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Precipitation and coarsening behavior of γ′ phase in CoNi-base superalloy under different aging treatments
Journal Pre-proof Precipitation and coarsening behavior of γ' phase in CoNi-base superalloy under different aging treatments Yong Guan, Yongchang Liu, Zongqing Ma, Huijun Li, Hongyao Yu PII:
S0042-207X(20)30084-1
DOI:
https://doi.org/10.1016/j.vacuum.2020.109247
Reference:
VAC 109247
To appear in:
Vacuum
Received Date: 19 December 2019 Revised Date:
2 February 2020
Accepted Date: 5 February 2020
Please cite this article as: Guan Y, Liu Y, Ma Z, Li H, Yu H, Precipitation and coarsening behavior of γ' phase in CoNi-base superalloy under different aging treatments, Vacuum, https://doi.org/10.1016/ j.vacuum.2020.109247. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Precipitation and coarsening behavior of γ′ phase in CoNi-base superalloy under different aging treatments
Yong Guana, Yongchang Liua∗, Zongqing Maa**, Huijun Lia, Hongyao Yub, c***
a. State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300354, P.R. China b. Beijing CISRI-GAONA Materials & Technology CO., LTD c. High Temperature Materials Research Institute, Central Iron & Steel Research Institute, P.R. China
Abstract: In order to investigate the influence of aging regimes on the coarsening behavior of γ′ precipitates in CoNi-base superalloy, single and double aging treatments were performed, respectively. For the superalloy after experiencing single aging treatment, the typical γ/γ′ microstructure with coherent γ′ embedded in the matrix is obtained and the coarsening kinetics of γ′ precipitates is in accordance with the Lifshitz-Slyozov-Wagner (LSW) theory. The corresponding coarsening rate increases owing to the addition of γ′ forming elements (Ti and Ni) and the calculated activation energy suggests that coarsening kinetics of γ′ is controlled by diffusion of Ni and Ti in the matrix for the investigated CoNi-base superalloy. Whereas for the superalloy after experiencing the double aging treatment, bimodal size distributions of γ′ precipitates are recognized and a decelerated coarsening of secondary γ′ precipitates in coarsening zone ∗
Co-corresponding authors. Tel & Fax: 0086-22-85356410 E-mail: [email protected] (Yongchang [email protected] (Hongyao Yu)
Liu), 1
[email protected]
(Zongqing
Ma),
is found as compared with the one after experiencing the single aging treatment, which could be attributed to the effect of primary γ′ precipitates. Key words: CoNi-base superalloy; Heat treatments; γ′ precipitates; Coarsening behavior
1. Introduction Recently, the discovery of a coherent L12-ordered γ′ precipitate Co3(Al, W) in Co-Al-W-base superalloys by Sato et al.[1] has drawn much attention because of the possibility to develop a new type of Co-base high temperature alloys. Similar to Ni-base superalloys strengthened by Ni3Al, Co-base superalloys consisting of the cuboidal γ′ embedded in γ matrix are promising candidate materials for high-temperature application in aviation industry[2, 3]. Therefore, there have been extensive studies focusing on the effect of alloying elements on γ′ solvus[4, 5] as well as the stability of γ′ phase[6, 7]. Since the γ′ precipitates in Co-Al-W-base superalloys are proved to be metastable by many researchers[6-10], the development of CoNi-base superalloy becomes the new focus because the substitutions of Ni for Co in Co-Al-W base alloys widens γ+γ′ two phase zone, widens the range of γ′ compositions, and also increases γ′ solvus temperature[11]. With the existence of stable γ′ in CoNi-base superalloy after long-term aging at 900 °C according to Qu et al., the tensile and compression property were also studied recently[12, 13]. Therefore, the studies in CoNi-base superalloy has become a new research direction. However, the coarsening behavior of CoNi-base superalloy is barely reported yet although the evolution of γ′ precipitates and their coarsening process are of crucial importance owing to the stability of γ/γ′ microstructure 2
at an elevated temperature depending on the resistance to coarsening of γ′ precipitates. The coarsening behavior of γ′ precipitates has been examined in the ternary Co-Al-W system by Meher et al.[14] and Azzam et al.[15], and the coarsening process was in accordance with the classical LSW coarsening model. Besides, further investigation suggests that alloying elements have a great influence on γ′ coarsening behavior in Co-Al-W-base superalloys[16]. Since the alloying elements, such as Cr, Ti, are usually added to Co-Al-W-base superalloys to guarantee the good oxidation resistance and mechanical properties[17, 18], the study of the kinetics of γ′ precipitates in multicomponent CoNi-base superalloys would be very meaningful to practical application. Heat treatment regime is critical to adjust precipitates and thus to optimize mechanical properties for the typical precipitation hardening alloys. For the majority of studies on Co-base superalloys, a single aging treatment was applied whereas the bimodal distribution of γ′ precipitates can be obtained by means of double aging treatment[19]. In Ni-base superalloys, it was reported that the effect of secondary γ′ precipitates on creep strength was larger than that of primary precipitates[20]. It can be predictable that the bimodal distribution of γ′ precipitates in Co-base superalloys would also have a great effect on their properties. Thereby, investigation on the coarsening process of the secondary γ′ precipitates in Co-base superalloys is also very important. In this study, single and double aging treatments were respectively carried out in multicomponent CoNi-base superalloys to investigate the γ′ precipitation as well as its coarsening behavior. 3
2. Materials and methods 2.1 Materials preparation Polycrystalline ingots of CoNi-base alloy were prepared by means of a ZG-25 vacuum induction furnace. The vacuum pressure of 0.5 Pa and the melting temperature of 1550 °C were used to improve metallurgical quality. The purity of all the raw materials was 99.99% by mass. The chemical compositions were shown in Table 1. 2.2 Heat treatment To dissolve primary γ′ precipitates, the ingots were encapsulated in quartz tubes backfilled with Ar gas and then solution treated at 1250 °C for 48 h, followed by air cooling. The homogenized samples were subsequently aged at 800 °C, 850 °C, 900 °C and 950 °C for 12 h, 48 h, 100 h and 200 h in a vertical furnace, and then quenched into water (single aging treatment). Double aging treatment was applied with the first-step aging at 1000 °C for 12 h, and the second-step aging at 800 °C for 12 h, 48 h, 100 h and 200 h, respectively. Every step was followed by quenching into water. The level of contamination elements, such as H, O and N in the sample before and after isothermal treatment was measured by the standard methods. Before the heat treatment, oxygen and nitrogen contents were measured to be about 3×10-6 and 5×10-6 by impulse heating-infrared and thermal conductivity method; phosphorus content was about 2×10-5 by n-butyl alcohol-chloroform extraction photometric method; hydrogen content was about 1×10-5 through impulse heating-thermal conductivity method; sulfur content was 2×10-6 by reducing distillation photometric method. After the heat treatment, oxygen, nitrogen content were about 2×10-6 and 4.5×10-6; phosphorus content 4
was about 1.5×10-5; hydrogen content was about 1×10-5; sulfur content was about 1×10-6. It reveals that the level of contamination elements has not obvious change during heat treatment, indicating the protective of Ar gas to the samples during heat treatment. As a result, the contamination elements barely change the microstructure and properties of aged samples. 2.3 Microstructure characterization The aged samples were mechanically ground, polished and etched using a Spar etchant solution (100 ml of distilled water, 100 ml of 32% HCl, 10 ml of 65% HNO3 and 0.3 ml of Spar etchant, with 1-methoxy-2-propanol as the main constituent) before examining microstructure evolution using the Hitachi S4800 scanning electron microscopy (SEM) operating at 5 kV voltages. In order to understand the partitioning behavior of alloying elements, the compositions of γ matrix and γ′ precipitates were respectively examined by means of energy dispersive X-ray spectroscopy (EDX) equipped in JEM-2100 F transmission electron microscope (TEM) after aged at 900 °C for 5, 24, 100 and 200 h. The TEM foils with a diameter of 3 mm were prepared by twin-jet polishing in a solution of 5 vol.% perchloric acid in ethanol at 30 V with a temperature of -30 °C. The mean γ′ precipitates radius, r, was estimated to be a/2, where a is the mean edge length of the cubic precipitates. The area fraction, Af , measured by Image Pro Plus software from the images was used to estimate the volume 3
fraction, Vf =A2 f of the γ′ precipitates, using the assumption that all γ′ precipitates were assumed to be cubes[16]. Vickers hardness was performed at room temperature with a 10 s dwell time at a load of 1 kg for single and double aged samples. 5
Table 1 Chemical compositions of CoNi-base superalloy (wt. %). Co
Ni
Al
W
Cr
Ti
C
Bal.
27.32
3.73
17.01
5.34
1.42
0.022
3. Results and discussion 3.1 γ′ precipitates evolution during single aging treatment The evolution of γ′ precipitates as aging time and temperature during single aging treatment was shown in Fig. 1. It is clear to see that γ′ homogeneously precipitates in the matrix, which is similar to Ni-base superalloys. It has been reported that undercooling has a great effect on microstructure evolution and mechanical properties in alloys because of the dependence of nucleation and growth of phases on undercooling[21, 22]. Therefore, the quantity of γ′ precipitates decreases with an increase aging temperature owing to the lower precipitate kinetics at a higher temperature. Besides, it is evident that the γ′ precipitates size, shape, and configuration significantly change with aging temperature and time.
6
Fig. 1. SEM micrographs after single aging treatment at 800 °C, 850 °C, 900 °C and 950 °C for 12 h, 48 h, 100 h and 200 h.
γ′ precipitates size increased with temperature owing to their higher growth rate at the higher temperature. The morphology of the γ′ precipitates is dependent on the competition between γ/γ′ interfacial energy and elastic strain energy arising from γ/γ′ lattice misfit. Xu et al.[23] suggests that γ′ evolved into a spherical shape at high aging temperature to reduce interfacial energy, while γ′ transformed into the cuboidal morphology at low aging temperature to reduce elastic strain energy in Ni-Co based superalloy. In this CoNi-base superalloy, it is clear to see that the edge of γ′ precipitates become less cubic when increasing the aging temperature. As the aging time increased, γ′ precipitates become more cubic and aligned owing to the elastic interactions among these precipitates. Besides, γ′ precipitates with T- or L-shape morphology were readily 7
seen at 800 °C, resulting from the impingement of the γ′ precipitates during the coarsening process.
Fig. 2. Evolution of (a) mean γ′ precipitate radius and (b) volume fraction during single aging treatment.
Mean precipitate radius and volume fraction of γ′ precipitates after single aging heat treatment were compared in Fig. 2. Fig. 2(a) indicated that the mean γ′ precipitate radius increases with aging time. It exhibited a faster growth rate at a higher aging temperature, suggesting that the growth rate of γ′ precipitate was relying on the aging temperature. The volume fraction of γ′ has a slightly increase as aging time prolonged except for the aging temperature of 950 °C. It indicates that the alloys mainly experienced the γ′ precipitates growth process before aged at 900 °C, resulting in an increase in γ′ volume fraction. When aged at a higher temperature, the precipitation of γ′ decreases while the discontinuous coarsening of γ′ precipitates happens with larger γ′ consuming the smaller ones, causing the decrease of γ′ volume fraction. Compared with that at 950 °C, the higher volume fraction of precipitates at the lower aging temperature resulting from higher solute supersaturation was predictable, as illustrated in Fig. 2(b). 3.2 Coarsening behavior of γ′ precipitates during single aging treatment 8
There have been extensive research works demonstrated that coarsening of γ′ precipitates in Ni-base superalloys follows the diffusion-controlled growth, presented by Lifshitz and Slyozov[24] and Wagner[25], which was characterized by the classical LSW coarsening model. Recently, the TIDC coarsening model[26] was proposed, which suggested that the solute diffusion at the matrix/precipitate interface is rate-limiting rather than solute diffusion through the disordered matrix. The temporal exponent of coarsening roughly determines the coarsening mechanism: it is expected to be 3 for LSW coarsening and 2 for TIDC coarsening[14]. In both cases the temporal evolution of average precipitate size r, as a function of aging time t, follows the power law: rnt -rn0 =K·t (1) where rt is the mean particle radius at time t, r0 is the mean particle radius at annealing time, t=0, n is the temporal exponent and K is coarsening rate constant. To determine the mechanism of coarsening of γ′ precipitates, the actual temporal exponent was obtained by plotting the logarithm of the mean precipitate radius (log r) against the logarithm of time (log t), as was shown in Fig. 3(a). The slope of linear fitting reveals the inverse of the temporal exponent, that is 0.34 for 800 °C, 0.31 for 850 °C, 0.32 for 900 °C and 0.29 for 950 °C (Table 2). During linear fitting, R2 is a statistical measure of how close the data are to the fitted regression line. The closer the value of R2 is to 1, the better the fit of the regression line to the observed values. It is clear to see that the temporal exponents agree very well with the LSW theory, indicating that LSW coarsening model is the dominant coarsening mechanism in the investigated CoNi-based superalloy, which is consistent with studies on Co-Al-W ternary system[14] 9
and Co-Ni-Al-W quaternary system[16]. Therefore, the temporal exponent, 3, was used to determine the rate constant, K for different aging temperatures. The plots of precipitate size raised to the third power (r3 ) vs. annealing time (t) were shown in Fig. 3(b). Linear fits to these plots yielded slopes that gave the rate constants, K=0.14×10-27 m3 s-1, 0.52×10-27 m3 s-1, 1.61×10-27 m3 s-1 and 3.58×10-27 m3 s-1 for 800 °C, 850 °C, 900 °C and 950 °C, respectively (Table 2). Obviously, the coarsening rate increases with an increase of aging temperature owing to the higher diffusion coefficients of the solute atoms required to form the γ′ phase at a higher temperature. Thus, the aging temperature is the primary cause for the observed differences in the coarsening rates.
Fig. 3. (a) Plots of log(r) vs. log(t) giving temporal exponents of γ′ precipitates, (b) plots showing linear fit of precipitate size (r3 in nm3) vs. aging time (t).
Compared with the observations in Co-10Al-10W ternary system with the measuring K to be 0.07 ×10-27 m3 s-1 and 1.41×10-27 m3 s-1 for 800 °C and 900 °C[14], faster coarsening rates were observed in this study, suggesting that alloying elements have a great impact on coarsening rates. The faster coarsening rates in this multi-components CoNi-base superalloy may be caused by addition of γ′ forming elements, such as Ti and Ni. To further explain the role of alloying elements, the partitioning behavior of which 10
as aging time when aged at 900 °C was shown in Fig. 4. The partitioning coefficient for γ′
γ
an alloying element X can be quantified in terms of the coefficient Kγ′/γ =CX /CX , where γ′
γ
CX and CX are the concentrations of the element X in γ′ and γ, respectively. It can be seen that Ni, Ti, Al and W distribute to the γ′ precipitates rather than the γ phase and stabilize the γ′ precipitates, whereas Co, Cr tend to partition to the γ matrix. This is consistent with the results in the previous studies on the partitioning behavior of alloying element in Co-Al-W base superalloy[27, 28]. Studies of impurity diffusion coefficients of γ′ forming elements in Co (Table 3) show that Ti which partitions very strongly to the γ′ phase has a greater diffusivity in Co than that of W in Co, thus resulting in the increase of coarsening rate.
Fig. 4. Partitioning behavior of alloying elements as aging time when aged at 900 °C.
Although the diffusivity of Ni in Co is lower than that of W in Co, the preferential partitioning of W to γ′ phase can be suppressed by a large addition of Ni, seen in Fig.4, which has also been proved by Eric et al.[29], who suggest that increasing the content of Ni increases the partitioning of Al and Ti to the γ′ and decrease that of W to γ′ phase. The partitioning of Ni to γ′ and its suppression to W shift composition of γ′ phase towards Ni3Al where the coarsening rate is controlled by diffusion of Al, which has a greater diffusivity than W. The forming of hybrid (Co, Ni)3(Al, W, Ti) with greater 11
compositional variability increases the coarsening rate of γ′ precipitates. Therefore, it is apparent that coarsening rate of γ′ is affected by γ′ forming elements. Table 2 Temporal exponent and rate constant for different aging temperatures based on linear fitting. Temperature(°C)
Temporal exponent (n-1)
Rate constant, K (m3 s-1)
800
0.34
0.14×10-27
850
0.31
0.52×10-27
900
0.32
1.61×10-27
950
0.29
3.58×10-27
Table 3 Frequency factor and activation energy of alloying elements in pure fcc-Co[30-32]. Element
Frequency factor (m2 s-1)
Activation energy (kJ mol-1)
Ni
2.75×10-5
270.3
Al
3.5×10-4
286
W
6.0×10-5
291
Ti
8.6×10-5
256.9
3.3 Determination of the activation energy for coarsening The coarsening rate constant, K, can be expressed as follows: K=
ΓV2m Dcm RT
(2)
where Γ is the surface energy per unit area of the matrix-particle phase boundary, D is the diffusion coefficient of the rate controlling solute in the matrix, cm is the equilibrium solubility of that solute in γ′ precipitate, Vm is the molar volume of the precipitate, R is the gas constant and T is the absolute temperature. The diffusion coefficient can be defined by: D=D0 exp
-Q RT
(3)
where D0 is a constant and Q represents the activation energy. Therefore, the 12
expression for the determination of activation energy can be obtained using Eqs. (2) and Eqs. (3): ln K
T cm
=Constant-
Q RT
(4)
Assuming that the cm /T term does not have a significant influence[33], the activation energy was calculated by plotting ln K versus 1⁄T using Eqs. (4) and data from Table 2. The activation energy Q was calculated to be 260 kJ mol-1 according to the linear fitting, as shown in Fig. 5. The value of R2 is very close to 1, indicating that the linear fitting results are reliable and valid for the calculation of coarsening activation energy. Our observation in activation energy is found to be lower than that of ternary Co-Al-W system, which was calculated to be 295 kJ mol-1[14]. It suggests that the coarsening kinetics of γ′ phase in multicomponent CoNi-base superalloy is different from the ternary Co-Al-W system.
Fig. 5. Plot of the rate constant lnK vs. reciprocal of absolute temperature 1⁄T.
In Co-Al-W ternary system, W is required in addition to Al to stabilize the γ′ phase and diffusion of W through the γ matrix is considered to be the rate-controlling step for γ′ coarsening owing to its significant lower diffusivity than that of Al. As for this investigated multicomponent CoNi-base alloy, it can be seen that the calculated 13
activation energy is obviously lower than the activation energy of W (291 kJ mol-1) but very close to that of Ni and Ti (Table 3), indicating that the diffusion of Ni and Ti through γ matrix may be the rate-controlling step during coarsening of γ′ precipitates. This difference between Co-Al-W ternary system may be attributed to the suppression of W partitioning to γ′ because of the addition of Ti and Ni. As the aging time prolonged, partitioning coefficient of Ti, Al and Ni have an increasing tendency while the partitioning trend of W to γ′ is opposite (Fig. 4). First-principle investigation indicates that Ti has a site preference for the face center Co site or the cube corner W site equally in γ′ phase[34]. Besides, the addition of Ni reduces the partitioning ratio of W. Therefore, with the increasing partitioning coefficient of Ni and Ti, the concentration of W in γ increases, leading to a decrease of partitioning coefficient of W. Therefore, in this multicomponent CoNi-base superalloy, Ni and Ti become the critical elements that determined the γ′ coarsening kinetics. In addition, the coarsening kinetics can be specified by using the calculated activation energy. Fig. 6 shows dependence of the mean size normalized by the activation energy, absolute temperature and gas constant on the aging time. Using linear regression analysis, the coarsening kinetic equation for coarsening of γ′ can be derived in the form: r3t -r30 =0.59×10-15 t exp -
14
260000 RT
(5)
Fig. 6. Determination of coarsening equation by plotting mean particle radius normalized by activation energy, aging temperature and gas constant vs. aging time.
3.4 Precipitation and coarsening of γ′ during double aging treatment 3.4.1 Evolution of γ′ precipitates during double aging treatment A duplex size distribution of γ′ precipitates was introduced by means of double aging treatment and the microstructure evolution was illustrated in Fig. 7. The larger precipitates formed at the end of the first-step aging (Fig. 7(a)) tended to exhibit spherical morphologies, and gradually transited to cuboidal morphologies as the second aging time prolonged (Fig. 7(b)-(c)). Numerous of fine cuboidal γ′ precipitates formed after implementing the second-step aging (Fig. 7(b)). Analogy with IN-738LC superalloy[35], the larger and the smaller γ′ precipitates were respectively called the primary γ′ and the secondary γ′ in this paper. It is clear that there was no formation of secondary γ′ precipitates after the first-step aging (Fig. 7(a)), which meant that the secondary γ′ precipitate only formed during the second-step aging treatment. Since the first-step aging was followed by quenching into the water, the γ matrix was in a supersaturated state at the beginning of the second-step aging. Therefore, when the 15
second-step aging was carried out, another nucleation of γ′ precipitates burst, leading to a duplex size distribution of γ′ precipitates.
Fig. 7. SEM micrographs after the (a)first-step and the second-step aging at 800 °C for (b) 12 h, (c) 48 h, (d) 100 h, (e) 200 h.
With the extension of the second-step aging time the primary γ′ precipitates became more cubic and its size distribution became less heterogeneous owing to the small primary γ′ in elliptical shape gradually dissolved (marked in red circles in Fig. 7 (b)). The number of secondary γ′ precipitates decreased with the second-step aging time while the evolution of secondary γ′ precipitates size was complex. It can be seen that in some places where the secondary γ′ precipitate was far from the primary γ′ precipitate, the secondary γ′ precipitates grew with increasing second-step aging time, as marked by 16
the red rectangular A, B, C and D in Fig. 7, while in some places the secondary γ′ precipitates adjacent to primary γ′ precipitates gradually dissolved or absorbed by the primary ones after prolonged aging time, as shown by the red rectangular D, F and G. These places were respectively called the coarsening zone and dissolution zone, as described in Ni-based superalloys with bimodal microstructure[36]. Careful calculation was made to evaluate the secondary γ′ precipitate sizes both in coarsening zone and dissolution zone, and the results were shown in Fig. 8(a). It revealed that the secondary precipitates in coarsening zone grew with aging time while the precipitate sizes in dissolution zone firstly increased and then decreased, which was not a feature of a classical coarsening regime.
Fig. 8. (a) Secondary γ′ precipitate sizes evolution in coarsening zone and dissolution zone, (b) volume fraction of total γ′ precipitates and secondary γ′ during the second-step aging.
Evolution of total volume fraction of γ′ precipitates (including primary and secondary γ′ precipitate) and the secondary γ′ precipitates were respectively shown in Fig. 8(b). The total volume fraction of γ′ precipitates at 800 °C remains about 43% as the second-step aging time prolonged, which is lower than that in single aging treatment because of the effect of first-step aging at a higher temperature. Whereas the volume 17
fraction of secondary γ′ precipitate increased from 30% to 35% as the second-step aging time extending from 24 h to 48 h and then dramatically decreased to 22% with increasing the second-step aging time to 100 h, indicating that more secondary γ′ precipitates dissolved when the second-step aging time exceeded 48 h. As the second-step aging time extending to 200 h, the volume fraction of secondary γ′ precipitates continue to decrease since more secondary γ′ precipitates in coarsening zone were merged by the primary ones, see Fig. 7(e). 3.4.2 Coarsening behavior of the secondary γ′ precipitates Since the γ′ precipitates in dissolution zone would be merged by primary γ′ precipitates, the coarsening behavior of the secondary γ′ precipitates in coarsening zone was discussed in this paper, which was shown in Fig. 9. The inverse of rate exponent, 0.29, was yielded from the slope of linear fitting, indicating that the coarsening behavior of secondary γ′ in coarsening zone basically followed the classical LSW theory. However, it was noticeable that the inverse of rate exponent was lower than that of single aging treatment at 800 °C, which suggested a decelerated coarsening of secondary γ′ precipitates. Apparently, in the proceeding of the second-step aging, the coarsening behavior of the secondary γ′ precipitates was clearly influenced by the primary γ′ precipitates. It seemed that although the alloy was in supersaturated state at the beginning of the second-step aging, the formation of primary γ′ precipitates has already consumed part of γ′ forming elements, resulting in a lower coarsening kinetics of secondary γ′ precipitates. The influence of primary γ′ precipitates was directly reflected in the secondary γ′ precipitate sizes in coarsening zone and volume fraction of 18
secondary γ′ precipitates, both of which were smaller than that of single aging treatment at 800 °C.
Fig. 9. Plot of log(radius) vs. log(the second-step aging time) for secondary γ′ precipitates coarsening kinetics in coarsening-zone.
3.5 Vickers hardness
Fig. 10. Vickers hardness evolution after single aging (800 °C) and double aging treatment.
Influence of aging regimes on Vickers hardness is presented in Fig. 10. As the age hardening alloys, the Vickers hardness first increases and then decrease with aging time prolong. This is mainly related to the γ′ size, morphology, and volume fraction. As for the samples subjected to single aging treatment, the γ′ volume fraction first increases and then barely changes with aging time while γ′ size increases, causing the 19
microhardness to increase first and then decrease. As for the samples subjected to double aging treatment, both the primary and secondary γ′ becomes cubic and the volume fraction of γ′ precipitates increased as aging time, which leads to an increase of elastic strain energy, resulting in a peak hardness after aging for 48 h. With the aging time further extended, the coarsening of γ′ and the decrease of γ′ volume fraction cause the decrease in microhardness. Double aging treatment leads to an increase in the hardness compared with the related single aging treatment. After double aging treatment for 1000 °C/12 h + 800 °C/48 h, the highest hardness of 401 HV is obtained, which is nearly 20 HV higher than single aging for 800 °C/48 h. Therefore, it is assured that the introduction of a duplex size distribution of γ′ precipitates in Co-base superalloys results in a higher hardness compared with the unimodal size distribution of γ′ precipitates. This might also lead to an improvement of mechanical properties owing to the duplex size distribution of γ′. The investigation on Ni-base superalloy reveals that the dislocations glide easily in the clean matrix channel while the existence of secondary γ′ prevents the dislocation motions in the matrix channel[20]. Therefore, the fine secondary γ′ precipitates distributed between the primary γ′ precipitates would also obstruct the deformation and then increase the strength in CoNi-base superalloys. Therefore, it can be predicted that the duplex size distribution of γ′ in Co-base superalloys would also have a great effect on its properties, which makes the study on the coarsening behavior of secondary γ′ more important and meaningful. 4. Conclusions 20
The precipitation of γ′ under different aging regimes and the corresponding coarsening kinetics in multi-component CoNi-base superalloy were investigated in this study. The following conclusions can be drawn: 1) During single aging treatment, the typical γ/γ′ microstructure was obtained. The coarsening behavior of γ′ precipitates was agreement with the classical LSW theory. Compared with the ternary Co-Al-W ternary system, the addition of Ti and Ni increased the coarsening rate. Diffusion of Ni and Ti was considered to be the rate-controlling step for γ′ coarsening in multi-components CoNi-base superalloy and the activation energy was calculated as 260 kJ mol-1. 2) Numerous secondary γ′ precipitates formed at the beginning of the second-step aging, resulting in the bimodal size distributions of γ′ precipitates. The coarsening behavior of secondary γ′ precipitate in coarsening zone obeyed the classical LSW theory with a lower rate exponent compared with the single-aging. A decelerated coarsening behavior was attributed to the effect of the primary γ′ precipitates. 3) A higher hardness was obtained in the duplex size distribution of γ′ microstructure. It is clear that the precipitation of γ′ and thus the mechanical properties can be adjusted through heat treatment regimes. Acknowledgements: The work was financially supported by the National Key R&D Program of China (Grant No.: 2017YFB0702901), National Natural Science Foundation of China (Grant Nos.: 51474156, 51604193, and U1660201) and the National High Technology
Research
and
Development
2015AA042504). 21
Program
of
China
(Grant
No.:
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[18] L. Shi, J.J. Yu, C.Y. Cui, X.F. Sun, Microstructural stability and tensile properties of a Ti-containing single-crystal Co–Ni–Al–W-base alloy, Materials Science and Engineering: A 646 (2015) 45-51. [19] D. Locq, M. Martin, C. Ramusat, F. Fossard, M. Perrut, γ′ Precipitation Study of a Co-Ni-Based Alloy, Metallurgical and Materials Transactions A 49(9) (2018) 3854-3864. [20] K. Kakehi, Effect of primary and secondary precipitates on creep strength of Ni-base superalloy single crystals, Materials Science and Engineering: A 278(1-2) (2000) 135-141. [21] H. Wang, P. Lü, X. Cai, B. Zhai, J. Zhao, B. Wei, Rapid solidification kinetics and mechanical property characteristics of Ni–Zr eutectic alloys processed under electromagnetic levitation state, Materials Science and Engineering: A 772 (2020) 138660. [22] P. Zhang, J. Chang, H. Wang, Transition from Crystal to Metallic Glass and Micromechanical Property Change of Fe-B-Si Alloy During Rapid Solidification, Metallurgical and Materials Transactions B (2019) 1-11. [23] L. Xu, C.G. Tian, C.Y. Cui, Y.F. Gu, X.F. Sun, Morphology evolution of unstableγ′ in Ni–Co based superalloy, Materials Science and Technology 30(8) (2014) 962-967. [24] I.M. Lifshitz, V.V. Slyozov, The kinetics of precipitation from supersaturated solid solutions, Journal of physics and chemistry of solids 19(1-2) (1961) 35-50. [25] C. Wagner, Theory of the aging of precipitates by dissolution-reprecipitation (Ostwald ripening), Z Elektrochem 65(7) (1961) 581-11. [26] A.J. Ardell, V. Ozolins, Trans-interface diffusion-controlled coarsening, Nat Mater 4(4) (2005) 309-16. [27] T. Omori, K. Oikawa, J. Sato, I. Ohnuma, U.R. Kattner, R. Kainuma, K. Ishida, Partition behavior of alloying elements and phase transformation temperatures in Co–Al–W-base quaternary systems, Intermetallics 32 (2013) 274-283. [28] I. Povstugar, P.-P. Choi, S. Neumeier, A. Bauer, C.H. Zenk, M. Göken, D. Raabe, Elemental partitioning and mechanical properties of Ti- and Ta-containing Co–Al–W-base superalloys studied by atom probe tomography and nanoindentation, Acta Materialia 78 (2014) 78-85. [29] E.A. Lass, D.J. Sauza, D.C. Dunand, D.N. Seidman, Multicomponent γ’-strengthened Co-based superalloys with increased solvus temperatures and reduced mass densities, Acta Materialia 147 (2018) 284-295. [30] C.E. Campbell, W.J. Boettinger, U.R. Kattner, Development of a diffusion mobility database for Ni-base superalloys, Acta Materialia 50(4) (2002) 775-792. [31] Y.-W. Cui, G. Xu, R. Kato, X.-G. Lu, R. Kainuma, K. Ishida, Interdiffusion and atomic mobility for face-centered cubic (FCC) Co-W alloys, Metallurgical and Materials Transactions A 44(4) (2013) 1621-1625. [32] T. Gómez-Acebo, B. Navarcorena, F. Castro, Interdiffusion in multiphase, Al-Co-Cr-Ni-Ti diffusion couples, Journal of phase equilibria and diffusion 25(3) (2004) 237-251. [33] R. MacKay, M. Nathal, γ′ coarsening in high volume fraction nickel-base alloys, Acta Metallurgica et Materialia 38(6) (1990) 993-1005. [34] M. Chen, C.-Y. Wang, First-principle investigation of 3d transition metal elements in γ′-Co3(Al,W), Journal of Applied Physics 107(9) (2010). [35] R. Moshtaghin, S. Asgari, Growth kinetics of γ′ precipitates in superalloy IN-738LC during long term aging, Materials & design 24(5) (2003) 325-330. [36] G. Boussinot, A. Finel, Y. Le Bouar, Phase-field modeling of bimodal microstructures in 23
nickel-based superalloys, Acta Materialia 57(3) (2009) 921-931.
Appendix: According to the LSW theory, average precipitate size r, as a function of aging time t, follows the power law: rnt -rn0 =K·t Since the solution treatment followed by air cooling was conducted before aging treatment, r0 can be ignored, leading to a simpler relationship: rnt =K·t The temporal exponent, n, were obtained by plotting log r-log t after taking the logarithm. Since the coarsening process conforms to LSW theory, n=3 was used to calculate the coarsening rate, K, by plotting r3 -t according to r3t =Kt. According to ln K
T cm
=Constant-
Q RT
and the obtained K values and the activation
energy was obtained by plotting ln K versus 1⁄T owing to the cm /T term does not have a significant influence.
24
Highlights Coarsening behavior of γ′ in single aging treatment is characterized by LSW model. Coarsening rate and activation energy of γ′ is related to γ′ forming elements. Bimodal size distribution of γ′ is introduced after double aging treatment. Evolution and coarsening of the secondary γ′ are affected by the primary γ′.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
S0042-207X(20)30084-1
DOI:
https://doi.org/10.1016/j.vacuum.2020.109247
Reference:
VAC 109247
To appear in:
Vacuum
Received Date: 19 December 2019 Revised Date:
2 February 2020
Accepted Date: 5 February 2020
Please cite this article as: Guan Y, Liu Y, Ma Z, Li H, Yu H, Precipitation and coarsening behavior of γ' phase in CoNi-base superalloy under different aging treatments, Vacuum, https://doi.org/10.1016/ j.vacuum.2020.109247. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Precipitation and coarsening behavior of γ′ phase in CoNi-base superalloy under different aging treatments
Yong Guana, Yongchang Liua∗, Zongqing Maa**, Huijun Lia, Hongyao Yub, c***
a. State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300354, P.R. China b. Beijing CISRI-GAONA Materials & Technology CO., LTD c. High Temperature Materials Research Institute, Central Iron & Steel Research Institute, P.R. China
Abstract: In order to investigate the influence of aging regimes on the coarsening behavior of γ′ precipitates in CoNi-base superalloy, single and double aging treatments were performed, respectively. For the superalloy after experiencing single aging treatment, the typical γ/γ′ microstructure with coherent γ′ embedded in the matrix is obtained and the coarsening kinetics of γ′ precipitates is in accordance with the Lifshitz-Slyozov-Wagner (LSW) theory. The corresponding coarsening rate increases owing to the addition of γ′ forming elements (Ti and Ni) and the calculated activation energy suggests that coarsening kinetics of γ′ is controlled by diffusion of Ni and Ti in the matrix for the investigated CoNi-base superalloy. Whereas for the superalloy after experiencing the double aging treatment, bimodal size distributions of γ′ precipitates are recognized and a decelerated coarsening of secondary γ′ precipitates in coarsening zone ∗
Co-corresponding authors. Tel & Fax: 0086-22-85356410 E-mail: [email protected] (Yongchang [email protected] (Hongyao Yu)
Liu), 1
[email protected]
(Zongqing
Ma),
is found as compared with the one after experiencing the single aging treatment, which could be attributed to the effect of primary γ′ precipitates. Key words: CoNi-base superalloy; Heat treatments; γ′ precipitates; Coarsening behavior
1. Introduction Recently, the discovery of a coherent L12-ordered γ′ precipitate Co3(Al, W) in Co-Al-W-base superalloys by Sato et al.[1] has drawn much attention because of the possibility to develop a new type of Co-base high temperature alloys. Similar to Ni-base superalloys strengthened by Ni3Al, Co-base superalloys consisting of the cuboidal γ′ embedded in γ matrix are promising candidate materials for high-temperature application in aviation industry[2, 3]. Therefore, there have been extensive studies focusing on the effect of alloying elements on γ′ solvus[4, 5] as well as the stability of γ′ phase[6, 7]. Since the γ′ precipitates in Co-Al-W-base superalloys are proved to be metastable by many researchers[6-10], the development of CoNi-base superalloy becomes the new focus because the substitutions of Ni for Co in Co-Al-W base alloys widens γ+γ′ two phase zone, widens the range of γ′ compositions, and also increases γ′ solvus temperature[11]. With the existence of stable γ′ in CoNi-base superalloy after long-term aging at 900 °C according to Qu et al., the tensile and compression property were also studied recently[12, 13]. Therefore, the studies in CoNi-base superalloy has become a new research direction. However, the coarsening behavior of CoNi-base superalloy is barely reported yet although the evolution of γ′ precipitates and their coarsening process are of crucial importance owing to the stability of γ/γ′ microstructure 2
at an elevated temperature depending on the resistance to coarsening of γ′ precipitates. The coarsening behavior of γ′ precipitates has been examined in the ternary Co-Al-W system by Meher et al.[14] and Azzam et al.[15], and the coarsening process was in accordance with the classical LSW coarsening model. Besides, further investigation suggests that alloying elements have a great influence on γ′ coarsening behavior in Co-Al-W-base superalloys[16]. Since the alloying elements, such as Cr, Ti, are usually added to Co-Al-W-base superalloys to guarantee the good oxidation resistance and mechanical properties[17, 18], the study of the kinetics of γ′ precipitates in multicomponent CoNi-base superalloys would be very meaningful to practical application. Heat treatment regime is critical to adjust precipitates and thus to optimize mechanical properties for the typical precipitation hardening alloys. For the majority of studies on Co-base superalloys, a single aging treatment was applied whereas the bimodal distribution of γ′ precipitates can be obtained by means of double aging treatment[19]. In Ni-base superalloys, it was reported that the effect of secondary γ′ precipitates on creep strength was larger than that of primary precipitates[20]. It can be predictable that the bimodal distribution of γ′ precipitates in Co-base superalloys would also have a great effect on their properties. Thereby, investigation on the coarsening process of the secondary γ′ precipitates in Co-base superalloys is also very important. In this study, single and double aging treatments were respectively carried out in multicomponent CoNi-base superalloys to investigate the γ′ precipitation as well as its coarsening behavior. 3
2. Materials and methods 2.1 Materials preparation Polycrystalline ingots of CoNi-base alloy were prepared by means of a ZG-25 vacuum induction furnace. The vacuum pressure of 0.5 Pa and the melting temperature of 1550 °C were used to improve metallurgical quality. The purity of all the raw materials was 99.99% by mass. The chemical compositions were shown in Table 1. 2.2 Heat treatment To dissolve primary γ′ precipitates, the ingots were encapsulated in quartz tubes backfilled with Ar gas and then solution treated at 1250 °C for 48 h, followed by air cooling. The homogenized samples were subsequently aged at 800 °C, 850 °C, 900 °C and 950 °C for 12 h, 48 h, 100 h and 200 h in a vertical furnace, and then quenched into water (single aging treatment). Double aging treatment was applied with the first-step aging at 1000 °C for 12 h, and the second-step aging at 800 °C for 12 h, 48 h, 100 h and 200 h, respectively. Every step was followed by quenching into water. The level of contamination elements, such as H, O and N in the sample before and after isothermal treatment was measured by the standard methods. Before the heat treatment, oxygen and nitrogen contents were measured to be about 3×10-6 and 5×10-6 by impulse heating-infrared and thermal conductivity method; phosphorus content was about 2×10-5 by n-butyl alcohol-chloroform extraction photometric method; hydrogen content was about 1×10-5 through impulse heating-thermal conductivity method; sulfur content was 2×10-6 by reducing distillation photometric method. After the heat treatment, oxygen, nitrogen content were about 2×10-6 and 4.5×10-6; phosphorus content 4
was about 1.5×10-5; hydrogen content was about 1×10-5; sulfur content was about 1×10-6. It reveals that the level of contamination elements has not obvious change during heat treatment, indicating the protective of Ar gas to the samples during heat treatment. As a result, the contamination elements barely change the microstructure and properties of aged samples. 2.3 Microstructure characterization The aged samples were mechanically ground, polished and etched using a Spar etchant solution (100 ml of distilled water, 100 ml of 32% HCl, 10 ml of 65% HNO3 and 0.3 ml of Spar etchant, with 1-methoxy-2-propanol as the main constituent) before examining microstructure evolution using the Hitachi S4800 scanning electron microscopy (SEM) operating at 5 kV voltages. In order to understand the partitioning behavior of alloying elements, the compositions of γ matrix and γ′ precipitates were respectively examined by means of energy dispersive X-ray spectroscopy (EDX) equipped in JEM-2100 F transmission electron microscope (TEM) after aged at 900 °C for 5, 24, 100 and 200 h. The TEM foils with a diameter of 3 mm were prepared by twin-jet polishing in a solution of 5 vol.% perchloric acid in ethanol at 30 V with a temperature of -30 °C. The mean γ′ precipitates radius, r, was estimated to be a/2, where a is the mean edge length of the cubic precipitates. The area fraction, Af , measured by Image Pro Plus software from the images was used to estimate the volume 3
fraction, Vf =A2 f of the γ′ precipitates, using the assumption that all γ′ precipitates were assumed to be cubes[16]. Vickers hardness was performed at room temperature with a 10 s dwell time at a load of 1 kg for single and double aged samples. 5
Table 1 Chemical compositions of CoNi-base superalloy (wt. %). Co
Ni
Al
W
Cr
Ti
C
Bal.
27.32
3.73
17.01
5.34
1.42
0.022
3. Results and discussion 3.1 γ′ precipitates evolution during single aging treatment The evolution of γ′ precipitates as aging time and temperature during single aging treatment was shown in Fig. 1. It is clear to see that γ′ homogeneously precipitates in the matrix, which is similar to Ni-base superalloys. It has been reported that undercooling has a great effect on microstructure evolution and mechanical properties in alloys because of the dependence of nucleation and growth of phases on undercooling[21, 22]. Therefore, the quantity of γ′ precipitates decreases with an increase aging temperature owing to the lower precipitate kinetics at a higher temperature. Besides, it is evident that the γ′ precipitates size, shape, and configuration significantly change with aging temperature and time.
6
Fig. 1. SEM micrographs after single aging treatment at 800 °C, 850 °C, 900 °C and 950 °C for 12 h, 48 h, 100 h and 200 h.
γ′ precipitates size increased with temperature owing to their higher growth rate at the higher temperature. The morphology of the γ′ precipitates is dependent on the competition between γ/γ′ interfacial energy and elastic strain energy arising from γ/γ′ lattice misfit. Xu et al.[23] suggests that γ′ evolved into a spherical shape at high aging temperature to reduce interfacial energy, while γ′ transformed into the cuboidal morphology at low aging temperature to reduce elastic strain energy in Ni-Co based superalloy. In this CoNi-base superalloy, it is clear to see that the edge of γ′ precipitates become less cubic when increasing the aging temperature. As the aging time increased, γ′ precipitates become more cubic and aligned owing to the elastic interactions among these precipitates. Besides, γ′ precipitates with T- or L-shape morphology were readily 7
seen at 800 °C, resulting from the impingement of the γ′ precipitates during the coarsening process.
Fig. 2. Evolution of (a) mean γ′ precipitate radius and (b) volume fraction during single aging treatment.
Mean precipitate radius and volume fraction of γ′ precipitates after single aging heat treatment were compared in Fig. 2. Fig. 2(a) indicated that the mean γ′ precipitate radius increases with aging time. It exhibited a faster growth rate at a higher aging temperature, suggesting that the growth rate of γ′ precipitate was relying on the aging temperature. The volume fraction of γ′ has a slightly increase as aging time prolonged except for the aging temperature of 950 °C. It indicates that the alloys mainly experienced the γ′ precipitates growth process before aged at 900 °C, resulting in an increase in γ′ volume fraction. When aged at a higher temperature, the precipitation of γ′ decreases while the discontinuous coarsening of γ′ precipitates happens with larger γ′ consuming the smaller ones, causing the decrease of γ′ volume fraction. Compared with that at 950 °C, the higher volume fraction of precipitates at the lower aging temperature resulting from higher solute supersaturation was predictable, as illustrated in Fig. 2(b). 3.2 Coarsening behavior of γ′ precipitates during single aging treatment 8
There have been extensive research works demonstrated that coarsening of γ′ precipitates in Ni-base superalloys follows the diffusion-controlled growth, presented by Lifshitz and Slyozov[24] and Wagner[25], which was characterized by the classical LSW coarsening model. Recently, the TIDC coarsening model[26] was proposed, which suggested that the solute diffusion at the matrix/precipitate interface is rate-limiting rather than solute diffusion through the disordered matrix. The temporal exponent of coarsening roughly determines the coarsening mechanism: it is expected to be 3 for LSW coarsening and 2 for TIDC coarsening[14]. In both cases the temporal evolution of average precipitate size r, as a function of aging time t, follows the power law: rnt -rn0 =K·t (1) where rt is the mean particle radius at time t, r0 is the mean particle radius at annealing time, t=0, n is the temporal exponent and K is coarsening rate constant. To determine the mechanism of coarsening of γ′ precipitates, the actual temporal exponent was obtained by plotting the logarithm of the mean precipitate radius (log r) against the logarithm of time (log t), as was shown in Fig. 3(a). The slope of linear fitting reveals the inverse of the temporal exponent, that is 0.34 for 800 °C, 0.31 for 850 °C, 0.32 for 900 °C and 0.29 for 950 °C (Table 2). During linear fitting, R2 is a statistical measure of how close the data are to the fitted regression line. The closer the value of R2 is to 1, the better the fit of the regression line to the observed values. It is clear to see that the temporal exponents agree very well with the LSW theory, indicating that LSW coarsening model is the dominant coarsening mechanism in the investigated CoNi-based superalloy, which is consistent with studies on Co-Al-W ternary system[14] 9
and Co-Ni-Al-W quaternary system[16]. Therefore, the temporal exponent, 3, was used to determine the rate constant, K for different aging temperatures. The plots of precipitate size raised to the third power (r3 ) vs. annealing time (t) were shown in Fig. 3(b). Linear fits to these plots yielded slopes that gave the rate constants, K=0.14×10-27 m3 s-1, 0.52×10-27 m3 s-1, 1.61×10-27 m3 s-1 and 3.58×10-27 m3 s-1 for 800 °C, 850 °C, 900 °C and 950 °C, respectively (Table 2). Obviously, the coarsening rate increases with an increase of aging temperature owing to the higher diffusion coefficients of the solute atoms required to form the γ′ phase at a higher temperature. Thus, the aging temperature is the primary cause for the observed differences in the coarsening rates.
Fig. 3. (a) Plots of log(r) vs. log(t) giving temporal exponents of γ′ precipitates, (b) plots showing linear fit of precipitate size (r3 in nm3) vs. aging time (t).
Compared with the observations in Co-10Al-10W ternary system with the measuring K to be 0.07 ×10-27 m3 s-1 and 1.41×10-27 m3 s-1 for 800 °C and 900 °C[14], faster coarsening rates were observed in this study, suggesting that alloying elements have a great impact on coarsening rates. The faster coarsening rates in this multi-components CoNi-base superalloy may be caused by addition of γ′ forming elements, such as Ti and Ni. To further explain the role of alloying elements, the partitioning behavior of which 10
as aging time when aged at 900 °C was shown in Fig. 4. The partitioning coefficient for γ′
γ
an alloying element X can be quantified in terms of the coefficient Kγ′/γ =CX /CX , where γ′
γ
CX and CX are the concentrations of the element X in γ′ and γ, respectively. It can be seen that Ni, Ti, Al and W distribute to the γ′ precipitates rather than the γ phase and stabilize the γ′ precipitates, whereas Co, Cr tend to partition to the γ matrix. This is consistent with the results in the previous studies on the partitioning behavior of alloying element in Co-Al-W base superalloy[27, 28]. Studies of impurity diffusion coefficients of γ′ forming elements in Co (Table 3) show that Ti which partitions very strongly to the γ′ phase has a greater diffusivity in Co than that of W in Co, thus resulting in the increase of coarsening rate.
Fig. 4. Partitioning behavior of alloying elements as aging time when aged at 900 °C.
Although the diffusivity of Ni in Co is lower than that of W in Co, the preferential partitioning of W to γ′ phase can be suppressed by a large addition of Ni, seen in Fig.4, which has also been proved by Eric et al.[29], who suggest that increasing the content of Ni increases the partitioning of Al and Ti to the γ′ and decrease that of W to γ′ phase. The partitioning of Ni to γ′ and its suppression to W shift composition of γ′ phase towards Ni3Al where the coarsening rate is controlled by diffusion of Al, which has a greater diffusivity than W. The forming of hybrid (Co, Ni)3(Al, W, Ti) with greater 11
compositional variability increases the coarsening rate of γ′ precipitates. Therefore, it is apparent that coarsening rate of γ′ is affected by γ′ forming elements. Table 2 Temporal exponent and rate constant for different aging temperatures based on linear fitting. Temperature(°C)
Temporal exponent (n-1)
Rate constant, K (m3 s-1)
800
0.34
0.14×10-27
850
0.31
0.52×10-27
900
0.32
1.61×10-27
950
0.29
3.58×10-27
Table 3 Frequency factor and activation energy of alloying elements in pure fcc-Co[30-32]. Element
Frequency factor (m2 s-1)
Activation energy (kJ mol-1)
Ni
2.75×10-5
270.3
Al
3.5×10-4
286
W
6.0×10-5
291
Ti
8.6×10-5
256.9
3.3 Determination of the activation energy for coarsening The coarsening rate constant, K, can be expressed as follows: K=
ΓV2m Dcm RT
(2)
where Γ is the surface energy per unit area of the matrix-particle phase boundary, D is the diffusion coefficient of the rate controlling solute in the matrix, cm is the equilibrium solubility of that solute in γ′ precipitate, Vm is the molar volume of the precipitate, R is the gas constant and T is the absolute temperature. The diffusion coefficient can be defined by: D=D0 exp
-Q RT
(3)
where D0 is a constant and Q represents the activation energy. Therefore, the 12
expression for the determination of activation energy can be obtained using Eqs. (2) and Eqs. (3): ln K
T cm
=Constant-
Q RT
(4)
Assuming that the cm /T term does not have a significant influence[33], the activation energy was calculated by plotting ln K versus 1⁄T using Eqs. (4) and data from Table 2. The activation energy Q was calculated to be 260 kJ mol-1 according to the linear fitting, as shown in Fig. 5. The value of R2 is very close to 1, indicating that the linear fitting results are reliable and valid for the calculation of coarsening activation energy. Our observation in activation energy is found to be lower than that of ternary Co-Al-W system, which was calculated to be 295 kJ mol-1[14]. It suggests that the coarsening kinetics of γ′ phase in multicomponent CoNi-base superalloy is different from the ternary Co-Al-W system.
Fig. 5. Plot of the rate constant lnK vs. reciprocal of absolute temperature 1⁄T.
In Co-Al-W ternary system, W is required in addition to Al to stabilize the γ′ phase and diffusion of W through the γ matrix is considered to be the rate-controlling step for γ′ coarsening owing to its significant lower diffusivity than that of Al. As for this investigated multicomponent CoNi-base alloy, it can be seen that the calculated 13
activation energy is obviously lower than the activation energy of W (291 kJ mol-1) but very close to that of Ni and Ti (Table 3), indicating that the diffusion of Ni and Ti through γ matrix may be the rate-controlling step during coarsening of γ′ precipitates. This difference between Co-Al-W ternary system may be attributed to the suppression of W partitioning to γ′ because of the addition of Ti and Ni. As the aging time prolonged, partitioning coefficient of Ti, Al and Ni have an increasing tendency while the partitioning trend of W to γ′ is opposite (Fig. 4). First-principle investigation indicates that Ti has a site preference for the face center Co site or the cube corner W site equally in γ′ phase[34]. Besides, the addition of Ni reduces the partitioning ratio of W. Therefore, with the increasing partitioning coefficient of Ni and Ti, the concentration of W in γ increases, leading to a decrease of partitioning coefficient of W. Therefore, in this multicomponent CoNi-base superalloy, Ni and Ti become the critical elements that determined the γ′ coarsening kinetics. In addition, the coarsening kinetics can be specified by using the calculated activation energy. Fig. 6 shows dependence of the mean size normalized by the activation energy, absolute temperature and gas constant on the aging time. Using linear regression analysis, the coarsening kinetic equation for coarsening of γ′ can be derived in the form: r3t -r30 =0.59×10-15 t exp -
14
260000 RT
(5)
Fig. 6. Determination of coarsening equation by plotting mean particle radius normalized by activation energy, aging temperature and gas constant vs. aging time.
3.4 Precipitation and coarsening of γ′ during double aging treatment 3.4.1 Evolution of γ′ precipitates during double aging treatment A duplex size distribution of γ′ precipitates was introduced by means of double aging treatment and the microstructure evolution was illustrated in Fig. 7. The larger precipitates formed at the end of the first-step aging (Fig. 7(a)) tended to exhibit spherical morphologies, and gradually transited to cuboidal morphologies as the second aging time prolonged (Fig. 7(b)-(c)). Numerous of fine cuboidal γ′ precipitates formed after implementing the second-step aging (Fig. 7(b)). Analogy with IN-738LC superalloy[35], the larger and the smaller γ′ precipitates were respectively called the primary γ′ and the secondary γ′ in this paper. It is clear that there was no formation of secondary γ′ precipitates after the first-step aging (Fig. 7(a)), which meant that the secondary γ′ precipitate only formed during the second-step aging treatment. Since the first-step aging was followed by quenching into the water, the γ matrix was in a supersaturated state at the beginning of the second-step aging. Therefore, when the 15
second-step aging was carried out, another nucleation of γ′ precipitates burst, leading to a duplex size distribution of γ′ precipitates.
Fig. 7. SEM micrographs after the (a)first-step and the second-step aging at 800 °C for (b) 12 h, (c) 48 h, (d) 100 h, (e) 200 h.
With the extension of the second-step aging time the primary γ′ precipitates became more cubic and its size distribution became less heterogeneous owing to the small primary γ′ in elliptical shape gradually dissolved (marked in red circles in Fig. 7 (b)). The number of secondary γ′ precipitates decreased with the second-step aging time while the evolution of secondary γ′ precipitates size was complex. It can be seen that in some places where the secondary γ′ precipitate was far from the primary γ′ precipitate, the secondary γ′ precipitates grew with increasing second-step aging time, as marked by 16
the red rectangular A, B, C and D in Fig. 7, while in some places the secondary γ′ precipitates adjacent to primary γ′ precipitates gradually dissolved or absorbed by the primary ones after prolonged aging time, as shown by the red rectangular D, F and G. These places were respectively called the coarsening zone and dissolution zone, as described in Ni-based superalloys with bimodal microstructure[36]. Careful calculation was made to evaluate the secondary γ′ precipitate sizes both in coarsening zone and dissolution zone, and the results were shown in Fig. 8(a). It revealed that the secondary precipitates in coarsening zone grew with aging time while the precipitate sizes in dissolution zone firstly increased and then decreased, which was not a feature of a classical coarsening regime.
Fig. 8. (a) Secondary γ′ precipitate sizes evolution in coarsening zone and dissolution zone, (b) volume fraction of total γ′ precipitates and secondary γ′ during the second-step aging.
Evolution of total volume fraction of γ′ precipitates (including primary and secondary γ′ precipitate) and the secondary γ′ precipitates were respectively shown in Fig. 8(b). The total volume fraction of γ′ precipitates at 800 °C remains about 43% as the second-step aging time prolonged, which is lower than that in single aging treatment because of the effect of first-step aging at a higher temperature. Whereas the volume 17
fraction of secondary γ′ precipitate increased from 30% to 35% as the second-step aging time extending from 24 h to 48 h and then dramatically decreased to 22% with increasing the second-step aging time to 100 h, indicating that more secondary γ′ precipitates dissolved when the second-step aging time exceeded 48 h. As the second-step aging time extending to 200 h, the volume fraction of secondary γ′ precipitates continue to decrease since more secondary γ′ precipitates in coarsening zone were merged by the primary ones, see Fig. 7(e). 3.4.2 Coarsening behavior of the secondary γ′ precipitates Since the γ′ precipitates in dissolution zone would be merged by primary γ′ precipitates, the coarsening behavior of the secondary γ′ precipitates in coarsening zone was discussed in this paper, which was shown in Fig. 9. The inverse of rate exponent, 0.29, was yielded from the slope of linear fitting, indicating that the coarsening behavior of secondary γ′ in coarsening zone basically followed the classical LSW theory. However, it was noticeable that the inverse of rate exponent was lower than that of single aging treatment at 800 °C, which suggested a decelerated coarsening of secondary γ′ precipitates. Apparently, in the proceeding of the second-step aging, the coarsening behavior of the secondary γ′ precipitates was clearly influenced by the primary γ′ precipitates. It seemed that although the alloy was in supersaturated state at the beginning of the second-step aging, the formation of primary γ′ precipitates has already consumed part of γ′ forming elements, resulting in a lower coarsening kinetics of secondary γ′ precipitates. The influence of primary γ′ precipitates was directly reflected in the secondary γ′ precipitate sizes in coarsening zone and volume fraction of 18
secondary γ′ precipitates, both of which were smaller than that of single aging treatment at 800 °C.
Fig. 9. Plot of log(radius) vs. log(the second-step aging time) for secondary γ′ precipitates coarsening kinetics in coarsening-zone.
3.5 Vickers hardness
Fig. 10. Vickers hardness evolution after single aging (800 °C) and double aging treatment.
Influence of aging regimes on Vickers hardness is presented in Fig. 10. As the age hardening alloys, the Vickers hardness first increases and then decrease with aging time prolong. This is mainly related to the γ′ size, morphology, and volume fraction. As for the samples subjected to single aging treatment, the γ′ volume fraction first increases and then barely changes with aging time while γ′ size increases, causing the 19
microhardness to increase first and then decrease. As for the samples subjected to double aging treatment, both the primary and secondary γ′ becomes cubic and the volume fraction of γ′ precipitates increased as aging time, which leads to an increase of elastic strain energy, resulting in a peak hardness after aging for 48 h. With the aging time further extended, the coarsening of γ′ and the decrease of γ′ volume fraction cause the decrease in microhardness. Double aging treatment leads to an increase in the hardness compared with the related single aging treatment. After double aging treatment for 1000 °C/12 h + 800 °C/48 h, the highest hardness of 401 HV is obtained, which is nearly 20 HV higher than single aging for 800 °C/48 h. Therefore, it is assured that the introduction of a duplex size distribution of γ′ precipitates in Co-base superalloys results in a higher hardness compared with the unimodal size distribution of γ′ precipitates. This might also lead to an improvement of mechanical properties owing to the duplex size distribution of γ′. The investigation on Ni-base superalloy reveals that the dislocations glide easily in the clean matrix channel while the existence of secondary γ′ prevents the dislocation motions in the matrix channel[20]. Therefore, the fine secondary γ′ precipitates distributed between the primary γ′ precipitates would also obstruct the deformation and then increase the strength in CoNi-base superalloys. Therefore, it can be predicted that the duplex size distribution of γ′ in Co-base superalloys would also have a great effect on its properties, which makes the study on the coarsening behavior of secondary γ′ more important and meaningful. 4. Conclusions 20
The precipitation of γ′ under different aging regimes and the corresponding coarsening kinetics in multi-component CoNi-base superalloy were investigated in this study. The following conclusions can be drawn: 1) During single aging treatment, the typical γ/γ′ microstructure was obtained. The coarsening behavior of γ′ precipitates was agreement with the classical LSW theory. Compared with the ternary Co-Al-W ternary system, the addition of Ti and Ni increased the coarsening rate. Diffusion of Ni and Ti was considered to be the rate-controlling step for γ′ coarsening in multi-components CoNi-base superalloy and the activation energy was calculated as 260 kJ mol-1. 2) Numerous secondary γ′ precipitates formed at the beginning of the second-step aging, resulting in the bimodal size distributions of γ′ precipitates. The coarsening behavior of secondary γ′ precipitate in coarsening zone obeyed the classical LSW theory with a lower rate exponent compared with the single-aging. A decelerated coarsening behavior was attributed to the effect of the primary γ′ precipitates. 3) A higher hardness was obtained in the duplex size distribution of γ′ microstructure. It is clear that the precipitation of γ′ and thus the mechanical properties can be adjusted through heat treatment regimes. Acknowledgements: The work was financially supported by the National Key R&D Program of China (Grant No.: 2017YFB0702901), National Natural Science Foundation of China (Grant Nos.: 51474156, 51604193, and U1660201) and the National High Technology
Research
and
Development
2015AA042504). 21
Program
of
China
(Grant
No.:
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Appendix: According to the LSW theory, average precipitate size r, as a function of aging time t, follows the power law: rnt -rn0 =K·t Since the solution treatment followed by air cooling was conducted before aging treatment, r0 can be ignored, leading to a simpler relationship: rnt =K·t The temporal exponent, n, were obtained by plotting log r-log t after taking the logarithm. Since the coarsening process conforms to LSW theory, n=3 was used to calculate the coarsening rate, K, by plotting r3 -t according to r3t =Kt. According to ln K
T cm
=Constant-
Q RT
and the obtained K values and the activation
energy was obtained by plotting ln K versus 1⁄T owing to the cm /T term does not have a significant influence.
24
Highlights Coarsening behavior of γ′ in single aging treatment is characterized by LSW model. Coarsening rate and activation energy of γ′ is related to γ′ forming elements. Bimodal size distribution of γ′ is introduced after double aging treatment. Evolution and coarsening of the secondary γ′ are affected by the primary γ′.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.