An analytical electron microscopy study of the role of La and Y during high-temperature oxidation of selected Ni-base alloys

Minor but critical additions of active elements such as Y to Al2O3-forming alloys and La to Cr2O3-forming alloys are known to have beneficial effects on their high-temperature oxidation resistance. Several mechanisms, sometimes opposing, particularly in the case of Al2O3-forming alloys have been proposed to explain the active element-effect. A number of extensive reviews have dealt with these mechanisms. In the case of Cr2O3-forming alloys, however, it is generally agreed that the role of active elements is to promote selective oxidation of Cr, reduce the scale growth rate, and improve its mechanical strength. For a better understanding of the active element-effect, it is essential to determine its atomic-scale distribution within the oxide scale. Using thin-foil analytical electron microscopy techniques, it has been shown that in some Al2O3-forming alloys and Cr2O3-forming alloys, Y tends to segregate at grain boundaries of the oxide scale resulting in modification of its transport properties and mechanical strength. Also, it has been suggested that other elements such as Ce and La may segregate to grain boundaries of oxide scale in Cr2O3-forming alloys. It was the objective of this study to determine the atomic-scale distribution of active elements in wrought Ni-base alloys of commercial grade using the techniques of analytical electron microscopy. Two alloys representing Al2O3-forming and Cr2O3-forming alloys were selected for the study. Haynes alloy no. 214 is a Y-containing alloy capable of developing Al2O3 scale upon exposure at temperature exceeding 1,000C and Haynes alloy no. 230 is a La-containing alloy protected by Cr2O3 scale