ON THE ROLE OF Mg ATOMS ON THE RECOVERY OF COLD-WORKED Al-5.5 AT, % Mg

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ON THE ROLE OF Mg ATOMS ON THE RECOVERY

OF COLD-WORKED Al-5.5 AT, % Mg

A. Ali, Z. Farid, R. Kamel, G. Said

To cite this version:

JOURNAL D E PHYSIQUE

CoZZoque C5, szlppZ6ment au nO1O, Tome 42, octobre 1981 page C5-313

ON THE ROLE OF Mg ATOMS ON THE RECOVERY OF COLD-WORKED A I - 5 . 5 A T , % M g

A.R. ALi, Z . F a r i d , R. Kamel and G. s a i d W

Physics Department, FaeuZty of Science, Cairo University, Giza, Egypt. * ~ h ~ s i c s Department, FacuZty of Education, Cairo University, Fagown, Egypt.

Abstract. Internal friction and thermo-mechanical analysis pointed towards the existence of.four annealing stages in

the recovery of cold-worked A1-5.5 a*.% Mg alloy in the temp-

erature range from 25 to 300°C. The first annealing stage,

activated by an energy of

0.63

annealing of vacancy-Mg couples formed during cold-working

.

By increasing the annealing temperature these vacancy-Mg couples started to form complex aggregates that led to the appearance o f an absorption band in the annealing spectrum,

The mechanisms responsible for these two recovery stages

were enhanced by pre-annealing. Recrystallization started

0

at temperatures about 110 C showing itself as another anneali-

ng band activated by an energy o f 1.42 eV. Mg solute atoms acting as impurity-pinning points,reta*ied the dislocation mobility and correspondingly the recrystallization process.

The binding energy between the Mg-solute atoms and disloc-

ations was found to be 0.22 eV. The annealing stage appearing

at higher temperature was charaterized by a large activation energy (1.62 eV) and was attributed t o the tendency of Mg atoms to form clusters in the Al-matrix.

I-Introduction.- It has been previously reported that the annealing spectra of pure A1,cold-worked at room temperature,compri.sed two recovery processes centered at about

60°c

1,

relatively free dislocations by a polygonization mechanism

activated by 0.92 eV, The second recovery process, attributed to

recrystallization,was activated by an energy of 1.2 eV,tending to higher values by increasing the impurity concentration in the Al- matrix( l).

For A1-Mg alloys, Thomas

( 3 )

ations, Also,it was indicated by Westwood and Broom ( 4 ) that the

observed strain ageing in this alloy corresponded likely to a

vacancy-dislocation interaction. However, in quenched Al-alloys

containing large amounts of Mg atoms

( 5

quenched vacancies remaining in solution were trapped by Mg atoms,

C5-3 14 JOURNAL DE PHYSIQUE

It is the aim of the present work t o study the role of Xg atoms o n the recovery o f cold-worked A1-5.5 at. % '*g alloy. Associated changes i n the internai Friction o f the rample and its macroscopic length were critically examined bo reveal the m e c h a i l i s m s involved during these recovery processes.

2-Experimental.- The test material w a s prepared from high-purity A1 and Mg by induction melting and suitable hamogeriization at 5 0 0 ~ ~ for 2 4 hours. Quenching from 5 0 0 ~ ~ t o room temperature with a

rate o f about10 3~ /sec w a s carried out t o bring the samples i n

metastable standard condition of solid solution. The material

was then shaped by extrusion into r a d s of 4 mm diameter. The rods were swaged at room temperature t o wires of 0.5 mm diameter.

Annealing was carried out by subjecting the test samples t o rectangular heat pulses by using a high heat capacity furnace maintained at the annealing temperature. T h e width o f resonance as determined by a micro-vibration detector was used t o find the internal friction. The change in the sample" macroscopic length was measured by thermo-mechanical analysis, TMA,technique(Heraeus

TA

gao

1.

7- Results.- Room temperature changes i n the internal friction of

pure A1 and A1-5.5 at.%Mg alloy were traced after successive

isochronal anneals o f deformed samples. Isochrones were effected

by heat pulses o f 1 0 minutes at temperatures successively

0

increasing i n steps o f 1 0 C.

Typical Q-' curves for cold-worked pure A1 and A1-5.5 at.% Mg

alloy are presented i n Fig.1, However, if the same maasurements

were carried out o n quenched alloy samples after pre-annealing at 400°c, an increase w a s found both in the rate o f recovery o f the first stage and in the peak height o f the second band, as clearly

depicted in fig.2ior AI-5.5 at.% Mg alloy. On the other hand,the

peak height o f the third recovery stage decreased with pre-anneal- ing. It could also be ncticed that the positions of the annealing stages were not affected by pre-annealing.

Thermo-mechanical analysis,

TMA,

oscopic iength of cold-worked A1-5.5 at.% Mg alloy with temperat-

ure as given in fig. 3(a). T h e first t w o annealing stages appea- red a s a decrease i n the macroscopic length ( c o n t r a c t i ~ n ~ , f o l l o w e d

by a linear increase w i t h temperature(expansion). The recovery

JOURNAL DE PHYSIQUE

PigSlThe annealing time dependence Fig.6: Plots relati

3

ME

%

.

Fig.7: Variation of log isochronal annealing time with I/T for diff- erent stages of annealing in cold- worked samples of pure ~ l ( 0 ) and ~1-5.5at.%W(. )

.

4,

Stage I.- This recovery stage occurred in the temperature range bet- ween 25 and 50°c. The annealing temperature had the effect of decre- asing the internal friction associated with a drop in the macroscop- ic length. It is.sxpected that vacancies created by cold-working or quenching to be present in the matrix in association with the solute

(6.7).

might

to a dislocation pinning effect due to migrating solute-vacancy pairs. This pinning action impeded free dislocations leading to the sup re as^

structure i n Al-matrix.Thus n o changes were expected to occur when subsequent heat treatments were made below this temperature. However quenching from 4 0 0 C C to room temperature would freeze extra vacanc- ies i n a metastable state approaching equilibrium during subsequent heat pulses.

The presently determined activation energy of this annealing

stage being

0.66

required for the migration o f vacancy-impurity pairs in A1(6,7).The- refore,the recovery o f this stage could be due to the migration o f these pairs to dislocations with the possible formation of complex aggregates at relatively high annealing temperatures.This suggested mechanism would anticipate a decrease i n the macroscopic length which was actually observed during t h e annealing process.

Stage 11.- This stage,denoted as I 1 in fig.1 for A1-5.5 at.%Mg,was

observed i n the temperature range from 5 0 to 1 1 0 ~ ~ . Apparently,this stage could also,be associated with the drop i n the macroscopic len- g t h a s seen i n fig.3. The mechanism involved i n this stage may be related t o the combined effect of the excessive growth of complex aggregates of vacancy-Mg pairs formed during the first annealing pro- cess and their subsequent dispersion at higher annealing temperatures, Both suggested processes were expected t o lead t o the contraction of the macroscopic length(9)as was actually observed in the present work. T h e increase in the peak height,due t o pre-annealing,could be related t o a n increase in the rate of formation o f complex aggreagates after

the matrix became relatively free from dis1ocations.The activation

energy due to this annealing stage was 0.73 eV(fig.7). This compares

with the energy required for core diffusion along dislocations in pu-

r e Al(lO).For this reason the mechanism controlling the recovery pro-

cess i n this stage i s reckoned to arise from the diffusion of Mg-ato- ms,probably along dislocations,from aggregates formed during the fir- st stage.

Stage 111.- The third annealing band appearing at temperatures above 1 1 0 ~ ~ could be attributed here to a recrystallization phenomenon(5)&

a s expected the peak height would decrease with pre-annealing.At the

C5-318 JOURNAL DE PHYSIQUE

As mentioned earlier,the controlling process i n this annealing

stage is climb which is essentially a self diffusion mechanism.The

experimental value o f activation energy 1.42 eV obtained for recrys-

tallization in cold-worked Al-worked A1-5.5 at.% Mg(see Fig.7)is in

good agreement with previously published data(3).Accounting for the

activation energy f o r recrystallization i n cold-worked pure A1 being

1.2 eV,the solute Mg-dislocation binding energy is easily seen t o be

0.22 eV. A value that agreed reasonably well with theoretical and

experimental value o f

0.28

0

rved i n the recovery spectra above 200 C as indicated i n fig.]. T h e activation energy entered in this process was e v a l u ~ t e d as 1.62 ex'. T h i s annealing stage could be ascribed to the tendency o f the solute Mg atoms t o create clusters in the matrix in the form o f (Al-Mg) particles,as previously described by Rothman et a1.(15).

T h e large activation energy o f 1.62 eV involved i n this process f o r the solute Mg atoms t o diffuse in A1 would entail that the rate o f clustering and zone formation would be relatively slow. Furtherm- ore, the solubility o f Mg i n A1 will be enhanced w i t h temperature increasing. T h i s causes clusters of (Al-Mg)particles t o become more

and more unstable. Increasing the temperature still further above

2 5 0 ~ ~ the formed clusters would go rapidly into solution.

Thus the diffusion o f the solute Mg atoms through the matrix would obviousl-y explain the rapid fall 2n internal friction,

5.

l ) R.Kamel,A.R.Ali and Z.Farid Phys.Stat.Soli(a)45,419(1978).

2) A.R.Ali and E.A.Naskedashvi1y Bull.Akad.Sci.GRS~,15(1974).

3) G.Thomas Phil.Mag. 4,1213(1959).

4 )

5) R.Kamel,A.R.Ali and Z.Farid Phys.Stat.Soli.(a)45,47(1978).

6)

7)

8)

9 )

Metals,Cambridge universiti(l969)

10)V.G.Van Bueren Imperfections i n Crystals ,(1960).

1ifG.Friede1,C.Boulanger and C.Crussard A ~ t a ~ e t . 3 , 3 8 0 ( 1 9 5 5 ) .

12)C.Dimitrov and 0,Dimitrov International Conference o n Vac-

ancies and Interstitials i n Met- ala,Germany,(i968),p.290.

13)G.Friedel Dislocations,~erg~mon,(lq64),

14)C.C.Smith and G.M.Leak Euovo Cimento 338,388(1976).

15)S.G.Rothman,N.L.Peterson,l.3. Phys.Stat.Soli.(b)63,K29(1974).