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Effect of γ′ content on the mechanical behavior of the WASPALOY alloy system
Materials Science and Engineering A308 (2001) 1 – 8 www.elsevier.com/locate/msea
Effect of g% content on the mechanical behavior of the WASPALOY alloy system Keh-Minn Chang, Xingbo Liu * Department of Mechanical & Aerospace Engineering, West Virginia Uni6ersity, Morgantown, WV 26506, USA Received 2 May 1999; received in revised form 12 December 2000
Abstract This article presents the research results of the effects of alloy chemistry, cooling rate and solution temperature on mechanical behavior, especially for the fatigue-crack growth of the WASPALOY alloy system. It is indicated that tensile strengths, both yield strength and ultimate tensile strength, increase with increasing Al + Ti content in the alloys. The solution temperature had almost no effect on the tensile properties of WASPALOY. Cooling rate showed almost no effect on the tensile properties of WASPALOY, but for the alloys with a higher Al+ Ti content, the yield strength of the alloys was improved with increasing cooling rate. The increase in precipitation content tremendously improved the stress rupture life of the alloys. The fatigue-crack resistance of the alloys can be improved by increasing the strengthening precipitate volume fraction, in spite of the variation of the cooling rate. The crack growth rates were time-dependent, as expected. The growth rate increased with decreasing loading frequency. However, a higher precipitate content showed a lower time dependence. © 2001 Elsevier Science B.V. All rights reserved. Keywords: g% content; Mechanical behavior; WASPALOY alloy system
1. Introduction WASPALOY has been extensively applied in different industries such as aircraft, chemical-plant equipment and petrochemical equipment. It can be considered as a conventional precipitation-strengthened superalloy. The major alloying additions are 19.5% Cr and 13.5 Co but no Fe. The precipitation phase upon aging is the meta-stable intermetallic compound of Ni3(Al, Ti), designed as g%, which has a ordered structure of L12. WASPALOY has a moderate low-temperature strength and an excellent creep strength at high temperatures. In the past several decades, the fatigue-crack growth behavior of high-temperature materials, especially superalloys, has received increasing attention because (1) the results of failure analyses indicate that fatigue is one of the major causes of failure in engineering structures, and (2) the fatigue life of engines is determined by the * Corresponding author. Tel.: +1-304-2933111; fax: + 1-3042936689. E-mail address: [email protected] (X. Liu).
initiation and the propagation behavior of the cracks [1,2]. Fabricated components usually contain geometric discontinuities, machining or assembly marks, or welding defects that facilitate fatigue-crack initiation. Therefore, it is broadly assumed that all parts contain pre-existing cracks. As a consequence, research on life prediction or service extension of superalloy components has been focused on their fatigue-crack growth characteristics. Several parameters have now been identified as playing significant roles in affecting the growth of fatigue cracks [1]. These are intrinsic parameters like alloy chemistry, heat treatment, microstructure, and elastoplastic behavior, mechanical factors such as the crack geometry, load amplitude, and stress load ratio, and physico-chemical parameters, including the nature, composition and temperature of the environment surrounding the crack tip. It has been revealed that crack growth is predominantly a cycle-dependent damage process with little frequency effect at very high frequencies. Crack growth is induced by irreversible crack tip plastic flow, which
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5093(00)02042-6
K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
2
Table 1 Chemical compositions of tested alloys (wt.%)
Mn Fe Si Cu Ni Cr Al Ti Mg Co Mo Nb Ta P B W Zr
HW-1
HW-2
B0.01 0.13 0.01 B0.01 68.37 14.48 1.50 2.90 B0.001 8.72 3.77 B0.01 0.01 B0.001 0.005 0.04 0.05
B0.01 0.11 B0.01 B0.01 66.92 14.81 1.78 3.51 B0.001 9.13 3.63 B0.01 B0.01 B0.001 0.008 0.05 0.05
HW-3
HW-4*
HW-5*
0.11
0.10
0.13
Balance 14.84 2.03 3.92
Balance 14.81 2.28 4.47
Balance 14.76 2.47 4.90
9.17 3.72
9.06 3.80
9.20 3.75
0.006
0.01
0.01
0.05
0.05
0.05
to low-frequency loading in air test conditions. These observations strongly support the conclusion that timedependent effects in superalloys are principally due to environmental degradation. The purpose of this paper was to investigate the effect of alloy chemistry, cooling rate and loading time on the mechanical behavior, especially for the fatigue-crack growth of the WASPALOY alloy system.
2. Experimental procedures
2.1. Experimental materials WASPALOY was selected as the basic research material, and various amounts of Al and Ti were added to the basic matrix. All the materials were melted in a vacuum: the chemical compositions are listed in Table 1. The main differences between the alloys were the contents of Al and Ti. In fact, to research the effect of g% on the properties of WASPALOY, the alloys were designed with different contents of Al and Ti, and then with the different content of g% strengthening phases. The HW-1 alloy had the standard WASAPLOY composition, while the others had higher contents of g%. The relationship between the content of g% and Al+Ti is shown in Table 2. The alloys were melted in a 40 lb (18 kg) VIM furnace and poured into F0.114 m round bars. These round bars were homogenized at 1200°C/24 h and extruded into 0.254× 0.254 m square bars.
Table 2 Designed Al and Ti content and that of precipitatea Alloy
Al (wt.%)
Ti (wt.%)
Al+Ti (wt.%) g% (wt.%)
HW-1 HW-2 HW-3 HW-4 HW-5
1.5 1.75 2.0 2.25 2.5
3.0 3.5 4.0 4.5 5.0
4.5 5.25 6.0 6.75 7.5
30 35 40 45 50
a The content of g% is the designed content. The alloys are designed based on the results calculated by PHACOMP software, which is used extensively in superalloy industry. The error is believed to be less than 5%.
2.2. Heat treatment
provides the driving force for crack growth [3,4]. At relatively low frequencies, crack growth includes timedependent processes, and fatigue-crack growth rates (FCGR) are strongly dependent on temperature, frequency, holding time and stress ratio. Time-dependent effects on high-temperature fatigue-crack growth behavior are generally ascribed to phenomena involving creep and/or environmental degradation processes [5– 10]. Furthermore, several studies on superalloys have demonstrated that high-temperature FCGR are decreased by several orders of magnitude [11 – 15] when the results of vacuum tests are compared with those due
To investigate the effect of solution temperature and cooling rate on the properties of the alloys, the experimental scheme shown in Table 3 was employed. It should be pointed out that the cooling rates shown in the table are the average slopes of the cooling curves from the solution temperature to 650°C. As indicated in Table 3, higher solution temperatures were employed with increasing contents of Al and Ti. Solution temperatures were determined by the metallography results, which took small coupons heattreated at different temperatures. The selection of
Table 3 Solution temperatures and cooling rates HW-1
Solution (°C) Cool (°C min−1)
HW-2
HW-3
HW-4
HW-5
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
1000 500
1000 200
1000 50
1050 500
1050 200
1050 50
1100 500
1100 200
1100 50
1125 500
1125 200
1125 50
1125 500
1125 200
1125 50
K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
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2.3. Mechanical tests 2.3.1. Tensile tests To research the effect of cooling rate and the content of g% on the tensile strengths and ductility of WASPALOY alloy system, a series of tensile tests were conducted at different test temperatures: room temperature, 538, 649 and 760°C. The dimensions (in millimeters) of the specimen are shown in Fig. 1.
Fig. 1. Dimensions of the tensile specimen.
Fig. 2. Effect of Al +Ti content on the tensile properties of the WASPALOY system at room temperature.
2.3.2. Stress-rupture tests Constant-loading stress-rupture tests of the alloys with different contents of Al+Ti (g%) were conducted at 760°C with an initial stress of 482 MPa. 2.3.3. Fatigue-crack propagation tests Fatigue-crack propagation tests with various wave forms (triangular wave with the cycle of 3S and 180S, trapezoid wave with 3+ 177S) were performed by employing the single-edge-notched (SEN) specimen. A reversible d.c. current was applied by a power supplier through the head of the specimen. The crack length was monitored by a pair of potential probes mounted on the front edge of the specimen gage across the pre-machined notch. The measured d.c. potential drop at any crack length was normalized and converted into the corresponding crack length by a single analytical relation, namely Johnson’s equation [8]:
A=
2W × cos − 1 y
n
cosh(yY/2W) cosh{(U/U0) cosh [cosh(yY/2W)/cos(yA0/2W)]} −1
(1) where A and A0 are the actual and initial crack lengths, respectively, U and U0 are updated and initial measured potential drops, respectively, and W and Y represent the specimen width and one half of the potential probe span, respectively.
3. Results Fig. 3. Effect of Al +Ti content on the tensile properties of WASPALOY at 650°C.
solution temperature was based on two criteria: (1) recrystallized grain structure; (2) limited grain growth. Thus, the grain sizes of all the alloys were almost the same, which is ASTM 8-9. The solution time for all the materials was 4 h, and the standard two-step aging treatment for WASPALOY, i.e. 850°C/4 h/AC + 760°C/16 h/AC, was used after the solution treatment.
3.1. Tensile properties Figs. 2 and 3 show the effect of Al+ Ti content on the tensile properties of the alloys at room temperature and 649°C. It can be seen that the tensile strengths, both yield strength and ultimate tensile strength, increased with increasing Al+ Ti content of the alloys. At the same time, the elongation of the alloys decreased with increasing Al+ Ti content. This is reasonable because the major strengthening mechanism of the alloy is the precipitation of g% phases. As the Al+ Ti content
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K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
PALOY. The strengths and elongation kept the same value as the solution temperature increased. It seems that there was a slightly increasing strength with increasing solution temperature. The tensile tests for the alloys were conducted at different temperatures, and the results with HW-2 are illustrated in Fig. 5. It is clear that the alloy maintained a good strength stability at a temperature lower than 649°C. At 760°C, the strength of the alloy was remarkably lost because of the solution of g% phases.
3.2. Stress rupture properties Fig. 4. Effect of solution temperature on the tensile properties of HW-1A at room temperature.
Fig. 5. Tensile properties of HW-2 at different test temperatures.
increased, the precipitation of g% also increased from 30 to 50%. Thus, the strength of the alloy was improved and the ductility was decreased. It should be noted that at an elevated temperature, the ductility change could be more complicated. The ductility of the alloy with 6% Al + Ti content was the highest. This should be investigated further and classified. The effect of solution temperature on the tensile properties of the HW-1 (WASPALOY) is shown in Fig. 4. It can be seen that the solution temperature had almost no effect on the tensile properties of WAS-
The effect of precipitation content on the stress rupture properties of the alloys was investigated at 760°C and 482 MPa. The results are shown in Fig. 6(a) and (b). It is clear that the increasing precipitation content tremendously improved the stress-rupture life of the alloys. The life of commercial WASPALOY was 13.9 h for a cooling rate of 500°C min − 1, while the life of HW-5A was 451.8 h. Under 760°C, most of the g% in WASPALOY had been dissolved into the matrix and caused a great loss of rupture strength. However, a large part of g% still remained in the alloy with a higher content of g% such as HW-5. Finally, the rupture life of HW-5 was much longer than that of HW-1 (WASPALOY). The effect of precipitation on ductility is more complicated and there is no clear trend of elongation with increasing precipitation content. Generally speaking, the cooling rate had an improvement effect on the stress-rupture life, but the effect was limited.
3.3. Fatigue-crack growth The low-cycle fatigue and fatigue-crack propagation behaviors play increasingly important roles in the alloy designation and performance during service life. The fatigue-crack propagation rates of the alloys are illustrated in Fig. 7. It is clear that the fatigue-crack resistance of the alloys can be improved by increasing the
Fig. 6. Effect of g% on the stress rupture properties of the WASPALOY system at 760°C, 482 MPa. (a) Rupture life. (b) Elongation.
K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
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Fig. 7. Fatigue-crack propagation of the alloys under various loading curves. (a) Cooling rate = 500°C min − 1. (b) Cooling rate =200°C min − 1. (c) Cooling rate = 50°C min − 1.
strengthening precipitate volume fraction, in spite of the variation in cooling rate. It can also be seen that fatigue-crack growth is a time-dependent behavior. The growth rate da/dN at 3S is much lower than that of 180S and 3+177S.
4. Discussion
4.1. Effect of cooling rate In most superalloys with a high precipitate content, the precipitation kinetics of the strengthening precipitate g% are too fast to prevent the formation of cooling precipitates after solution treatments. These precipitates play an important role in the mechanical behavior of the alloy [16,17]. It is shown in Fig. 8 that there is almost no effect of cooling rate on the tensile properties of WASPALOY, which has been aged after the solution treatment. However, for alloys with a higher Al+ Ti content, the cooling rate plays an important role in the strength of the alloys. It is demonstrated in Fig. 9 that the yield strength of the higher precipitates content alloys is improved with increasing cooling rate. The higher the precipitate volume fraction, the stronger is the cooling-rate dependence. The improvement in strength represents the real hardening effect caused by increasing the precipitate content. The cooling precipi-
Fig. 8. Effect of cooling rate on the tensile properties of HW-1 (WASPALOY) at room temperature.
Fig. 9. Effect of cooling rate on the yield strengths of the alloys at room temperature.
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K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
precipitates, an interlocking mechanism can occur to make cracking even more difficult. As a result, the fatigue-crack resistance is improved by the g% precipitates.
4.3. Time-dependent fatigue-crack propagation beha6iors
Fig. 10. Effect of precipitate content on the fatigue-crack growth rate at DK=33 MPa* m and 649°C.
tates that contribute to a minimum hardening effect are mostly suppressed by the fast cooling, and then there remains the larger amount of ‘‘effective g% precipitation’’ during the aging procedure.
4.2. Precipitate content Fatigue-crack propagation rates under various loading waveforms as a function of the precipitate volume fraction are shown in Fig. 10. It can be seen that the fatigue-crack resistance is clearly improved by increasing the precipitate content. Generally speaking, the cooling precipitates can form homogeneously in the grains and heterogeneously along the grain boundaries. There is a non-equilibrium reaction between the cooling precipitates and grain boundaries, and the resulting microstructure plays a predominant role in the fatiguecrack growth behavior of the alloys. When a g% precipitate nucleates at the grain boundary, it becomes coherent with one grain but incoherent with the other. The precipitate particle appears to grow at the incoherent interface toward the adjacent grain boundary, and the kinetics is accelerated by the grain boundary diffusion. Since the chromium solubility is very low in the g% phase, excess chromium atoms will be repelled out of the precipitate growth front. As a result, the grain boundaries are locally enriched in chromium, thus enhancing the oxidation resistance when cracking occurs along the grain boundaries. Our research results in In718 from Auger analyses have confirmed such a chromium-enrichment phenomenon [18]. The other effect is that the g% precipitates along grain boundaries can become the barrier of the crack growth and deflect the crack-growth path during the propagation of the cracks. If precipitate growth into the adjacent grain exceeds the inter-particle spacing of
Low cycle fatigue (LCF) of turbine disk alloys is determined by the initiation and the propagation of fatigue cracks. Metallurgical control of fatigue-crack propagation in high-strength superalloys becomes feasible only through a clear understanding of the fatigue-cracking mechanism, as well as the microstructure/property relationships. Linear elastic fracture mechanics (LEFM) has been extensively employed to study the fracture properties of high-strength superalloys [19]. The stress-intensity factor is an appropriate parameter to describe the complete stress field at the tip of crack. In case of cyclic loading, the fatigue-crack growth rate, da/dN, in most high-strength superalloys can be correlated by the stress intensity factor, DK. As proposed by Speidial [20], any differences in da/dN observed for materials under a cycle-dependent fatigue-crack condition, e.g. room temperature and vacuum, are mainly the effects of the elastic modulus. Normalizing DK by the elastic modulus can unify fatigue-crack growth rates measured from a wide variety of alloy systems. Testing parameters, such as frequency and waveform, play a negligible role in determining the da/dN for a fixed DK. In fact, the stage II fatigue-crack growth rate is usually described by the Paris power law: da = C× (DK)n dN
(2)
where C and n are constants related to materials. At a temperature higher than 500°C, superalloys start to show a time-dependent component of FCP; DK is no longer the only parameter controlling da/dN. An acceleration of da/dN is observed for a given DK when the fatigue-cycle frequency is decreased below a certain value. Pure time-dependent crack growth at elevated temperatures has been described as creep-crack growth. However, the effect of the environment must be considered if the test is performed in air, and the most important effect of the air is the oxidation in front of the crack tip. Since the grain boundary is the weakening part of materials and the fast channel for the oxygen diffusion, it has indicated that the Stress Assisted Grain Boundary Oxidation (SAGBO) plays the most important role on the environmental effect [12]. The time-dependent fatigue-crack growth rate then can be rewritten as:
da da = dNair dN
+
fat-air
da dt
K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
×t
crp-air
+[Interaction-term]fat-crp
=
da dN
+ vac-fat
da dt
(3)
×t +[Inter-term]SAFGBO-fat crp-vac
+ [Inter-term]SAGBO-crp +[Inter-term]fat-crp where (da/dN)fat-air is the crack growth rate in air caused by fatigue, (da/dt)crp-air is the crack growth rate in caused by creep, t is the cycle period, [Inter-term]SAGBO-fat, is the effect of SAGBO on the fatigue-crack growth rate, and [Inter-term]SAGBO-crp is the effect of SAGBO on the creep crack growth. It has been indicated that the time-dependent high-temperature crack propagation rates of many materials in vacuum [11,13,14], as well as inert gas environments [12,15], are substantially lower that in air. Thus, neglecting the effect of creep, the Eq. (3) can be rewritten as:
da dN
= fat-air
or
da dN
= fat-air
da dN
da dN
+[Inter-term]SAGBO-fat
(4)
×[Inter-term]SAGBO-fat.
(5)
fat-vac
fat-vac
Fig. 11 shows the fatigue-crack growth rate of the HW-2B alloy under different loading waveforms. The
Fig. 11. Crack growth rate vs. DK for the HW-2B alloy at 649°C.
7
stress-intensity factor, DK, is found to correlate the crack growth rates satisfactorily. The general trend is that the growth rate increases with increasing DK. However, the crack growth rates are obviously time-dependent. The growth rate increases with decreasing loading frequency. The growth rate for the 3+177S is higher than that of 180S, although they both have the same loading period. Fig. 12 shows the relationship between precipitate content and the power coefficient, n, fitting in the Paris Law. It can be seen that the g% content has little effect on the coefficient at the frequency of 1/3 Hz, but plays an important role at the trapezoid waveform of 3+177S. However, the higher precipitate content shows a lower time dependence. From Eq. (2), the (da/dN)fat-vac will remain constant for a fixed DK. Substituting into Eq. (5), it can be seen that the time dependence of the fatigue-crack growth rate is mainly caused by the interaction between SAGBO and the pure fatigue. The interaction between fatigue and oxidation in front of crack tip is two-way. At first, the tension stress in front of the crack tip accelerates the diffusion of oxygen along the grain boundaries and then increases the content of oxygen. However, the appearance of oxygen in front of the crack tip will cause a damage zone, caused by the combination of mechanical and chemical effects. As a result, the fatigue-crack growth rate increases. The final fatigue-crack growth rate is the dynamic equilibrium between the diffusion of oxygen and the ‘‘pure fatiguecrack propagation’’. The formation of the damage zone in front of crack tip is the key to understanding the time-dependent cracking behavior in air [16]. The necessary requirements for forming a damage zone include temperature, time, stress intensity and environment. Consider hold-time fatigue cycles (such as 3+ 177S) applying in a crack in air. During the hold time, the crack may or may not grow, depending on the incubation period at the test temperature. Nevertheless, a damage zone is built up in front of the crack tip. The formation of damage zone is at a speed equivalent to the sustained loading crack growth rate. After the hold time, the crack undergoes the subsequent cyclic stress intensity (unloading and then loading). A substantial crack increment results from fast crack growth in the damaged zone. The physical process of crack growth under time-dependent conditions includes damage zone build-up, accelerated cyclic crack growth, and sustained loading crack growth.
5. Conclusions
Fig. 12. Fitting coefficient, n, in Paris Law as a function of precipitate content.
The tensile strengths, both yield strength and ultimate tensile strength, increase with increasing Al+Ti content in the alloys. At the same time, the elongation of the alloys decreases with increasing Al+Ti content.
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K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
The alloy maintains a good strength stability at a temperature lower than 649°C. At a temperature of 760°C, the strength of the alloy is remarkably lost because of the solution of g% phases. The solution temperature has almost no effect on the tensile properties of WASPALOY The cooling rate shows almost no effect on the tensile properties of WASPALOY, which is aged after solution treatment. However, for the alloys with a higher Al+ Ti content, the cooling rate plays an important role in the strength of the alloys. It is demonstrated that the yield strength of the higher precipitates content alloys is improved with increasing cooling rate. The higher the precipitate volume fraction, the stronger is the cooling rate dependence. The increase in precipitation content tremendously improves the stress-rupture life of the alloys. However, the effect of precipitation on the ductility is more complicated, and there is no clear trend of elongation with increasing precipitation content. The fatigue-crack resistance of the alloys can be improved as the increasing of strengthening precipitate volume fraction, in spite of the variation in cooling rate. The crack-growth rates are clearly time-dependent. The growth rate increases with decreasing loading frequency. However, the higher precipitate content shows a lower time dependence.
.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
[20]
L. Garimella, P. Liaw, D. Klarstrom, JOM 49 (1997) 67–71. M. Cao, F. Gabrielli, R. Pelloux, Res. Mech. 17 (1986) 163–177. A. Palazotto, J. Mercer, Eng. Frac. Mech. 35 (1990) 967–986. Z. Mei, C. Krenn, J. Morris, Jr., Met. Mat. Trans. A 26A (1995) 2063– 2073. M. Stroosnijder, V Guttmann, J.H.W. de Wit, Met. Mat. Trans. A 26A (1995) 2103– 2110. A. McEvily, J.L. Velazquez, Met. Trans. A 23A (1992) 2211– 2221. H. Smith, D. Michel, Met. Trans. A 17A (1986) 370–374. I. Vasatis, R. Pelloux, Met. Trans. A 16A (1985) 1515–1520. L. Korusiewicz, J. Dong, M. Kumosa, Scripta Met. Mat. 29 (1993) 573– 578. E. Fleury, L. Remy, Mat. Sci. Eng. A167 (1993) 23–30. D. Duquette, M. Gell, Met. Trans. 2 (1971) 1325–1331. G. Webster, Mat. Sci. Tech. 3 (1987) 716– 725. S. Lynch, Acta Met. 36 (1988) 2639– 2661. R. Bricknell, D. Woodford, Met. Trans. A 12A (1981) 1673– 1680. J. Davidson, J. Tien, Met. Trans. A 12A (1981) 865–876. K. Chang, M. Henry, M. Benz, JOM December (1990) 29–35. K. Chang, Acta Met. Sinica (Engl. Lett.) 9 (1996) 467–471. J. Dong, X. Liu, X. Xie, R. Thompson, Acta Met. Sinica (Engl. Lett.) 10 (1997) 510– 515. W. Carpenter, B. Kang, K. Chang, in: E. Loria (Ed.), Superalloys 718, 625, 706 and Various Derivatives, TMS, Warrendale, PA, 1997, pp. 679– 688. M. Speidel, in: P. Sahm, M. Speidel (Eds.), High Temperature Materials in Gas Turbines, Elsevier, New York, 1974, pp. 207– 251.
Effect of g% content on the mechanical behavior of the WASPALOY alloy system Keh-Minn Chang, Xingbo Liu * Department of Mechanical & Aerospace Engineering, West Virginia Uni6ersity, Morgantown, WV 26506, USA Received 2 May 1999; received in revised form 12 December 2000
Abstract This article presents the research results of the effects of alloy chemistry, cooling rate and solution temperature on mechanical behavior, especially for the fatigue-crack growth of the WASPALOY alloy system. It is indicated that tensile strengths, both yield strength and ultimate tensile strength, increase with increasing Al + Ti content in the alloys. The solution temperature had almost no effect on the tensile properties of WASPALOY. Cooling rate showed almost no effect on the tensile properties of WASPALOY, but for the alloys with a higher Al+ Ti content, the yield strength of the alloys was improved with increasing cooling rate. The increase in precipitation content tremendously improved the stress rupture life of the alloys. The fatigue-crack resistance of the alloys can be improved by increasing the strengthening precipitate volume fraction, in spite of the variation of the cooling rate. The crack growth rates were time-dependent, as expected. The growth rate increased with decreasing loading frequency. However, a higher precipitate content showed a lower time dependence. © 2001 Elsevier Science B.V. All rights reserved. Keywords: g% content; Mechanical behavior; WASPALOY alloy system
1. Introduction WASPALOY has been extensively applied in different industries such as aircraft, chemical-plant equipment and petrochemical equipment. It can be considered as a conventional precipitation-strengthened superalloy. The major alloying additions are 19.5% Cr and 13.5 Co but no Fe. The precipitation phase upon aging is the meta-stable intermetallic compound of Ni3(Al, Ti), designed as g%, which has a ordered structure of L12. WASPALOY has a moderate low-temperature strength and an excellent creep strength at high temperatures. In the past several decades, the fatigue-crack growth behavior of high-temperature materials, especially superalloys, has received increasing attention because (1) the results of failure analyses indicate that fatigue is one of the major causes of failure in engineering structures, and (2) the fatigue life of engines is determined by the * Corresponding author. Tel.: +1-304-2933111; fax: + 1-3042936689. E-mail address: [email protected] (X. Liu).
initiation and the propagation behavior of the cracks [1,2]. Fabricated components usually contain geometric discontinuities, machining or assembly marks, or welding defects that facilitate fatigue-crack initiation. Therefore, it is broadly assumed that all parts contain pre-existing cracks. As a consequence, research on life prediction or service extension of superalloy components has been focused on their fatigue-crack growth characteristics. Several parameters have now been identified as playing significant roles in affecting the growth of fatigue cracks [1]. These are intrinsic parameters like alloy chemistry, heat treatment, microstructure, and elastoplastic behavior, mechanical factors such as the crack geometry, load amplitude, and stress load ratio, and physico-chemical parameters, including the nature, composition and temperature of the environment surrounding the crack tip. It has been revealed that crack growth is predominantly a cycle-dependent damage process with little frequency effect at very high frequencies. Crack growth is induced by irreversible crack tip plastic flow, which
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5093(00)02042-6
K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
2
Table 1 Chemical compositions of tested alloys (wt.%)
Mn Fe Si Cu Ni Cr Al Ti Mg Co Mo Nb Ta P B W Zr
HW-1
HW-2
B0.01 0.13 0.01 B0.01 68.37 14.48 1.50 2.90 B0.001 8.72 3.77 B0.01 0.01 B0.001 0.005 0.04 0.05
B0.01 0.11 B0.01 B0.01 66.92 14.81 1.78 3.51 B0.001 9.13 3.63 B0.01 B0.01 B0.001 0.008 0.05 0.05
HW-3
HW-4*
HW-5*
0.11
0.10
0.13
Balance 14.84 2.03 3.92
Balance 14.81 2.28 4.47
Balance 14.76 2.47 4.90
9.17 3.72
9.06 3.80
9.20 3.75
0.006
0.01
0.01
0.05
0.05
0.05
to low-frequency loading in air test conditions. These observations strongly support the conclusion that timedependent effects in superalloys are principally due to environmental degradation. The purpose of this paper was to investigate the effect of alloy chemistry, cooling rate and loading time on the mechanical behavior, especially for the fatigue-crack growth of the WASPALOY alloy system.
2. Experimental procedures
2.1. Experimental materials WASPALOY was selected as the basic research material, and various amounts of Al and Ti were added to the basic matrix. All the materials were melted in a vacuum: the chemical compositions are listed in Table 1. The main differences between the alloys were the contents of Al and Ti. In fact, to research the effect of g% on the properties of WASPALOY, the alloys were designed with different contents of Al and Ti, and then with the different content of g% strengthening phases. The HW-1 alloy had the standard WASAPLOY composition, while the others had higher contents of g%. The relationship between the content of g% and Al+Ti is shown in Table 2. The alloys were melted in a 40 lb (18 kg) VIM furnace and poured into F0.114 m round bars. These round bars were homogenized at 1200°C/24 h and extruded into 0.254× 0.254 m square bars.
Table 2 Designed Al and Ti content and that of precipitatea Alloy
Al (wt.%)
Ti (wt.%)
Al+Ti (wt.%) g% (wt.%)
HW-1 HW-2 HW-3 HW-4 HW-5
1.5 1.75 2.0 2.25 2.5
3.0 3.5 4.0 4.5 5.0
4.5 5.25 6.0 6.75 7.5
30 35 40 45 50
a The content of g% is the designed content. The alloys are designed based on the results calculated by PHACOMP software, which is used extensively in superalloy industry. The error is believed to be less than 5%.
2.2. Heat treatment
provides the driving force for crack growth [3,4]. At relatively low frequencies, crack growth includes timedependent processes, and fatigue-crack growth rates (FCGR) are strongly dependent on temperature, frequency, holding time and stress ratio. Time-dependent effects on high-temperature fatigue-crack growth behavior are generally ascribed to phenomena involving creep and/or environmental degradation processes [5– 10]. Furthermore, several studies on superalloys have demonstrated that high-temperature FCGR are decreased by several orders of magnitude [11 – 15] when the results of vacuum tests are compared with those due
To investigate the effect of solution temperature and cooling rate on the properties of the alloys, the experimental scheme shown in Table 3 was employed. It should be pointed out that the cooling rates shown in the table are the average slopes of the cooling curves from the solution temperature to 650°C. As indicated in Table 3, higher solution temperatures were employed with increasing contents of Al and Ti. Solution temperatures were determined by the metallography results, which took small coupons heattreated at different temperatures. The selection of
Table 3 Solution temperatures and cooling rates HW-1
Solution (°C) Cool (°C min−1)
HW-2
HW-3
HW-4
HW-5
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
1000 500
1000 200
1000 50
1050 500
1050 200
1050 50
1100 500
1100 200
1100 50
1125 500
1125 200
1125 50
1125 500
1125 200
1125 50
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2.3. Mechanical tests 2.3.1. Tensile tests To research the effect of cooling rate and the content of g% on the tensile strengths and ductility of WASPALOY alloy system, a series of tensile tests were conducted at different test temperatures: room temperature, 538, 649 and 760°C. The dimensions (in millimeters) of the specimen are shown in Fig. 1.
Fig. 1. Dimensions of the tensile specimen.
Fig. 2. Effect of Al +Ti content on the tensile properties of the WASPALOY system at room temperature.
2.3.2. Stress-rupture tests Constant-loading stress-rupture tests of the alloys with different contents of Al+Ti (g%) were conducted at 760°C with an initial stress of 482 MPa. 2.3.3. Fatigue-crack propagation tests Fatigue-crack propagation tests with various wave forms (triangular wave with the cycle of 3S and 180S, trapezoid wave with 3+ 177S) were performed by employing the single-edge-notched (SEN) specimen. A reversible d.c. current was applied by a power supplier through the head of the specimen. The crack length was monitored by a pair of potential probes mounted on the front edge of the specimen gage across the pre-machined notch. The measured d.c. potential drop at any crack length was normalized and converted into the corresponding crack length by a single analytical relation, namely Johnson’s equation [8]:
A=
2W × cos − 1 y
n
cosh(yY/2W) cosh{(U/U0) cosh [cosh(yY/2W)/cos(yA0/2W)]} −1
(1) where A and A0 are the actual and initial crack lengths, respectively, U and U0 are updated and initial measured potential drops, respectively, and W and Y represent the specimen width and one half of the potential probe span, respectively.
3. Results Fig. 3. Effect of Al +Ti content on the tensile properties of WASPALOY at 650°C.
solution temperature was based on two criteria: (1) recrystallized grain structure; (2) limited grain growth. Thus, the grain sizes of all the alloys were almost the same, which is ASTM 8-9. The solution time for all the materials was 4 h, and the standard two-step aging treatment for WASPALOY, i.e. 850°C/4 h/AC + 760°C/16 h/AC, was used after the solution treatment.
3.1. Tensile properties Figs. 2 and 3 show the effect of Al+ Ti content on the tensile properties of the alloys at room temperature and 649°C. It can be seen that the tensile strengths, both yield strength and ultimate tensile strength, increased with increasing Al+ Ti content of the alloys. At the same time, the elongation of the alloys decreased with increasing Al+ Ti content. This is reasonable because the major strengthening mechanism of the alloy is the precipitation of g% phases. As the Al+ Ti content
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PALOY. The strengths and elongation kept the same value as the solution temperature increased. It seems that there was a slightly increasing strength with increasing solution temperature. The tensile tests for the alloys were conducted at different temperatures, and the results with HW-2 are illustrated in Fig. 5. It is clear that the alloy maintained a good strength stability at a temperature lower than 649°C. At 760°C, the strength of the alloy was remarkably lost because of the solution of g% phases.
3.2. Stress rupture properties Fig. 4. Effect of solution temperature on the tensile properties of HW-1A at room temperature.
Fig. 5. Tensile properties of HW-2 at different test temperatures.
increased, the precipitation of g% also increased from 30 to 50%. Thus, the strength of the alloy was improved and the ductility was decreased. It should be noted that at an elevated temperature, the ductility change could be more complicated. The ductility of the alloy with 6% Al + Ti content was the highest. This should be investigated further and classified. The effect of solution temperature on the tensile properties of the HW-1 (WASPALOY) is shown in Fig. 4. It can be seen that the solution temperature had almost no effect on the tensile properties of WAS-
The effect of precipitation content on the stress rupture properties of the alloys was investigated at 760°C and 482 MPa. The results are shown in Fig. 6(a) and (b). It is clear that the increasing precipitation content tremendously improved the stress-rupture life of the alloys. The life of commercial WASPALOY was 13.9 h for a cooling rate of 500°C min − 1, while the life of HW-5A was 451.8 h. Under 760°C, most of the g% in WASPALOY had been dissolved into the matrix and caused a great loss of rupture strength. However, a large part of g% still remained in the alloy with a higher content of g% such as HW-5. Finally, the rupture life of HW-5 was much longer than that of HW-1 (WASPALOY). The effect of precipitation on ductility is more complicated and there is no clear trend of elongation with increasing precipitation content. Generally speaking, the cooling rate had an improvement effect on the stress-rupture life, but the effect was limited.
3.3. Fatigue-crack growth The low-cycle fatigue and fatigue-crack propagation behaviors play increasingly important roles in the alloy designation and performance during service life. The fatigue-crack propagation rates of the alloys are illustrated in Fig. 7. It is clear that the fatigue-crack resistance of the alloys can be improved by increasing the
Fig. 6. Effect of g% on the stress rupture properties of the WASPALOY system at 760°C, 482 MPa. (a) Rupture life. (b) Elongation.
K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
5
Fig. 7. Fatigue-crack propagation of the alloys under various loading curves. (a) Cooling rate = 500°C min − 1. (b) Cooling rate =200°C min − 1. (c) Cooling rate = 50°C min − 1.
strengthening precipitate volume fraction, in spite of the variation in cooling rate. It can also be seen that fatigue-crack growth is a time-dependent behavior. The growth rate da/dN at 3S is much lower than that of 180S and 3+177S.
4. Discussion
4.1. Effect of cooling rate In most superalloys with a high precipitate content, the precipitation kinetics of the strengthening precipitate g% are too fast to prevent the formation of cooling precipitates after solution treatments. These precipitates play an important role in the mechanical behavior of the alloy [16,17]. It is shown in Fig. 8 that there is almost no effect of cooling rate on the tensile properties of WASPALOY, which has been aged after the solution treatment. However, for alloys with a higher Al+ Ti content, the cooling rate plays an important role in the strength of the alloys. It is demonstrated in Fig. 9 that the yield strength of the higher precipitates content alloys is improved with increasing cooling rate. The higher the precipitate volume fraction, the stronger is the cooling-rate dependence. The improvement in strength represents the real hardening effect caused by increasing the precipitate content. The cooling precipi-
Fig. 8. Effect of cooling rate on the tensile properties of HW-1 (WASPALOY) at room temperature.
Fig. 9. Effect of cooling rate on the yield strengths of the alloys at room temperature.
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K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
precipitates, an interlocking mechanism can occur to make cracking even more difficult. As a result, the fatigue-crack resistance is improved by the g% precipitates.
4.3. Time-dependent fatigue-crack propagation beha6iors
Fig. 10. Effect of precipitate content on the fatigue-crack growth rate at DK=33 MPa* m and 649°C.
tates that contribute to a minimum hardening effect are mostly suppressed by the fast cooling, and then there remains the larger amount of ‘‘effective g% precipitation’’ during the aging procedure.
4.2. Precipitate content Fatigue-crack propagation rates under various loading waveforms as a function of the precipitate volume fraction are shown in Fig. 10. It can be seen that the fatigue-crack resistance is clearly improved by increasing the precipitate content. Generally speaking, the cooling precipitates can form homogeneously in the grains and heterogeneously along the grain boundaries. There is a non-equilibrium reaction between the cooling precipitates and grain boundaries, and the resulting microstructure plays a predominant role in the fatiguecrack growth behavior of the alloys. When a g% precipitate nucleates at the grain boundary, it becomes coherent with one grain but incoherent with the other. The precipitate particle appears to grow at the incoherent interface toward the adjacent grain boundary, and the kinetics is accelerated by the grain boundary diffusion. Since the chromium solubility is very low in the g% phase, excess chromium atoms will be repelled out of the precipitate growth front. As a result, the grain boundaries are locally enriched in chromium, thus enhancing the oxidation resistance when cracking occurs along the grain boundaries. Our research results in In718 from Auger analyses have confirmed such a chromium-enrichment phenomenon [18]. The other effect is that the g% precipitates along grain boundaries can become the barrier of the crack growth and deflect the crack-growth path during the propagation of the cracks. If precipitate growth into the adjacent grain exceeds the inter-particle spacing of
Low cycle fatigue (LCF) of turbine disk alloys is determined by the initiation and the propagation of fatigue cracks. Metallurgical control of fatigue-crack propagation in high-strength superalloys becomes feasible only through a clear understanding of the fatigue-cracking mechanism, as well as the microstructure/property relationships. Linear elastic fracture mechanics (LEFM) has been extensively employed to study the fracture properties of high-strength superalloys [19]. The stress-intensity factor is an appropriate parameter to describe the complete stress field at the tip of crack. In case of cyclic loading, the fatigue-crack growth rate, da/dN, in most high-strength superalloys can be correlated by the stress intensity factor, DK. As proposed by Speidial [20], any differences in da/dN observed for materials under a cycle-dependent fatigue-crack condition, e.g. room temperature and vacuum, are mainly the effects of the elastic modulus. Normalizing DK by the elastic modulus can unify fatigue-crack growth rates measured from a wide variety of alloy systems. Testing parameters, such as frequency and waveform, play a negligible role in determining the da/dN for a fixed DK. In fact, the stage II fatigue-crack growth rate is usually described by the Paris power law: da = C× (DK)n dN
(2)
where C and n are constants related to materials. At a temperature higher than 500°C, superalloys start to show a time-dependent component of FCP; DK is no longer the only parameter controlling da/dN. An acceleration of da/dN is observed for a given DK when the fatigue-cycle frequency is decreased below a certain value. Pure time-dependent crack growth at elevated temperatures has been described as creep-crack growth. However, the effect of the environment must be considered if the test is performed in air, and the most important effect of the air is the oxidation in front of the crack tip. Since the grain boundary is the weakening part of materials and the fast channel for the oxygen diffusion, it has indicated that the Stress Assisted Grain Boundary Oxidation (SAGBO) plays the most important role on the environmental effect [12]. The time-dependent fatigue-crack growth rate then can be rewritten as:
da da = dNair dN
+
fat-air
da dt
K.-M. Chang, X. Liu / Materials Science and Engineering A308 (2001) 1–8
×t
crp-air
+[Interaction-term]fat-crp
=
da dN
+ vac-fat
da dt
(3)
×t +[Inter-term]SAFGBO-fat crp-vac
+ [Inter-term]SAGBO-crp +[Inter-term]fat-crp where (da/dN)fat-air is the crack growth rate in air caused by fatigue, (da/dt)crp-air is the crack growth rate in caused by creep, t is the cycle period, [Inter-term]SAGBO-fat, is the effect of SAGBO on the fatigue-crack growth rate, and [Inter-term]SAGBO-crp is the effect of SAGBO on the creep crack growth. It has been indicated that the time-dependent high-temperature crack propagation rates of many materials in vacuum [11,13,14], as well as inert gas environments [12,15], are substantially lower that in air. Thus, neglecting the effect of creep, the Eq. (3) can be rewritten as:
da dN
= fat-air
or
da dN
= fat-air
da dN
da dN
+[Inter-term]SAGBO-fat
(4)
×[Inter-term]SAGBO-fat.
(5)
fat-vac
fat-vac
Fig. 11 shows the fatigue-crack growth rate of the HW-2B alloy under different loading waveforms. The
Fig. 11. Crack growth rate vs. DK for the HW-2B alloy at 649°C.
7
stress-intensity factor, DK, is found to correlate the crack growth rates satisfactorily. The general trend is that the growth rate increases with increasing DK. However, the crack growth rates are obviously time-dependent. The growth rate increases with decreasing loading frequency. The growth rate for the 3+177S is higher than that of 180S, although they both have the same loading period. Fig. 12 shows the relationship between precipitate content and the power coefficient, n, fitting in the Paris Law. It can be seen that the g% content has little effect on the coefficient at the frequency of 1/3 Hz, but plays an important role at the trapezoid waveform of 3+177S. However, the higher precipitate content shows a lower time dependence. From Eq. (2), the (da/dN)fat-vac will remain constant for a fixed DK. Substituting into Eq. (5), it can be seen that the time dependence of the fatigue-crack growth rate is mainly caused by the interaction between SAGBO and the pure fatigue. The interaction between fatigue and oxidation in front of crack tip is two-way. At first, the tension stress in front of the crack tip accelerates the diffusion of oxygen along the grain boundaries and then increases the content of oxygen. However, the appearance of oxygen in front of the crack tip will cause a damage zone, caused by the combination of mechanical and chemical effects. As a result, the fatigue-crack growth rate increases. The final fatigue-crack growth rate is the dynamic equilibrium between the diffusion of oxygen and the ‘‘pure fatiguecrack propagation’’. The formation of the damage zone in front of crack tip is the key to understanding the time-dependent cracking behavior in air [16]. The necessary requirements for forming a damage zone include temperature, time, stress intensity and environment. Consider hold-time fatigue cycles (such as 3+ 177S) applying in a crack in air. During the hold time, the crack may or may not grow, depending on the incubation period at the test temperature. Nevertheless, a damage zone is built up in front of the crack tip. The formation of damage zone is at a speed equivalent to the sustained loading crack growth rate. After the hold time, the crack undergoes the subsequent cyclic stress intensity (unloading and then loading). A substantial crack increment results from fast crack growth in the damaged zone. The physical process of crack growth under time-dependent conditions includes damage zone build-up, accelerated cyclic crack growth, and sustained loading crack growth.
5. Conclusions
Fig. 12. Fitting coefficient, n, in Paris Law as a function of precipitate content.
The tensile strengths, both yield strength and ultimate tensile strength, increase with increasing Al+Ti content in the alloys. At the same time, the elongation of the alloys decreases with increasing Al+Ti content.
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The alloy maintains a good strength stability at a temperature lower than 649°C. At a temperature of 760°C, the strength of the alloy is remarkably lost because of the solution of g% phases. The solution temperature has almost no effect on the tensile properties of WASPALOY The cooling rate shows almost no effect on the tensile properties of WASPALOY, which is aged after solution treatment. However, for the alloys with a higher Al+ Ti content, the cooling rate plays an important role in the strength of the alloys. It is demonstrated that the yield strength of the higher precipitates content alloys is improved with increasing cooling rate. The higher the precipitate volume fraction, the stronger is the cooling rate dependence. The increase in precipitation content tremendously improves the stress-rupture life of the alloys. However, the effect of precipitation on the ductility is more complicated, and there is no clear trend of elongation with increasing precipitation content. The fatigue-crack resistance of the alloys can be improved as the increasing of strengthening precipitate volume fraction, in spite of the variation in cooling rate. The crack-growth rates are clearly time-dependent. The growth rate increases with decreasing loading frequency. However, the higher precipitate content shows a lower time dependence.
.
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