Effect of high pressure on interdiffusion in Cu-Zn alloys at temperatures near the melting point
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I.
INTRODUCTION
T H E R E is little doubt that diffusion in copper and Cu-Zn alloys occurs by the vacancy mechanism as pointed out by many investigations 1-6 since the time of Kirkendall's experiment. 7 But, the contribution of the monovacancy and divacancy to diffusion of this system is obscure. One of the experiments which throws light on the above problem is the effect of high pressure on diffusion. The authors have reported the interdiffusion coefficients D in the a-phase of Cu-Zn system under high pressure in the temperature range from 1108 to 1223 K. 8 The activation volumes obtained from the pressure dependence of the diffusion coefficients are (4.5 to 5.0) x 10 -6 m3/mol, which indicate diffusion of zinc in these alloys occurs predominantly by the monovacancy mechanism at 1108 to 1223 K. At a temperature near the melting point of this alloy, the magnitude of the monovacancy and divacancy contributions to diffusion is of interest and has not yet been investigated. The purposes of this paper are (a) to determine the diffusion coefficients in the a-phase of the Cu-Zn system under high pressure in the temperature range from 1256 to 1377 K, and (b) to describe the diffusion mechanism near the melting point from the determination of the activation volumes.
II.
EXPERIMENTAL PROCEDURES
B. Diffusion under Atmosphere (0.1 MPa) The bars (8 mm 4~ x 80 mm) were made from the alloys by lathing and their surfaces were metallographically polished by 0.3/zm alumina powder. The block of pure copper (24 x 25 x 70 mm 3) was made, and then a hole (8 mm ~b x 70 mm) was centrally drilled in this block. The drilled surface of the block was mechanically polished by 6/0 emery papers. 0.3 ~m alumina particles were placed on the surface of the bars of alloys as the inert markers by painting methanol containing 0.3/xm particles and drying. The bars of alloys with the alumina particles were inserted into the hole in the block of pure copper, and then they were quickly rolled to about 7.5 mm in thickness at 1123 K for bonding the alloy to copper. The rolling as the bonding method breaks up the oxide films at the interface between pure copper and alloy to the fine pieces and distributes them over a much larger area, and then the clean interface appears as pointed out by Murphy. 9 The rolled metals were cut into 5 mm lengths to serve as the diffusion couples. The bonding interfaces of the diffusion couples were examined by an electron probe microanalyzer (EPMA). It was found that diffusion during the bonding treatments was negligibly small. The diffusion couples were annealed at 1233 to 1313 K under the flow of high purity argon gas in order to prevent the oxidation and evaporation of zinc, and cooled in ice water. The temperatures and times of diffusion annealing are listed in Table I.
A. Alloy Preparations The pure copper and the copper base alloys of 4.9 and 28.5 at. pct Zn were prepared from 99.99 wt pct purity copper and 99.996 wt pet purity zinc in an induction melting furnace in an argon atmosphere. After hot and cold rolling,
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