PREDICTING SUBSURFACE ENRICHMENT/DEPLETION PROCESSES DURING HIGH-TEMPERATURE OXIDATION OF ALLOY 625 THIN FOILS

摘要

For its high creep resistance the commercial nickel-base alloy 625 (UNS N06625) relies on solid solution strengthening in combination with precipitation hardening by formation of δ-Ni_3Nb and (Ni,Mo,Si)_6C precipitates during high-temperature service. In oxidizing environments the alloy forms a slow growing, continuous, and well adherent chromia layer on the material surface which protects the alloy against rapid oxidation attack. The growth of the chromia base oxide scale results in oxidation-induced chromium depletion in the subsurface zone of the alloy. SEM analyses of the cross-sectioned specimens revealed that this process results in formation of a wide subsurface zone in which the mentioned strengthening phases are dissolved, in spite of the fact that both phases do not contain substantial amounts of chromium. The cross-sectional analyses revealed that, in parallel to the formation of a precipitate depleted zone, a thin, continuous layer of niobium-rich intermetallic precipitates formed in the immediate vicinity of the scale/alloy interface. Thermodynamic computer modeling (Thermo-Calc) along with diffusion simulation software (DICTRA) was used to predict the oxidation-induced subsurface phase enrichment and depletion during oxidation (acronym SPEDO) in alloy 625 as a function of time. The modeling results are compared with the experimental findings concerning oxide scale and subsurface depletion layer formation obtained by scanning electron microscopy (SEM/EDX/WDX). The subsurface phase enrichment/depletion effect during oxidation is found to be substantially less pronounced in thin foils than in thick specimens of alloy 625. It is illustrated that this effect is related to the smaller reservoirs of the scale-forming element chromium as well as that of the 5-Ni3Nb phase stabilizing element niobium. As the thinner specimens become more rapidly depleted in chromium resulting in flatter chromium depletion profiles, a much weaker driving force for uphill-diffusion of Nb towards the scale/alloy interface leads to a substantially less pronounced δ-phase enrichment/depletion.