Effect of heat treatments on phase chemistry of the Nickel-Base superalloy SRR 99
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I.
INTRODUCTION
AMONG the properties that control high-temperature performance of nickel-base alloys, phase chemistry and stability of the y ' phase are of major importance. The effect of heat treatments on phase chemistry of SRR 99 has been investigated by Hopgood et al. using field-ion microscopy (FIM) and EDX methods. Ill They conclude that the composition of the y' phase is largely unchanged by heat treatments at 1100 ~ and 870 ~ except for Cr, whereas the composition of the matrix is strongly dependent on heat treatment. Slight changes of composition measured at the y / y ' interfaces together with differences in precipitate distribution are believed to strongly influence the mechanical properties. Partition ratios P -- c~,/c~ and site preferences SP = Pn/PI, with PI being the probability for an element to occupy a face-center site (Ni site) and PII that for an element to occupy a corner site (A1 site) of the L12 unit cell, have been measured by several authors. ~2-7j Guard and Westbrook t2i found that in Ni3A1, elements Co and Cu substitute for Ni sites; Ti, Si, V, and Mn substitute for A1 sites; and Fe, Cr, and Mo have equal preference for both sites. They suggested that the partition behavior of an element is due to its atomic size. Elements nearest in size to either Ni or A1 have the greatest solubility in the y' phase. Karg et al.t3] showed that Nb, Ta, Ti, and V substitute for A1 sites, while Cr, Mo, and W are distributed on both Ni and A1 sites. However, the addition of further elements can change the partitioning behavior, as has been shown by Loomis et al. [4] Partition ratios of 15 nickel-base commercial alloys were determined by Kriege and Baris tSJ and showed that in the sequence of decreasing solubility, Ti, Ta, and A1 are strong y'-forming elements, while Cr, Co, and W are matrix-forming elements. A study on site preferences
R. SCHMIDT, Dipl.-Phys., and M. FELLER-KNIEPMEIER, Professor, are with the Institut fOr MetaUforschung, Technische Universitat Berlin, BH 18, Stral3e des 17. Juni 135, D-1000 Berlin 12. Manuscript submitted May 17, 1991. METALLURGICAL TRANSACTIONS A
(SPs) of the elements in the y ' phase of CMSX2 was performed by Blavette and c o - w o r k e r s . [6,7] In this work, the distribution of elements in SRR 99 after four different heat treatments is determined in order to detect the influence of growth and annealing conditions on phase compositions.
II.
EXPERIMENTAL
A. Material and Heat Treatments The nominal composition of SRR 99 is listed in Table I. Specimens were analyzed after solution treatment only (crystal A), after single- and double-stage annealing (crystals B and C), and after overaging (crystal D). The different heat treatments applied are given in Table II. Foils for transmission electron microscopy were cut in (001) orientation and thinned by electropolishing using a solution of two parts perchloric acid in eight parts ethanol. Typical TEM micrographs representing the heattreated specimens A, B, C, and D are shown in Figure 1. B. Optimizing the Experimental Co
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