Microstructural degradation in the subsurface layer of the nickel base alloy 718 upon high-temperature oxidation

Abstract

ABSTRACT The focus of this study is on assessing the chromium-depletion occurring in the subsurface layer associated with the formation of chromium oxide upon high-temperature exposure. As this is a critical aspect for the lifetime of many components made of nickel base alloys, a magnetic technique was developed that allowed assessing the chromium-depleted zone in a non-destructive manner. The samples made from alloy 718 were pre-oxidised in the temperature range from 700°C to 1200°C and then characterised both magnetically and by scanning electron microscopy. Analysis of the data obtained demonstrated that the magnetic measurements are capable of revealing the effect of oxidation temperature on microstructural degradation in the near subsurface layer. Specifically, analysis of the 3rd harmonic of the eddy current signal allowed inferring the oxidation temperature and the width of the chromium-depleted layer.