High temperature crack growth behavior in a precipitate-hardened nickel base superalloy under constant KI conditions

Scripta METALLURGICA et MATERIALIA

Vol. 29, pp. 573-578, 1993 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

HIGH TEMPERATURE CRACK GROWTH BEHAVIOR IN A PRECIPITATE-HARDENED NICKEL BASE SUPERALLOY UNDER CONSTANT K 1 CONDITIONS L. Korusiewicz, J. Ding, M. Kumosa Department of Materials Science and Engineering Oregon Graduate Institute of Science & Technology Beaverton, Oregon 97006-1999, USA

(Received

December

8, 1992)

Introductiqn Nickel base superaUoys are typically used in high temperature components of gas turbines. A new precipitate(ppt)hardened cast superalloy has been developed to satisfy the needs for structural component applications in advanced aircraft engines. These applications require an adequate understanding of crack extension mechanisms due to creep deformation and damage in the crack tip region in response to sustained or slowly varying loads. In this study, high temperature crack growth behavior in the nickel base superalloy under constant K~ (Mode I) condition was investigated. The experiments were performed at 6500C in air. Material The test material was a recently developed nickel-base superalloy with enhanced nickel, cobalt and tantalum contents. In this alloy, in addition to the fcc 3' matrix, there are three principal precipitate phases: gamma prime(.),'), gamma double prime(')'") and delta(~). This alloy utilizes the 3'" phase as the principal strengthening phase instead of 3'% which is the common strengthening phase for most of the superalloys. By changing casting parameters and postcast heat treatment procedures, different grain sizes and precipitate distributions can be created in the alloy considered. Two groups of samples with significantly different microstructures, based on fine and coarse grains, were tested. Exneriment Constant K specimens, also known as tapered double cantilever beam specimens have been designed specifically for crack extension studies(I,2). For these specimens, the experimental and numerical findings show the compliance to be linearly related to crack length over a range governed by the angle of taper. As a consequence, the stress intensity factor, K:, is constant when load is maintained at the same level. A number of constant K specimens were tested at various applied loads to determine the relationship between the stress intensity at the crack tip and crack growth rate. The potential drop technique(3) was employed to measure crack length during testing. In addition, the entire crack growth process was monitored by an optical traveling microscope. The stress intensity factor was calculated from the following relationship:

x;p [

dc]

(1)

where P is the load, E is the Young's modulus, B is the specimen thickness and C is the specimen compliance. In the present study, compliance of the double cantilever beam specimen and the functional relationship between compliance and crack length were determined experimentally. Artificial cracks (slots) were utilized to accelerate the 573 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.

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calibration test and to define crack lengths in an unambiguous manner. The typical procedures consisted of cutting the slot by an abrasive wheel. After a compliance reading was taken, the procedure was repeated until the desired range of artificial crack was covered. An experimental set-up(4) was used to initiate a crack at the notch root in the specimen (without fatigue precracking) and to carry out the crack growth tests. The temperature distribution on the surface of the specimens was measured by three independent thermocouples. The experimental data obtained from the tests, namely potential drop, applied load, temperature and crack opening displacement, was stored and re-processed on a PC with the "Solus" system.

Results and Discussion Crack growth behavior in the fine grained materiah The crack propagation rate, da/dt, versus the applied stress intensity factor, KI, is shown in Fig. 1. It can be seen from Fig. 1 that similar to fatigue, there appears to be a threshold stress intensity factor, K,, below which cracks do not grow. The value of the threshold stress intensity factor for the investigated alloy is lower than 26MPav/m. Fig.2 shows the crack incubation time as a function of Kx. It is apparent that the incubation time increases rapidly when the stress intensity factor is approximately 24.5MPav/m. This confirms the existence of the threshold in the experiments conducted. For further calculations, K~ was assumed to be equal to 24.4MPav/m. The presence of a crack growth threshold indicates that the crack growth rate should be controlled by an effective driving force(5) G~ = (K~2 - K~h2)/Eand can be expressed as da/dt = cG~. In this case, the crack growth law is given by the following expression (Fig.3): da = 1.122 * 10-1°(Kt2-K~) L!56 dt

(2)

The exponent 1.156 in equation 2 is close to the value of 1.025 obtained for Alloy 718 tested at 650°C under similar crack growth conditions(5), and is close to 1. This validates the application of the effective driving force Gc as the controlling parameter for the crack growth case considered. Figures 4 and 5 show the two most characteristic crack tip configurations in the free grained samples. The microstructure of the alloy is a combination of ppts (mainly ~ plates)-rich grain boundary areas and nearly ppts-free grain interiors (light contrast). It can be seen from these figures that there is a damage zone ahead of the crack tip which consists of several microcracks initiated at U3, interfaces and v-grain boundaries, and broken ~-plates. During the crack propagation process the mieroeracks coalesce to form the main crack. It is evident that the &rich regions around grain boundaries are the favorable initiation spots for fracture of this particular alloy. This effect can be related to the lower deformation ability of ~-rich regions as compared with the grain interiors.

Crack growth behavior in the coarse grained material: The coarse grained material was investigated in the same manner as the fine grained material. For this alloy, however, attempts to initiate cracks at the notch by gradual loading up to - 90MPav/m (see Fig.6) in two of the specimens (TAP1 and TAP2) failed entirely. Therefore, a fatigue precrack of 0.5mm was introduced in the third specimen (Fig.6 - specimen TAP3). For this specimen, the crack propagation rates under three different constant Kx values were very low and the final crack length was 3.8ram after 390h of testing. Very often, the crack growth was temporarily stopped and significant crack tip blunting (in comparison with the fine grained material) occurred which was evident from optical microscopy and by a decrease in the potential drop signal. For this specimen, there was an indication of creep deformation in the crack tip region. Fig.7 shows a crack profile of the coarse grain alloy. The microstructure of this material is based on a combination of ppt-rich inter-dendritic and ppt-free intra-dendritic areas. It can be seen from Fig.7 that microcracks always initiate in the inter-dendritic regions, as indicated by points B, C and E. They are arrested in the intra-dendritic areas (points A and D). It seems that the higher resistance to crack propagation in the intra
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Conclusions

1. There exists a threshold stress intensity factor, K~, in the fine grained material, below which the cracks will not initiate and propagate at 650°C under constant KI conditions. 2. The effective driving force G~ can be used to describe the crack growth behavior in the fine grained material. 3. The coarse grained material exhibits much higher resistance to fracture than the fine grained material. 4. The crack growth process in the superalloy considered is strongly affected by both the grain size and the distribution of precipitates. Aeknowledeements

The authors are grateful to the Bureau of Mines in Albany, OEDD, Oregon Lottery, General Electric Aircraft Engines and Precision Castparts Corporation for their financial support. The authors are indebted to Prof. W.E. Wood without whose support the work would not have been possible. References

1. B.F. Brown(Ed.), Stress-Corrosion Cracking in High Strength Steels and in Titanium and Aluminum Alloys, Naval Research Laboratory, Washington,D.C.(1972) 2. H.H. Ottens and C.J. Lof, Engineering Fracture Mechanics, 6, 573 (1974) 3. A. Saxena, Engineering Fracture Mechanics, 13,741 (1980) 4. L. Korusiewicz, J. Ding, M.S. Kumosa, Micro- and Macro-Structural Aspects of Fatigue Crack Growth Behavior in Structural Casting Superalloys, Annual Progress Report to PCC and GE, MSE Oregon Graduate Institute of Science & Technology (1991) 5. C.D. Liu, Y.F, Han, M.G. Yah, M.C. Chaturvedi, Engineering Fracture Mechanics, 41,229 (1992)

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CRACK GROWTH

FIG. 5. Crock tip region in the fine grained alloy.

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