Effect of Grain Boundary Structure on Grain Boundary Diffusivities in the Au/Ag System
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EFFECT OF GRAIN BOUNDARY STRUCTURE ON GRAIN BOUNDARY DIFFUSIVITIES INTHE AU/AG SYSTEM QING MA AND R.W.BALLUFFI Massachusetts Institute of Technology, Cambridge, MA ABSTRACT Grain boundary chemical diffusivities for a series of symmetric [0011 tilt boundaries in the Au/Ag system were measured by the surface accumulation method using newly developed thin-film multi-crystal specimens, in which the grain boundaries feeding the accumulation surface were all of the same type. Possible effects due to segregation at the grain boundaries and surfaces were avoided. CSL boundaries of low-L ( I.e., 5, 13, 17, 25) and also more general boundaries with tilt angles between the low-!: orientations were selected. The diffusivities were found to vary monotonically with tilt angle ( i.e., no cusps at low-Vs were found) in a manner consistent with the Structural Unit model. INTRODUCTION During the last few decades, a number of studies has been made of the effect of grain boundary structure on grain boundary diffusion [1]. However, the basic characteristics of the effect for high angle boundaries remain unclear. One of the current issues is the possible existence of cusps in the grain boundary diffusion parameter, 8Db, versus misorientation angle, 0, curve for [001] tift boundaries at special misorientations corresponding to low-E short-period boundaries. (Here, Db = boundary diffusivity, 8 = boundary width.) Figurel displays the result of a Ag grain boundary self-diffusion experiment reported recently by Sommer, et al. [2] showing a monotonic relationship between 8Db and 0. On the other hand, Aleshin, al al. [3] measured the lumped chemical grain boundary diffusion parameter s8Db versus misorientation and observed deep cusps at several special misorlentations (Fig. 2). This parameter includes the segregation factor, s, which itself is also a function of boundary structure. Segregation effects, rather than variation in the 8Db parameter may therefore have been responsible for the presence of the cusps in the latter case, and special efforts must be made to test this possibility. In this report, we present measurements of chemical grain boundary diffusivities In the Au/Ag system for a series of symmetric [001] tilt boundaries using a newly developed multi-crystal surface accumulation method. In this method (illustrated in Fig. 3) lattice diffusion is frozen out, and during the diffusion annealing the Ag atoms diffuse from the source surface to the accumulation surface through the transverse grain boundaries. 40
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Fig. 1 Grain boundary self-diffusion of Ag along [001] tilt grain boundaries, after Sommer Dial. [2].
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Fig. 2 Grain boundary chemical diffusion along [0011 tilt grain boundaries in AI/Zn system, after Aleshin glal. [3].
Mat. Res. Soc. Symp. Proc. Vol. 209. §1991 Materials Research Society
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The chemical composition of both surfaces is monitored by means of Auger s
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