The influence of sulfur on stress-rupture fracture in inconel 718 superalloys
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I. INTRODUCTION
THE process of ductile fracture in steels comprises the growth and coalescence of microvoids, which nucleate initially on second-phase particles, i.e., nonmetallic inclusions and carbides.[1,2] Void or cavity formation can be affected by the presence of impurities. Small impurity concentrations can cause monolayer coverages of interfaces through various segregation mechanisms such as equilibrium segregation.[3–6] Two problems associated with segregation and interface cohesion of interest to the present study are (1) grain-boundary impurity segregation, leading to a loss of grain-boundary cohesion, and (2) formation of cavities on the grain boundary and at particles. It was suggested[7,8] that the reduction of grain-boundary and free-surface energies by impurity segregation could lead to reduced grain-boundary cohesion at elevated temperatures. Grain-boundary impurities are also expected to strongly affect the grain-boundary self-diffusion rate. Middleton[9] has suggested that the major effect of impurities on cavity nucleation is through their effect on the density and size distribution of nonadherent grain-boundary particles. However, the effects of impurity elements on creep and stress-rupture fracture are not well established in the literature. Some studies show no effect,[10] others show that impurities are deleterious,[11] and others show that impurities can be beneficial.[12,13] Surface and interfacial segregation has been studied extensively in binary and ternary alloys.[14–16] Commercial steels and superalloys are complex systems with many elemental species, and, while impurity segregation has been widely studied in the former, comparatively little is known about the superalloys. Although little is known about interfacial segregation in superalloys, clean superalloy production is at J.X. DONG, Associate Professor, and X.S. XIE, Professor, are with the High Temperature Materials Testing and Research Laboratory, Department of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. R.G. THOMPSON, Professor, is with the Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham, AL 35294. Manuscript submitted March 25, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
the vanguard of the superalloy industry, and cleanliness has been basically considered in two categories: inclusions and detrimental-element control.[17] Sulfur and phosphorus are generally regarded as the most common impurities and detrimental elements in superalloys. Much has been published on the detrimental effects of sulfur segregation to grain boundaries in superalloys and its prevention by the addition of gettering elements (Ti, Zr, Hf, La, and Mg) which form primary sulfides.[18,19] Early work by Merica and Waltenberg attributed sulfur effects to the formation of a Ni-Ni3S2 eutectic film, but more-recent studies suggest that a low-melting grain-boundary phase is not necessary for high-temperature intergranular fail
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