The Sulfidation Attack of a Nickel-Base Alloy at Intermediate Temperatures
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INTRODUCTION
CREEP and fatigue crack growth in nickel-base alloys at homologous temperatures on the order of 0.5 has become a subject of considerable interest in recent years. Many studies have shown that the environment can play a very active role in the fracture behavior. Oxygen in the environment, for example, can produce 100 fold increases in the crack rate in some alloys compared to tests in vacuum or inert environment. Sulfidizing environments have been less widely studied, but the evidence to date shows that minor amounts of sulfidizing species in the environment can also produce severe degradation of the properties of high strength superalloys.~'2'3 Softer, heat resistant alloys usually are less severely affected. 4'5'6 Pronounced environmental effects are commonly associated with intergranular crack growth. In many cases the results suggest that the environment does not cause a new type of fracture, but instead accelerates inherent cracking processes that occur independent of the environment. This enhancement often appears to be the result of diffusion of an embrittling species such as oxygen or sulfur into the grain boundaries ahead of the crack tip. Calculations show that diffusion rates, at temperatures above about 500 ~ are generally rapid enough to keep pace with the observed crack growth rates. 7 It has been observed in many instances that exposing an unstressed sample to the aggressive environment often produces no signs of corrosion damage and no degradation in properties when the specimen is tested. Thus, some minimum level of strain or strain rate evidently may be required to produce a deleterious interaction with the environment. The simplest explanation for this observation is that protective films on the surface must be ruptured before an interaction with the environment can occur, but mechanical effects could also accelerate the grain boundary diffusivity or the interactions between the environment and grain boundary microstructure. S. FLOREEN, Research Fellow, and R.H. KANE, Principal Metallurgist, are with Inco Alloy Products Company Research Center, Sterling Forest, Suffern, NY 10901. This paper is based upon a presentation delivered at the symposium "Effects of Environment on Elevated Temperature Mechanical Properties" held at the February, 1982 meeting of TMS-AIME in Dallas, Texas, under the sponsorship of the Corrosion and Environmental Effects Joint Committee (TMS/MSD).
METALLURGICAL TRANSACTIONS A
To explore these questions further some simple tests were carried out in which smooth specimens of INCOLOY* *Trademark of the Inco family of companies.
alloy 800 were strained various amounts in creep or fatigue in a helium-2 pct SO2 environment and then examined. This relatively soft alloy was selected for study because it possesses considerable ductility and can be deformed substantial amounts without failure. The sulfidizing environment was used to improve the sensitivity of energy-dispersive X-ray spectroscopy (EDS) to precipitation at the grain boundary, since small levels of su
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