Hydrogen induced lattice expansion in nickel
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INTRODUCTION
IT is well known that hydrogen atoms occupy interstitial sites in a metal and produce an expansion of the surrounding lattice. The influence this expansion will have on activated processes is determined by the magnitude of the excess volume per metal atom, AV/I), or, expressed in molar quantities, by the partial molal volume (PMV). Hence, a knowledge of the PMV in specific metals is important in understanding hydrogen behavior, such as trapping, segregation, and embrittlement. Understandably, there have been numerous measurements of hydrogen-induced lattice expansion in metals with high hydrogen solubilities (e.g. Nb, Pd, and other hydride forming metals), but relatively few in low solubility metals. Peisl j has recently written an excellent review article in which he has compiled a number of hydrogen PMV measurements. The values of lattice expansion reported in his paper range from about 0.1 to 0.2 at. vol. Only one reference was cited for Ni 2 from which a value of 0.28 at. vol was deduced. However, this measurement was made in the nickel hydride phase formed by cathodic charging to high fugacity, and not in the solid solution phase of nickel. Baranowski, et al. 3 compared the volume expansions in Pd, several Pd alloys, a Cu-Ni alloy, and Ni measured at very large hydrogen concentrations. They found a volume increment of 2.88 .~3 per H atom for hydrogen concentrations less than about 0.7 to 0.8 H/Me (hydrogen to metal ratio). This is the basis of the 2.9 ,~3 rule" in Peisl's review. ~A Ni value due to Majchrzak4 at 0.9 H/Me deviated from the above rule, as did data in Pd at the higher hydrogen concentrations. In later work, Krukowski and Baranowski5 measured H-induced expansion in fcc Ni-Mn alloys and found volume increments of 2.5 ,~3 for all of the alloys above a hydrogen content of - 0.3 H/Me. Recent hybrid (lattice defect-quantum chemical) calculations by Baskes, et al., 6 which include electronic effects, found the volume expansion created by an interstitial H atom in Ni to be < 0.1 at. vol. This leads to very weak (-0.1 eV) calculated binding energies to a number of defects. Measurements of hydrogen binding energies to various traps in Ni 7 agree qualitatively with the calculations. However, both the experimental and theoretical results are not consistent with the large value of lattice relaxation mentioned above. G.J. THOMAS is Member Technical Staff, Sandia National Laboratories, Livermore, CA 94550. W.D. DROTNING is Member Technical Staff, Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted June 10, 1982. METALLURGICALTRANSACTIONS A
The present paper describes our measurements of the hydrogen-induced lattice expansion in Ni. Data were also obtained for hydrogen in Nb as a verification of our procedure. The relatively low solubility and high diffusivity of hydrogen in nickel must be considered when attempting these measurements. Our experiments differ in three important ways from the previous determination in N i - (a) the use of gas-phase charging, (b) measurement
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