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Temperature Dependence of Deformation Behavior in a Co–Al–W-Base Single Crystal Superalloy

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Temperature Dependence of Deformation Behavior in a Co–Al–W-Base Single Crystal Superalloy Materials Science & Engineering A 620 (2015) 36–43 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea Temperature dependence of deformation behavior in a Co–Al–W-base single crystal superalloy L. Shi, J.J. Yu n, C.Y. Cui, X.F. Sun Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China article info abstract Article history: Tensile properties of a single-crystal Co–Al–W–Ni–Cr–Ta alloy with low tungsten content have been Received 18 June 2014 studied within the temperatures ranging from 20 to 1000 1C at a constant strain rate of 1.0 Â 10 À4 sÀ1. Received in revised form The alloy exhibits comparable yield strength with that of Co–Al–W-base alloys containing more 17 September 2014 tungsten. From 600 1Cto8001C, a yield strength anomaly is observed, probably due to the cross-slip Accepted 18 September 2014 of superdislocations from the octahedral plane to the cube plane. TEM analysis demonstrates that Available online 26 September 2014 stacking faults (SFs) appear both in γ channels and γ0 precipitates in a wide temperature range. These SFs Keywords: are responsible for the obvious strain hardening observed in stress–strain curves. From room – – Co Al W-base alloy temperature to 900 1C, the deformation is dominated by dislocations shearing γ0 particles. At 1000 1C, Tensile behavior 0 the main deformation mechanism is dislocations bypassing γ particles. Dislocation structures & 2014 Elsevier B.V. All rights reserved. Stacking fault 1. Introduction that the addition of Cr makes against the improvement of γ0 solvus temperature. With 8 at% Cr additions, the oxidation resistance of Nickel-base superalloys, possessing exceptional mechanical this new type Co-base superalloys is approaching the level of properties due to the well known strengthening of L12 type MAR-M 509 at 800 1C [6]. Recent investigations by Shinagawa γ0 precipitates, are widely used for manufacturing aircraft and et al. [7] indicate that substitution of Ni for W can stabilize the γ0 power-generation engine turbines. Recently, Co-base superalloys phase and slightly increase the γ0 solvus temperature, which is 0 strengthened by γ (Co3(Al,W)) with L12 structure have gained beneficial to decrease the density. Besides, a combined addition of substantial interest. A series of experimental [1–7] and computa- Cr and Ni to ternary Co–Al–W system can improve the γ0 solvus tional [8,9] efforts have been done to study the effects of alloying temperature [5]. It can be seen that the combination of alloying elements on the microstructure and mechanical property of the elements greatly affects the γ0 solvus temperature of Co–Al–W- new Co-base alloys, suggesting that Co3(Al,W) has some simila- base alloys. It is interesting to know the effect of a combined rities with that of Ni3Al and can be practically used as the addition of Ni, Cr and Ta on the microstructure and property of Co– strengthening phase of Co-base superalloys. Al–W alloy system. Thus, in the present study, a Co–Al–W–Ni–Cr– However, in the Co–Al–W-base alloys, large amount of W is Ta alloy system is tentatively designed, whose tungsten amount is 0 added to stabilize Co3(Al,W), leading to a high density. The γ merely half of those reported Co-base alloys [3,4], and attentions solvus temperature is relatively lower compared with that of Ni- are paid to the microstructure and tensile properties of the alloy base superalloys, a big restriction on high temperature applica- and characterizing the main deformation microstructures with the tions. Efforts have been dedicated to improve the two-phase γ/γ0 aim of comparison to those of Co-base alloys containing high microstructural stability at elevated temperature. Ta is effective to amount of tungsten. improve the γ0 solvus temperature in ternary Co–Al–W system [3,4]. The γ0 solvus temperature of Co–9Al–8W–2Ta–2Cr (at%) alloy is above 1050 1C which is slightly lower than that of Co– 2. Experimental procedure 9Al–8W–2Ta (at%) alloy [3], while the γ0 solvus temperature of Co– 7.8Al–7.8W–2Ta–4.5Cr (at%) alloy is only 960 1C [4].InaCo–7.5Al– The nominal composition (at%) of the alloy studied is as follows: 7W–xCr (x¼13, 17, 21, at%) alloy system [5], the γ0 solvus Al10,W5,Ni17,Cr6,Ta2.7,balancebyCo(namedas5W).The temperature is decreasing as the Cr content increasing. It seems master alloy was melted in a vacuum induction furnace, and then directionally solidified into [001] single crystal rods by Bridgman n Corresponding author. Tel.: þ86 24 2397 1713. technique at a constant withdraw rate of 6 mm/min. The melting 0 E-mail address: [email protected] (J.J. Yu). point and γ solvus temperature of the alloy were determined by http://dx.doi.org/10.1016/j.msea.2014.09.074 0921-5093/& 2014 Elsevier B.V. All rights reserved. L. Shi et al. / Materials Science & Engineering A 620 (2015) 36–43 37 Differential Thermal Analysis (DTA) under high purity Ar atmosphere other Co-base superalloys with γ/γ0 microstructure and the Ni- with a heating rate of 10 1C/min. The heat treatments were carried base superalloy CMSX-4 are also given in Table 1. It can be seen out as follows: 1310 1C/10 h, furnace coolingþ1000 1C/36 h, air cool- that the solidus and liquidus of Co-base superalloys are higher ingþ750 1C/24 h, air cooling. Heat-treated samples were polished than those of Ni-base superalloys. This suggests that there is a in a solution of 42 ml H3PO4þ34 ml H2SO4þ24 ml H2Oat10V. possibility for greater temperature capability compared to Ni-base The microstructure was analyzed using a Scanning Electron Micro- alloys. However, the γ0 solvus temperature of Co-base alloys is still scope (SEM). The volume fraction and size distribution of γ0 much lower than that of Ni-base superalloy CMSX-4. By comparing precipitates were analyzed by image analyzer. Co–9Al–9W (at%) alloy and Co–7.3Al–6.8W (at%) alloy, the γ0 Tensile specimens with a nominal 35 mm gage length and a solvus temperature is decreased from 985 1C to 854 1C, indicating diameter of 5 mm were machined from heat-treated samples. a lower W content depresses the stability of Co3(Al,W). With large Tensile tests were conducted at a strain rate of 1 Â 10 À4 sÀ1 from amount of Ni additions, the γ0 solvus temperature increases room temperature to 1000 1C with the crystal growth direction slightly. As mentioned earlier, alloying with certain amount of Cr parallel to the tension loading direction. During the test, the will result in the decrease of γ0 solvus temperature. Possessing a temperature variation was maintained within 72 1C. At least higher amount of W and Ta, the γ0 solvus temperature of Co–7.8Al– two identical specimens were tested at each temperature. A 7.8W–4.5Cr–2Ta (at%) alloy is lower than that of Co–9.9Al–4.8W– JMS-6301F field-emission scanning electron microscope (SEM) 1.8Ta (at%) alloy, probably associated with the addition of Cr. Thus, was used to observe the fractures. Transverse sections of the simple substitution of Ni for W or alloying with certain amount of fractured specimens were cut into discs with 0.5 mm in thickness. Cr in Co–Al–W–Ta alloy system is not valid to improve the γ0 These discs were polished to 50 μm, and then subjected to twin- solvus temperature. The 5W alloy exhibits a relative higher γ0 jet polishing in a solution of methanol with 5 vol% perchloric acid solvus temperature and lower density compared with those of Co– at À30 1C and 18–20 V. A JEM 2100 Transmission Electron Micro- 8.8Al–9.8W–2Ta (at%) alloy, suggesting that alloying with high Ni scope (TEM) was used for dislocations analysis. and high Ta can overwhelm negative Cr effect as well as the negative low W effect. The SEM micrographs and frequency size distribution of γ0 3. Results precipitates of the heat-treated sample are shown in Fig. 1. The 5W alloy is only constituted of γ and γ0 phases. The γ0 precipitates 〈 〉 3.1. Microstructures exhibit cuboidal morphology, aligned along the 100 direction, which is similar to that of the typical Ni-base superalloys. It is The transformation temperatures measured by DTA are given possible to note that the size distribution is close to a Gaussian γ0 in Table 1. For comparison, the transformation temperatures of distribution (Fig. 1b). The average size of precipitates is about 310 nm. Based on the results of image analyzer, the γ0 volume Table 1 fraction of the alloy is about 65%. Liquidus, solidus, γ0-solvus temperatures and density of the investigated alloy, together with those of other Co-base and Ni-base superalloys. 3.2. Tensile behavior Alloy Transformation Density temperature (1C) (g cmÀ3) Fig. 2 shows true stress–strain curves of the alloy tested at Solidus Liquidus γ0 solvus different temperatures. Crystal orientations of four single-crystal bars used in the present study are about 51,61,31,81 away from 5W 1395 1426 1100 9.32 [001], respectively. It can be seen that the alloy exhibits different – – –– Co 7.3Al 6.8W (at%) [5] 854 9.18 tensile behavior over the experimental temperature ranges. That is Co–9.2Al–9W (at%) [1] 1441 1466 985 9.54 Co–8.8Al–9.8W–2Ta (at%) [4] 1407 1451 1079 49.54 at room temperature, a strong strain hardening phenomenon is Co–7.3Al–7.2W–20.2Ni (at%) [5] –– 881 9.29 observed.
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