Effects of cobalt concentration on the relative resistance to octahedral and cube slip in nickel-base superalloys
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INTRODUCTION
THE deformation behaviors
of L12 intermetallic compounds and Ni-base superalloys strengthened by the L12 3/'phase are complex. This subject has been recently reviewed in depth by Pope and Ezz. I~l The present study is concerned with an effect of the alloying element Co on one aspect of the deformation behavior of ~/-strengthened Ni-base superalloys. It is well known that single crystals of L12 intermetallics and 3/-strengthened superalloys can deform at elevated temperature by slip on the cube, rather than octahedral, plane when stressed in directions near (111). In the ordered fcc L12 phase, slip occurs by pairs of total dislocations necessarily connected by an antiphase boundary. The large Burgers vector for cube slip is compensated by a relatively lower antiphase boundary energy on the cube plane than on the octahedral plane. Thus, in the L12 phase, the critical resolved shear stress, CRSS, for cube slip is relatively closer to that for octahedral slip than in the disordered fcc structure. Still, it was surprising to observe evidence of primary cube slip at room temperature in an advanced single crystal superalloy, tzl No study of the effects of composition on the relative CRSS for octahedral and cube slip has been performed. However, several individual alloys have been studied with respect to yielding behavior as a function of crystal orientation. Analysis of these data suggested that Co concentration may have an important effect on the relative CRSS for octahedral and cube slip. These earlier results will be discussed together with the present results of compressive yield tests on crystals of alloys with two different Co levels in both of two different alloy bases, having axes near [001] and [111] at 760 *C, a temperature common to many previous investigations.
GENE E. BOBECK is with the Department of Metallurgical and Mining Engineering, University of Idaho, Moscow, ID 03843. R.V. MINER is Deputy Chief, Analytical Science Branch, NASA Lewis Research Center, 21000 Brookpark Road, Cleveland, OH 44135. Manuscript submitted December 7, 1987.
METALLURGICALTRANSACTIONS A
MATERIALS AND PROCEDURES
A few years ago, studies were conducted on many alloy systems to see if Co usage could be reduced because of the high price then demanded by the producing nations. Cast single crystals of two alloys with different Co concentrations based on MAR-M247 were provided by Rolls Royce, Ltd., Darby, England. MAR-M247 is a development of the Martin Marietta Corporation. The crystals measured about 11 by 5.5 by 1.1 cm. Chemical analyses are listed in Table I together with the nominal composition of MARM247. The other data presented in Table I describing the constitution of the phases in the alloys will be discussed later. It may be seen that the concentrations of the solute elements in the two alloys differ significantly only for Co, being either 5 or 10 wt pct. The two alloys were heat treated slightly differently. It was determined by Roils Royce that the 5Co alloy should be solution treated at 1320 ~ 20
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