Delayed alumina scale spallation on Rene'N5+Y: moisture effects and acoustic emission

Abstract

The single crystal superalloy Rene'N5 (with or without Y-doping and hydrogen annealing) was cyclically oxidized at 1150 °C for 1000 h. After considerable scale growth (≥500 h), even the adherent alumina scales formed on Y-doped samples exhibited delayed interfacial spallation during subsequent water immersion tests, performed up to 1 year after oxidation. Spallation was characterized by weight loss, the amount of spalled area and acoustic emission response. Hydrogen annealing (prior to oxidation) reduced spallation both before and after immersion, but without measurably reducing the bulk sulfur content of the Y-doped alloys. The duration and frequency of sequential, co-located acoustic emission events implied an interfacial crack growth rate at least 10 −3 m s −1, but possibly higher than 10 2 m s −1. This is much greater than classic moisture-assisted slow crack growth rates in bulk alumina (10 −6 to 10 −3 m s −1), which may still have occurred undetected by acoustic emission. An alternative failure sequence is proposed: an incubation process for preferential moisture ingress leads to a local decrease in interfacial toughness, thus allowing fast fracture driven by stored strain energy.