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Portevin-Le Chatelier effect in a Ni–Cr–Mo alloy containing ordered phase with Pt2Mo-type structure at room temperature
Materials Science & Engineering A 650 (2016) 317–322
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Portevin-Le Chatelier effect in a Ni–Cr–Mo alloy containing ordered phase with Pt2Mo-type structure at room temperature Liang Yuan a,n, Rui Hu a,n, Jinshan Li a, Xiaoqing Zhang b, Yan’an Yang b a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China Xi’an Filter Metal Materials Co., Ltd., Xi’an 710072, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 18 October 2015 Accepted 19 October 2015 Available online 20 October 2015
Serrated flow behavior or the Portevin-Le Chatelier (PLC) effect in a Ni–Cr–Mo alloy containing ordered phase was investigated at uniaxial tensile and nanoindentation tests at room temperature. Results demonstrate that the periodic arrangement of atoms for nano-sized ordered phase with Pt2Mo-type structure obtained by ageing treatment at 600 °C, induces the appearance of an embedded serration (a small serration is embedded in two adjacent large serrations) in the alloy during uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1. The behavior characteristic of small serration is almost independent on strain rate, but that of large serration is significantly dependent on strain rate. Both the stress drop (Δs) of the large serration and the interval (tw) between adjacent large serrations increase with decreasing strain rate from 10 3 to 10 4 s 1. Moreover, a single serration also appears in load-displacement curve of aged sample at loading rate of 10 3 s 1. Both formation of order-disorder transformation-induced twins and twinning of ordered phase itself are responsible for the occurrence of the embedded serrations. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ordered phase Ni–Cr–Mo alloy Serration Order-disorder transformation Twinning
1. Introduction Ni–Cr–Mo-based superalloys have attracted increasing interest in the chemical and nuclear industry because of an excellent combination of corrosion properties and mechanical properties [1,2]. It has been reported [3–5] that the ordered phases precipitate through order-disorder transformation in Ni–Cr–Mo superalloys. These nano-sized ordered phases play a dominate role in enhancing the mechanical properties of Ni–Cr-based superalloys [3,5,6]. It is interesting to note that a preferential deformation mode occur from the {111}〈110〉fcc slip systems to the {111}〈112〉fcc twinning systems due to formation of ordered phases (Pt2Mo-type, DO22 and D1a) in the alloys [7,8]. As a result, the combination of high strength and reasonable ductility is obtained in the alloys. It has been reported that, however, appearance of twins can induce the alloy to exhibit plastic instability during tensile tests [9,10], i.e. serrated flow or the Portevin-Le Chatelier (PLC) effect [11–22]. On the other hand, a number of nano-sized precipitations strengthened alloys exhibit plastic instability or the PLC effect during plastic deformation [16]. In other words, nanosized precipitates play a dominate role in enhancing plastic instability. The PLC effect in a Ni–Cr–Mo alloy containing ordered n
Corresponding authors. Fax: þ 86 29 88460294. E-mail addresses: [email protected] (L. Yuan), [email protected] (R. Hu).
http://dx.doi.org/10.1016/j.msea.2015.10.070 0921-5093/& 2015 Elsevier B.V. All rights reserved.
phase is therefore expected to appear during plastic deformation due to the effect of twins and nano-sized phases. In this work, an embedded serration has been observed in a Ni–23Cr–16Mo (wt%) alloy (named as Hastelloy C2000 alloy) containing ordered phase with Pt2Mo-type structure during uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1. In addition, the serrated flow phenomenon (a single serration) also appears in load-displacement curve of aged sample at loading rate of 10 3 s 1. It is well known that the PLC effect has an important impact on ductility and fatigue life of the alloys [23,24], hence, there is an ongoing interest to understand the occurrence of this phenomenon. Until now, several models have been proposed to explain the mechanisms of the PLC effect observed in various kinds of alloys since the phenomenon was first observed in Al-Cu alloys by Portevin and Le Chatelier in 1923 [25]. These models include: (a) dynamic strain aging (DSA) [11,17–20], (b) shearing of precipitates [12], (c) stacking faults (SF) effect [14], (d) phase transformation-induced effect [15] and (e) diffusioncontrolled pseudo-locking mechanism [16]. The aforementioned models are associated with pinning and freeing of mobile dislocations. In other words, dynamic interactions among dislocations and/or solute atoms, precipitates, ordered phases and SF induce appearance of the serrated flow in various kinds of alloys, such as AlLi [16], FeMnAl steel [15], NiCrFe [11] and NiCoCr [13]. However, effect of twins, especially order-disorder transformation-induced twins on the PLC effect in
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Ni–Cr–Mo-based alloys is barely reported although induction of the PLC effect by twins has been mentioned by other authors [9,10]. In this study, uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1 were carried out on an aged Ni– 23Cr–16Mo (wt%) alloy containing ordered phases with Pt2Motype structure. Moreover, nanoindentation tests at room temperature with two different loading rates of 5 10 2 and 10 3 s 1 were also carried out in order to probe the nano-sized ordered phase influences on the serrated response. Then, we described the behavior characteristics of the serrations induced by twinning both matrix and ordered phase itself. Finally, we also discussed the occurrence mechanism of this PLC effect.
2. Materials and experimental procedures The experimental alloy, with the nominal chemical composition of Ni–22.66Cr–15.80Mo (wt%) with 1.53Cu, 0.75Fe, 0.24Al, 0.23Mn, 0.20Co, 0.02Si 0.003P, 0.001S and 0.001C as minority alloying elements, was supplied by the Haynes International Inc. USA. The plate samples were solution treated at 1150 °C for 2 h followed by water quenched. Subsequently, the samples were aged at 600 °C for 500 h then air cooled in order to precipitate ordered phase in the alloy matrix. Uniaxial tensile tests were performed on the ageing-treated plates with a cross-section of 6 2 mm2 and a gauge length of 25 mm, using an ASTM-E8M machine at room temperature with strain rates of 10 3 and 10 4 s 1. Nanoindentation tests were conducted at room temperature using two different loading rates of 5 10 2 and 10 3 s 1, respectively, and a 30 s dwell at the peak load was implemented to allow for viscoelastic relaxation prior to
unloading. The unloading rate was 1.5 times of the corresponding loading rate. The spacing between the discrete indentations was set to be greater than 100 μm to minimize the interaction of strain gradients between different indentations. The foils for transmission electron microscope (TEM) observation were cut from samples perpendicular to the uniaxial tensile direction. A twin-jet electropolisher with a polishing solution of 5% perchloric acid and 95% ethanol was used to make thin foils. TEM observations were carried out on a Tecnai G2 F30 operated at 300 kV.
3. Results and discussion Fig. 1 shows TEM dark-field (DF) image of aged samples and the corresponding SAED patterns. It can be seen that spherical particles are uniformly dispersed in the alloy matrix (Fig. 1a), the average size of which is determined as 15–21 nm (inset in Fig. 1a) by the software program ImageJ and its volume fraction is calculated to be approximately 8.6% by a formula used by Lu et al. [5]. The characteristic reflections at the [001] (Fig. 1b), [112] (Fig. 1c) and [110] positions (Fig. 1d), respectively, suggest that the nanosized particles have a Pt2Mo-type structure [6,26]. Moreover, the quantitative chemical analysis on the phases demonstrated that they approximately contain 18.93Cr, 22.73Mo and 58.34Ni (wt%) and correspond to 22.88Cr, 14.89Mo and 62.23Ni (at%), suggesting that the phase is Mo-rich as compared to the matrix. Hence, it is identified as being Ni2(Cr, Mo) stoichiometry, which is consistent with result reported by Lu et al. [5]. The stress–strain curves of the as-quenched and aged samples are presented in Fig. 2. The stress–strain curves of the as-quenched samples at strain rates of 10 3 and 10 4 s 1 display a normal behavior characteristic, i.e. a smooth curve feature (Fig. 2a). In the
Fig. 1. TEM dark-field (DF) image (a) of aged samples and the corresponding selected area electron diffraction (SAED) patterns in [001] (b), [112] (c) and [110] zone-axis (d), respectively, and the inset in (a) shows the size distribution of particle phases.
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Fig. 2. Tensile engineering stress–strain curves of samples at room temperature with strain rates of 10 3 and 10 4 s 1 for (a) as-quenched samples and (b) aged samples, and (c) an enlarged drawing of “Section A” in (b).
aged samples, however, the serrated stress–strain curves are observed at strain rates of 10 3 and 10 4 s 1 (Fig. 2b). This indicates that the serrated flow phenomenon is associated with precipitation of ordered phase in the aged samples. It is well known [13] that serrated flow is generally seen to occur when a critical plastic strain, ε is reached. The strain corresponding to the peak of the first serrations was taken as the critical plastic strain, ε, for onset of serrations. It can be seen from Fig. 2b that the ε decreases from ε1 E1.5% to ε2 E1.3% with increasing strain rate from 10 4 to 10 3 s 1. Furthermore, this peculiar γ matrix and Pt2Mo-type phase dual microstructure result in a promising combination of mechanical properties including high strength ( 1000 MPa) and reasonable ductility ( 60%) in ageing-treated sample compared to the sample in as-quenched condition ( 750 MPa). Strain rate sensitivity (m) of the flow stress at a given temperature (T) and strain (ε) is estimated using the data from the stress–strain curves through following expression (1) [27]:
m=
log(σ2/σ1) . log (ε1/ε2)ε, T
(1)
where s1 and s2 are the flow stresses at ε1 and ε2 respectively. The strain rate sensitivity (m) of the flow stress is calculated as following: m ¼ 0.27 o0, i.e. the PLC effect exhibits the negative strain rate sensitivity (nSRS) feature, which suggests that type of the serration in the alloy is an inverse PLC effect [13,27]. In order to better describe behavior characteristic of the serrations, an enlarged drawing of “Section A” in Fig. 2b is shown in Fig. 2c. It is found that some small serrations also appear between two
adjacent large serrations (marked by dashed circle in Fig. 2c). Moreover, the stress drop of large serration at 10 4 s 1 (B1-B2) is greater than that at 10 3 s 1 (A1-A2), i.e. Δs1 4 Δs2, and the interval between large serrations for the former (B2-B4) is also greater than that for the latter (A2-A4), i.e. tw1 E2tw2. It is well known [28,29] that high-resolution load-displacement capabilities of nanoindentation make it an effective tool for probing nano-sized perturbations, such as plastic instability or strain localization. Therefore, nanoindentation experiments at room temperature were also carried out on both samples in order to more carefully probe the microstructural (nano-sized ordered phases) influences on the serrated mechanical response since the nano-sized ordered phases are uniformly dispersed in the aged alloy matrix (Fig. 1a). Fig. 3a shows the nanoindentation response of the as-quenched and aged samples at indentation strain rate of 5 10 2 s 1. In this case, it is found that both as-quenched and aged sample exhibit normal the load-displacement curve feature. When indentation strain rate drop to 10 3 s 1, however, for aged sample, loading curve became serrated after reaching a load. In contrast, no serration or discontinuities are observed in as-quenched sample (Fig. 3b). This phenomenon is consistent with reported by Li et al. [30] that some alloys exhibit serrations in the load-depth curves during room temperature instrumented indentation. The result of loading curve further indicated that precipitation of ordered phase in the aged samples can enhance the serrated flow behavior. On the other hand, it is clearly (inset table in Fig. 3a) that the elasticity modulus (E) of the aged samples is larger than that of the as-quenched samples, as well as the
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Fig. 3. Load-displacement curve of samples at room temperature with different load rates for (a) 5 10 2 s 1 and (b) 10 3 s 1.
hardness of the former is larger than that of the latter under the same indentation depth of 1500 nm. Thus, it could be concluded that this nano-sized ordered phases affect significantly mechanical properties of the alloy, which is consistent with the result in Fig. 2. In order to clarify the formation mechanism of the serrations, the microstructures of the as-quenched and aged samples after different tensile states are characterized by TEM, as shown in
Fig. 4. In the as-quenched samples, high dislocation density is displayed in the matrix (Fig. 4a and b). In the aged samples containing Ni2(Cr, Mo) phase, however, a large amount of deformation twin appears instead of dislocations whether tensile strain rate is 10 3 s 1 (Fig. 4c) or 10 4 s 1 (Fig. 4d). This indicates that orderdisorder transformation can lead to change of deformation mode from slip into twinning, which is consistent with the result
Fig. 4. TEM images of samples after uniaxial tensile tests at room temperature, (a) and (b) as-quenched sample with strain rate of 10 3 s 1 and 10 4 s 1, respectively, (c) and (d) aged samples with strain rate of 10 3 s 1 and 10 4 s 1, respectively.
L. Yuan et al. / Materials Science & Engineering A 650 (2016) 317–322
obtained by Tawancy et al. [7,8]. Hence, it is reasonably assumed that the formation of twins might be responsible for appearance of the serrations. In fact, the phenomenon of twins-induced serrations has been slightly mentioned by other authors in some alloys. For example, Li et al. [10] reported that twins induce formation of serration at cryogenic temperature, and Lu et al. [9] also found that high rate sensitivity of flow stress appear in pure copper resulting from induction of nano-sized twins. Since it is generally accepted that [11,18,20] the occurrence of serrated flow can be attributed to dynamic pinning and freeing of dislocations, we supposed that this serration exhibited in the stress–strain curves is induced by dynamic strain ageing (DSA), which is based on the interactions between diffusing solute atoms and mobile dislocations over the range of plastic flow. The activation energy of serrated flow can be calculated using the expression (2) [27]:
εC(m + β) = Kε exp(Q /RT ).
(2)
where εC is the critical strain for serrations, ε is the strain rate, m and β are an exponent related to the vacancy concentration and the density of mobile dislocations with plastic strain, respectively, K is a constant, Q is the activation energy, R is the gas constant and T is the absolute temperature. Consequently, the value of Q
321
calculated by the expression (2) is approximately 158 kJ/mol, which is not consistent with the results in literatures that the mean value of Q in both the normal DSA and the inverse DSA regime is calculated to be 46 and 143 kJ/mol, respectively, in Ni–Cr– Mo-based superalloys [27,31,32]. It is apparently that dynamic aging of both diffusing solute atoms and mobile dislocations does not govern this serration in the C2000. The analysis above rather indicates that twinning is responsible for appearance of serration in this alloy. The detailed study of an enlarged drawing in Fig. 2c enables us to develop a good understanding of the serrated flow process. For example, in the first stage (B1-B2), a deformation twin of the matrix is completed when a critical shear stress (ε) is reached, and an abrupt drop in stress is attributed to relaxation of tensile stress. The stage (B2-B3) corresponds to stress rises rapidly with developing strain during plastic deformation. The stage (B3-B4) corresponds to the continued accumulation of tensile stress until the ε requirements for triggering the next deformation twin reach. In this stage, a second round of twinning is initiated when the stress is sufficiently large. Repeat of the twinning of the matrix and the accumulation of the ε leads to the appearance of the serrations until tensile failure of sample. The twinning of the matrix has been clearly displayed in Figs. 4c–d and 5a–c. On the other hand,
Fig. 5. TEM DF images (a, b) of aged samples tensile-failed at room temperature with strain rate of 10 4 s 1. The inset in (a) and (b) show [110] zone-axis electron diffraction pattern and morphology of dislocation on surface of ordered phase, respectively. A schematic (c) illustrating the arrangement of the matrix, ordered phase and the corresponding twin spots in inset in (a). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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twinning of ordered phase itself displayed clearly in DF image (marked by pink arrows in Fig. 5a) and a schematic (Fig. 5c), has occurred since it is sheared by twinning of the matrix during plastic deformation. It can be deduced that twinning of ordered phase itself leads to the appearance of small serrations. The inset in Fig. 1a shows that the average size (15–21 nm) of ordered phase is less difference, and this is the reason why stress drop of small serrations is quite similar. The tw as considered an incubation time of twin nucleation is connected to strain rate. The smaller strain rate is, the longer tw is, hence, tw1 4tw2, as shown in Fig. 2c. The stage (B1-B2) corresponds to a nucleation-growth process of deformation twin in the matrix. It can be seen from an enlarged drawing in Fig. 2b that the corresponding stress at strain rate of 10 4 s 1 (s1 ¼525 MPa) is higher than that of 10 3 s 1 (s2 ¼505 MPa) when the first serration appear. Therefore, the mobile atomic layers for the former are more than that for the latter due to a certain shearing stress (γ E0.707) in the fcc matrix when twin happened. As a result, the width of deformation twin in the alloy matrix after tensile failure at strain rate of 10 4 s 1 is larger than that of 10 3 s 1, which is consistent with result in Fig. 4c and d. The wider twin can lead to the larger the stress relaxation, thereby stress drop of large serrations at strain rate of 10 4 s 1 is higher than that of 10 3 s 1. Moreover, it is found that the uncut-ordered phases still exist in the fractured matrix after tensile tests (marked by dash line circle in Fig. 5b), suggesting that the shearing of ordered phase by twinning of the matrix is completed consecutively rather than instantaneously. On the other hand, it is found that some dislocations appear on surface of uncut-ordered phases (the inset in Fig. 5b), indicating that dislocation slip might also participate in the plastic deformation during tensile tests. As a result, an antiphase boundary (APB) may be introduced within the precipitates upon a dislocation pair cutting through ordered coherent precipitates, inducing occurrence of serration in alloys due to a diffusion-controlled pseudo-locking mechanism reported by Ovri et al. [16]. On the other hand, formation of ordered phase decreases the stacking fault energy (SFE) of the matrix, which have an impact on both twinning of the matrix and twin nucleus stability. Therefore, further work is needed to clarify the effect of these factors on the PLC effect.
4. Conclusion In the studied alloy, nano-sized ordered phase is obtained by ageing at 600 °C for 500 h. This peculiar ordered phase with Pt2Mo-type structure results in appearance of an embedded serration (a small serration is embedded in two adjacent large serrations) accompanied by high strength ( 1000 MPa) and reasonable ductility ( 60%) in the alloy during uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1. Behavior characteristic of small serration is almost independent on strain rate, but that of large serration significantly dependent on strain rate due to a different induced mechanism. Both the stress drop (Δs) of the large serration and the interval (tw) between adjacent large serrations increase with decreasing strain rate from
10 3 to 10 4 s 1. Moreover, the load-displacement curve at room temperature for aged sample containing ordered phase also exhibits the serrated flow phenomenon (a single serration) when loading rate decreases from 5 10 2 s 1 to 10 3 s 1, suggesting that nano-sized Ni2(Cr, Mo) phases can enhance the PLC effect. Both formation of order-disorder transformation-induced twins and twinning of ordered phase itself are responsible for the occurrence of this embedded serrations, which generally arise from dynamic pinning and freeing of dislocations at intermediate temperature. Meanwhile, an antiphase boundary (APB) introduced by a dislocation pair cutting through ordered precipitates may also affect occurrence of the embedded serration.
Acknowledgments The authors are grateful for the financial support of the National High Technology Research and Development Program of China (No. 2013AA031004), the National Natural Science Foundation of China, China (No. 51171150) and the Program of Introducing Talents of Discipline to Universities (No. B08040).
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Portevin-Le Chatelier effect in a Ni–Cr–Mo alloy containing ordered phase with Pt2Mo-type structure at room temperature Liang Yuan a,n, Rui Hu a,n, Jinshan Li a, Xiaoqing Zhang b, Yan’an Yang b a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China Xi’an Filter Metal Materials Co., Ltd., Xi’an 710072, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 18 October 2015 Accepted 19 October 2015 Available online 20 October 2015
Serrated flow behavior or the Portevin-Le Chatelier (PLC) effect in a Ni–Cr–Mo alloy containing ordered phase was investigated at uniaxial tensile and nanoindentation tests at room temperature. Results demonstrate that the periodic arrangement of atoms for nano-sized ordered phase with Pt2Mo-type structure obtained by ageing treatment at 600 °C, induces the appearance of an embedded serration (a small serration is embedded in two adjacent large serrations) in the alloy during uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1. The behavior characteristic of small serration is almost independent on strain rate, but that of large serration is significantly dependent on strain rate. Both the stress drop (Δs) of the large serration and the interval (tw) between adjacent large serrations increase with decreasing strain rate from 10 3 to 10 4 s 1. Moreover, a single serration also appears in load-displacement curve of aged sample at loading rate of 10 3 s 1. Both formation of order-disorder transformation-induced twins and twinning of ordered phase itself are responsible for the occurrence of the embedded serrations. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ordered phase Ni–Cr–Mo alloy Serration Order-disorder transformation Twinning
1. Introduction Ni–Cr–Mo-based superalloys have attracted increasing interest in the chemical and nuclear industry because of an excellent combination of corrosion properties and mechanical properties [1,2]. It has been reported [3–5] that the ordered phases precipitate through order-disorder transformation in Ni–Cr–Mo superalloys. These nano-sized ordered phases play a dominate role in enhancing the mechanical properties of Ni–Cr-based superalloys [3,5,6]. It is interesting to note that a preferential deformation mode occur from the {111}〈110〉fcc slip systems to the {111}〈112〉fcc twinning systems due to formation of ordered phases (Pt2Mo-type, DO22 and D1a) in the alloys [7,8]. As a result, the combination of high strength and reasonable ductility is obtained in the alloys. It has been reported that, however, appearance of twins can induce the alloy to exhibit plastic instability during tensile tests [9,10], i.e. serrated flow or the Portevin-Le Chatelier (PLC) effect [11–22]. On the other hand, a number of nano-sized precipitations strengthened alloys exhibit plastic instability or the PLC effect during plastic deformation [16]. In other words, nanosized precipitates play a dominate role in enhancing plastic instability. The PLC effect in a Ni–Cr–Mo alloy containing ordered n
Corresponding authors. Fax: þ 86 29 88460294. E-mail addresses: [email protected] (L. Yuan), [email protected] (R. Hu).
http://dx.doi.org/10.1016/j.msea.2015.10.070 0921-5093/& 2015 Elsevier B.V. All rights reserved.
phase is therefore expected to appear during plastic deformation due to the effect of twins and nano-sized phases. In this work, an embedded serration has been observed in a Ni–23Cr–16Mo (wt%) alloy (named as Hastelloy C2000 alloy) containing ordered phase with Pt2Mo-type structure during uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1. In addition, the serrated flow phenomenon (a single serration) also appears in load-displacement curve of aged sample at loading rate of 10 3 s 1. It is well known that the PLC effect has an important impact on ductility and fatigue life of the alloys [23,24], hence, there is an ongoing interest to understand the occurrence of this phenomenon. Until now, several models have been proposed to explain the mechanisms of the PLC effect observed in various kinds of alloys since the phenomenon was first observed in Al-Cu alloys by Portevin and Le Chatelier in 1923 [25]. These models include: (a) dynamic strain aging (DSA) [11,17–20], (b) shearing of precipitates [12], (c) stacking faults (SF) effect [14], (d) phase transformation-induced effect [15] and (e) diffusioncontrolled pseudo-locking mechanism [16]. The aforementioned models are associated with pinning and freeing of mobile dislocations. In other words, dynamic interactions among dislocations and/or solute atoms, precipitates, ordered phases and SF induce appearance of the serrated flow in various kinds of alloys, such as AlLi [16], FeMnAl steel [15], NiCrFe [11] and NiCoCr [13]. However, effect of twins, especially order-disorder transformation-induced twins on the PLC effect in
318
L. Yuan et al. / Materials Science & Engineering A 650 (2016) 317–322
Ni–Cr–Mo-based alloys is barely reported although induction of the PLC effect by twins has been mentioned by other authors [9,10]. In this study, uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1 were carried out on an aged Ni– 23Cr–16Mo (wt%) alloy containing ordered phases with Pt2Motype structure. Moreover, nanoindentation tests at room temperature with two different loading rates of 5 10 2 and 10 3 s 1 were also carried out in order to probe the nano-sized ordered phase influences on the serrated response. Then, we described the behavior characteristics of the serrations induced by twinning both matrix and ordered phase itself. Finally, we also discussed the occurrence mechanism of this PLC effect.
2. Materials and experimental procedures The experimental alloy, with the nominal chemical composition of Ni–22.66Cr–15.80Mo (wt%) with 1.53Cu, 0.75Fe, 0.24Al, 0.23Mn, 0.20Co, 0.02Si 0.003P, 0.001S and 0.001C as minority alloying elements, was supplied by the Haynes International Inc. USA. The plate samples were solution treated at 1150 °C for 2 h followed by water quenched. Subsequently, the samples were aged at 600 °C for 500 h then air cooled in order to precipitate ordered phase in the alloy matrix. Uniaxial tensile tests were performed on the ageing-treated plates with a cross-section of 6 2 mm2 and a gauge length of 25 mm, using an ASTM-E8M machine at room temperature with strain rates of 10 3 and 10 4 s 1. Nanoindentation tests were conducted at room temperature using two different loading rates of 5 10 2 and 10 3 s 1, respectively, and a 30 s dwell at the peak load was implemented to allow for viscoelastic relaxation prior to
unloading. The unloading rate was 1.5 times of the corresponding loading rate. The spacing between the discrete indentations was set to be greater than 100 μm to minimize the interaction of strain gradients between different indentations. The foils for transmission electron microscope (TEM) observation were cut from samples perpendicular to the uniaxial tensile direction. A twin-jet electropolisher with a polishing solution of 5% perchloric acid and 95% ethanol was used to make thin foils. TEM observations were carried out on a Tecnai G2 F30 operated at 300 kV.
3. Results and discussion Fig. 1 shows TEM dark-field (DF) image of aged samples and the corresponding SAED patterns. It can be seen that spherical particles are uniformly dispersed in the alloy matrix (Fig. 1a), the average size of which is determined as 15–21 nm (inset in Fig. 1a) by the software program ImageJ and its volume fraction is calculated to be approximately 8.6% by a formula used by Lu et al. [5]. The characteristic reflections at the [001] (Fig. 1b), [112] (Fig. 1c) and [110] positions (Fig. 1d), respectively, suggest that the nanosized particles have a Pt2Mo-type structure [6,26]. Moreover, the quantitative chemical analysis on the phases demonstrated that they approximately contain 18.93Cr, 22.73Mo and 58.34Ni (wt%) and correspond to 22.88Cr, 14.89Mo and 62.23Ni (at%), suggesting that the phase is Mo-rich as compared to the matrix. Hence, it is identified as being Ni2(Cr, Mo) stoichiometry, which is consistent with result reported by Lu et al. [5]. The stress–strain curves of the as-quenched and aged samples are presented in Fig. 2. The stress–strain curves of the as-quenched samples at strain rates of 10 3 and 10 4 s 1 display a normal behavior characteristic, i.e. a smooth curve feature (Fig. 2a). In the
Fig. 1. TEM dark-field (DF) image (a) of aged samples and the corresponding selected area electron diffraction (SAED) patterns in [001] (b), [112] (c) and [110] zone-axis (d), respectively, and the inset in (a) shows the size distribution of particle phases.
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Fig. 2. Tensile engineering stress–strain curves of samples at room temperature with strain rates of 10 3 and 10 4 s 1 for (a) as-quenched samples and (b) aged samples, and (c) an enlarged drawing of “Section A” in (b).
aged samples, however, the serrated stress–strain curves are observed at strain rates of 10 3 and 10 4 s 1 (Fig. 2b). This indicates that the serrated flow phenomenon is associated with precipitation of ordered phase in the aged samples. It is well known [13] that serrated flow is generally seen to occur when a critical plastic strain, ε is reached. The strain corresponding to the peak of the first serrations was taken as the critical plastic strain, ε, for onset of serrations. It can be seen from Fig. 2b that the ε decreases from ε1 E1.5% to ε2 E1.3% with increasing strain rate from 10 4 to 10 3 s 1. Furthermore, this peculiar γ matrix and Pt2Mo-type phase dual microstructure result in a promising combination of mechanical properties including high strength ( 1000 MPa) and reasonable ductility ( 60%) in ageing-treated sample compared to the sample in as-quenched condition ( 750 MPa). Strain rate sensitivity (m) of the flow stress at a given temperature (T) and strain (ε) is estimated using the data from the stress–strain curves through following expression (1) [27]:
m=
log(σ2/σ1) . log (ε1/ε2)ε, T
(1)
where s1 and s2 are the flow stresses at ε1 and ε2 respectively. The strain rate sensitivity (m) of the flow stress is calculated as following: m ¼ 0.27 o0, i.e. the PLC effect exhibits the negative strain rate sensitivity (nSRS) feature, which suggests that type of the serration in the alloy is an inverse PLC effect [13,27]. In order to better describe behavior characteristic of the serrations, an enlarged drawing of “Section A” in Fig. 2b is shown in Fig. 2c. It is found that some small serrations also appear between two
adjacent large serrations (marked by dashed circle in Fig. 2c). Moreover, the stress drop of large serration at 10 4 s 1 (B1-B2) is greater than that at 10 3 s 1 (A1-A2), i.e. Δs1 4 Δs2, and the interval between large serrations for the former (B2-B4) is also greater than that for the latter (A2-A4), i.e. tw1 E2tw2. It is well known [28,29] that high-resolution load-displacement capabilities of nanoindentation make it an effective tool for probing nano-sized perturbations, such as plastic instability or strain localization. Therefore, nanoindentation experiments at room temperature were also carried out on both samples in order to more carefully probe the microstructural (nano-sized ordered phases) influences on the serrated mechanical response since the nano-sized ordered phases are uniformly dispersed in the aged alloy matrix (Fig. 1a). Fig. 3a shows the nanoindentation response of the as-quenched and aged samples at indentation strain rate of 5 10 2 s 1. In this case, it is found that both as-quenched and aged sample exhibit normal the load-displacement curve feature. When indentation strain rate drop to 10 3 s 1, however, for aged sample, loading curve became serrated after reaching a load. In contrast, no serration or discontinuities are observed in as-quenched sample (Fig. 3b). This phenomenon is consistent with reported by Li et al. [30] that some alloys exhibit serrations in the load-depth curves during room temperature instrumented indentation. The result of loading curve further indicated that precipitation of ordered phase in the aged samples can enhance the serrated flow behavior. On the other hand, it is clearly (inset table in Fig. 3a) that the elasticity modulus (E) of the aged samples is larger than that of the as-quenched samples, as well as the
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Fig. 3. Load-displacement curve of samples at room temperature with different load rates for (a) 5 10 2 s 1 and (b) 10 3 s 1.
hardness of the former is larger than that of the latter under the same indentation depth of 1500 nm. Thus, it could be concluded that this nano-sized ordered phases affect significantly mechanical properties of the alloy, which is consistent with the result in Fig. 2. In order to clarify the formation mechanism of the serrations, the microstructures of the as-quenched and aged samples after different tensile states are characterized by TEM, as shown in
Fig. 4. In the as-quenched samples, high dislocation density is displayed in the matrix (Fig. 4a and b). In the aged samples containing Ni2(Cr, Mo) phase, however, a large amount of deformation twin appears instead of dislocations whether tensile strain rate is 10 3 s 1 (Fig. 4c) or 10 4 s 1 (Fig. 4d). This indicates that orderdisorder transformation can lead to change of deformation mode from slip into twinning, which is consistent with the result
Fig. 4. TEM images of samples after uniaxial tensile tests at room temperature, (a) and (b) as-quenched sample with strain rate of 10 3 s 1 and 10 4 s 1, respectively, (c) and (d) aged samples with strain rate of 10 3 s 1 and 10 4 s 1, respectively.
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obtained by Tawancy et al. [7,8]. Hence, it is reasonably assumed that the formation of twins might be responsible for appearance of the serrations. In fact, the phenomenon of twins-induced serrations has been slightly mentioned by other authors in some alloys. For example, Li et al. [10] reported that twins induce formation of serration at cryogenic temperature, and Lu et al. [9] also found that high rate sensitivity of flow stress appear in pure copper resulting from induction of nano-sized twins. Since it is generally accepted that [11,18,20] the occurrence of serrated flow can be attributed to dynamic pinning and freeing of dislocations, we supposed that this serration exhibited in the stress–strain curves is induced by dynamic strain ageing (DSA), which is based on the interactions between diffusing solute atoms and mobile dislocations over the range of plastic flow. The activation energy of serrated flow can be calculated using the expression (2) [27]:
εC(m + β) = Kε exp(Q /RT ).
(2)
where εC is the critical strain for serrations, ε is the strain rate, m and β are an exponent related to the vacancy concentration and the density of mobile dislocations with plastic strain, respectively, K is a constant, Q is the activation energy, R is the gas constant and T is the absolute temperature. Consequently, the value of Q
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calculated by the expression (2) is approximately 158 kJ/mol, which is not consistent with the results in literatures that the mean value of Q in both the normal DSA and the inverse DSA regime is calculated to be 46 and 143 kJ/mol, respectively, in Ni–Cr– Mo-based superalloys [27,31,32]. It is apparently that dynamic aging of both diffusing solute atoms and mobile dislocations does not govern this serration in the C2000. The analysis above rather indicates that twinning is responsible for appearance of serration in this alloy. The detailed study of an enlarged drawing in Fig. 2c enables us to develop a good understanding of the serrated flow process. For example, in the first stage (B1-B2), a deformation twin of the matrix is completed when a critical shear stress (ε) is reached, and an abrupt drop in stress is attributed to relaxation of tensile stress. The stage (B2-B3) corresponds to stress rises rapidly with developing strain during plastic deformation. The stage (B3-B4) corresponds to the continued accumulation of tensile stress until the ε requirements for triggering the next deformation twin reach. In this stage, a second round of twinning is initiated when the stress is sufficiently large. Repeat of the twinning of the matrix and the accumulation of the ε leads to the appearance of the serrations until tensile failure of sample. The twinning of the matrix has been clearly displayed in Figs. 4c–d and 5a–c. On the other hand,
Fig. 5. TEM DF images (a, b) of aged samples tensile-failed at room temperature with strain rate of 10 4 s 1. The inset in (a) and (b) show [110] zone-axis electron diffraction pattern and morphology of dislocation on surface of ordered phase, respectively. A schematic (c) illustrating the arrangement of the matrix, ordered phase and the corresponding twin spots in inset in (a). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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twinning of ordered phase itself displayed clearly in DF image (marked by pink arrows in Fig. 5a) and a schematic (Fig. 5c), has occurred since it is sheared by twinning of the matrix during plastic deformation. It can be deduced that twinning of ordered phase itself leads to the appearance of small serrations. The inset in Fig. 1a shows that the average size (15–21 nm) of ordered phase is less difference, and this is the reason why stress drop of small serrations is quite similar. The tw as considered an incubation time of twin nucleation is connected to strain rate. The smaller strain rate is, the longer tw is, hence, tw1 4tw2, as shown in Fig. 2c. The stage (B1-B2) corresponds to a nucleation-growth process of deformation twin in the matrix. It can be seen from an enlarged drawing in Fig. 2b that the corresponding stress at strain rate of 10 4 s 1 (s1 ¼525 MPa) is higher than that of 10 3 s 1 (s2 ¼505 MPa) when the first serration appear. Therefore, the mobile atomic layers for the former are more than that for the latter due to a certain shearing stress (γ E0.707) in the fcc matrix when twin happened. As a result, the width of deformation twin in the alloy matrix after tensile failure at strain rate of 10 4 s 1 is larger than that of 10 3 s 1, which is consistent with result in Fig. 4c and d. The wider twin can lead to the larger the stress relaxation, thereby stress drop of large serrations at strain rate of 10 4 s 1 is higher than that of 10 3 s 1. Moreover, it is found that the uncut-ordered phases still exist in the fractured matrix after tensile tests (marked by dash line circle in Fig. 5b), suggesting that the shearing of ordered phase by twinning of the matrix is completed consecutively rather than instantaneously. On the other hand, it is found that some dislocations appear on surface of uncut-ordered phases (the inset in Fig. 5b), indicating that dislocation slip might also participate in the plastic deformation during tensile tests. As a result, an antiphase boundary (APB) may be introduced within the precipitates upon a dislocation pair cutting through ordered coherent precipitates, inducing occurrence of serration in alloys due to a diffusion-controlled pseudo-locking mechanism reported by Ovri et al. [16]. On the other hand, formation of ordered phase decreases the stacking fault energy (SFE) of the matrix, which have an impact on both twinning of the matrix and twin nucleus stability. Therefore, further work is needed to clarify the effect of these factors on the PLC effect.
4. Conclusion In the studied alloy, nano-sized ordered phase is obtained by ageing at 600 °C for 500 h. This peculiar ordered phase with Pt2Mo-type structure results in appearance of an embedded serration (a small serration is embedded in two adjacent large serrations) accompanied by high strength ( 1000 MPa) and reasonable ductility ( 60%) in the alloy during uniaxial tensile tests at room temperature with strain rates of 10 3 and 10 4 s 1. Behavior characteristic of small serration is almost independent on strain rate, but that of large serration significantly dependent on strain rate due to a different induced mechanism. Both the stress drop (Δs) of the large serration and the interval (tw) between adjacent large serrations increase with decreasing strain rate from
10 3 to 10 4 s 1. Moreover, the load-displacement curve at room temperature for aged sample containing ordered phase also exhibits the serrated flow phenomenon (a single serration) when loading rate decreases from 5 10 2 s 1 to 10 3 s 1, suggesting that nano-sized Ni2(Cr, Mo) phases can enhance the PLC effect. Both formation of order-disorder transformation-induced twins and twinning of ordered phase itself are responsible for the occurrence of this embedded serrations, which generally arise from dynamic pinning and freeing of dislocations at intermediate temperature. Meanwhile, an antiphase boundary (APB) introduced by a dislocation pair cutting through ordered precipitates may also affect occurrence of the embedded serration.
Acknowledgments The authors are grateful for the financial support of the National High Technology Research and Development Program of China (No. 2013AA031004), the National Natural Science Foundation of China, China (No. 51171150) and the Program of Introducing Talents of Discipline to Universities (No. B08040).
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