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Notch sensitivity has been connected with the presence of a film-like s-phase morphology in the grain boundaries of cast test bars of Alloy 718 of a problem material in which premature fracture occurred during testing of smooth stress rupture bars at 650 “C. A heat treatment designed to produce a plate-like S-phase morphology at the grain boundaries improved the material dramatically. Comparison with a reference material indicated the that the presence of coarse MC-carbides (0.9NbO.lTiC) at the grain boundaries attributed to the notch sensitivity problem. The degradation of the material by the carbides seems to be due to a considerable swelling action during a selective oxidation of these carbides. Additionally, a formation of a liquid phase at the carbides was confirmed. The volume increase of the selectively oxidized carbides was demonstrated by a an experiment in which two pieces of cast Alloy 718 with polished surfaces facing one another were pressed together and exposed to air at 650 “C for one hour. Introduction Stress rupture testing of smooth bars at 649 “C (l200 OF) at 621 MPa (90.000 Psi) for 20 hours is considered as a severe production test for Alloy 718 structural castings. For practical reasons such testing is performed on ‘cast to size’ bars rather than on specimens excised from the integral parts of the castings. The grain size of such ‘cast to size’ test bars is however finer than the grain size of most of the bulk material in a large structural casting and the testing of such bars is consequently of limited value. It is still valued as an indicator of proper process control. Due to a change of the thermal conditions during the casting process of the very large compressor rear frame for the General Electric CF6-80C aircraft engine most of the test bars failed the twenty hours test. The structural material of the casting was however not degraded which was confiied by testing performed on excised material from all frames were the ‘cast to size’ bars did not meet the requirements. Superalloys 718,625 and Various Derivatives Edited by Edward A. Lxia The Minerals, Metals & Materials Society, 1991 603 Although the practical problem had been resolved there was an interest to understand why most of the ‘cast to size’ test bars failed and an investigation was initiated at General Electric (GE), Engineering Materials Technology Laboratory (EMTL) in Evendale, Cincinnati, Ohio. In this investigation, however, no major differences could be found between failed and nonfailed bars. Grain size and dendritic arm spacing were the same. The only difference found was the microhardness ratio between interdendritic and dendritic core regions. A small but consistently larger hardness ratio was found in the failed bars. The fracture surface revealed a very pronounced intergrannlar fracture which emphasized the importance of the grain boundaries. By closer examination a film-like S-phase was found in all boundaries close to the fracture surfaces. In the non-failed bars such delta fihns were much less frequent. By consulting the extensive literature on Alloy 718 and similar alloys it is obvious that there are different problems and material mechanisms associated with this type of high temperature intergranular weakness. Main areas are notch sensitivity, crack growth and creep, inhomogeneous slip band deformation versus homogeneous deformation, ductility, environment, intergranular oxidation, chromium content, incipient melting of Laves phase and MC-carbides, microstructure and grain boundary morphology as well as heat treatment schedules. Most of the research has been performed on wrought material but the high temperature grain boundary weakness is a common feature with cast material. In table I, listing research contributions, the above areas are given as subheadings. The basic understanding has been elusive but recent research at GE has been fruitful due to the growing insight (1) (K.-M. Chang 1990). A recent review (2) (L. A. James 1989) covers the fatigue crack growth from many phenomenological points of view. TableIResearchAreasReIatedtoHighTempemture blk7.pU4bW~tmdl-iStOf~ (3)MmriarMdJ~es1964,(4)EEiseLrteil~(5)Wa~pndHoll1%~(6)Rrcymond1%~(7)W~onI97~ (8)MulletrmdDonachie1975,(9)Menidr1974 (1O)XiietaL 1988 (7)Wfion lwy (ll)hh?ickl974, (12)Smith mulhfiil97~ (13)Fhwn 198Q (14)SadammdaandShohinian 1980, (1S)SandezxetaL 198z (l4)ClavelandPineau 1982, (17)PedmnrmdPineaul9~ (18)CGnetaL 1984d(19)1989,(2O)lkmbupjetd 1985and(21)19& (22)chrmgl985mrd(l)l~(23)Jraesrmd~~~1~(~)~ondImmm~nl~(25)KouletaL 1% [26)chfwwdil989 Slivbandvemrr homoRsreour&+onnationmeehmrinn~toaerificolsizeofthehmdeninav~ (~)PmJonisetoLl965),((28)cDzmmulpinemc1~(7)wilron1~(29)Ob~etaL 1974, (11)h4enick1974and(9)197~ (14)clavelandpinemr 1% (1S)SanderetaL 1981,(30)uaVeletd 1980 (21)ThambumjetaL 19% (31)Matrhionnietal. 1988, (26)&hatuwedil989 DUCtili& (4)Eiselstein 1%5, (32)iUrnyhandManim1%9,(7)~on 1973 (9)Mekkl976 EnvircMmentasakevfoctor (13)Fknwn 1984 (14)SadaIwh andShahi&n I& (l7)PednmandPheaul~ (20)llwmbumjetd 1985and(2l)ls!& (22) cYlaJlg1985,(33)AdieuetaL 1989 CYtmmiumwntent (22)chlmg19~ (39)IolwetaL 1989 604 (40) Kiwmvsky et aL 1989, (41) Thomron and Gena~lu 1983, (42) Cieskzk et al 1989, (43) Bouse 1989 cmbon andcarbides (3) Maniar and James 1%4, (4) Eirektein 1%5, (5) Wagtw and Hall 1%5, (44) Kinnan and Wwington 1970, (45) Sbvup and R@ese I%!$ (32) Muzyka and Mark IWS, (46) Bamasvamy et aL 1972, (11) Menick 1974, (12) Smith and Miilll97& (47) Jackman et aL 198Q (48) Lerch 1982 (17) Pe&vn and Pineau 19g2, (34) Ratchet andRemy 1932 and (35) 1983, (41) llwmson and Gencuiu 1983, (49) Moyer 1984, (21) Thmnbunaj et al. 1% (1) Chang 1990, (SO) Mius and Blackbum 19% hBmsbudu?e and Smin boundarymomhokxy (4) Eisektebz 1965, (6) Raymond 1967, (51) Boesch and Cana& 1%9, (44) Kbman and WmrLtgron 1970. (9) Menick 1976 (52) Bouse and Schitke 1980, (14) Sakmmda and Shinian 1984 (18) Chen et aL 1984 and (19) 1989, (49) Moyer 1984, (20) Thambw@ et CrL 1985 and (21) 1986j (22) Gang 1985 and (1) 199Q (23) James and Mills 1985, (53) Zhang et aL 1987 (54) Bmoks and Bnk’gw 198& (24) Koul and lmmari&on 198& (25) Kind et aL 19865 (10) Xi et aL 1% (43) Bouse 19m, (55) Gmkon and Radavkb 1989, (42) CTeskak et a.? 1989, (56) Gou et al. 1989, (39) Joner et aL 19es. (40) fiorovsky et aL 1989, (57) Kmuger 1989, (SO) Milt3 and Bkkbwn 19% Effectdf%heat-andsohtibntemrxmtun3 (4) Eisektein l%S, (5) Wagxer and Hall 1%5, (7) Wilson 1974 (8) Muller and Donachie 1975, (9) Menick 1976 (12) Smith and Miill197& (14) Sadammdk and Shahinian 1984 (20) lkmbumj et al 1985 and (21) 19% (22) Chang 1985 and (I) lm, (24) Koul and Jmmarigeon 1% (50) Mii lmnd Bkxkhum 1989s There seems to be two opposing views on the effect 01 S-phase on the mechanical properties. The frost and for a long time prevailing view (6,10,23,44,50.51,57) is that the S-phase degrades the mechanical properties, especiahy the low temperature fracture toughness and the low cycle fatigue properties. The main reason for this degradation is that the S-phase is niobium-rich (Ni3Cb) and the surrounding matrix will consequently be depleted on this element during the precipitation. Since the main hardening precipitate, the Y”, is of the same chemical composition as the Sphase it will later during the age hn*dening be difficult to get proper hardening close to th:, S-precipitates, a zone denuded in 7” will develop (6,44). Accordingly, this zone will be weak. The second view is that the presence of the S-phase at the grain boundaries is good for the high temperature stress rupture and crack growth properties (9, 10, 14, 18, 20, 21,24,25, 53, 54, 57). This latter view seems at fust glance to be in conflict with the first view as well as with our observations. The S-phase generally precipitates along specific habitus (5) in the matrix by which a platelie morphology is produced. S-phase of film-like morphology is often reported as ‘grain boundary 6’ (54) which makes sense since it consists of individual particles though very closely spaced which is revealed at higher magnification. Such ‘films’ precipitates in the tower part of the temperature region where S-phase precipitates. At a higher temperature, the S -phase grain boundary nucleuses grow along the favorable habitus into the surrounding grain matrixes by which the film-like morphology gradually disappears. In cast alloys such nucleation and growth at grain boundaries is enhanced by high niobium concentrations (55). If the depletion of the niobium close to the d-phase and the consequent weakness in this zone is taken into consideration any S’film’ morphology will provide for an easy crack path while a morphology with S-phase plates extending into the surrouuding grains will produce a tortuous path. As a consequence the latter type of morphology should improve the mechanical properties of the material and this point of view was to be the working hypothesis for the further investigations performed at Volvo Flygmotor AB in Sweden. 605

Grain Boundary &#948;-Phase Morphologies, Carbides and Notch Rupture Sensitivity of Cast Alloy 718

Grain Boundary δ-Phase Morphologies, Carbides and Notch Rupture Sensitivity of Cast Alloy 718