Role of temperature and strain rate on the hydrogen-induced intergranular rupture in alloy 600
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1/8/04
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Role of Temperature and Strain Rate on the Hydrogen-Induced Intergranular Rupture in Alloy 600 J. CHÊNE and A.M. BRASS Tensile tests were conducted at various temperatures (77 to 550 K) and strain rates (105 to 101 s1) in order to study the effects of hydrogen on the ductility loss and the intergranular fracture of hydrogencharged (32 wt ppm) tensile specimens of alloy 600. The H-induced intergranular cracking was shown to require H segregation to grain boundaries (GBs) during plastic deformation. The concordance between the temperature/strain rate domains, where H-induced intergranular rupture of alloy 600 is observed and those of H transport by dislocations, is in favor of a major influence of this mechanism of H transport on the intergranular rupture of H-charged alloy 600 in the 180 to 500 K temperature range. The possible contribution of this mechanism to intergranular stress corrosion cracking (IGSCC) of alloy 600 in the pressurized water reactor (PWR) environment is discussed.
I. INTRODUCTION
THE numerous recent studies on the intergranular stress corrosion cracking (IGSCC) susceptibility of alloy 600 in pressurized water reactor (PWR) environments[1–7] suggest a role of hydrogen in the cracking mechanism, even if it may affect differently the initiation and the propagation of the crack. The dependence of the SCC susceptibility on the amount of H dissolved in the pressurized water[1] has been related to the effect of the H partial pressure on the stability and on the physico-chemical properties of oxide films formed on the surface, suggesting a possible effect of H on the crack initiation. Recent permeation measurements[3,8,9] show both a significant absorption of hydrogen in iron- and nickel-base alloys exposed to primary water and a strong barrier effect on H absorption of the oxide films grown in the environment. This barrier effect is also confirmed by the much larger hydrogen concentrations measured in samples undergoing plastic deformations, which induce local damage in the oxide film, while exposed to primary water.[4,5,10,11] All these observations suggest a local enhancement of H absorption, namely, at the crack tip, as a consequence of deformation-induced damage in the film. Strong similarities have also been observed between the IGSCC and the H-induced intergranular rupture of alloy 600.[6,7,12,13] The propagation of SCC cracks is thus considered to occur as the consequence of H-deformation interactions.[2,3,6,14] The most widely proposed cracking mechanisms are either H-induced softening[14,15,16] or H-induced decohesion related with H segregation at grain boundaries (GBs).[7] The GB segregation may either result from H trapping to GBs or to defects at the GB (e.g., precipitates)[7,17] or from a straininduced redistribution of hydrogen to the boundaries.[13] This later phenomenon may involve the strain-assisted transport
J. CHÊNE and A.M. BRASS, Senior Research Scientists, are with CNRS, Laboratoire de Physico-Chimie de l’Etat Solide, Université Paris-Sud, 91405 O
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