Oxygen-Induced Dynamic Embrittlement in Nickel-Base Superalloys
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Oxygen-Induced Dynamic Embrittlement in Nickel-Base Superalloys C. J. McMahon, Jr. and W. M. Kane Department of Materials Science and Engineering University of Pennsylania Philadelphia, PA 19104 ABSTRACT The phenomenon of dynamic embrittlement involves the stress-induced diffusion of a surfaceadsorbed embrittling element into grain boundaries, leading to time-dependent decohesion along these boundaries. Here, the state of our understanding of this generic type of brittle fracture is reviewed, with the focus on cracking of nickel-base superalloys caused by oxygen, including recent and new results on cracking in bicrystals, thermo-mechanical processing to reduce the susceptibility to dynamic embrittlement, and quench cracking. INTRODUCTION Dynamic embrittlement is an important type of brittle fracture that is time-dependent, in contrast to the classical high-speed types exemplified by the rupture-to-cleavage transition and intergranular embrittlement in steels. Although dynamic embrittlement is fairly widespread in structural alloys, it has received relatively little attention, compared with the classical types of brittle fracture. It is not yet widely recognized as a generic type, and the systematic study of its mechanism is still in the early stages. The present paper deals with this phenomenon in the context of Ni-base alloys of the types used in gas turbines and aircraft engines, where it is known to be a life-limiting factor [1]. While it is generally understood to be associated with oxygen in the atmosphere, the mechanism by which it occurs is still considered by some to be a matter for debate. BACKGROUND WORK IN STEELS Our involvement with this phenomenon began with a study of stress-relief cracking in alloy steels. This is a case of sulfur-induced cracking along prior austenite grain boundaries in steels that have been heated to a temperature in the 1200-to-1300˚C range and cooled fairly rapidly, as in the heataffected zone of a weldment [2,3]. During the heating there is some dissolution of sulfides, followed by segregation of sulfur to the austenite grain boundaries and then re-precipitation of less-stable sulfides on a fine scale in the boundaries as the steel cools. When residual or imposed stresses relax by creep at
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500-600˚C, sharp intergranular cracks can grow out from cavities that form at grain-boundary particles. The mechanism of cracking is the stress-induced diffusion of surface-adsorbed sulfur into grain boundaries and the slow decohesion of these boundaries [2,4]. The reason that this type of cracking has been called “dynamic embrittlement” [5,6], is that the embrittlement process occurs during the application of stress, rather than before. An alloy of Cu-8%Sn loaded at 265˚C was studied as a model of the steel to test the generality of the mechanism; it was found to crack in essentially the same way as the steel [7]. The tin, a surface-active embrittling element, played the same role as sulfur does in the steels. EMBRITTLEMENT BY OXYGEN In the two examples cited above, the sour
