Mg content.
on Mg
is strongly and Since the range
of temperature of the recovery is in enough agreement with that observed after quenching, the hypothesis of introduction their
of vacancies
trapping
by
by
magnesium
cold-working
atoms
and
appears
very
plausible. The proportionality
of Ap at 240°C to the magnesium
content, and hence the absence of a saturation
effect,
suggest that the number of vacancies generated during a constant deformation to
accept
confirmed
is proportional
this
deduction,
by
experiments
however,
to Mg content ; it
should
performed
at
be
lower
temperatures. 8.
DISCUSSION
The interpretation given to the reported can be summarized as follows: (a) Vacancies
can be introduced
aluminium-magnesium
results
in aluminium
alloys by quenching
and
or cold-
working; (b) At room temperature, pure aluminium
mobility
is very noticeable,
to cluster or precipitate
of vacancies
in
so that they tend
very quickly;
(c) In presence of magnesium
they can be trapped
and remain in excess; (d) The equilibrium by annealing t-0 mg%
at.
Fra. 4. Some results of Fig. 2 reported versus Mg content: (I) and (II) refer to cold-worked samples; (III) is their difference and is the decrement due to recrystallization; (IV) refers to quenched samples. The decrements of (I) and (IV) should be due to vacancies.
have
been carried
out on samples
100°C;
can be reached
at this temperature,
vacancies can be set free from their traps or molecules (magnesium-vacancy)
and can precipitate
on disloca-
tions or grain boundaries. This picture explains concisely,
as we have seen, all
the reported results. In support
Measurements
concentration
at about
reported
other
of the same view, independent
Perryman(4-5)
proofs,
which
may
has be
used after quenching and was carried out about two months after the cold-working.
summarized as follows: (1) The precipitation of Mg in supersaturated Al-Mg alloys at 100°C is much quicker if the sample has been cold-worked (the trapped vacancies, generated by
* However values in the range between 1.5-2.3 eV could be these appear to be very large to be considered extimed; physically significant.
cold-working, increase the speed of diffusion); (2) At room temperature, the presence of magnesium increases the recovery of cold-worked aluminium (due
cold-worked 70 per cent R.A. by drawing; the annealing procedure has been kept similar to that
204
ACTA
METALLURGICA,
to the vacancies which can speed the climb of dislocations); on the contrary, at higher temperatures (~200°C) the presence of magnesium, whioh now is free from vacancies, slows down the creep of aluminium. (3) The density of a cold-worked Al-Mg (2.9 per cent wt. Mg) is increased by annealing, which is in agreement with the decrease in the concentration of vacancies; (4) The recovery of resistivity by annealing for a given deformation is larger, the larger the magnesium contents: we also have had occasion to verify this point (Fig. 4), and our results are in good agreement with Perryman’s. In conelusion, we have a great number of facts which can be easily explained by assuming the existence of an interaction between vacancies and solute atoms; we may hold, therefore, that the hypothesis of the existence of vacancies and of their interaction with solute magnesium appears strongly conned. We have attempted also to look for other interpretations of the reported results, but at present with negative results; for example, one could try to explain the increase of resistivity by quenching, observed in the Al-Mg alloys, by a dispersion of Mg atoms in solution and the subsequent decrease in annealing by strain-ageing {assuming that the contribution of Mg to resistivity is lower if atoms are on dislocation). Although such a small contribution of this kind could exist, similar to that observed in strain-ageing of C and N in Fe,(i3) it is improbable that it might be preponderant in this case, as is shown by the fact that the increase of resistivity at room temperature due to the quenching Al-Mg alloys is of the same order of that observed in pure aluminium at liquid-air temperature. * We call attention to the fact that from the preceding results it is only possible to deduee that, in presence of magnesium, vacancies are trapped in excess at room temperature; the precise way this happens can be only object of conjecture and the formation of magnesium-vacancy couples can be accepted only as a tentative mechanism. However, the nature of the interaction between magnesium and vacancies would probably be principally elastic in origin, due to the larger radius of magnesium atoms. Unfortunately, from the present results it is not possible to deduce the value of the interaction energy, due to the lack of a correct interpretation of the results of Fig. 3. * See footnote on p. 200.
VOL.
6,
1958 9. CONCLUSIONS
If we accept the assumption that the quenching defects observable in pure alum~ium are vacancies (or divacancies), the reported results, together with those reported by Perryman on cold-worked Al-Mg alloys, show the existence of an appreciable interaction between vacancies and solute magnesium atoms in aluminium. This is deduced p~cipa~y from the following facts: (1) No appreciable recovery of resistivityis observed at room temperature aft.er quenching in presence of a sufficient concentration of Mg (this is observed in pure aluminium) ; (2) A relatively large increase of resistivity is observed by quenching, in presence of magnesium; (3) B recovery of the same order of such increase is observed by annealing (after quenching) at about 100°C; (4) In the same range of temperature, within which it is possible to observe the recovery after quenching, a larger recovery after bold-working is observed, the greater the magnesium content. Note add& in proof Just after the present paper was sent for publication, the attention of the Authors was drawn to quite recent results of Westwood and Broom (Acta Met. 5,249 (1957)) on the strain ageing of pure Al-Mg alloys. Their results show clearly that in these alloys a movement, increased by vacancies, of Mg atoms takes place at about room temperature. We can deduce, therefore, that an interpretation of our results in terms of a simple trapping action of vacancies by Mg atoms, forming motionless couples, would probably be incorrect. Instead, we might think that vacancies reach dislocations with Mg atoms at room temperature, but they can only be absorbed (in some complex way) at about lOO”C, at which temperature we find the decrease of resistivity. It might be that this fact could be explained if M.g atoms occupy jogs on dislocations and thereby hamper climbing. REFERENCES 1. T. BROOM Advunc. Phys. 3, 26 (1954). 2. F. SEITZ Acta Cvpt. 3, 355 (1950). 3. TV. M. LOMER and A. H. CO~RELL Phil.Mag. 48, 711 (1955). 4. E. C. W. PERRYHAN J. Metal+ N.Y. 8, 1247 (1956). 5. E. C. W. PERRYXAN Actu Met. 8, 412 (1955). 6. R. MADDIN~~~A.H.COTTRELL Phil.Maa.48.735 (1955). 7. M. LEVY and M. METZGER Phil. Msg. 46, 1021 (i955): 8. M. WINTENBERCER C.R. Acad.Sci., Pari~.242,128(1956). 9. C. PANSERI,F. GATTO and T. FEDERIGRI Acta Mat. 5, 50 (1957). 10.W.~ESORBO&~~~.T~NBULL Bull. Amer.Phgs.Soc.2, 262 (1957). 11. F. J. BRADSHAW and S. PEARSON Phil.Maa. ” 1._ 812 (1957).
12. J. MOLENAAR snd A. W. ARTS N&we, Lond. 166, 690 f19.501. 13. k. H.'COTTRELLand A. T. CHURCHMAN J. Iron St. Inst. 162, 271 (1949).
BETWEEN SOLUTE MAGNESIUM VACANCIES IN ALUMINIUM*
C. PANSERI,t A study by resistivity
F. GATTOt
measurements
and
T.
ATOMS
AND
FEDERIGHIt
at 20°C has been carried out on the effects of small magnesium
concentrations (to a magnesium of 1.08 per cent at. fr. Mg) on the annealing out of vacancies frozen-in by quenching sluminium wires (quenching medium: brine at 2°C). The following results are observed: (1) a concentration as small as about 0.13 per cent at. fr. of Mg atoms is sufficient nearly to suppress the annealing out of vacancies which is observed in pure aluminium at room temperature; (2) in the meantime we have an increase of resistivity of the sample which is sensitive to the quenching speed; (3) this increase is annealed out in isochronal annealing in the range These facts are easily interpreted if we assume, in agreement with a point of view advanced SO-120°C. recently by Perryman in the case of the examination of other properties of Al-Mg alloys, that magnesium atoms in aluminium are able to trap vacancies at room temperature and that only at about 80-120°C vacancies can free themselves from traps. A preliminary study of the isothermal annealing out of vacancies in the presence of magnesium has shown that the phenomenon is complex and not well understood. Finally, some additional experiments on the isochronal recovery of resistivity of cold-worked Al-Mg alloys have shown that in the same range in which it is possible to observe the recovery after quenching, a This result is in agreement with the larger recovery, the greater the magnesium content, is observed. view of generation of vacancies by cold-working.
INTERACTION
ENTRE
LACUNES
ET ATOMES L’ALUMINIUM
DE
MAGNESIUM
DISSOUS
DANS
Des mesures de resistivite ont Bte effect&es it 20°C en vue d’etudier les effets de concentrations faibles de magnesium (jusqu’a 1,68%) sur la restauration des laounes gelees par trempe dans des fils d’aluminium (trempe dans la saumure It 2%). Les resultats suivants ont Bte obtenus; (1) une concentration aussi basse que 0,13% de magnesium est suf%lsante pour supprimer presque completement la restauration des lacunes qui est observee dans l’aluminium pur aux temperatures ordinaires; (2) on observe une augmentation de resistivite dependant de la vitesse de trempe; (3) cet accroissement se restaure par un traitement isochrone entre 80 et 120°C. Ces observations peuvent s’interpreter facilement si l’on suppose, en accord avec le point de vue de Perryman It propos d’autres proprietes des alliages Al-Mg, que les atomes de magnesium sont capables de bloquer des lacunes it la temperature ordinaire et qu’entre 80 et 12O”C, ces lacunes se lib&rent de leurs pieges. Une etude preliminaire de la restauration des lacunes en presence de magnesium a montre que le phenomene est complexe et encore ma1 compris. Enfln des experiences supplementaires sur la restauration de la resistivite d’alliages AI-Mg Bcrouis ont montre que dans certaines conditions la restauration est d’autant plus grande que la teneur en magnesium est forte. Ce resultat est en accord avec l’idee de la formation de lacunes par l’ecrouissage.
WECHSELWIRKUNG
ZWISCHEN
GEL&TEN MAGNESIUM-ATOMEN IN ALUMINIUM
UND
LEERSTELLEN
Die Erholung eingefrorener Leerstellen in Aluminium in Gegenwart kleiner Magnesiumgehalte (bis zu 1,68 at.% Mg) wurde durch Widerstandsmessungen bei 20°C untersucht, mit folgenden Ergebnissen: (1) Bereits 0,13 at. o/0 Mg reichen aus, um das Ausheilen der Leerstellen, welches in reinem Aluminium bei Raumtemperatur auftritt, fast vollstandig zu unterdrticken. (2) An Stelle dessen bekommt man einen Widerstandsanstieg, der von der Abschreck-Geschwindigkeit abhangt. (3) Dieser Anstieg heilt im isochronen Aufheizverfahren zwischen 80”-120°C aus. Diese Tatsachen lessen sich durch die Annahme deuten, die such Perryman fiir andere Eigenschaften von Al-Mg-Legierungen gemacht hat, dass namlich Mg-Atome in Aluminium Leerstellen bei Raumtemperatur binden kiinnen, und dass sich diese erst bei 80” bis 12O’C losreissen k&men. Das isotherme Erholungsverhalten ist in Gegenwart von Magnesium sehr komplex und noch schlecht verstanden. Schliesslioh wurde noch die isochrone Erholung des Widerstandes kalt bearbeiteter Al-Mg-Legierungen untersucht, mit dem Ergebnis, dass im gleichen Bereich wie beim Abschrecken eine starke Erholung zu beobachten ist, welche mit wachsendem Magnesium-Gehalt zunimmt. Das ist mit der Vorstellung einer Leerstellenerseugung bei Verformung im Einklang.
* Received July 17, 1957. t Istituto Sperimentale dei Metalli ACTA
METALLURGICA,
VOL.
6,
Leggeri, MARCH
Divisione 1958
Ricerche, 198
Via Dell8
Post8
8-10,
Milano,
Italy.
PANSERI,
GATT0
AND
FEDERIGHI:
It is currently assumed that vacancies can be introduced in metals in several ways, as quenching from high ~mperat~e, cold-working and irradiation(r) and that eventually they can be trapped by impurity atoms.c1-3) In the present paper, results obtained in a research carried out to confirm the existence of an interaction between vacancies and solute magnesium atoms in aluminum are reported. The existence of such interaction was recently advanced by Perryman,(4y5) who reported that several properties of cold-worked Al-Mg alloys (on which we will come later into discussion) are easily explained by such assumption. In this research, excess of vacancies has been introduced principally by quenching and in some additional experiments by cold-working; the excess of vacancies has been evaluated by accurate resistivity measurements at room temperature. The possibility of quenching-in defects in aluminium was proved recently;t6-‘l~ since these quenched defects appertr to be thermally generated and activated, the interpretation in terms of vacancies (or groups of vacancies) appears to be the most straightforward; in this paper such interpretation is accepted a,s a working hypothesis. Vacancies appear to conserve a great mobility in pure aluminium at room temperature, so to rapidly cluster or precipitate on grain boundaries or on dislocations, when an excess is produced.(6-i2) In preliminary experiments, however, it was found that the presence of some kind of solute atoms can slow down the rate of room-temperature recovery of resistivity in quenohed aluminium, which is interpreted as due to the precipitation or clustering of vacancies. For this purpose, solute magnesium atoms seemed to be very efficient, and this fact might again be ascribed to the existence of an interaction between vacancies and solute magnesium atoms, in agreement with Perryman’s view: due to this interaction, vacancies could be trapped by solute magnesium atoms remaining in excess in respect to the equilibrium conoentration. To support this view, however, it was considered necessary to acquire some more experimental information. 1. SECTIONS
OF THE
RESEARCH
Measurements have been carried out on four Al-Mg alloys, marked A, B, C, D, prepared with aluminium 99.995 per cent and pure magnesium; the composition (in atomic fraction) were: B = pure aluminium, B = 0.13 per cent Mg, G = 0.42 per cent Mg and D = 1.68 per cent Mg; solubility of Mg in Al, at room temperature, was not surpassed.
Mg ATOMS
AND
VACANCIES
IN
Al
199
To understand the various stages of the research, it must be noted that the preliminary experiments had shown that by quench~g from high temperature it is possible to observe: (a) a recovery of resistivity after quenching, at room temperature; (b) a “permanent” i.ncrease of resistivity, that is, an increase of resistivity with respect to the value before quenching (this increase must be considered permanent only at room temperature). The research was divided, therefore, in several sections: (I) Study of room-temperature recovery and of the permanent increase of resistivity after quenching from 550°C; these experiments show clearly the trapping of vacancies by solute ma~e~um atoms; (II) Study of recovery of the permanent increase of resistivity by annealing at increasing temperatures, after quenching from 550°C; these experiments show in what range of temperature (NW-120°C) vacancies can be set free from magnesium atoms and the equilibrium concentration re-establish~; (III) Study of the effect of quenching speed from 550% on permanent increase of resistivity (this study was carried out on alloy C only); (IV) Study of isothermal recovery of the permanent increase of resistivity at temperatures in the range of 75 to lZO”C, to know the rate of precipitation of vacancies (this study was carried out on alloy B only) ; (V) Study of recovery of resistivity after coldworking, by annealing, as in (II) (this study was carried out to make clear the interaction between magnesium and vacancies generated in cold-working). Since in Sections (I)-(IV) of the research samples were slways annealed 1 hr at 570% before the performance of the experiments, it has been assumed that density of dislooation should remain practically unchanged subsequently; the variation of resistivity has been interpreted therefore as a varia,tion of concentration of puntiform defects. This may be considered correct for pure aluminium and for alloy B; in this last case concentration of ma’gnesium can be estimated to be small enough to prevent appreciable clustering, so that the contribution of magnesium atoms to resistivity may be considered constant. For alloys C and D, though precipitation phenomena are to be excluded, the presence of clustering cannot be, a priori, rejected. 2. EXPERIMENTAL
PART
Owing to the very small resistivity variations, these were always computed from resistance variations of wire samples ($ 1.56 mm, length 120 cm), with soldered contacts.
ACTA
200
To
simplify
manual
operations,
METALLURGICA,
samples
wound as lamp spirals and supported
VOL.
1958
The relatively
were
by an insulating
6,
observed
large resistivity
in pure aluminium
variation
appears
which is
to be notably
Quenching operations were carried out by rod. manual extraction of samples from furnace and by
reduced by the presence of about 0.13 per cent at. Mg
rapid
of Mg.
immersion
about 2°C). In Sections
in the
cooling
medium
was carried out in an air furnace;
20°C by a potentiometric
were taken;
were carried
out at
system, using an oil-stirred
to eliminate only,
to
a Wenner-type
use a different
out
current direction resistance-time;
rapidly,
thermal
recovery
(Section
of resistance,
procedure:
parasitic
study
at room temperature
the rapid variation were carried
and practically
suppressed
by a higher concentration
due to a trapping of vacancies operated by magnesium, we must expect by
was used.
Normal precautions quenching
of samples
in Section (IV) an
bath to assure a constant temperature;
e.m.f.
at
If we assume that the absence of variation is really (II) and (V) the annealing
oil-bath was employed. All resistance measurements
potentiometer
(brine
after
(I)) due to
quenching,
inverting
and hence plotting
As a matter of fact, experience has pointed out that
the first alloys, and less for the others (this fact is connected probably with a small variation of Mg content due to its preferential
oxidation).
TABLE 1. Permanent increase of resistivity at room temperature, by quenching from 550°C
the from
the results were
0.103
A
2.8 -10.0 -10.5 -11.0
R 0.42 1.68
3. RECOVERY AT ROOM AFTER QUENCHING
Typical quenching
results from
for
the
TEMPERATURE (SECTION I)
recovery
550°C are reported
at
20°C
in Fig.
1.
after The
differences Ap, expressed for its small value in rnp fi cm, has been
computed
by taking
in
two diagrams of
true results have been computed
the mean value of the two diagrams; satisfactorily reproducible.
of resistivity
such increase exists as reported in Table 1. Such data have been reproducible well enough for aluminium and
measurements each time
increase of resistivity
respect to the value before quenching.
it was necessary to
resistance
a permanent
that is an increase
the values
of
each
sample 1 hr after quenching as reference value.
The reported permanent increases appear to be very interesting from two standpoints: (1) The permanent increase of Mg alloys, which is relatively large, is nearly independent from Mg concentration;
this fact shows that about 0.1 per cent at.
fr. Mg is sufficient to trap nearly all the vacancies,
in
agreement with the very small variation of resistivity observed after quenching (Fig. l);* (2) A small permanent pure aluminium the following
increase appears to exist for
also;
we shall return to this fact paragraph.
in
4. ANNEALING AFTER QUENCHING (SECTION II)
Keeping
in mind the preceding
considerations,
we
may expect that by annealing the quenched samples of Al-Mg alloys at increasing temperature, the permanent increase of resistivity
due to the quenching
action can be reduced.
min FIG. 1. Time variation of resistivity at 20°C after quenching from 550°C for Al and Al-Mg alloys. The reference value in ordinate (zero value) is the value of each sample 1 hr after quenching.
* It is easy to note that the observed permanent increase in presence of Mg (-10-11 rnp R cm) is much higher than the observable recovery reported for pure Al (Fig. 1) at 20°C; this is not a contradiction, because by the adopted technique the first measurement has been carried out only 40 set aftell quenching, and so we have had a noticeable recovery. Our subsequent air-liquid measurements on quenched alumiuium (to be published) have shown that the increase of resistivity by quenching from 55O”C, is about 10-l 1 rnp fi cm, hence of the expected value, in agreement with the given interpretation.
PANSERI,
GATT0
AND
FEDERIGHI:
Mg ATOMS
AND
0
VACANCIES
IN
Al
201
0
Fig. 2. (a): resistivity variation at 20°C of quenched samples, after annealing + br at increasing temperatures. The reference value in ordinate is the value of each sample 1 hr after quenching. (b): as above, for cold-worked samples (70 per cent R.A.). The reference value in ordinate is the value of each sample two months after cold-working.
Typical results are reported in Fig. 2(a). As a matter of fact in presence of magnesium, a sensible recovery appears to exist in a range of temperatures between 80 and 120°C; the magnitude of the recovery is of the same order as the permanent increase observed at room temperature by quenching, in full agreement with the proposed interpretation of trapping of vaca,neies: in pure aluminium, vacancies can rapidly precipitate at room temperature; ‘in the presence of magnesium, vacancies are trapped at room temperature, so we observe no recovery of resistivity; such a recovery is, however, observed at higher temperatures, when vacancies are set free from magnesium atoms, or when solute-vacancy couples have acquired a greater mobility to precipitate on dislocations or on grain- boundaries. The sma,Upermanent increase of resistivity of pure aluminium (Table 1) is partially reduced by annealing in a range from 150-170°C; no recovery is observed in t,his range in presence of magnesium; therefore this phenomenon must be considered peculiar of the recovery of vacancies in pure aluminium and its analysis will be done in an appropriate paper. * It is interesting to note in Fig. 2 that annealing of 4 hr at about 200°C is sufficient for recovery of all
the quenched vacancies. This fact suggests evnluating the magnitude of Ap due to the quenched vacancies, as the difference between the value of resistivity after quenching and the value of the same sample after annealing at a convenient temperature, i.e. Q hr at 240°C (instead of taking Ap as the difference between the value before and after quenching which can be altered by the preferential oxidation of magnesium). Such procedure has been always adopted in the following. 5. EFFECT
OF QUENCHING (SECTION III)
SPEED
We must expect the perma.nent increase of resistivity by quenching in presence of magnesium to be sensitive to quenching speed, due to the variation in the number of trapped va,cancies. Experiments have been carried out on alloy C only, a.nd results are reported in Table 2. * We anticipate, however, that our air-liquid measurements on pure aluminium will con&m that the recovery of A,, due to the quenching is annealed out in two stages, the first of which takes place at about room temperature (see Fig. 1) and the second at about 160°C (see Fig. 2). The last stage, which is not observed in presence of Mg, is probabIy due to the annealing out of clustered vacancies.
ACTA
202
METALLURGICA,
VOL.
6,
1958
min
FIG. 3. Isothermal annealing of alloy B after quenching from 550°C the resistivity
The
effect
of
(pi, pt and pa we, respectively, of the sample after quenching, after time t at the annealing temperature, and after & final annealing of 4 hr at 240°C).
quenching
speed
air-cooling
or furnace-cooling
vacancies;
on the contrary,
is very
giving
evident,
no trapping
brine-quenching
of
appears
in about the same time are nearly equally effective in trapping vacancies. The possibility of
trapping
all vacancies
sufficient concentration
of magnesium
obtained in respect to the quenching in water at 17°C.
method
the
Intermediate
vacancies
to trap probably
all the vacancies,
results
are
as no increase is
obtained
by
less
drastic
for
studying
in aluminium
energy
by
a
suggests a new
of
formation
by quenching
of
from various
quenching. It is interesting to note, however, that quenching in
temperatures, viz. it should be possible to substitute the freezing action of low temperatures (to immobilize
water at 50°C appears to be as effective as quenching in brine, in trapping nearly all vacancies. This fact
vacancies) by the trapping action of magnesium. In this way it should be possible t’o conduct measure-
can be related to the reduced mobility
ments at room temperature.
Al-Mg
alloys due to interacting
of vacancies in
action of magnesium
atoms; it is probable that when the temperature of the sample is reduced to about 100°C during the quenching action,
all vacancies
are trapped
by magnesium
as
shown in the preceding sections. The meaning of this fact is that all quenching media that are able to reduce the temperature
of the sample to about 100°C
TABLE 2. Influence of cooling media from 550°C on the increase of resistivity at 20°C of alloy C Cooling medium Brine at 3°C Water at 17OC Water at 50°C Oil at 28°C Air-jet Still air Furnace
rn/_8Klccm 10.5 10.5 9.0 2.9 0.2 0 0
6. ISOTHERMAL
Results peratures
(SECTION
of isothermal
annealing
of alloys
are reported curves,
ANNEALING
QUENCHING
at several
tem-
B, after quenching from 55O”C,
in Fig.
results have
AFTER IV)
3.
To normalize
been expressed
by
the various (pt - pR)/
(PQ - PR) where PD PQ and pR are respectively the resistivity of the sample at time t, after quenching, and after a final annealing at 240°C. The curves appear to be very complex; they are not exponential, and an attempt to prove that they were curves of some order n has given negative results; they are not therefore like the recovery curves observed in quenched pure aluminium, which we have found to be approximately of order two. Similarly, they are not curves of strain-ageing type.
PANSERI,
GATT0
Besides, it was impossible energy computing,
AND
FEDERIGHI:
Mg
Typical
to deduce an activation
as usual, the time necessary
for a
given variation, because in the usual diagram vs. l/T a straight line is not obtained.*
log t
The conclusion the
equilibrium
presence of magnesium
of vacancies
ANNEALING
other experimental
AFTER
recovery
therefore by
of resistivity
in
Fig. 2(b).
270°C due to recrystallization,
It
is
a recovery
content.
In Fig. 4 the decrement together
in
80 to 13O”C, whose magnitude
increases with magnesium
lization;
COLD-WORKING
Ap evaluated
at 240°C
with the total variation
of Mg
evaluated
if it was possible after
cold-working,
to a
in the same range as observed
after quenching, which t,akes place at temperatures low enough to be distinguished from resistivity due to recrystallization.
the
while the last is scarcely dependent
content, the Ap observed in recovery
V)
to know
annealing
are reported
203
Al
to observe, as distinct from the decrease at
content,
It is currently assumed that vacancies can be introduced in metals by cold-working; it has seemed demonstrate,
results
IN
at 360°C and with the variation due only to recrystal-
(SECTION
interesting
VACANCIES
(by the data of Fig. 2) is reported in function
results are perhaps necessary. 7.
about
in the
is not a simple phenomenon,
and for it& correct interpretation
possible
AND
the range from about
therefore is that the return towards concentration
ATOMS
decrea,se of
linearly dependent
on Mg content.
on Mg
is strongly and Since the range
of temperature of the recovery is in enough agreement with that observed after quenching, the hypothesis of introduction their
of vacancies
trapping
by
by
magnesium
cold-working
atoms
and
appears
very
plausible. The proportionality
of Ap at 240°C to the magnesium
content, and hence the absence of a saturation
effect,
suggest that the number of vacancies generated during a constant deformation to
accept
confirmed
is proportional
this
deduction,
by
experiments
however,
to Mg content ; it
should
performed
at
be
lower
temperatures. 8.
DISCUSSION
The interpretation given to the reported can be summarized as follows: (a) Vacancies
can be introduced
aluminium-magnesium
results
in aluminium
alloys by quenching
and
or cold-
working; (b) At room temperature, pure aluminium
mobility
is very noticeable,
to cluster or precipitate
of vacancies
in
so that they tend
very quickly;
(c) In presence of magnesium
they can be trapped
and remain in excess; (d) The equilibrium by annealing t-0 mg%
at.
Fra. 4. Some results of Fig. 2 reported versus Mg content: (I) and (II) refer to cold-worked samples; (III) is their difference and is the decrement due to recrystallization; (IV) refers to quenched samples. The decrements of (I) and (IV) should be due to vacancies.
have
been carried
out on samples
100°C;
can be reached
at this temperature,
vacancies can be set free from their traps or molecules (magnesium-vacancy)
and can precipitate
on disloca-
tions or grain boundaries. This picture explains concisely,
as we have seen, all
the reported results. In support
Measurements
concentration
at about
reported
other
of the same view, independent
Perryman(4-5)
proofs,
which
may
has be
used after quenching and was carried out about two months after the cold-working.
summarized as follows: (1) The precipitation of Mg in supersaturated Al-Mg alloys at 100°C is much quicker if the sample has been cold-worked (the trapped vacancies, generated by
* However values in the range between 1.5-2.3 eV could be these appear to be very large to be considered extimed; physically significant.
cold-working, increase the speed of diffusion); (2) At room temperature, the presence of magnesium increases the recovery of cold-worked aluminium (due
cold-worked 70 per cent R.A. by drawing; the annealing procedure has been kept similar to that
204
ACTA
METALLURGICA,
to the vacancies which can speed the climb of dislocations); on the contrary, at higher temperatures (~200°C) the presence of magnesium, whioh now is free from vacancies, slows down the creep of aluminium. (3) The density of a cold-worked Al-Mg (2.9 per cent wt. Mg) is increased by annealing, which is in agreement with the decrease in the concentration of vacancies; (4) The recovery of resistivity by annealing for a given deformation is larger, the larger the magnesium contents: we also have had occasion to verify this point (Fig. 4), and our results are in good agreement with Perryman’s. In conelusion, we have a great number of facts which can be easily explained by assuming the existence of an interaction between vacancies and solute atoms; we may hold, therefore, that the hypothesis of the existence of vacancies and of their interaction with solute magnesium appears strongly conned. We have attempted also to look for other interpretations of the reported results, but at present with negative results; for example, one could try to explain the increase of resistivity by quenching, observed in the Al-Mg alloys, by a dispersion of Mg atoms in solution and the subsequent decrease in annealing by strain-ageing {assuming that the contribution of Mg to resistivity is lower if atoms are on dislocation). Although such a small contribution of this kind could exist, similar to that observed in strain-ageing of C and N in Fe,(i3) it is improbable that it might be preponderant in this case, as is shown by the fact that the increase of resistivity at room temperature due to the quenching Al-Mg alloys is of the same order of that observed in pure aluminium at liquid-air temperature. * We call attention to the fact that from the preceding results it is only possible to deduee that, in presence of magnesium, vacancies are trapped in excess at room temperature; the precise way this happens can be only object of conjecture and the formation of magnesium-vacancy couples can be accepted only as a tentative mechanism. However, the nature of the interaction between magnesium and vacancies would probably be principally elastic in origin, due to the larger radius of magnesium atoms. Unfortunately, from the present results it is not possible to deduce the value of the interaction energy, due to the lack of a correct interpretation of the results of Fig. 3. * See footnote on p. 200.
VOL.
6,
1958 9. CONCLUSIONS
If we accept the assumption that the quenching defects observable in pure alum~ium are vacancies (or divacancies), the reported results, together with those reported by Perryman on cold-worked Al-Mg alloys, show the existence of an appreciable interaction between vacancies and solute magnesium atoms in aluminium. This is deduced p~cipa~y from the following facts: (1) No appreciable recovery of resistivityis observed at room temperature aft.er quenching in presence of a sufficient concentration of Mg (this is observed in pure aluminium) ; (2) A relatively large increase of resistivity is observed by quenching, in presence of magnesium; (3) B recovery of the same order of such increase is observed by annealing (after quenching) at about 100°C; (4) In the same range of temperature, within which it is possible to observe the recovery after quenching, a larger recovery after bold-working is observed, the greater the magnesium content. Note add& in proof Just after the present paper was sent for publication, the attention of the Authors was drawn to quite recent results of Westwood and Broom (Acta Met. 5,249 (1957)) on the strain ageing of pure Al-Mg alloys. Their results show clearly that in these alloys a movement, increased by vacancies, of Mg atoms takes place at about room temperature. We can deduce, therefore, that an interpretation of our results in terms of a simple trapping action of vacancies by Mg atoms, forming motionless couples, would probably be incorrect. Instead, we might think that vacancies reach dislocations with Mg atoms at room temperature, but they can only be absorbed (in some complex way) at about lOO”C, at which temperature we find the decrease of resistivity. It might be that this fact could be explained if M.g atoms occupy jogs on dislocations and thereby hamper climbing. REFERENCES 1. T. BROOM Advunc. Phys. 3, 26 (1954). 2. F. SEITZ Acta Cvpt. 3, 355 (1950). 3. TV. M. LOMER and A. H. CO~RELL Phil.Mag. 48, 711 (1955). 4. E. C. W. PERRYHAN J. Metal+ N.Y. 8, 1247 (1956). 5. E. C. W. PERRYXAN Actu Met. 8, 412 (1955). 6. R. MADDIN~~~A.H.COTTRELL Phil.Maa.48.735 (1955). 7. M. LEVY and M. METZGER Phil. Msg. 46, 1021 (i955): 8. M. WINTENBERCER C.R. Acad.Sci., Pari~.242,128(1956). 9. C. PANSERI,F. GATTO and T. FEDERIGRI Acta Mat. 5, 50 (1957). 10.W.~ESORBO&~~~.T~NBULL Bull. Amer.Phgs.Soc.2, 262 (1957). 11. F. J. BRADSHAW and S. PEARSON Phil.Maa. ” 1._ 812 (1957).
12. J. MOLENAAR snd A. W. ARTS N&we, Lond. 166, 690 f19.501. 13. k. H.'COTTRELLand A. T. CHURCHMAN J. Iron St. Inst. 162, 271 (1949).