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Prior studies of Alloy 600 and A!loy X-750 have shown the existence of a maximum in stress coqosion cracking (SCC) susceptibility in high temperature water (e.g., at 360°C), when testing is conducted over a range of dissolved (i.e., aqueous) hydrogen (Hz) concentrations. It has also been shown that this maximum in SCC susceptibility tends to occur in proximity to the nickehickel oxide (NiNiO) phase transition, suggesting that oxide phase stability may affect primary water SCC (PWSCC) resistance. Previous studies have estimated the Ni/NiO transition using thermodynamic calculations based on free energies of formation for NiO and H20. The present study reports experimental measurements of the Ni/NiO transition performed using a contact electric resistance (CER) instrument. The CER is capable of measuring the surface resistance of a metal to determine whether it is oxide-covered or oxide-f?ee at a given condition. The transition aqueous hydrogen (Hz) concentration corresponding to the Ni/NiO equilibrium was measured at 288, 316, 338 and 360°C using high purity Ni specimens. The results showed an appreciable deviation (;.e., 7 to 5 8 scc Hzkg H20) between the measured NiNiO transition and the theoretical Ni/NiO transition previously calculated using free energy data from the Journal of Solution Chemistry. The CERmeasured position of the N m i O transition is in good agreement with the maxima in PWSCC susceptibility at 338 and 360°C. The measured Ni/NiO transition provides a reasonable basis for estimating the aqueous H? level at which the maximum in SCC susceptibility is Iikely to be observed at temperatures lower than 338 to 360"C, at which SCC tes& are time-consuming to perform. Limited SCC data are presented which are consistent with the observation that SCC susceptibility is maximized near the NdNiO transition at 288°C.

Miniature, nonstandard, tensile-type specimens have been developed for use in radiation effects experiments at high energy neutron sources where the useful radiation volume is as small as a few cubic centimeters. The end result of our development is a 12.7-mm-long sheet-type specimen, with a 5.1-mm-long, 1.0-mm-wide gage section, which is fabricated from 0.25-mm-thick sheet stock by a punching technique. Data obtained using miniature specimens of an austenitic, a ferritic and a precipitation hardened alloy are in good agreement with data obtained using much larger specimens. This finding indicates that miniature tensile specimen data may be used for engineering design purposes and that miniature tensile specimen technology may be applicable to fields other than the study of radiation effects. This paper reviews the miniature specimen technology which was developed and reports the data which were obtained.

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