Hydrogen-Induced Enhancement of Interdiffusion in Cu-Ni Diffusion Couples

Abstract

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.

Hydrogen-Vacancy Interactions in Fe-C Alloys

“…For low ½Va tot (in this regime in which ½H tot $ 0:1½C tot ), carbon interstitials are the prevalent PDC (C I zone) and no binding between H and C is found to occur. Clusters containing both H and C, such as [3] and given the relatively small radius of the hydrogen atom as compared to bcc Fe vacancies and interstitials, we conjecture that these small Va x H z PDCs should be highly mobile, therefore having important macroscopic consequences which can be tested experimentally. Direct calculation of these PDC migration mechanisms and lowest energy barriers will be a topic of future studies.…”

“…Furthermore, the well-known problem of hydrogeninduced degradation of the mechanical properties of metals has had significant impact in Fe-rich alloys such as hardened steels that suffer severe embrittlement [4,5]. Experimental and theoretical evidence [1][2][3][4][5][6][7] suggests that one unifying theme behind these hydrogen-mediated effects is the strong interaction between hydrogen impurities and other point defects in the material, and the subsequent microstructural changes that occur as a result of that interaction. Although a rich experimental literature detailing macroscopic effects exists, fundamental understanding of the interactions of hydrogen with point defect clusters (PDCs) in metals is limited.…”

“…In the past few decades, a variety of fundamentally new phenomena has been observed in metals and their alloys under hydrogen-rich conditions, such as large volume contractions in body-centered cubic (bcc) -Fe [1,2], and the enhancement of diffusion at metal-metal junctions [3]. Furthermore, the well-known problem of hydrogeninduced degradation of the mechanical properties of metals has had significant impact in Fe-rich alloys such as hardened steels that suffer severe embrittlement [4,5].…”

Measurement of diffusion process of iron atoms under high pressure of hydrogen by time-domain analysis of nuclear resonant scattering of X-rays

“…Main application of the technique is for the hydrogen-induced enhancement of diffusion in the subsurface region. Hydrogen-induced enhancement of atomic diffusion has extensively been studied for bulk materials by Fukai and coworkers [1][2][3][4]. In those studies, hydrogen-induced enhancement of inter-diffusion was confirmed in the phase separation process of binary alloys as well as in the concentration profiles of diffusion couples [1][2][3].…”

Thermodynamics of impurity-enhanced vacancy formation in metals

“…7,8 Later, SAV formation was also seen in Au, Nb, Cr, Co Pt, Mn, and other metals and metal alloys. [9][10][11][12][13][14][15][16][17] The origin of SAV formation lies in the lowering of the system's total energy. The main mechanism of impurity assisted vacancy formation is now commonly believed to be the vacancy formation energy decrease due to the impurity trapping.…”