Abstract
The nickel based superalloy IN718 is known to be prone to hydrogen sensitivity, causing degradation of its mechanical properties. Therefore, during mechanical testing of hydrogen charged samples, a well-defined hydrogen distribution is essential to better understand the influence of hydrogen on dislocation movement and plasticity behavior. The possibility of charging cylindrical specimens of IN718 with hydrogen using cathodic charging is investigated. The method is based on an electro-chemical process using a molten salt electrolyte. The resulting hydrogen concentration is measured for various radii, and it is shown that the anisotropic diffusion coefficient resulting from electromigration, inherent in the charging method, must be taken into account as it has a major impact on the charging parameters of IN718. Also, no evidence of degassing during storage is found. Further, changes in surface roughness were examined by SEM, and only limited surface degradation is observed, which is not considered to significantly affect the results.
Highlights
Inconel 718, or IN718 is a commercial nickel-chromium-iron based superalloy, which shows thermal and mechanical stability with maintained yield strength and fracture toughness in the temperature range of approx. −250 to 650 °C [1]
The resulting hydrogen concentration is measured for various radii, and it is shown that the anisotropic diffusion coefficient resulting from electromigration, inherent in the charging method, must be taken into account as it has a major impact on the charging parameters of IN718
The fracture surface morphology of IN718 subjected to hydrogen embrittlement, studied with the use of high resolution electron microscopes [7,8], gives general observations indicating localized ductile fracture
Summary
Inconel 718, or IN718 is a commercial nickel-chromium-iron based superalloy, which shows thermal and mechanical stability with maintained yield strength and fracture toughness in the temperature range of approx. −250 to 650 °C [1]. The fracture surface morphology of IN718 subjected to hydrogen embrittlement, studied with the use of high resolution electron microscopes [7,8], gives general observations indicating localized ductile fracture. Based on these observations, Hicks and Altstetter [7] suggested hydrogen-enhanced localized plasticity (HELP) as the mechanism of hydrogen embrittlement in IN718. Hicks and Altstetter [7] suggested hydrogen-enhanced localized plasticity (HELP) as the mechanism of hydrogen embrittlement in IN718 This mechanism, originally proposed by Birnbaum [9], describes the interaction between the solute atmosphere of hydrogen and the dislocations in the material. Other mechanisms generally describing hydrogen embrittlement, e.g. hydrogen-induced decohesion or hydride formation [12], would in IN718 require concentration levels of both stress and hydrogen that only, if at all, would be found near a crack-tip
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