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An investigation on notch embrittlement of a Ni-base superalloy
ScriptaMetallurgicaet Mate&l& Vol. 32. No. 8. pp. 1145-l148.1995 Copyright8 1995Elsevia ScienceLtd Printedin the USA. Au rights -ed 095&716X/95$9.50+ .OO
0956-716X(95)00115-8
AN INVESTIGATION
ON NOTCH EMBRITTLEMENT
OF A Ni-BASE SUPERALLOY
S. Srinivas’, D.V.V. Satyanarayana’, D. Gopikrishna” and M.C.Pandey’ ‘Defence Metallurgical Research Laboratory, Kanchanbagh Hyderabad-500 258, India “Mishra Dhatu Nigam, Kanchanbagh, Hyderabad-500 258, India (Received May 26, 1994) (Revised Ndvember 4, 1994) Introduction Superalloy ‘718, which is strengthened by y’ and y” precipitates, is one of the most widely used nickel based superalloys for high temperature applications [l-4]. The alloy also contains incoherent delta phase and MC type of carbides, and in rare cases the Laves phase and minor phases like borides [5]. The performance of this alloy depends on the relative amount of all these phases. Due to its prominent overall mechanical properties up to 650” C and fairly good formability, the alloy has found extensive usage in gas turbine disc applications. In these applications, the notch stress rupture performance is extremely important. In service, high temperature components are often subjected to a complex stress system which may vary from a uniaxial to a multi-axial stress state [6]. Circumferentially notched round bars have been used to investigate the creep rupture behaviour of other superalloys. The experimental results obtained in Inconel X-750 [6] and Nimonic 80A [7] have shown that the creep damage mechanisms were strongly influenced by the state of stress imposed by the notch. Several stu’dies on superalloy 718 [g-10] at high temperature showed variations in notch rupture life, depending on the size, morphology and distribution of delta phase. This paper reports the investigation carried out to determine the factors responsible for inconsistent notch rupture behaviour in superalloy 718. Exnerimental
Procedure
Specimens for this study were obtained from a heat that was VIMWAR processed and upset forged pancakes produced by Mishra Dhatu Nigam, Hyderabad. The nominal composition of the superalloy 718 used in this investigation is in wt % : C - 0.03, Cr - 17 7, Ni - 53, MO - 3 1, Ti - I 01, Al - 0.46, Nb - 5.23, Fe- balance. A number of test bars, were solution treated at 955* C/l h/water quenched and aged at 720” C/8 h, tirnace cooled to 620” C/8 h, air cooled and subsequently machined to combination smooth and notched (Kt = 3.2) stress rupture specimens, as per the dimensions shown in Fig 1 These specimens were tested at 650* C and at a constant load of 690 MPa. The rupture lives of a few specimens failed in the notched portion were found to be inferior (
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Vol. 32, No. 8
confirmed by EPMA as carbides containing 80.50 wt% Nb and 8.70 wt% Ti. Examination of the microstructure in the specimens that failed in the notch portion showed little or no amount of delta phase in certain regions and these were associated with extensive grain boundary cavitation (Fig. 5) The microstructure of the specimens that failed in the smooth region exhibited uniformly dispersed tine delta phase and absence of any grain boundary cavities (Fig 6). Discussion
The presence of carbide clusters at the fracture surface in the specimens that failed in the notched portions indicates that heavy segregation of carbides is one of the factors responsible for notch embrittlement Recently Nix [I 11 reviewed the mechanisms of high temperature intergranular creep fracture and pointed out that cavity nucleation at stress concentrations is very much pronounced under multi-axial stress states, usually produced at notches and at other geometrical irregularities. The results shdw that the carbides have promoted local stress concentration and plastic strain incompatibility between the matrix and the hard carbides resulting in early cavity initiation and fracture (Fig. 2a) It may also be noted in Fig 2b that intergranular mode of fracture in the specimens that failed in the notched portion was due to extensive grain boundary cavitation in the lean delta phase region as shown in Fig. 5 On the other hand, transgranular dimple fracture in the specimens that failed in the smooth portion indicates that the finely dispersed delta phase altered the fracture process from an intergranular fracture to a ductile dimple fracture mode. Thus, it is believed that the presence of delta phase impeded the grain boundary sliding and arrested cracks from further extension or they may have inhibited link-up and growth of cavities along grain boundaries. These observations are similar to those of other investigators [S-IO, 121 who have reported a loss of notch rupture lives due to the absence of delta phase at the grain boundaries. It is, therefore, believed that the combined effect of the presence of carbide clusters and the lean delta phase regions led to notch embrittlement This work shows that there was considerable segregation of elements like Nb, Ti, and C in the alloy leading to inhomogeneity and inconsistent notch rupture behaviour Coliclusions
The notch embrittlement of superalloy 718 is attributed to the combined effect of clustered (Nb. Ti) carbides and the absence of an optimum amount of delta phase at the grain boundaries Acknowledgements
We would like to express our gratitude to Mr S L.N Acharyulu, Director, DMRL for permission to publish this paper and to Dr. D Banerjee and Mr S. N Jha for many useful discussions References 1. 2. 3. 4. 5 6. 7. 8. 9. 10. 11. 12
E. A. Loria, Journal of Metals, 44, 33 (1992) J. M. Oblak, D. F. Paulonis, and D S. Duvall, Met Trans , 5, 143 (1974) I. Kirman and D. H. Warrington, J Iron Steel Inst , 99, 1264 (1967). I. Kirman and D. H Warrington, Met Trans., I, 2667 ( 1970) D D. Krueger, Superalloy 718 - Metallurgy and applications. Ed 1 E A Loria, The Minerals Metals and Materials Society, p 279 (1989). M. C. Pandey, A. K Mukherjee and D. M. R Taplin, J. ofMat Sci., 20, 1201 (1985) M. S. Loveday and B. F. Dyson, Mechanical Behaviour ofMaterials. Ed , K J Miller and R F Smith, ICM 3, 2, p. 213, Pergamon Press, Oxford (1980). James F. Muller and Mathew J. Donachie Jr, Met Trans A, GA, 2221 (1975). Donald R. Muzyka and G. N Maniar, Met. Eng. Quart , 9. 23 (1969). J P. Stroup and R. A. Heacox, Journal of Metals, 21, 46 (1969) W. D. Nix, Mechanical Behaviour of Materials-V, Ed, M.G Yan et al., ICM-5, 2, p 1383. Pergamon Press, Oxford (1987). C Q. Zhang, R. W. Zhang and M G. Yan, Mechanical Behaviour of Materials-V, Ed., M G Yan et al 1 ICM-5, 1, p. 1149, Pergamon Press, Oxford (1987).
Vol. 32, No. 8
NOTCHFMBRRTLEMENT
Fig. 1. Combination smooth- and- notched test stress- rupture specimen (dimensions are in mm.)
Fig.2. Fractographs of a specimen that failed in the notch portion showing (a) clustered particles and b) intergranular mode of fracture.
Fig. 3. Fractograph of a specimen that failed in the smooth portion showing transgranular mode of fracture.
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t k”-
ul z
W t-
z -.. Fig. 4. EDAX pattern of the clustered particles shown in Fig. 2a.
Fig. 5 Optical micrograph showing grain boundary cavities in the lean delta phase region of a specimen that failed in the notch portion.
Fig. 6. Optical micrograph showing uniformly distributed fine delta phase in a specimen that failed in the smooth portion.
0956-716X(95)00115-8
AN INVESTIGATION
ON NOTCH EMBRITTLEMENT
OF A Ni-BASE SUPERALLOY
S. Srinivas’, D.V.V. Satyanarayana’, D. Gopikrishna” and M.C.Pandey’ ‘Defence Metallurgical Research Laboratory, Kanchanbagh Hyderabad-500 258, India “Mishra Dhatu Nigam, Kanchanbagh, Hyderabad-500 258, India (Received May 26, 1994) (Revised Ndvember 4, 1994) Introduction Superalloy ‘718, which is strengthened by y’ and y” precipitates, is one of the most widely used nickel based superalloys for high temperature applications [l-4]. The alloy also contains incoherent delta phase and MC type of carbides, and in rare cases the Laves phase and minor phases like borides [5]. The performance of this alloy depends on the relative amount of all these phases. Due to its prominent overall mechanical properties up to 650” C and fairly good formability, the alloy has found extensive usage in gas turbine disc applications. In these applications, the notch stress rupture performance is extremely important. In service, high temperature components are often subjected to a complex stress system which may vary from a uniaxial to a multi-axial stress state [6]. Circumferentially notched round bars have been used to investigate the creep rupture behaviour of other superalloys. The experimental results obtained in Inconel X-750 [6] and Nimonic 80A [7] have shown that the creep damage mechanisms were strongly influenced by the state of stress imposed by the notch. Several stu’dies on superalloy 718 [g-10] at high temperature showed variations in notch rupture life, depending on the size, morphology and distribution of delta phase. This paper reports the investigation carried out to determine the factors responsible for inconsistent notch rupture behaviour in superalloy 718. Exnerimental
Procedure
Specimens for this study were obtained from a heat that was VIMWAR processed and upset forged pancakes produced by Mishra Dhatu Nigam, Hyderabad. The nominal composition of the superalloy 718 used in this investigation is in wt % : C - 0.03, Cr - 17 7, Ni - 53, MO - 3 1, Ti - I 01, Al - 0.46, Nb - 5.23, Fe- balance. A number of test bars, were solution treated at 955* C/l h/water quenched and aged at 720” C/8 h, tirnace cooled to 620” C/8 h, air cooled and subsequently machined to combination smooth and notched (Kt = 3.2) stress rupture specimens, as per the dimensions shown in Fig 1 These specimens were tested at 650* C and at a constant load of 690 MPa. The rupture lives of a few specimens failed in the notched portion were found to be inferior (
1145
1146
NOTCH EMBRlTI’L.EMENT
Vol. 32, No. 8
confirmed by EPMA as carbides containing 80.50 wt% Nb and 8.70 wt% Ti. Examination of the microstructure in the specimens that failed in the notch portion showed little or no amount of delta phase in certain regions and these were associated with extensive grain boundary cavitation (Fig. 5) The microstructure of the specimens that failed in the smooth region exhibited uniformly dispersed tine delta phase and absence of any grain boundary cavities (Fig 6). Discussion
The presence of carbide clusters at the fracture surface in the specimens that failed in the notched portions indicates that heavy segregation of carbides is one of the factors responsible for notch embrittlement Recently Nix [I 11 reviewed the mechanisms of high temperature intergranular creep fracture and pointed out that cavity nucleation at stress concentrations is very much pronounced under multi-axial stress states, usually produced at notches and at other geometrical irregularities. The results shdw that the carbides have promoted local stress concentration and plastic strain incompatibility between the matrix and the hard carbides resulting in early cavity initiation and fracture (Fig. 2a) It may also be noted in Fig 2b that intergranular mode of fracture in the specimens that failed in the notched portion was due to extensive grain boundary cavitation in the lean delta phase region as shown in Fig. 5 On the other hand, transgranular dimple fracture in the specimens that failed in the smooth portion indicates that the finely dispersed delta phase altered the fracture process from an intergranular fracture to a ductile dimple fracture mode. Thus, it is believed that the presence of delta phase impeded the grain boundary sliding and arrested cracks from further extension or they may have inhibited link-up and growth of cavities along grain boundaries. These observations are similar to those of other investigators [S-IO, 121 who have reported a loss of notch rupture lives due to the absence of delta phase at the grain boundaries. It is, therefore, believed that the combined effect of the presence of carbide clusters and the lean delta phase regions led to notch embrittlement This work shows that there was considerable segregation of elements like Nb, Ti, and C in the alloy leading to inhomogeneity and inconsistent notch rupture behaviour Coliclusions
The notch embrittlement of superalloy 718 is attributed to the combined effect of clustered (Nb. Ti) carbides and the absence of an optimum amount of delta phase at the grain boundaries Acknowledgements
We would like to express our gratitude to Mr S L.N Acharyulu, Director, DMRL for permission to publish this paper and to Dr. D Banerjee and Mr S. N Jha for many useful discussions References 1. 2. 3. 4. 5 6. 7. 8. 9. 10. 11. 12
E. A. Loria, Journal of Metals, 44, 33 (1992) J. M. Oblak, D. F. Paulonis, and D S. Duvall, Met Trans , 5, 143 (1974) I. Kirman and D. H. Warrington, J Iron Steel Inst , 99, 1264 (1967). I. Kirman and D. H Warrington, Met Trans., I, 2667 ( 1970) D D. Krueger, Superalloy 718 - Metallurgy and applications. Ed 1 E A Loria, The Minerals Metals and Materials Society, p 279 (1989). M. C. Pandey, A. K Mukherjee and D. M. R Taplin, J. ofMat Sci., 20, 1201 (1985) M. S. Loveday and B. F. Dyson, Mechanical Behaviour ofMaterials. Ed , K J Miller and R F Smith, ICM 3, 2, p. 213, Pergamon Press, Oxford (1980). James F. Muller and Mathew J. Donachie Jr, Met Trans A, GA, 2221 (1975). Donald R. Muzyka and G. N Maniar, Met. Eng. Quart , 9. 23 (1969). J P. Stroup and R. A. Heacox, Journal of Metals, 21, 46 (1969) W. D. Nix, Mechanical Behaviour of Materials-V, Ed, M.G Yan et al., ICM-5, 2, p 1383. Pergamon Press, Oxford (1987). C Q. Zhang, R. W. Zhang and M G. Yan, Mechanical Behaviour of Materials-V, Ed., M G Yan et al 1 ICM-5, 1, p. 1149, Pergamon Press, Oxford (1987).
Vol. 32, No. 8
NOTCHFMBRRTLEMENT
Fig. 1. Combination smooth- and- notched test stress- rupture specimen (dimensions are in mm.)
Fig.2. Fractographs of a specimen that failed in the notch portion showing (a) clustered particles and b) intergranular mode of fracture.
Fig. 3. Fractograph of a specimen that failed in the smooth portion showing transgranular mode of fracture.
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NOTCHEMBFSITLEMENT
Vol. 32. No. 8
t k”-
ul z
W t-
z -.. Fig. 4. EDAX pattern of the clustered particles shown in Fig. 2a.
Fig. 5 Optical micrograph showing grain boundary cavities in the lean delta phase region of a specimen that failed in the notch portion.
Fig. 6. Optical micrograph showing uniformly distributed fine delta phase in a specimen that failed in the smooth portion.