Diffusion Along Migrating and Stationary Grain Boundaries: The Al-Zn System
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DIFFUSION ALONG MIGRATING THE Al-Zn SYSTEM
W. GUST*, M.B. HINTZ *Max-Planck-Institut Metallkunde, **Department
AND STATIONARY
GRAIN BOUNDARIES:
40, R. LUCIC , AND B. PREDEL* far Metallforschung and Institut
SeestraBe 92, D-7000 Stuttgart of Metallurgical Engineering,
gical University,
Houghton,
MI 49931,
1, FRG; Michigan
far Technolo-
USA
ABSTRACT The discontinuous precipitation reaction has been studied in an Al- 2 8a/o Zn solid solution. It appears that the diffusion rates determined by us for migrating grain boundaries are of the same order of magnitude as the values determined by Hd~ner (1974) for stationary grain boundaries. To the same statement have led results from the discontinuous coarsening reaction in an Al-29a/o Zn alloy (Fournelle et al., 1982) as well as from the eutectoid reaction in an Al-6oa/o Zn alloy (Cheetham et al., 1971).
INTRODUCTION Discontinuous precipitation tions has been hitherto investigated
in
massive Al-Zn solid soluonly by Pawlowski 71. 6
L J.
The grain boundary diffusion values s DB (s segregation factor, ý grain boundary width, DB grain boundary diffusion coefficient) obtained are lying, however, - exactly as the values obtained by Butler et al. in thin foils which can be subjected to 2 , 3• TEM investigations - too low. Therefore, the reaction has been again investigated by us in an Al-28a/o Zn alloy (Fig. 1). The alloy (according to the weighing-in Al-28.4a/o Zn) has been produced from 3N8 Al and 4N5 Zn, afterwards homogenized for about 14 d at 673 K and then quenched in water. Etching has been effected using newly prepared concentrated solution of NaOH at about 333 K (30-120 s). The effect of etching is better, if the sample is heated, beforehand, in hot water. The eutectoid Al-Zn system is charcterized by two metastable miscibility gaps (R-phase, GPII-zones). Our precipitation investigations are hinting at a third metastable miscibility gap (GPI-zones). One point of support of the GPI miscibility gap lies at 2 8 a/ 0 Zn and 310 K (Fig. 2). One point of support belonging to the R miscibility gap is lying, according to our findings, at Zn and 367 K, through which the hitherto 5 8 a/ 0 quoted retrograde course of the solvus line is corrected. The occurrence of the metastable phases implies a complicated precipitation behaviour. Before, through discontinuous precipitation, the equilibrium phase (8) is formed, in the supersaturated solid solution the metastable phases (GPI, GPII, R) are appearing in of which are the manner of Guinier-Preston zones, the structures cleared up as yet. not entirely
Mat. Res. Soc. Symp. Proc. Vol. 21 (1984) OElsevier Science Publishing Co., Inc.
514
1,
S
a A*
M, LM
pIa) FIG. 1. Discontinuous precipitation at grain boundaries (GB). 303 K; (b) 28 min 404 K, lamellar structure. LM (a) 17 h optical micrograph, SEM scanning electron micrograph. 700
150
600
100
337 K v = 4.2
9
10 O mls
500
50 315K v =7.3 x 10-cm/s
400 0
I
0
I
I
5
10
I
15
Mean seam width b as a FIG. 3. function of the annealing time t.
300
At.% Zn
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