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In-situ formed heterogeneous grain structure in spark-plasma-sintered CoCrFeMnNi high-entropy alloy overcomes the strength-ductility trade-off
Journal Pre-proof In-situ formed heterogeneous grain structure in spark-plasma-sintered CoCrFeMnNi high-entropy alloy overcomes the strength-ductility trade-off Feilong Jiang, Cancan Zhao, Dingshan Liang, Weiwei Zhu, Yiwen Zhang, Shuai Pan, Fuzeng Ren PII:
S0921-5093(19)31411-X
DOI:
https://doi.org/10.1016/j.msea.2019.138625
Reference:
MSA 138625
To appear in:
Materials Science & Engineering A
Received Date: 16 September 2019 Revised Date:
30 October 2019
Accepted Date: 1 November 2019
Please cite this article as: F. Jiang, C. Zhao, D. Liang, W. Zhu, Y. Zhang, S. Pan, F. Ren, In-situ formed heterogeneous grain structure in spark-plasma-sintered CoCrFeMnNi high-entropy alloy overcomes the strength-ductility trade-off, Materials Science & Engineering A (2019), doi: https://doi.org/10.1016/ j.msea.2019.138625. 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. © 2019 Published by Elsevier B.V.
In-situ formed heterogeneous grain structure in spark-plasma-sintered CoCrFeMnNi high-entropy alloy overcomes the strength-ductility trade-off
Feilong Jianga,b, Cancan Zhaoa, Dingshan Lianga,c, Weiwei Zhua,d, Yiwen Zhanga, Shuai Pana,b, Fuzeng Rena,*
a
Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, China
b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, China
c
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China d
Institute of Applied Physics and Materials Engineering, Faculty of Science & Technology, University of Macau, Macau, China
Corresponding Author. E-mail: [email protected]
1
Abstract The conflict between strength and ductility has been a longstanding dilemma for metallic alloys. Herein, we present a facile strategy to overcome the strength-ductility trade-off of face-center-cubic high-entropy alloys (HEAs) via the in-situ formation of heterogeneous grain structure, using the high local temperature gradients generated during spark plasma sintering coupled with the ‘sluggish diffusion’ of HEAs. The fabricated CoCrFeMnNi HEA has a heterogeneous structure over a large range of grain sizes from 140 nm to 5 µm, which enables a gigapascal yield strength and a uniform elongation of 9.5%. The mechanism responsible for the associated mechanical properties was analyzed.
Keywords: Heterogeneous grain structure; high-entropy alloy; mechanical properties
2
Introduction High-entropy alloys (HEAs) with multi-principal elements have been received considerable research interests in recent fifteen years [1, 2]. Among the HEAs, the equimolar face-centered cubic (fcc) CoCrFeMnNi has been one of the most widely investigated HEAs, owing to its outstanding ductility and exceptional fracture toughness at cryogenic temperatures [3-6]. However, the coarse-grained (CG; grain size d > 1 µm) CoCrFeMnNi alloy with high tensile ductility exhibits low yield strength at room temperature, usually below 500 MPa, which hinders its practical applications at room or elevated temperatures [7].
Refining the grain size down to submicron- or nanometer-scale can significantly improve yield strength, but inevitably accompany the expense of ductility due to the constrained dislocation activity [8]. In particular, when the tensile yield strength (σ0.2) reaches over 1 GPa, the common dilemma of strength-ductility trade-off stands out [9-11]. To overcome this strength-ductility trade-off, Sun et al. [12, 13] fabricated an ultrafine-grained (UFG) HEA with grain size of ~ 500 nm via a serial processing, including hot forging, cold rolling and controlled annealing, and obtained σ0.2 of 888 MPa with UE of 21%. On this basis, they further pre-strained the HEA at 77 K and obtained σ0.2 of 1.07 GPa with a UE of 15.3% [14].
Besides the abovementioned strategy, the alloys with heterogeneous structure can also achieve an excellent combination of high strength and ductility [15-20]. For example, the heterogeneous structure of CrCoNi MEA was obtained through a serial processing, including homogenizing, cold-rolling, annealing and quenching, and achieved σ0.2 of 1.15 GPA and UE of 22% [16]. Wu et al. [17] reported a heterogeneous-structures-architecting strategy through cold-rolling and intermediate-temperature-annealing of arc-melted Al0.1CoCrFeNi HEA, and 3
achieved a yield strength of 711 MPa and a UE of 30.3%. Mechanical alloying (MA) can force the elemental powder mixture into the formation of a single-phase nanostructured solid solution for the alloy systems with low/moderate enthalpy of mixing. The mechanically alloyed nanostructured powder densified by spark plasma sintering (SPS) may yield heterogeneous structure, as demonstrated in conventional metals [21, 22] and in medium-entropy alloys [7], owing to the fast heating rate and high local temperature gradients which modify locally the recrystallization response of the severely deformed structure during sintering.
In this context, herein, we describe a feasible strategy that overcomes the strength-ductility trade-off of fcc HEAs via the in-situ formation of heterogeneous grain structure utilizing the high local temperature gradients generated during SPS coupled with the ‘sluggish diffusion’ of HEAs. The classical fcc CoCrFeMnNi HEA was selected as our model system. We have fabricated the HEA using a combination of MA and SPS, and systematically investigated the phase, microstructure, and mechanical properties.
2. Experimental procedure Commercially pure Co, Cr, Fe, Mn and Ni (> 99.9 wt%) elemental powders were mixed with an equiatomic ratio. The powder mixture was subjected to high energy ball milling with hardened steel balls (ball-to-powder weight ratio = 5:1) using a SPEX 8000D at ambient temperature in an argon glove box for 12 h. Then, the ball-milled powder was packed into a graphite die with an inner diameter of 15 mm and densified by SPS (SPS-211Lx, Fuji Electronic Industrial Co., Ltd.) at 1000 °C and 45 MPa for 5 min with a vacuum pressure of ~ 5 Pa.
4
The “dog-bone-shaped” tensile specimens were cut from the as-sintered cylinders using electrical discharge machining. Uniaxial tensile tests were performed at ambient temperature, with an initial quasi-static strain rate of 5×10-4 s-1. Stereo Digital Image Correlation (DIC) method was used to monitor the deformation of the tensile specimens (with strain resolution of 0.005%). A fine black speckle pattern was first painted on a white undercoat on the test section, and then, the optical images of the specimen were captured during tensile deformation, and finally the images were analyzed using the VIC-2D system to compute the specimen surface’s displacements. σ0.2, ultimate tensile strength (σUTS), UE, and εf were determined from the obtained stress-strain curves. Three independent specimens were tested to confirm the reproducibility and a representative curve was provided.
The phase composition of both powders and bulk specimen were identified by X-ray diffraction (XRD) pattern recorded on a Rigaku Smartlab-9KW diffractometer with a Cu-Kα radiation (45 kV, 200 mA). The composition distribution and microstructure of the spark-plasma-sintered (SPSed) CoCrFeMnNi HEA were examined using a field-emission scanning electron microscope (SEM; MIRA 3, TESCAN, Czech Republic) equipped with backscattering diffraction electron (BSE) detector, energy dispersive X-ray spectroscopy (EDX; AZtec EDX system, Oxford Instruments, UK) and electron backscattering diffraction (EBSD; Nordlys Max2, Oxford Instruments, UK). The EBSD measurements were performed with accelerating voltage of 20 kV, working distance of 15 mm and step size of 40 nm. The grain size was calculated using the diameter of the circle equivalent to the grain section area, which was automatically measured by the Channel 5 software in accordance with the Standard ISO/DIS 13067. Ion images (yielding channel contrast) of the microstructure of the specimens before and after tensile tests were examined using focused ion beam (FIB; Helios NanoLabTM 600i, FEI, USA). The deformation microstructure of HEA near the fracture 5
surface was also characterized by transmission electron microscopy (TEM and high resolution TEM (FEI Tecnai G2 F30 S-TWIN) operated at 300 kV.
3. Results and discussion Fig. 1 shows the XRD patterns of the starting powder mixture, 12 h ball-milled powder and the SPSed CoCrFeMnNi HEA. In contrast to the XRD pattern of the starting powder mixture which contained five constituent elements (Fig. 1a), a single-phase fcc solid-solution was formed after 12 h of ball milling (Fig. 1b). The ball-milled powder shows large agglomerates in the size of 46 ± 20 µm and uniform distribution of Co, Cr, Fe, Mn, and Ni (Fig. 2). The single-phase fcc structure was preserved in the SPSed bulk alloys, with a lattice parameter of 3.599 Å. The XRD pattern of the bulk HEA shows sharp reflections with high intensity, indicating grain growth and residual strain release during SPS.
Fig. 1 XRD patterns of the starting powder mixture of Co, Cr, Fe, Mn and Ni (a), 12 h-ball milled powder (b) and the SPSed CoCrFeMnNi HEA (c). 6
Fig. 2 SEM image of the 12 h-ball milled Co, Cr, Fe, Mn and Ni powder mixture (a) and corresponding EDX elemental maps (b-f).
The SPSed CoCrFeMnNi HEA shows the relative density of 98.12 (± 0.16)%. BSE image shows uniform contrast, and the corresponding EDX elemental maps show uniform distribution of the constituent elements. These results confirm that no composition segregation was present in the HEA. Fig. 3a shows the microstructure of the as-sintered HEA. A considerable number of twins are present, due to the low stacking energy of the alloy. Fig. 3b shows the EBSD inverse pole figure (IPF) map of the HEA. Statistical analysis on the grain size distribution is presented in Fig. 3c. The grain size of the HEA ranges from 140 nm to 5 µm (with an average of 430 nm), which can be divided into two levels: 1) the ultrafine grains (100 nm ≤ d ≤1 µm), with number fraction fUFG = 95.1% and area fraction AUFG= 55.6%; and 2) the coarse grains (1 µm < d ≤ 5 µm), with fCG = 4.9% and area fraction ACG= 44.4%. The kernel average misorientation (KAM) map (Fig. 3d), which is associated with the geometrically necessary dislocations (GNDs), indicates local misorientations due to the lattice distortion. In the as-sintered HEA, the lattice distortion of UFG is relatively larger than 7
the CGs. Statistical KAM distribution (Fig. 3e) shows the maximum of 2° and an average of 0.33°.
Fig. 3 Characterization the microstructure of the CoCrFeMnNi HEA fabricated by mechanical alloying and SPS. (a) an ion image; (b) EBSD IPF-Z (Z//ND) map; (c) grain size distribution; (d) KAM map; (e) distribution of KAM corresponding to (d).
A representative tensile stress-strain curve of the SPSed CoCrFeMnNi HEA is presented in Fig. 4a. Uniform elongation (UE) is a better measure of ductility for small samples because they are insensitive to the sample size [23, 24]. It is known that the UE is governed by the strain-hardening rate (Θ = dσT/dεT, where σT and εT are true stress and true strain, respectively). According to Considére criterion [15, 16, 23, 25], when Θ < σT, necking is predicted to occur. The determined yield strength σ0.2 = ~1003 MPa, σUTS = ~1074 MPa, UE = 9.5%, εf = 16.5%. Fig. 4b plots the true stress and strain-hardening rate as a function of the 8
true strain, which illustrates the strain hardening behavior of the alloy. The strain hardening rate first decreased and then the HEA showed a transient hardening at true strains ranging from ~1.7 to ~2.8%. Such discontinuous yielding in elastoplastic transition should be attributed to back-stress strengthening induced by uneven stress distribution [15-18, 20, 26]. With increasing strain, the strain hardening rate decreased and necking occurred at a true strain of about 9.1%, as indicated by the arrow in Fig. 4b. A comparison of σ0.2 and UE with other previously reported CoCrFeMnNi HEAs (Fig. 4c) [3-5, 11, 12, 14, 27, 28]. CG CoCrFeMnNi HEAs have high ductility, but low yield strength (σ0.2 < 500 MPa). In contrary, nanostructured HEAs exhibit high yield strength but low ductility. UFG HEAs provide a better combination of strength and ductility. Herein, using a facile approach which does not need complex processing, we obtained a heterogeneous grain structure of CoCrFeMnNi HEA with yield strength of over 1 GPa and UE of 9.5%, which overcomes the longstanding dilemma of strength-ductility trade-off.
9
Fig. 4 (a) A representative tensile stress-strain curve of the CoCrFeMnNi HEA (the upper inset shows the sample dimension (with unit in mm) used for the tensile tests); the lower inset is the local strain levels after tensile test); (b) true stress and strain-hardening rate as a function of true strain; (c) a comparison of yield strength (σ0.2) versus UE for the single-phase fcc CoCrFeMnNi HEA reported in literatures.
To unravel the mechanism of such a good combination of high strength and ductility, we also performed detailed characterization of the fractograph and the microstructure after tensile test. The fractograph (Fig. 5a) shows a typical ductile dimpled fracture by submicron-void coalescence. Plastic deformation and numerous slip lines inside the grains were located in the slip trace of the sample surface (300 µm away from the fracture) (Fig. 5b). A variation in the height of the surface steps was observed from the grain interior to grain boundaries, suggesting that slip is initially triggered in the center of the grains away from the boundaries [29]. Apparently, the coarse grains shows activation of multiple slip systems. Cross-sectional ion image also shows (Fig. 5c) that the coarse grains underwent more severe deformation than the ultrafine ones, indicating that the coarse grains contribute more to the plasticity, and the grain-boundary strengthening mechanism produced by the UFGs contributed more to the strength [15, 16]. Fig. 5d shows IPF map with high angle grain boundaries (HAGBs) of the CrCoFeMnNi HEA after tensile test (~ 300 µm away from the fracture). The grains were obviously elongated along the tensile direction (TD), but no appreciable deformation twins were formed. Despite that the critical stress for twinning in the CoCrFeMnNi HEA was reported to be 720 (± 30) MPa, it should be noted that this critical stress was resolved in HEA with homogeneous structure [5]. The flow stress required to form deformation twins also depends on the temperature [5, 30] and grain size [31]. The smaller grain size and higher temperature would result in a low twinning activity. Our result is consistent with previous observations in the HEAs with bimodal structure [4, 30]. It should 10
also be noted that due to the small thickness of deformation twins, the individual twins may not be reliably detected by EBSD due to resolution limit [4, 14, 32]. A comparison of the local misorientation maps before (Fig. 3e) and after (Fig. S2) tensile tests shows that the lattice distortion inside the coarse grains is obviously increased and the KAM shows a larger maximum (3°) and average values (0.61°), to accommodate the mismatch of active slip systems among neighboring regions of individual grains [33]. After the tensile test, profuse dislocations can be found in the grain interior (Fig. 5e). High-resolution TEM analyses show that a high density of stacking faults were observed in the ultrafine-grains (Fig. 5f). The high density of stacking faults can be attributed to the low SFE of fcc HEA, which inhibits the dynamic recovery of dislocations during the deformation process at room temperature [12].
Fig. 5 (a) A SEM image of the fracture morphology after tensile test; (b)-(d) are SEM image, ion image and IPF map of the sample surface after tensile test (~ 300 µm away from the fracture); (e) a bright filed TEM image of the sample obtained from the site near the fracture surface; (f) a typical HRTEM image of an ultrafine grain showing a high density of stacking faults. 11
We now analyze the fundamental physical origin for the excellent combination of high strength-ductility in the present CoCrFeMnNi HEA with a heterogeneous structure. As is known, an intrinsic drawback of homogeneous structure is the trade-off dilemma: the increase of strength by grain refinement inevitably accompanies a drastic reduction of ductility, due to the mechanical instability and saturated work hardening in UFG materials [34]. For polycrystalline materials, the grain boundary strengthening could be calculated by the Hall-Petch relation [35] which shows the yield strength σ is related to the average grain size d by the equation: σ = σ0 + kd-1/2
(1)
where σ0 is the frictional stress resisting the glide of dislocations and k is a constant. The frictional stress σ0 and the Hall-Petch constant k of CoCrFeMnNi HEA at 293 K have been reported to be 125 MPa and 494 MPa µm-1/2 in Ref. [4] and 188 MPa and 497 MPa µm-1/2 in Ref. [36], respectively. Using these available data and the measured average grain size (430 nm) by EBSD (Fig. 3b), the maximum estimated yield strength σ of the HEA with homogeneous structure is only 514 MPa, which is much lower than the measured yield strength (σ0.2 = ~1003 MPa) of the one with heterogeneous structure in the present study. Thus, the excellent combination of strength and ductility in the present CoCrFeMnNi HEA should be attributed to extra hardening induced by microstructural heterogeneity. In comparison with homogeneous counterpart, the heterogeneous grain structure would lead to nonhomogeneous plastic deformation and induce strain gradient and large internal stresses [16, 19, 26]. At the early stage of plastic deformation, the soft domains with coarse grains will first initiate dislocation slip to produce plastic strain but the hard domains with ultrafine grains remain elastic. Due to the low SFE of the present fcc CoCrFeMnNi HEA, the full dislocation could dissociate into partials to form stacking faults as the twin nuclei [37], as observed in Fig. 5f. The low SFE provides a low threshold stress of twinning formation and 12
facilitates the transformation from stacking faults to deformation twins [38, 39]. At the low strain level, deformation twins can only be found in coarse grains. For those ultrafine grains, dislocations are emitted and piled up at the domain boundaries (see Fig. 5e). The excess GNDs will be generated due to strain incompatibility, to accommodate the strain gradients [16, 26]. The pile-up of GNDs generates the long-range stress (back-stress) towards the dislocation source and thus, will countervail the forward motion of dislocation and effective stress required for activating the dislocation sources. This will make the synergetic strengthening to increase the overall yield strength of the material [19, 34]. At the later stage of plastic deformation, both the soft and hard domains deform plastically but the soft ones sustain much larger strains than the hard ones, and thereby, producing strain partitioning. The generated strain gradients at the domain boundaries will increase with increasing strain partitioning, and thus, produce back-stress work hardening. Such back-stress work hardening will help to prevent necking during tensile testing and consequently improve ductility. This explains the excellent combination of high strength−ductility of the present single-phase FCC HEA with heterogeneous grain structure. Finally, it should also be noted that there may also be strengthening contribution from the minor amount of impurity oxides formed during the sintering process at high temperature, due to the high affinity to oxygen of Cr and Mn and the relatively low vacuum pressure [40]. In addition, it is worth to mention the present SPSed HEA is not fully dense. The presence of micro/nanopores can make the experimentally measured ductility lower than the intrinsic one of the HEA.
4. Conclusion In summary, a bulk CoCrFeMnNi HEA was fabricated by a combination of high energy ball milling and SPS. High energy ball milling forced into the formation of a single fcc 13
solid-solution. The SPSed CoCrFeMnNi HEA shows heterogeneous structure with grain size ranging from 140 nm to 5 µm. Such in-situ formed heterogeneous structure enables a gigapascal yield strength and UE of 9.5%, overcoming the strength-ductility trade-off. The present findings should provide deep insights into developing high-performance HEAs.
Supporting Information Fig. S1-2 are included.
Acknowledgements This work was financially supported by the Fundamental Research Program of Shenzhen
(Grant
No.
JCYJ20170412153039309)
and
Guangdong
Innovative
&
Entrepreneurial Research Team Program (No. 2016ZT06C279).
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0921-5093(19)31411-X
DOI:
https://doi.org/10.1016/j.msea.2019.138625
Reference:
MSA 138625
To appear in:
Materials Science & Engineering A
Received Date: 16 September 2019 Revised Date:
30 October 2019
Accepted Date: 1 November 2019
Please cite this article as: F. Jiang, C. Zhao, D. Liang, W. Zhu, Y. Zhang, S. Pan, F. Ren, In-situ formed heterogeneous grain structure in spark-plasma-sintered CoCrFeMnNi high-entropy alloy overcomes the strength-ductility trade-off, Materials Science & Engineering A (2019), doi: https://doi.org/10.1016/ j.msea.2019.138625. 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. © 2019 Published by Elsevier B.V.
In-situ formed heterogeneous grain structure in spark-plasma-sintered CoCrFeMnNi high-entropy alloy overcomes the strength-ductility trade-off
Feilong Jianga,b, Cancan Zhaoa, Dingshan Lianga,c, Weiwei Zhua,d, Yiwen Zhanga, Shuai Pana,b, Fuzeng Rena,*
a
Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, China
b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, China
c
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China d
Institute of Applied Physics and Materials Engineering, Faculty of Science & Technology, University of Macau, Macau, China
Corresponding Author. E-mail: [email protected]
1
Abstract The conflict between strength and ductility has been a longstanding dilemma for metallic alloys. Herein, we present a facile strategy to overcome the strength-ductility trade-off of face-center-cubic high-entropy alloys (HEAs) via the in-situ formation of heterogeneous grain structure, using the high local temperature gradients generated during spark plasma sintering coupled with the ‘sluggish diffusion’ of HEAs. The fabricated CoCrFeMnNi HEA has a heterogeneous structure over a large range of grain sizes from 140 nm to 5 µm, which enables a gigapascal yield strength and a uniform elongation of 9.5%. The mechanism responsible for the associated mechanical properties was analyzed.
Keywords: Heterogeneous grain structure; high-entropy alloy; mechanical properties
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Introduction High-entropy alloys (HEAs) with multi-principal elements have been received considerable research interests in recent fifteen years [1, 2]. Among the HEAs, the equimolar face-centered cubic (fcc) CoCrFeMnNi has been one of the most widely investigated HEAs, owing to its outstanding ductility and exceptional fracture toughness at cryogenic temperatures [3-6]. However, the coarse-grained (CG; grain size d > 1 µm) CoCrFeMnNi alloy with high tensile ductility exhibits low yield strength at room temperature, usually below 500 MPa, which hinders its practical applications at room or elevated temperatures [7].
Refining the grain size down to submicron- or nanometer-scale can significantly improve yield strength, but inevitably accompany the expense of ductility due to the constrained dislocation activity [8]. In particular, when the tensile yield strength (σ0.2) reaches over 1 GPa, the common dilemma of strength-ductility trade-off stands out [9-11]. To overcome this strength-ductility trade-off, Sun et al. [12, 13] fabricated an ultrafine-grained (UFG) HEA with grain size of ~ 500 nm via a serial processing, including hot forging, cold rolling and controlled annealing, and obtained σ0.2 of 888 MPa with UE of 21%. On this basis, they further pre-strained the HEA at 77 K and obtained σ0.2 of 1.07 GPa with a UE of 15.3% [14].
Besides the abovementioned strategy, the alloys with heterogeneous structure can also achieve an excellent combination of high strength and ductility [15-20]. For example, the heterogeneous structure of CrCoNi MEA was obtained through a serial processing, including homogenizing, cold-rolling, annealing and quenching, and achieved σ0.2 of 1.15 GPA and UE of 22% [16]. Wu et al. [17] reported a heterogeneous-structures-architecting strategy through cold-rolling and intermediate-temperature-annealing of arc-melted Al0.1CoCrFeNi HEA, and 3
achieved a yield strength of 711 MPa and a UE of 30.3%. Mechanical alloying (MA) can force the elemental powder mixture into the formation of a single-phase nanostructured solid solution for the alloy systems with low/moderate enthalpy of mixing. The mechanically alloyed nanostructured powder densified by spark plasma sintering (SPS) may yield heterogeneous structure, as demonstrated in conventional metals [21, 22] and in medium-entropy alloys [7], owing to the fast heating rate and high local temperature gradients which modify locally the recrystallization response of the severely deformed structure during sintering.
In this context, herein, we describe a feasible strategy that overcomes the strength-ductility trade-off of fcc HEAs via the in-situ formation of heterogeneous grain structure utilizing the high local temperature gradients generated during SPS coupled with the ‘sluggish diffusion’ of HEAs. The classical fcc CoCrFeMnNi HEA was selected as our model system. We have fabricated the HEA using a combination of MA and SPS, and systematically investigated the phase, microstructure, and mechanical properties.
2. Experimental procedure Commercially pure Co, Cr, Fe, Mn and Ni (> 99.9 wt%) elemental powders were mixed with an equiatomic ratio. The powder mixture was subjected to high energy ball milling with hardened steel balls (ball-to-powder weight ratio = 5:1) using a SPEX 8000D at ambient temperature in an argon glove box for 12 h. Then, the ball-milled powder was packed into a graphite die with an inner diameter of 15 mm and densified by SPS (SPS-211Lx, Fuji Electronic Industrial Co., Ltd.) at 1000 °C and 45 MPa for 5 min with a vacuum pressure of ~ 5 Pa.
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The “dog-bone-shaped” tensile specimens were cut from the as-sintered cylinders using electrical discharge machining. Uniaxial tensile tests were performed at ambient temperature, with an initial quasi-static strain rate of 5×10-4 s-1. Stereo Digital Image Correlation (DIC) method was used to monitor the deformation of the tensile specimens (with strain resolution of 0.005%). A fine black speckle pattern was first painted on a white undercoat on the test section, and then, the optical images of the specimen were captured during tensile deformation, and finally the images were analyzed using the VIC-2D system to compute the specimen surface’s displacements. σ0.2, ultimate tensile strength (σUTS), UE, and εf were determined from the obtained stress-strain curves. Three independent specimens were tested to confirm the reproducibility and a representative curve was provided.
The phase composition of both powders and bulk specimen were identified by X-ray diffraction (XRD) pattern recorded on a Rigaku Smartlab-9KW diffractometer with a Cu-Kα radiation (45 kV, 200 mA). The composition distribution and microstructure of the spark-plasma-sintered (SPSed) CoCrFeMnNi HEA were examined using a field-emission scanning electron microscope (SEM; MIRA 3, TESCAN, Czech Republic) equipped with backscattering diffraction electron (BSE) detector, energy dispersive X-ray spectroscopy (EDX; AZtec EDX system, Oxford Instruments, UK) and electron backscattering diffraction (EBSD; Nordlys Max2, Oxford Instruments, UK). The EBSD measurements were performed with accelerating voltage of 20 kV, working distance of 15 mm and step size of 40 nm. The grain size was calculated using the diameter of the circle equivalent to the grain section area, which was automatically measured by the Channel 5 software in accordance with the Standard ISO/DIS 13067. Ion images (yielding channel contrast) of the microstructure of the specimens before and after tensile tests were examined using focused ion beam (FIB; Helios NanoLabTM 600i, FEI, USA). The deformation microstructure of HEA near the fracture 5
surface was also characterized by transmission electron microscopy (TEM and high resolution TEM (FEI Tecnai G2 F30 S-TWIN) operated at 300 kV.
3. Results and discussion Fig. 1 shows the XRD patterns of the starting powder mixture, 12 h ball-milled powder and the SPSed CoCrFeMnNi HEA. In contrast to the XRD pattern of the starting powder mixture which contained five constituent elements (Fig. 1a), a single-phase fcc solid-solution was formed after 12 h of ball milling (Fig. 1b). The ball-milled powder shows large agglomerates in the size of 46 ± 20 µm and uniform distribution of Co, Cr, Fe, Mn, and Ni (Fig. 2). The single-phase fcc structure was preserved in the SPSed bulk alloys, with a lattice parameter of 3.599 Å. The XRD pattern of the bulk HEA shows sharp reflections with high intensity, indicating grain growth and residual strain release during SPS.
Fig. 1 XRD patterns of the starting powder mixture of Co, Cr, Fe, Mn and Ni (a), 12 h-ball milled powder (b) and the SPSed CoCrFeMnNi HEA (c). 6
Fig. 2 SEM image of the 12 h-ball milled Co, Cr, Fe, Mn and Ni powder mixture (a) and corresponding EDX elemental maps (b-f).
The SPSed CoCrFeMnNi HEA shows the relative density of 98.12 (± 0.16)%. BSE image shows uniform contrast, and the corresponding EDX elemental maps show uniform distribution of the constituent elements. These results confirm that no composition segregation was present in the HEA. Fig. 3a shows the microstructure of the as-sintered HEA. A considerable number of twins are present, due to the low stacking energy of the alloy. Fig. 3b shows the EBSD inverse pole figure (IPF) map of the HEA. Statistical analysis on the grain size distribution is presented in Fig. 3c. The grain size of the HEA ranges from 140 nm to 5 µm (with an average of 430 nm), which can be divided into two levels: 1) the ultrafine grains (100 nm ≤ d ≤1 µm), with number fraction fUFG = 95.1% and area fraction AUFG= 55.6%; and 2) the coarse grains (1 µm < d ≤ 5 µm), with fCG = 4.9% and area fraction ACG= 44.4%. The kernel average misorientation (KAM) map (Fig. 3d), which is associated with the geometrically necessary dislocations (GNDs), indicates local misorientations due to the lattice distortion. In the as-sintered HEA, the lattice distortion of UFG is relatively larger than 7
the CGs. Statistical KAM distribution (Fig. 3e) shows the maximum of 2° and an average of 0.33°.
Fig. 3 Characterization the microstructure of the CoCrFeMnNi HEA fabricated by mechanical alloying and SPS. (a) an ion image; (b) EBSD IPF-Z (Z//ND) map; (c) grain size distribution; (d) KAM map; (e) distribution of KAM corresponding to (d).
A representative tensile stress-strain curve of the SPSed CoCrFeMnNi HEA is presented in Fig. 4a. Uniform elongation (UE) is a better measure of ductility for small samples because they are insensitive to the sample size [23, 24]. It is known that the UE is governed by the strain-hardening rate (Θ = dσT/dεT, where σT and εT are true stress and true strain, respectively). According to Considére criterion [15, 16, 23, 25], when Θ < σT, necking is predicted to occur. The determined yield strength σ0.2 = ~1003 MPa, σUTS = ~1074 MPa, UE = 9.5%, εf = 16.5%. Fig. 4b plots the true stress and strain-hardening rate as a function of the 8
true strain, which illustrates the strain hardening behavior of the alloy. The strain hardening rate first decreased and then the HEA showed a transient hardening at true strains ranging from ~1.7 to ~2.8%. Such discontinuous yielding in elastoplastic transition should be attributed to back-stress strengthening induced by uneven stress distribution [15-18, 20, 26]. With increasing strain, the strain hardening rate decreased and necking occurred at a true strain of about 9.1%, as indicated by the arrow in Fig. 4b. A comparison of σ0.2 and UE with other previously reported CoCrFeMnNi HEAs (Fig. 4c) [3-5, 11, 12, 14, 27, 28]. CG CoCrFeMnNi HEAs have high ductility, but low yield strength (σ0.2 < 500 MPa). In contrary, nanostructured HEAs exhibit high yield strength but low ductility. UFG HEAs provide a better combination of strength and ductility. Herein, using a facile approach which does not need complex processing, we obtained a heterogeneous grain structure of CoCrFeMnNi HEA with yield strength of over 1 GPa and UE of 9.5%, which overcomes the longstanding dilemma of strength-ductility trade-off.
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Fig. 4 (a) A representative tensile stress-strain curve of the CoCrFeMnNi HEA (the upper inset shows the sample dimension (with unit in mm) used for the tensile tests); the lower inset is the local strain levels after tensile test); (b) true stress and strain-hardening rate as a function of true strain; (c) a comparison of yield strength (σ0.2) versus UE for the single-phase fcc CoCrFeMnNi HEA reported in literatures.
To unravel the mechanism of such a good combination of high strength and ductility, we also performed detailed characterization of the fractograph and the microstructure after tensile test. The fractograph (Fig. 5a) shows a typical ductile dimpled fracture by submicron-void coalescence. Plastic deformation and numerous slip lines inside the grains were located in the slip trace of the sample surface (300 µm away from the fracture) (Fig. 5b). A variation in the height of the surface steps was observed from the grain interior to grain boundaries, suggesting that slip is initially triggered in the center of the grains away from the boundaries [29]. Apparently, the coarse grains shows activation of multiple slip systems. Cross-sectional ion image also shows (Fig. 5c) that the coarse grains underwent more severe deformation than the ultrafine ones, indicating that the coarse grains contribute more to the plasticity, and the grain-boundary strengthening mechanism produced by the UFGs contributed more to the strength [15, 16]. Fig. 5d shows IPF map with high angle grain boundaries (HAGBs) of the CrCoFeMnNi HEA after tensile test (~ 300 µm away from the fracture). The grains were obviously elongated along the tensile direction (TD), but no appreciable deformation twins were formed. Despite that the critical stress for twinning in the CoCrFeMnNi HEA was reported to be 720 (± 30) MPa, it should be noted that this critical stress was resolved in HEA with homogeneous structure [5]. The flow stress required to form deformation twins also depends on the temperature [5, 30] and grain size [31]. The smaller grain size and higher temperature would result in a low twinning activity. Our result is consistent with previous observations in the HEAs with bimodal structure [4, 30]. It should 10
also be noted that due to the small thickness of deformation twins, the individual twins may not be reliably detected by EBSD due to resolution limit [4, 14, 32]. A comparison of the local misorientation maps before (Fig. 3e) and after (Fig. S2) tensile tests shows that the lattice distortion inside the coarse grains is obviously increased and the KAM shows a larger maximum (3°) and average values (0.61°), to accommodate the mismatch of active slip systems among neighboring regions of individual grains [33]. After the tensile test, profuse dislocations can be found in the grain interior (Fig. 5e). High-resolution TEM analyses show that a high density of stacking faults were observed in the ultrafine-grains (Fig. 5f). The high density of stacking faults can be attributed to the low SFE of fcc HEA, which inhibits the dynamic recovery of dislocations during the deformation process at room temperature [12].
Fig. 5 (a) A SEM image of the fracture morphology after tensile test; (b)-(d) are SEM image, ion image and IPF map of the sample surface after tensile test (~ 300 µm away from the fracture); (e) a bright filed TEM image of the sample obtained from the site near the fracture surface; (f) a typical HRTEM image of an ultrafine grain showing a high density of stacking faults. 11
We now analyze the fundamental physical origin for the excellent combination of high strength-ductility in the present CoCrFeMnNi HEA with a heterogeneous structure. As is known, an intrinsic drawback of homogeneous structure is the trade-off dilemma: the increase of strength by grain refinement inevitably accompanies a drastic reduction of ductility, due to the mechanical instability and saturated work hardening in UFG materials [34]. For polycrystalline materials, the grain boundary strengthening could be calculated by the Hall-Petch relation [35] which shows the yield strength σ is related to the average grain size d by the equation: σ = σ0 + kd-1/2
(1)
where σ0 is the frictional stress resisting the glide of dislocations and k is a constant. The frictional stress σ0 and the Hall-Petch constant k of CoCrFeMnNi HEA at 293 K have been reported to be 125 MPa and 494 MPa µm-1/2 in Ref. [4] and 188 MPa and 497 MPa µm-1/2 in Ref. [36], respectively. Using these available data and the measured average grain size (430 nm) by EBSD (Fig. 3b), the maximum estimated yield strength σ of the HEA with homogeneous structure is only 514 MPa, which is much lower than the measured yield strength (σ0.2 = ~1003 MPa) of the one with heterogeneous structure in the present study. Thus, the excellent combination of strength and ductility in the present CoCrFeMnNi HEA should be attributed to extra hardening induced by microstructural heterogeneity. In comparison with homogeneous counterpart, the heterogeneous grain structure would lead to nonhomogeneous plastic deformation and induce strain gradient and large internal stresses [16, 19, 26]. At the early stage of plastic deformation, the soft domains with coarse grains will first initiate dislocation slip to produce plastic strain but the hard domains with ultrafine grains remain elastic. Due to the low SFE of the present fcc CoCrFeMnNi HEA, the full dislocation could dissociate into partials to form stacking faults as the twin nuclei [37], as observed in Fig. 5f. The low SFE provides a low threshold stress of twinning formation and 12
facilitates the transformation from stacking faults to deformation twins [38, 39]. At the low strain level, deformation twins can only be found in coarse grains. For those ultrafine grains, dislocations are emitted and piled up at the domain boundaries (see Fig. 5e). The excess GNDs will be generated due to strain incompatibility, to accommodate the strain gradients [16, 26]. The pile-up of GNDs generates the long-range stress (back-stress) towards the dislocation source and thus, will countervail the forward motion of dislocation and effective stress required for activating the dislocation sources. This will make the synergetic strengthening to increase the overall yield strength of the material [19, 34]. At the later stage of plastic deformation, both the soft and hard domains deform plastically but the soft ones sustain much larger strains than the hard ones, and thereby, producing strain partitioning. The generated strain gradients at the domain boundaries will increase with increasing strain partitioning, and thus, produce back-stress work hardening. Such back-stress work hardening will help to prevent necking during tensile testing and consequently improve ductility. This explains the excellent combination of high strength−ductility of the present single-phase FCC HEA with heterogeneous grain structure. Finally, it should also be noted that there may also be strengthening contribution from the minor amount of impurity oxides formed during the sintering process at high temperature, due to the high affinity to oxygen of Cr and Mn and the relatively low vacuum pressure [40]. In addition, it is worth to mention the present SPSed HEA is not fully dense. The presence of micro/nanopores can make the experimentally measured ductility lower than the intrinsic one of the HEA.
4. Conclusion In summary, a bulk CoCrFeMnNi HEA was fabricated by a combination of high energy ball milling and SPS. High energy ball milling forced into the formation of a single fcc 13
solid-solution. The SPSed CoCrFeMnNi HEA shows heterogeneous structure with grain size ranging from 140 nm to 5 µm. Such in-situ formed heterogeneous structure enables a gigapascal yield strength and UE of 9.5%, overcoming the strength-ductility trade-off. The present findings should provide deep insights into developing high-performance HEAs.
Supporting Information Fig. S1-2 are included.
Acknowledgements This work was financially supported by the Fundamental Research Program of Shenzhen
(Grant
No.
JCYJ20170412153039309)
and
Guangdong
Innovative
&
Entrepreneurial Research Team Program (No. 2016ZT06C279).
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.