Short Notes K25

phys. stat. sol. (a) 2l, K25 (1974)

Subject classification: 10.1; 21: 21.1

Research Institute for Non-Ferrous Metals (a) and Institute for General Physics, Lordnd Eotvos University (b), Budapest

Anneal Hardening in Dilute A1-Cr Solid Solutions

BY E. KOVACS-CSEThNYI (a) and I. KOVACS (b)

It is known that in some cold-worked copper alloys the yield stress does not de-

crease but increases up to the recrystallization temperature during isochronal heat

treatment (1). This phenomenon is called anneal hardening (2, 3). The purpose of

this note is to report the existence of this effect in dilute A1-Cr solid solutions.

The alloys were prepared from 99.999% pure A1 and 99,99% Cr using master

alloys. The cast rods of 12 mm diameter, cooled down so rapidly that solidified

as solid solutions, were cold-drawn to wires of 1 mm diameter.

The wire samples obtained were used for the determination of the yield s t ress ,

d o, perature as a function of temperature of isochronal heat treatment. The annealing

a t room temperature and for resistivity measurements a t liquid nitrogen tem-

TPG) - Fig. 1

.-.--.--.-. 100 200 300 400

TPCl - Fig. 2

Fig. 1. The yield s t r e s s as a function of the annealing temperature. 0 0.01 at% Cr; a 0.03 at% Cr; A 0.2 at% Cr: + 0.4 at% C r

Fig. 2. The resistivity as a function of the annealing temperature for the same A1-Cr alloys

K26 physica status solidi (a) 21

time was 1 h at each temperature.

The results are shown in Fig. 1 and 2. It is clear that a significant anneal har-

dening takes place in these alloys. The explanation of this effect is probably in con-

nection with the following. It was shown previously that in the A1-Cr alloy the solid

solution state remains after cold-work, too (4). This fact is shown by the large re-

sistivity (Fig.2) and is likely a consequence of the very large activation energy of

diffusion of C r in A1 (5). There is, however, a strong size effect of C r atoms in

the aluminium matrix (6) which can cause a strong interaction between dislocations

and Cr atoms. To make this interaction effective it is necessary to raise the tem-

perature above 100 C where the rate of diffusion becomes large enough for leading

to the formation of Cottrell clouds around the dislocations. This process takes place

without clustering, which is shown clearly by the constancy of the resistivity in the

temperature range of the yield s t r e s s increase. A detailed study of the phenomenon

is presently carried out.

0

References

(1) T. J. KOPPENAAL and M. E. FINE, Trans. AIME 221. 1178 (1961). (2) R. W. CAHN, Physical Metallurgy, North-Holland Publ. Co., Amsterdam 1970

(p. 1139).

(3) H. WARLIMONT, R o c . 3rd Internat. Conf. Strength of Metals and Alloys,

(4) F. J. KEDVES, E. KOVACS-CSETiiNYI, and L. GERGELY, phys. stat.so1. (a)

Cambridge 1973, Vol. 1 (p. 554).

561 (1972).

(5) A. TONEJC, Phil. Mag. g, 753 (1973). (6) H.W. KING, J. Mater. Sci. 1, 79 (1966).

(Received November 8, 1973)