Short-fatigue-crack growth in a nickel-base superalloy at room and elevated temperature

Int J Fatigue 13 No 2 (1991) pp 133-138

Short-fatigue-crack growth in a nickel-base superalloy at room and elevated temperature J . C . Healy, L. G r a b o w s k i and C.J. Beevers

An optical system, developed to monitor short-crack growth at elevated temperature, was used to study the fatigue behaviour of Waspaloy. Tests conducted at 19 °C and 500 °C revealed that the dominant mechanism of crack formation was slip band cracking. Crack formation was also associated with coarse carbide particles within the matrix. The dominant failure mechanism at 19 °C and 500 °C was one of mixed mode-I and -II fracture, with the mode-II shear displacements giving rise to nonclosure induced by surface roughness. Oxide- and plasticity-induced non-closure processes made only a minor contribution to the overall growth process. Shortfatigue-crack growth measured at R = 0.1 was faster at 500 °C than at 19 °C at an equivalent value of ~KI. This was attributed to a change in slip character from highly planar to one involving increasing amounts of cross slip at 19°C and 500°C, respectively.

Key words: crack formation; slip band cracking; short cracks; temperature effects

In the quest to develop new materials capable of operating at elevated temperatures there is a need to understand the principle failure mechanisms governing component life. In the gas turbine compressor engine this reduces to understanding fatigue failure under extreme conditions of temperature and stress. With the current drive to produce cleaner materials of high integrity and surface quality there is an increasing need to study the fatigue process down to crack lengths approaching the microstructure. In recent years many attempts have been made to describe fatigue crack growth in this regime, 1-7 but very few investigators have managed to extend their studies to realistic engine temperatures. 6,z The purpose of the current investigation is to address this problem by studying short-crack growth in Waspaloy at room and elevated temperatures.

Material The material used in this investigation is Waspaloy, whose microstructure is shown in Fig. 1. The material was in disc form and had been given the following heat treatments: (1) (2) (3)

1115 °C for 4 h, oil quenched, 800 °C for 4 h, air cooled, 760 °C for 16 h, air cooled.

Its nominal composition and mechanical properties measured at 19 °C and 500 °C are given in Table 1.

Experimental Specimen geometry Two types of specimen were used to investigate fatigue behaviour at room and elevated temperature. In the study of 'long'-crack growth, a corner notch geometry was employed as shown in Fig. 2(a); and in the study of 'short'-crack growth, a shallow notch (Kt = 1.03) geometry was chosen as shown in Fig. 2(b). Prior to testing the surface of each specimen was mechanically polished to a finish of 0.25 ~m using both silicon carbide and diamond paste. Etching was then achieved using Kalling's reagent No 2.

Fatigue testing Constant-amplitude fatigue testing was carried out in laboratory air using an Amsler Vibrophore machine operating at a frequency of 100 Hz. At 19 °C, testing was conducted at load ratios (R) of 0.1 and 0.5. Long-crack thresholds were achieved by adopting a conventional load shedding technique down to growth rates (da/dN) approaching 10 -s mm/cycle. Subsequent propagation was then monitored to produce post-threshold crack growth. Short-crack testing was achieved under a constant load range at a maximum bulk stress level of 0.75% to 0.80% proof stress (ps). Surface crack lengths (2c) ranged from 20 ~m to 400 ~m. In both studies, cellulose acetate replicas were used to record the surface crack length. A two-stage process involving precracking and load shedding at 19 °C prior to elevated-temperature fatigue testing was used. At 500 °C long- and short-fatigue-crack studies were conducted as above, at a relative bulk stress level for short cracks ranging from 0.8 to 1.2 of the proof stress

0142-1123/91/020133-06 © 1991 Butterworth-Heinemann Ltd Int J Fatigue March 1991

133

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Table 1. Nominal composition and mechanical properties of Waspaloy Composition (wt.%) Cr 19.5

Co 13.5

Mo 4.2

AI 1.4

Ti 3.0

Fe C Ni 1.0 0.06 Balance

Mechanical properties

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Fig. 2 The test geometry: (a) corner notched fatigue specimen, (b) short-fatigue-crack specimen

inclined at between 30 ° and 60 ° to the stress axis, Fig. 3(a). On these planes high levels of critically resolved stress are to be expected. The formation of other cracks was associated with TiC carbides, Fig. 3(b).

Fatigue studies at 19 °C Temperature (°C) 19 500

0.2% ps

Ultimate tensile strength

931 830

1351 1231

measured at 500 °C. Surface crack lengths were measured using an optical system similar to that of Larsen.S Images of the fatigue surface were assessed directly through a small silica quartz window of a resistance furnace containing the specimen. These were then examined using an image analyser.

Growth rates and stress analysis

Fatigue data generated at positive stress ratios at ambient temperature are presented in Fig. 4. Comparison of the longand short-crack data revealed the classic 'short-crack effect',

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In both the room- and elevated-temperature studies crack shapes were consistent and dependent on geometry. Longcrack growth was reflected in a crack shape defined by a sector of 90 ° (quarter penny) and short-crack growth was typified by a semicircular crack shape (half-penny). In both cases the aspect ratio 2c/a = 2, where a relates to the throughsection crack length. Growth rates were determined by dividing successive crack depths by the number of cycles over which each increment of crack growth had occurred. When there was no apparent growth, a rate of 10 -s ram/cycle was assigned to indicate temporary arrest. Mode-I stress intensity solutions for short- and longcrack growth were calculated using the solutions of Pickard. 9

Results Initiation Freely initiated cracks in the shallow notched specimens occurred predominantly in the larger grains along slip bands

134

Fig. 3 Crack formation sites in Waspaloy at 19 °C: (a) slip band cracking, (b) carbide cracking

Int J Fatigue March 1991

5(b) shows the non-closure of crack faces and local contact points of an unloaded crack that was grown at 500 °C.

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Fatigue studies at 500 °C

R=05 10-5

The fatigue crack growth data generated at 500 °C are presented in Fig. 6. The maximum bulk stress level for short cracks ranged from 0.8 to 1.2 of the ps at 500 °C. Comparison of the data revealed similar trends to those exhibited by the material at 19 °C, but to a less pronounced effect. At values of AK1 less than 8 MPa V/m, short-crack growth was intermittent, ceasing to grow below 6 MPa X/m. At higher stress intensity ranges the trend in short-crack propagation was one of increasing growth rate with increased AK1.

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Fig. 4 Fatigue crack growth in Waspaloy at 19 °C

with short-crack growth rates occurring at stress intensity levels below the AKI threshold at R = 0.1 and 0.5. Scatter in the data was found to decrease with increasing AK1 from a minimum value at AK1 = 1.3 MPa X/m to a maximum value at AK1 = 16.8 MPa X/m. Variability in the short-crack data was attributed to crack tip/microstructural interactions, with deflections occurring at inclusions, annealing twins, intersecting slip systems, and grain boundaries, Fig. 5(a). The observation of a short crack in Fig. 5(a) at minimum load shows regions of the crack that have not fully closed, thus indicating the existence of roughness-induced closure. Similar observations were also obtained from tests at 500 °C. Figure

A comparison of the growth generated at 19 °C and 500 °C produced the scatter diagram shown in Fig. 7. Close examination of this plot revealed two trends. At higher AKI values the growth rates measured at 500 °C were marginally greater than those at 19 °C, and the combined data normalized to a convergent trend of minimum scatter at the higher AKt values. Close examination of the fracture profile at 19 °C revealed that crack path deflections were dependent on slip character,

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Int J F a t i g u e M a r c h 1991

102

A/( I ( MPa ,/'~) Fig. 7 A comparison of short-crack data

135

Fig. 8(a). At 500 °C the fracture profile was less faceted and smoother (macroscopically planar), Fig. 8(b).

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The data presented in Fig. 7 were also analysed in terms of AK211, AK~ and AKN, where

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Comparison of the data, Fig. 9, revealed that none of these parameters proved any more successful than AK~. This was fortunate, allowing direct comparison of the current data with those of other investigators in both the short- and longcrack regimes.

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Initiation

Fatigue crack formation at 19 °C and 500 °C in Waspaloy was predominantly associated with slip band cracking along

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136

planes orientated at between 30° and 60° to the maximum stress direction. This mechanism has been reported by others 7"1°-12 who have observed slip band cracking on the (110} (111) system for a number of precipitation-strengthened alloys. The formation of propagating cracks as the consequence of carbide cracking can be argued in terms of the local stress state. 13 If there exists a significant difference in moduli between the carbide and matrix, local tensile stresses may form in the matrix during the cooling process. Consequently, whilst the carbide may crack early in the fatigue process subsequent propagation is dominated by microstructural effects.

Int J Fatigue March 1991

Fatigue

crack growth

Short-crack g r o w t h at 19°C, R = O. 1 From the short-/long-crack comparison of data given in Fig. 4, it appears that the data merge at short crack lengths (2c) approaching nine grain diameters. This is similar to that reported by other investigators 14'*s who found the transition length to be approximately 10 times the microstructural element size. Whilst there exists general disagreement with respect to the physical parameters determining the transition crack length, there is commonality in the belief that once the crack samples the microstructure as a continuum, the kinetics of short-crack growth diminish to those associated with longcrack propagation. Comparison of short- and long-crack growth data measured at R = 0.1 and 0.5, respectively, clearly demonstrated that in the case of Waspaloy short cracks grew faster than long cracks at the same nominal mode-I stress intensity range. This effect has been observed by other investigators in other nickel-base superalloys. 1-7 The reason for this trend has been attributed to the absence of crack closure as observed by Newman and Beevers5 in Astraloy and Nimonic 901. In these materials multigrained short cracks exhibited similar growth rates to those of longcrack AKe, data. This trend was evident up to surface crack lengths of 250/~m. Newman and Beevers also found that short cracks forming in single grains exhibited a lower intrinsic resistance to growth in comparison to a multiple grained crack front because of the ease of slip. In the case of a single grain on the surface of a specimen, crack propagation in association with planar slip is easily achieved owing to the absence of any substantial constraint on the surface. In the long-crack case, however, the situation is more difficult because of the multigrained crack front having to accommodate a diversity of slips. McCarver and Ritchie2 reported similar trends in Rene 95, which contained a necklace microstructure. The AKI threshold for short cracks was found to be 60% lower than for long cracks at R = 0.1. Both sets of data did, however, normalize at R = 0.8. The discrepancy between long- and short-crack growth at low R-ratios was rationalized in terms of closure induced by fracture surface roughness arising from growth along the crystallographic planes. Similar results have also been reported by Soniak and Remy, ~6 who found that short-crack growth could be rationalized in terms of AKeff. In both these studies conclusions were drawn from throughthickness short cracks in which the geometrical constraints were the same as those of the long crack. With this condition of similitude, it logically follows that the closure experienced by a crack tip is continuous in its development with crack length. At short lengths, where the 'wake' beyond the crack tip is limited, a small closure contribution is experienced. Conversely at long crack lengths a larger crack 'wake' leads to a higher closure contribution. The applicability of this type of short-crack configuration in describing the growth of freely initiated surface cracks in plane specimens is questionable when the results are compared with the work of others.l'3 Brown et al 1 failed to rationalize the differences in long- and short-crack growth in Waspaloy and Astraloy tested at R = 0.8 and R = 0.1. Similar trends were observed by Kendall et al 3 in fine-, coarse- and mixedgrained structures of Astraloy. In an endeavour to explain this discrepancy it was suggested that incorrect estimates of crack shape would produce invalid stress intensities. These authors argued that at very short crack lengths, crack shape

Int J Fatigue March 1991

is strongly influenced by neighbouring grains. Consequently, an equilibrium shape is not achieved until several grains have been encompassed by the crack front. In response to this statement, re-evaluation of their data by the current authors revealed that even if a wide range of a / c ratios are used, the data could not be rationalized in terms of the stress intensity range. In the current investigation roughness-induced closure was found to be a function of crack tortuosity and mode-II shear, Fig. 5. At short crack lengths these effects were found to be minimal when the crack path lay through similarly orientated grains. Conversely, significant differences in slip texture produced greater crack tortuosity and mode-II shear displacements of the crack paths. Roughness-induced closure was also found to operate at long crack lengths irrespective of stress ratio but its effectiveness on crack growth was reduced as the stress ratio was increased. From this result it is clear that the discrepancy between short- and long-crack growth at 19 °C was the result of a combination of factors, which include a lower intrinsic-closure-free short-crack threshold, differences in closure behaviour, and material constraint. Scatter within the short-crack data was attributed to the strong influence of microstructural effects. Short-crack growth at 500°C, R = O. 1

Short-crack propagation at 500 °C exhibited similar growth trends to those at 19 °C, Fig. 6. At low values of stress intensity the occurrence of mode-II shear deflections gave rise to a limited crack path tortuosity and temporary crack arrest. At higher stress intensities a relatively flat fracture surface was evident, which resulted in growth rates approaching longcrack growth rates. The occurrence of a flatter fracture surface was attributed to more homogeneous slip behaviour and local plasticity effects. In support of these findings Hudak et al z reported similar trends in Waspaloy and Astraloy at 600 °C. The information presented in Fig. 10 shows the results obtained in the present investigation and those obtained by Hudak et al 7 for Waspaloy and course-grained Astraloy at 600 °C. The results obtained by Hudak et al represented a relatively small data sample for Waspaloy at 600 °C and the data were obtained at much higher maximum stress to ps ratios, 1.5-1.8, than in the present investigation, 0.8-1.2. The results, however, are complimentary and support the general trend of a somewhat higher growth rate for short cracks at 500 °C compared with 19 °C. 10-4

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Fig. 10 Comparison of short-crack growth at elevated temperatures in nickel-based superalloys

137

Investigations by other researchers, 6 who studied the fatigue behaviour of IN-100 at room and elevated temperatures, revealed very different trends. At room temperature, short- and long-crack growth exhibited similar trends to that of Waspaloy. However, at 600 °C both the data sets agreed irrespective of crack length. The difference in behaviour between the two temperatures was explained in terms of the fracture mode. At 19 °C, short-crack growth was transgranular and highly faceted, but at 600 °C it adopted the long-crack intergranular mode of fracture.

Comparison of short-crack growth at 19 °C and 5 0 0 ° C , R = 0.7

A comparison of short-crack growth at 19 °C and 500 °C between 6 MPa V'm and 17 MPa ~v/m revealed marginally higher growth rates with increase in temperature, Fig. 7. This was attributed to a combination of factors including CTOD, fracture mode and closure. The effect of increasing C T O D (through a decrease in strength with an increase in temperature) appeared to have no significant effect on growth rates at 500 °C compared with 19 °C. The occurrence of a more macroscopically planar fracture surface at 500 °C in comparison to 19 °C suggested that the difference in growth rates may be attributed to a change in growth mechanism, as reported by Hudak e t al. 7 In their studies short-crack fracture profiles in Waspaloy and Astraloy were reported to be a function of slip homogeneity, which increased with temperature as a consequence of enhanced cross slip and climb. Similar observations have also been reported by other investigators, 17,1s who have conducted longcrack growth studies at 19 °C, 550 °C and 650 °C in Waspaloy. The change in fracture profile from highly faceted to a relatively smooth surface would be accompanied by a reduction in closure at elevated temperature. Although it was shown that a roughness-induced closure mechanism was operating at both temperatures, the flatter fracture surface at 500 °C is likely to have a smaller degree of asperity contact between opposing fracture faces. No direct observations were made of oxide asperities and it has to be concluded that even at 500 °C there was no substantial contribution to the closure process from oxidation products.

(7)

References 1. 2. 3.

4.

5.

6.

7.

8.

9.

10.

11. 12. 13. 14.

Conclusions (1)

(2) (3)

(4)

(5)

(6)

138

The dominant mechanism of fatigue crack formation in Waspaloy at 19 °C and 500 °C was slip band cracking. Cracks formed in this manner were successful in producing failure within the material. Fatigue crack formation also occurred as a consequence of carbide cracking within the matrix. The dominant mode of fatigue crack propagation occurring at short and long crack lengths was one of mixed mode-I and -II fracture. Mode-II displacements led to a mismatch of the fracture surface resulting in non-closure of the crack tip. The occurrence of oxideinduced closure was not significant. Short-fatigue-crack growth rates at R = 0.1 were marginally faster at 500 °C than at 19 °C at an equivalent fiJ(1. Higher growth rates at 500 °C in comparison to those at 19 °C were related to slip character and fracture surface topology. In varying the maximum stress from 0.85 to 1.2 of the 0.2% ps at 500 °C, there was no applied stress effect

on short-fatigue-crack growth rates measured at R = 0.1, when plotted in terms of the AK]. Short-crack growth at 500 °C was faster than longcrack growth at an equivalent AK] when measured at R = 0.1.

15. 16.

17.

18.

Brown, C.W., King, J.E. and Hicks, M.A. Met Sci 18 (1984) pp 374-380 McCarver J.F. and Ritchie, R.O. Mater Sci Eng 55 (1982) pp 63-67 Kendall, J.M., King, J.E., Woollin, P. and Knott, J.F. Proc Mtg Aerospace Materials, Lucern, Switzerland, 1987 (MPR Publishing Services, Shrewsbury, UK, 1988) pp 7.1-7.12 Newman, P. and Beavers, C.J. Fatigue "84, Birmingham, UK, 1984 Ed C.J. Beevers (Engineering Materials Advisory Services, Warley, UK, 1984) Vol II pp 785-796 Newman, P.T. and Beavers, C.J. Proc Conf on Small Fatigue Cracks, 1986 Eds R. Ritchie and J. Lankford (The Metallurgical Society, Warrendale, PA, 1986) pp 97-116 Cook, S., Lankford, J. and Sheldon, C.P. 'Research on growth of microcracks in nickel-base superalloys' Technical Report AFML-TR-78-133 (South West Research Institute, Texas, 1978) Hudak, S.J. Jr, Davidson, D.L., Chan, K.S., Howland, A.C. and Walsch, M.J. 'Growth of small cracks in aeroengine disc materials' Technical Report AFWAL-TR-88-4090 (Materials Laboratories, Wright-Patterson Air Force Base, Ohio, 1988) Larsen, J.M. Fracture Mechanics, ASTM STP 905 Eds J.H. Underwood et al (American Society for Testing and Materials, 1986) pp 226-238 Pickard, A.C. 'The application of 3-dimensional finite element methods to fracture mechanics and fatigue life prediction' (Engineering Materials Advisory Services, Warley, UK, 1986) Ch 4 pp81-116 Grabowski, L. and Beavers, C.J. Annual Report, Project 2077/D148 XR/RAE(P) (Birmingham University, UK, 1986) Merrick, H.F. Metall Trans 5 (1974) pp 891-897 Larch, B.A., Jayaraman, N. and Antolovich, S.D. Mater Sci Eng 66 (1984) pp 151-166 Woolin, P. 'Fatigue in nickel-superalloys' PhD thesis (Cambridge University, UK, 1989) Kendall, J.M., Grabowski L. and King, J.E. 'Materials development in turbomachinery, design' Proc 2nd Parsons International Turbine Conf, Cambridge, 1988 Eds D.M.R. Taplin, J.F. Knott and M.H. Lewis (Parsons Press, Dublin, 1989) pp 231-240 Taylor, D. and Knott, J.F. Fatigue Fract Eng Mater Struct 4 (1981) pp 147-155 Soniak, F. and Ramy, L. The Behaviour of Short Fatigue Cracks, EGF Publ 1 Eds K. Miller and E. Del Los Rios (Mechanical Engineering Publications, UK, 1986) pp 133-142 Clavel, M., Levaillent, C. and Pineau, A. Creep-Fatigue Interactions Eds R.M. Pelloux and N.S. Stoloff (Metallurgical Society of AIME, Warrendale, PA, 1979) pp 24-45 Clavel, M. and Pineau, A. Mater Sci Eng 55 (1982) pp 157-171

Authors J.C. Healy and C.J. Beevers are from the IRC in Materials for High Performance Applications, and the School of Metallurgy and Materials, University of Birmingham, P.O. Box 363, Birmingham, B15 2TT, UK. L. Grabowski is from Rolls Royce plc, P.O. Box 31, Derby, DE2 8BJ, UK.

Int J Fatigue M a r c h 1991