Grain Boundary Chemistry of Alloy 718-Type Alloys

Abstract

Grain boundary chemistries of 718-type alloys doped with B, P, S, and/or C were studied to determine the segregation behavior of these elements. Specimens were intergranularly embrittled by hydrogen charging. These embrittled specimens were fractured in the ultra-high vacuum of a scanning Auger microprobe. Surface chemistries and depth profiles of these freshly fractured specimens were taken. Typical features analyses were grain boundary surfaces, fractured matrix surfaces, carbidematrix interfaces, Laves-matrix interfaces, cleaved carbide surfaces, and sulfide-matrix interfaces. It was found that B segregated uniformly to grain boundary surfaces as did P and S when these elements were present as intentional additives. Sulfur segregated to carbide and Laves phases interfaces even when the sulfur level was kept as low as possible. No strong evidence of competition between these elements for grain boundary segregation was found. Implications of these results relative to element partitioning during solidification, constitutional liquation of carbides and the mechanism of intergranular liquation cracking. Introduction The bulk chemistry of 718-type alloys is formulated to serve various purposes such as: ease of processing, high temperature solid solution strength, 7' and 7 precipitation for age hardening, and carbides for grain size control. Apart from the bulk chemistry, the chemistry of the grain boundaries is considered important to prevent stress rupture and B and Zr are added to strengthen the boundaries. While the structureproperty relationships for the bulk alloy have been studied extensively, similar studies on the grain boundary and precipitate interfaces have not been published. This paper presents the results of a study on the effects of minor alloying and impurity elements, and processing on the interfacial chemistry of alloy 718. The effects of certain non-metallic impurities (S and P) and the minor alloying additions (C and B) on the mechanical properties of superalloys are well documented; however, these elements are also suspected of significant intergranular effects yet superdl~y~ 718,625 and Various Derivatives Edited by Edward A. LX% The Miner&, Metals & Materials society, 1991 53 information on the actual interfacial chemistries involved is limited.(l,2,3) The development of Auger electron spectroscopy (AES) techniques allows for compositional analysis of the top few atomic layers of a solid surface. This makes it possible to determine, experimentally, interfacial chemistries when measurable interfacial surface areas can be exposed by means of freshly fracturing samples in a ultra-high vacuum environment. Because of the metallurgical design of the nickel superalloys, intergranular fracture areas are difficult to obtain without altering the interfacial chemistry of the material. Previous AES studies of superalloys have utilized either hot stages or high temperature tensile devices which are known to cause interfacial segregation. With hydrogen embrittlement of 718, caused by large concentrations of interstitially absorbed or adsorbed hydrogen, a reversible method is available to produce fracture separation at grain boundary surfaces and r-matrix/ second phase interfaces without altering the segregation behavior of the elements found there.(4) Much has been published on detrimental effects of sulfur segregation to grain boundaries in high nickel superalloys and its prevention by the addition of gettering elements (Ti, Zr, Hf, La, and Mg) which form primary sulfides.(3,5) Early work by Merica and Waltenberg attributed sulfur's effects to the formation of a Ni-NiaS2 eutectic film but more recent studies suggest that a low melting grain boundary phase is not necessary for high temperature intergranular failure.(6) Less information has been reported on the embrittling of superalloy grain boundaries by phosphorus, however, the information available supports the hypothesis that phosphorus segregates to grain boundaries. grain It has also been reported that impurity enriched boundaries decrease intergranular embrittlement by hydrogen.(7,8,9) Both carbon and boron are intentional minor additions to 718. Residual carbon in the form of MC carbides have been identified cellularly and intragranularly in many commercial nickel-iron base superalloys including 718. Because carbon is used as a refining agent to deoxidize and desulfurize the charge and at the same time influences the fluidity and castability of liquid metal, carbon levels must be carefully controlled. Boron in the range of 0.003-0.030% is standardly added to improve stress rupture properties and hot workability of these alloys.(lO) Direct evidence for the atomic segregation of boron at some grain boundaries and for boride precipitation at others has been reported. The beneficial effects of boron are believed to be due to its ability to reduce grain boundary diffusion reactions and so delay the formation of denuded zones. Given the various and important relationships between impurity and minor alloy additions and intergranular material properties it is important to understand the interactions of these.elementL