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I.

INTRODUCTION

IN the design of most nickel-base superalloys, high strength at elevated temperature is of primary importance. Since failure during creep at high temperature occurs predominantly by intergranular fracture, the nature of grain boundary phases and the grain boundary chemistry plays a significant role. The segregation of impurities and trace elements to grain boundaries is a phenomenon which is now well documented, ~'2 and numerous studies have shown that creep and tensile properties are affected by the presence of trace elements. 1'3 However, not all trace elements are necessarily detrimental; many either improve or have no effect on material properties. Holt and Wallace 3 have recently classified the most common trace dements according to whether they have detrimental or beneficial effects, as shown in Table I. Among these, O and S can cause embrittlement during high temperature deformation of nickel-base alloys, 3 even at low ppm levels. For instance, an 80 pet loss of high temperature ductility has been observed for Ni-S alloys when the S content is increased from 10 to 20 ppm. 4 Similar drastic losses in ductility caused by O have also been reported for nickel-base superalloys when low ppm O level is introduced either by processing 5 or during high temperature air exposure. 6 These impurity elements must therefore be controlled either by removal during melting or alloying additions. The content of dissolved gases such as O, N, and H can be reduced by melting the alloy in vacuum using vacuum induction melting (VIM) techniques.7'8 In VIM an O content on the order of 20 to 50 ppm can be reached by reduction E COSANDEY,formerlyResearchAssociate, HenryKrumb Schoolof Mines, Columbia University, New York, NY 10027, is now Assistant Professor, Department of Mechanics and Materials Science, Rutgers University,P. O, Box 909, Piscataway,NJ 08854. D. LI, Visiting Scholar, and J. K. TIEN, Professorof Metallurgy,are with HenryKrumbSchoolof Mines, Columbia University, New York, NY 10027. F. SCZERZENIE, Staff Metallurgist, is with Special Metals Corporation, New Hartford, NY 13413. This paper is based on a presentation made at the symposium"The Role of Trace Elements and Interfaces in Creep Failure" held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorshipof The Mechanical Metallurgy Committeeof TMS-AIME. METALLURGICALTRANSACTIONSA

with carbon, but further O refinement and S removal can be achieved only through the addition of refining aids such as Ca, Mg, and lanthanide elements via the formation of oxides and sulfides. Lanthanide or rare earth elements (REM) are very attractive refining elements since they possess the lowest free energy for oxide and sulfide formation. 9'1~ Results have indicated that O and S refining following REM additions result in an improved ductility for both the hot workability and creep behavior in nickel-base alloys. 12-19 Results of a recent study 19 on the effect of various Ce additions on the final O and S contents