Internal friction in hydrogen-charged CrNi and CrNiMn austenitic stainless steels
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I.
INTRODUCTION
THE internal friction (IF) technique can provide valuable information about the state of hydrogen atoms and their mobility in austenitic steels. As soon as hydrogen atoms cause, in some way, local deviation from the cubic distortion in face-centered cubic (fcc) lattice, the Snoeklike relaxation concerned with diffusion jumps of hydrogen atoms has to occur. The activation enthalpy of the Snoeklike peaks obtained from IF spectra corresponds to that of hydrogen migration in the respective neighborhood of substitutional solutes. Such a mechanism, once unambiguously proven, could be used in the following as an important source of information on the local hydrogen migration, in addition to the average hydrogen diffusivity data obtained from the permeation measurements. Another interesting possibility provided by IF technique lies in the measurement of hysteretic damping caused by acoustic emission during hydrogen-induced cracking or any other dynamical dislocation processes accompanied with the absorption of energy of mechanical vibrations. A number of IF measurements have been performed on hydrogenated iron-base fcc alloys including austenitic stainless steels (e.g., References 1 through 12). Peterson et al.[~,2] were the first to observe in IF spectrum of AISI 310 type austenitic steel two hydrogen-induced peaks located at 205
V.G. GAVRILJUK, Professor and Head, and A.V. TARASENKO, Scientific Researcher, are with the Department of Alloy Steels, Institute for Metal Physics, Kiev 252142, Ukraine. H. H.~.NNINEN, Professor and Head, and K. ULLAKKO, Scientific Researcher, Laboratory of Engineering Materials, are with the Helsinki University of Technology, SF-02150 Espoo, Finland. S.Yu. SMOUK, formerly Young Scientific Researcher, Institute for Metal Physics, is Postgraduate Student, Laboratory of Engineering Materials, Helsinki University of Technology. Manuscript submitted March 14, 1995. METALLURGICALAND MATERIALS TRANSACTIONS A
and 225 K for frequency of 80 kHz. They attributed the first peak to an atom-pair relaxation mechanism (Snoeklike relaxation), where a hydrogen atom constitutes one-half of the pair and either a nickel or chromium atom is the other half. The second peak was believed to result from hydrogen-dislocation interaction in analogy to the hydrogen coldwork peak in body-centered cubic (bcc) iron and ferritic steel (Snoek-Krster (SK) relaxation). From the half-width of the first peak measured at one frequency or from the frequency shift of the peak temperature (two frequencies have been used), the authors t2j evaluated the enthalpy of activation to be equal to about 0.53 to 0.56 + 0.13 eV, which is in the limits of the data scatter for hydrogen diffusion in CrNi austenitic steels, 0.52 to 0.67 eV, obtained from hydrogen permeation studiesY 3j However, in the following, this peak was never observed in IF spectra of austenitic steels measured at frequencies of 1.0 Hz or 0.5 kHz, when it had to be located at about 150 and 175 K, respectively, if the value of the activation enthalpy was corr
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