Discontinuous precipitation at [001] twist boundaries in Cu-0.75 wt pct Be alloy bicrystals
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INTRODUCTION
DISCONTINUOUS precipitation (DP) is a heterogeneous reaction that leads to the formation of a lamellar structure behind a grain boundary advancing into a supersaturated solid solution (␣0). The lamellar structure consists of the new precipitated phase (␥) and the solute-depleted matrix phase (␣), which are not in complete equilibrium regarding the chemical composition. The advancing grain boundary, called the reaction front, provides a short-circuit path of solute transport.[1] Thus, it has been shown that the analysis of the steady-state DP reaction kinetics offers an indirect, but convenient, method of measuring the boundary chemical diffusivity triple products s␦Db (where s, ␦, and Db are the segregation factor, boundary thickness, and chemical grainboundary diffusion coefficient, respectively),[1] e.g., using the model of Petermann and Hornbogen.[2] This approach has already been applied in a number of binary systems, such as Cu-Be,[3] Zn-Cu,[4] Cd-Ag,[5] etc. In a previous article,[6] we have studied systematically the nucleation and growth of DP at various [001] symmetric tilt boundaries in orientation-controlled bicrystals of a Cu-0.75 wt pct Be alloy. We have found that there is a good correlation between both the incubation period for cell formation and the cell growth rate and the boundary energy. That is, higher-energy boundaries have a shorter incubation period and a faster growth rate. Furthermore, it has been shown that plots of the activation energy, Qb , and pre-exponential factor, (s␦Db)0, of grain-boundary diffusion against misorientation display several peaks, and the positions of these peaks are in agreement with those of cusps in the boundary energy vs misorientation diagram. In this work, the formation and growth of DP are investigated for [001] twist boundaries in Cu-0.75 wt pct Be alloy
bicrystals. In addition, the values of Qb and (s␦Db)0 are evaluated as a function of the misorientation angle. II. EXPERIMENTAL A Cu-0.75 wt pct Be alloy for this investigation was prepared by melting together electrolytic Cu (99.99 pct purity) and Cu-3.81 wt pct Be master alloy in a high-purity alumina crucible under an Ar atmosphere. Bicrystals of this alloy having various [001] twist boundaries with misorientation angles ⫽ 10 to 45 deg were produced in a high-purity graphite mould by the Bridgman method using two-seed crystals. Hereafter, the grain boundary with the misorientation angle will be expressed as boundary. After confirming by Laue analysis that the bicrystals had the designated twist boundaries, they were homogenized at 1123 K and then spark-cut into specimen pieces (1 ⫻ 15 ⫻ 15 mm). The surface of the specimens is perpendicular to the twist boundaries. The specimens were solution-treated at 1073 K for 1 hour in a vacuum, quenched into ice water, and then aged at 523 to 733 K in a vacuum. Quantitative metallographic measurements were performed by optical microscopy (OM) and scanning electron microscopy (SEM). The cell width, L, and interlamellar spacing, , were determined by
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