Drastic enhancements of the interdiffusion were observed in Cu-Ni diffusion couples when samples were heated under high hydrogen pressures ͑ϳ5 GPa͒. Interdiffusion coefficients measured between 600 800 ± C were increased by ϳ10 4 times on the Ni-rich end and by ϳ10 times on the Cu-rich end. The observation is explained in terms of superabundant vacancy formation in the presence of interstitial hydrogen atoms. [S0031-9007(98)06440-0] PACS numbers: 66.30.Ny, 61.72.Ji A few years ago we discovered a formation of superabundant vacancies in Ni and Pd, amounting to as much as ϳ10 at. %, when specimens were heated under high hydrogen pressures [1,2]. Subsequent work on less hydrogen absorbing metals, Al [3] and Mo [4], also showed enhanced vacancy concentrations in hydrogen-charged samples amounting to, respectively, 8 and 12 orders of magnitude higher than in vacuum under corresponding p, T conditions. The origin of this superabundant vacancy formation was ascribed to the lowering of the formation energy of a vacancy by trapping hydrogen atoms on its neighboring interstitial sites [4][5][6][7].Since the diffusion of metal atoms generally proceeds via a vacancy mechanism, a concomitant enhancement of the metal-atom diffusion can be expected, and was indeed indicated in the hydrogen-induced acceleration of phase separations in some Pd alloys [8][9][10][11]. In a Pd 0.8 Rh 0.2 alloy, for example, the interdiffusion coefficient at 600 ± C was roughly estimated to be ϳ10 7 times larger under 5 GPa of hydrogen pressure than in vacuum [10].The present work is intended to demonstrate unambiguously the hydrogen-induced enhancement of the interdiffusion, by examining its composition and temperature dependence more quantitatively, using diffusion couples of Cu-Ni.A specimen consisted of a pair of Cu and Ni disks, each 2.0 mm f 3 0.1 mm in size, 99.9% and 99.99% in purity, respectively. It was enclosed in an NaCl container (a hydrogen sealant) together with an internal hydrogen source ͑LiAlH 4 ͒, and placed at the center of a highpressure cell. Details of the sample cell design were given elsewhere [12]. The cell, a cube of 8 mm edge length, was compressed from all six sides equally by tungsten carbide anvils of top faces measuring 6 mm 3 6 mm, using a cubic-anvil press Oz.F1 of our laboratory.

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