Deformation modes of a Ni-base superalloy under compression

Scripta METALLURGICA et MATERIALIA

Vol. 29, pp. 117-122, 1993 Printed in the U.S.A.

DEFORMATION

MODES OF A Ni-BASE UNDER COMPRESSION

Pergamon Press Ltd. All rights reserved

SUPERALLOY

Nho-Kwang Park, Byung-Hoon Kim and Sang-Lae Lee* Korea Institute of Machinery & Metals, 66 Sangnarndong, Changwon, Kyungnam * Dept. of Metallurgical Engineering, Pusan National University, Kumjungku, Pusan, Republic of Korea (Received March 2, 1993) (Revised April 19, 1993) Introduction A forging process is widely used for nickel base superalloys to make gas turbine parts. The quality of the final products depends on the quality of the ingot and its processing. Strain localization, which often occurs during forging, deteriorates the soundness of the forged products, due to their inhomogeneous microstructure and mechanical properties. Strain localization occurs when the deformation is unstable for a given test or material condition. A therroomechanical approach has often been employed to assess the strain localization behavior under tension or compression loading [1-3]. Under the condition of axisymmetric compression, bulging occurs when the deformation is stable and friction and die chilling are not insignificant, and shearing occurs when the deformation is unstable. The appearance of shearing under axisymmetrie deformation conditions may be attributed to factors such as friction, tool misalignment, etc [4]. However, we have experienced stable deformation of bulging mode transforming to the unstable deformation of shearing mode under axisymmetric compression depending on test conditions for a given test set-up. The intent of this study is to elucidate how the deformation mode of hot isostaticaUy consolidated Rene 95 containing high volume fraction of gamma prime is affected by grain size and test parameters under compression loading. For the present test conditions, the deformation mode can be grouped into two, bulging and shearing, and the deformation mode map, which reveals the conditions for each mode, is constructed. Exoerirnent Rene 95 powders of average particle size of 37 ~tm and 87 p.m were encapsulated and vacuum-degassed in a stainless steel can, followed by hot isostatic pressing at 1100 oC/100 MPa for 3 h. This process provides a sound compact of theoretical density. The compacts were machined into the red-type specimens of 5 mm dia. x 10 mm long by electro-discharge machining. Compression tests were conducted at constant strain rates under vacuum ~ 0.1 Pa. The specimen temperature is maintained constant by using a thermocouple welded to the center surface of the specimen and a R.F. heating coil as a heating source. Boron nitride powder was applied to both ends of the specimen as a lubricant to reduce the friction between the specimen and dies. Strain rates were varied within 10-3 ~ 100 s -1 and test temperatures within 950 ~ 1150 oc. All the other test paranmtcrs were maintained constant. The deformation mode is classified by the shape of the deformed specimens. The shearing, which is hereaffter denoted by S, is defined by unstable deformation localized at the plane at an angle of 45 ° with respect to the loading axis, and the bulging, which is hereafter denoted by B, is defined by the stable deformation concentrated near the center of the specimen due to the friction and die chilling. When a clearcut cannot be made between S and B from the specimen configuration, M is used. An attempt is made to assess the flow localization behavior based on the proposal by Jonas et al. [2]. Since the shearing is considered in the present study, the flow localization factor, ct, is obtained from the stress-strain curves at the strain of 0.5 by the equation, ot = ~/'/m where ~' = [8(1n~)/~]¢ and m = [5(lna)/8(ln£)]e,T. The optical and TEM metallography were conducted on the longitudinal cross-section of compression specimens.

i17 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.

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Resul~ Figure 1 shows a typical microstructure of Rene 95 after HIP'ing. A bimodal distribution of gamma prime precipitates is noticed; the coarse precipitates are pinned on grain boundaries and fine precipitates are evenly distributed inside the grains. The average grain size of HIP'ed samples depends on the initial particle size. The grain sizes of 6 I.tm and 15 Ixm were obtained from the particle sizes of 37 lam and 87 lain respectively. Stressstrain curves are obtained from the HIP'ed Rene 95, and a selection of results are shown in Figures 2 and 3 for the two grain sizes. The flow stress shows a peak at the strain of < 0.1, above which the flow stress decreases with strain. The extent of stress reduction at high strains becomes greater with increasing strain rate, or with decreasing temperature. The level of the flow stress increases with decreasing temperature, increasing strain rate and grain size. The deformation mode of compressed specimens varies with test parameters such as grain size, strain rate and temperature. The deformation modes, S and B, which stand for shearing and bulging respectively, are identified by the shape of specimens at the compressive strain of 0.5. Figure 4 summarizes the experimental observations on the deformation mode in the plot of inverse absolute temperature vs strain rate. S tends to occur at high strain rates and low temperatures, and the demarcation line between S and B shifts to low strain rates and high temperatures with increasing grain size. Discussion The flow softening behavior after a stress peak, as shown in Figures 2 and 3, may be attributed either to adiabatic heating following strain localization or to dynamic recrystallization. Other factors such as dynamic recovery and formation of microcracks at prior particle boundaries may also induce flow softening or load drop in the stressstrain curve. However, those factors would exert a minor effect on the flow curves, resulting in a small change in the slope of vectorial flow curves without inducing such a big change from strain hardening to strain softening. When flow localization occurs, the deformation tends to concentrate upon macroscopic shear planes regardless of orientation relationship between neighboring grains in the deforming polycrystalline bodies. Strain localization results in the formation of shear bands which cross over many grains [5,6]. The specimen would be reduced in height and be increased in diameter uniformly throughout the specimen under compression load, if the temperature is maintained evenly and no friction is allowed between the specimen and dies. From a practical point of view, die chilling and friction between forging stock and dies are unavoidable and bulging occurs during deformation. In the present study, experimental set-up is similar to the practical forging process, and the stable deformation in the compressed specimen leads to bulging (B) and the unstable deformation to shearing (S). Figure 4 summarizes the test conditions for the deformation modes, S and B. It is to be noted that the domain for B is enlarged and the demarcation line between S and B is shifted to low temperature and high strain rates with decreasing grain size. The slope of the boundary equation between the domains of S and B does not vary with grain size. It is further to be noted that the demarcation line is well matched with the flow localization factor ec of 5 - 10, even though some experimental error in measuring the ¢z should be allowed. Similar behavior has been noticed in other materials [7]. Assuming that the strain rate is related to the temperature by an Arrhenius equation t = Aexp(-Q/RT), one can obtain the activation energy for the S mode from Fig.4. The activation energy is calculated to be 950 kJ/mole regardless of the grain size. The measured activation energy is too high to be related to any of the microdeformation mechanisms. The activation energy for the self-diffusion in nickel is about 250 kJ [8]. The activation energy decreases with decreasing temperature, depending on the active deformation mechanism; i.e. dislocation climb and cross slip, dislocation glide over short range obstacles [9]. The activation energy for hot working is normally much greater than that for self-diffusion in metals and alloys prone to re,crystallization [10]. From the strain rate dependence of the peak stress in the flow curves, activation energies for hot working of Rene 95 are measured to be 680 - 950 kJ/mole, depending on the strain rate [11]. The upper bound is close to the present value obtained from the demarcation line between B and S. The reason for the similarity in activation energies measured by two different methods may be attributed to the fact that S

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tends to occur under certain temperature and strain rates due to localized polyslip without dynamic recrystallization. This view is corroborated by the optical metallography as shown in Fig.5, which reveals prominent dynamic recrystallization in bulged specimens, not in sheared specimens. The activation energy for S is an intrinsic material property and is independent of grain size. The finer grain size, however, enhances the forgeability of HIP'ed Rene 95 by enlarging the domain for bulging. This is quite understandable, since dynamic re,crystallization occurs preferentially near the grain boundaries. The similarity in the activation energy regardless of the grain size has a practical significance in forging processes, since a guideline for the stable deformation conditions can be drawn for various grain sizes. Conclusion 1. A deformation mode map which delineates the temperature and strain rate conditions for shearing and bulging mode of deformation is constructed for HIP'ed Rene 95. The deformation tends to be dominated by shearing with increasing strain rate and grain size, and with decreasing temperature. 2. The demarcation line between shearing and bulging, corresponding to the strain localization factor ~ = 5-10, produces an apparent activation energy of 950 kJ/mole regardless of the grain size, which is close to the activation energy for the hot working. Acknowledgements This work was supported by the Ministry of Science and Technology. The authors would also like to thank Dr. S. I. Oh of Dept. of Mechanical Design and Production Engineering, Seoul National University for his comments on the stability of deformation. References 1. E. W. Hart, Acta Met. 15, 351 (1967). 2. J. J. Jonas, R. A. Holt and C. E. Coleman, Acta Met. 24, 911 (1976). 3. S. I. Oh, S. L. Semiatin and J. J. Jonas, Metall. Trans., 23A, 963 (1992). 4. S. L. Semiatin and J. J. Jonas, Formability and Workability of Metals, ASM, Metals Park, OH 44073 (1984) 5. S. L. Semiatin, G. D. Lahoti and S. I. Oh, Materials Behavior under High Stress and Ultrahigh Loading Rates, 119 (1983). 6. M. Hatherly and A. S. Malin, Scripta Met. 18,449 (1984). 7. S. L. Semiatin and G. D. Lahoti, Met. Trans. A. 12A, 1705 (1981) 8. H. Oikawa, T. Kato and S. Karashima, Trans. JIM, 14, 389 (1973). 9. A. G. Evans and R. D. Rawlings, Phys. Stat. Sol. 34, 9 (1969) 10. J. J. Jonas, C. M. Sellars and W. J. McG. Tegart, Met. Rev. 14, 1 (1969) 11. N. K. Park and B. H. Kim, 5th Conf. on Mechanical Behavior of Materials, 95 (1991)

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FIG. 1. TEM Micrograph of Rene95. HIP'edat ll00*C/100MPa/3 h. 1200

1200 T=IO00:C 1----o-- T=1050~C I " r--]loo~c ii T=I1513"C I

I000 800

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~ 600

m 400

m 400

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0

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T= 95o'c T=I000"C T=I050"C T=II00°C T=II5O*C

~-- 800

600

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(b)

1000

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i

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0.6

0.6

TRUE STRAIN

STRAIN

FIG. 2. True Stress vs. True Strain. Grain Size = 6 lain. (a) 10"2S "1 (b) 10°S "l

1200 1

. .

[

•

T= 950°C

tit)

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~1000 t

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,

, _ t ' T,scc T=IO0~'C

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,

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FIG. 3. True Stress vs. True Strain. Grain Size = 15 ttm. (a) 1 0 2 S "1

(b) 1 0 ° S t

0;3 STRAIN

014

015

0.6

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8.17

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S

121

S

S

S 16.4

S

%

7.85

B 2.0

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B 0.9

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S 156.3

S 320.1

S 443.4

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S 6.9

S 7.3

S 136.2

S 66.6

7.56

B 0~.7~"" ,,,S 3.9

S 489.9

S 332.0

7.28

B

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FIG. 4, Deformation Mode Map. Numbers in The Plot Representing Strain Localization Factor, ct. (a) Grain Size = 6 lam. (b) Grain Size = 15 lam.

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FIG. 5. Optical Micrographs of Compressed Rene 95. Grain Size = 15 ~m. (a) 1150°C, 10"2S .l (d) 1150 °C, 10°S-i (b) 1050 °C, 10 "2S .l (¢) 1050 °C, 10 °S-1 (c) 950°C, 10"2S "l (f) 950°C, 10°S "1

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