Mierostructural changes during annealing of cast Ni-Cr-Hf alloys P. Acosta P. Adeva G. Caruana M. Aballe In order to gain knowledge of the nickel-rich corner of the Ni-Cr-Hf phase diagram, microstructures of cast alloys and their evolution towards equilibrium conditions have been followed by optical and scanning electron microscopy, microprobe analysis, and quantitative metallography. The evolution of as-cast microstructures involves the development of a NirHf plate-like precipitate in two- phase alloys, and various morphological changes in three-phase alloys. As-cast structures are compared with those obtained by arc melting. Direct processing of the scanning electron microst'ope image has proved to be a valuable tool for microstructural characterization of the transforming phases. MSTl34e @ 1986 The Institute of Metals. Manuscript received 14 August 1985; infinalform 4 November 1985. The authors are in the Centro Nacional de Investigaciones Me talltrgicas, Madrid, Spain. lntroduction Some superalloys.and heat resisting alloys contain up to 3'5 wt-/" hafniuml to provide additional strength from the formation of one or more of the intermetallic compounds (the characteristics of which have been reviewed by Kaufman and Nesor2) and to improve the consistency of intermediate creep properties. The MAR-M004 and MAR- M007 alloys have been developed to this end.3 On the other hand, hafnium has a high affinity for carbon and oxygen, and its presence can easily lead to the formation of carbides and oxides. An example of this affinity has been shown by Yunrong and Yulina who monitored the increase in hafnium carbide at the expense of the NirHf-phase during the prolonged heat treatment of a hafnium-bearing alloy. The ternary Ni-Cr-Hf system is not fully charactertzed, although Nash and Wests have described in detail some isothermal sections. In the nickel-rich corner, arrd at 1273 and 1473 K, several phase fields have been delineated for the equilibrium state (Figs. I and 2). One aspect which remains to be more fully studied is the evolution towards equilibrium on annealing the as-cast structure. This could be important for superalloys used in the as-cast state where service life is strongly influenced by the microstructural changes at elevated temperatures. Experimental procedure Alloys were selected on the basis of previous works to study structures containing y-phase (nickel-base solid solution) plus one or more of the phases NisHf, Ni7Hf2, and a (chromium-base solid solution). The nominal compositions of the alloys which are considered in this paper are shown in Table 1. Of these, alloys I and 2 were prepared by the authors, and alloys 3 and 4, prepared by Nash and West6 and supplied in the as-cast state after arc melting, were used for comparison. Starting materials were 99.9/"Ni pellets, electrolytic purity Cr, and 99.9%Hf (including 3/,Zr). Before melting the required compositions, a 50Ni-50Cr master alloy was prepared, and this was subsequently melted with adequate amounts of Ni and Hf to obtain the final alloys. A Balzers induction furnace VSG 02 was used, and the alloys were melted in a high-purity alumina crucible under an argon atmosphere and were allowed to solidify in the crucible. Weight losses after melting were negligible. The alloys examined in the as-cast state showed a uniform structure and a small number of inclusions in the areas close to the external surface. Samples were prepared by cutting the casting, and thermal treatments were carried out in silica capsules under partial argon atmosphere at 1273 and 1373K for times between 8 and 168h. Conventional metallographic preparation for optical examination included mechanical polishing and etching in a reagent consisting of ll/"HF @0vol.-\),20%HNO3, and 70\ water (in vol.-/"). Scanning electron microscopy (SEM) and microanalysis was carried out on unetched samples for microanalysis, and etched ones for quantitative metallo- graphy (QM). A JEOL JXA5OA microprobe interfaced to a Kontron IBAS2 image analyser was used for micro- structural characterization and QM of the samples.7 Some observations by transmission electron microscopy (TEM) were made on cast samples using a JEOL 1008 apparatus. Microprobe data were corrected using a ZAF procedure developed by the authors and carried out using an HP 85 microcomputer.s The number of fields examined for QM work was at least24, the corresponding images being stored on disc and further processed for measurement of the volume fraction of the y-phase Y,, and the interface area per unit volume between the 7- and NirHf-phases S,, respectively. Vickers microhardness measurements were also carried out, with 100 g load and 5 s indentation time. a+ NirHfr+NirHf lsothermal (Ref.5) N i7Hf2 section of Hf,at.-% Ni-Cr-Hf system at 1273K "\o .b /+NirHf, Materials Science and Technology June 1986 Vol.2 539