Cryogenic deformation mechanism of CrMnFeCoNi high-entropy ... · cryogenic mechanical properties...

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Original Article Cryogenic deformation mechanism of CrMnFeCoNi high-entropy alloy fabricated by laser additive manufacturing process Zengcheng Qiu a, b , Chengwu Yao a, b , Kai Feng a, b, c, ** , Zhuguo Li a, b, * , Paul K. Chu c a Shanghai Key Laboratory of Materials Laser Processing and Modication, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China c Department of Physics, Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, Special Administrative Region article info Article history: Received 19 December 2017 Received in revised form 31 January 2018 Accepted 1 February 2018 Available online 8 February 2018 Keywords: High-entropy alloys Laser additive manufacturing Cryogenics Dislocation density Deformation twinning abstract Well-formed equimolar CrMnFeCoNi high-entropy alloy (HEA) bulk samples with good tensile properties are fabricated by laser additive manufacturing (LAM) processing. To elucidate the deformation mechanism, tensile tests are performed on at 77 K and 293 K and interrupted at different strains. Electron backscatter diffraction and X-ray diffraction indicate that the large initial dislocation density introduced by LAM processing increases the yield strength signicantly and dislocation motion is the dominant deformation mechanism. In addition, deformation twinning is a large addition at large strain levels under cryogenic conditions. These two mech- anisms and their interaction produce the excellent mechanical properties of bulk HEA. © 2018 The Author. Publishing Services provided by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction High-entropy alloys (HEAs), a new type of alloys, was intro- duced by Yeh et al., in 2004 [1]. HEAs generally dened as alloys composed of 5 or more alloying elements in equiatomic or near- equiatomic ratios are unique in that they have simple solution structures (mostly FCC or BCC) instead of intermetallic phases due to the high congurational entropies. Recent research reveals that the unique structure of HEAs may produce high strength [2] and hardness [3], outstanding wear resistance [4], nobler corrosion resistance [5], as well as excellent electrical and magnetic proper- ties [6]. Among the various kinds of HEAs, the equiatomic CrMnFeCoNi alloy, one of the most widely investigated since 2004 [7], has attracted much attention in many aspects, including the microstructure and phases [8e10], recrystallization [11], lattice strain [12], sluggish diffusion [13], oxidation [14] and corrosion properties [5]. Lately, it's surprisingly found that CrMnFeCoNi alloy exhibits fascinating cryogenic mechanical properties [15,16] consequently spurring much research activity [17e19]. Its strength and ductility increase dramatically with decreasing tem- perature in addition to outstanding fracture toughness at room temperature that remains high even at 77 K. These exceptional cryogenic mechanical properties are attributed to nanoscale deformation twinning that occurs under cryogenic conditions at large strain scale besides planar-slip dislocation which is the fundamental deformation mechanism at room temperature. In most researches, deformation twinning has been observed at 77 K at ~20% strain [16] but is absent at 273 K [15,16,19]. However, according to G. Laplanche's study [18], the critical strain for defor- mation twinning is ~7.4% at 77 K and ~25% at 293 K while the critical tensile stress is ~720 MPa. Despite all that, the deformation mechanism transitioning from planar-slip dislocation activity to deformation twinning is fundamental to a good outstanding of the mechanical properties. In fact, deformation twinning leads to a continuous high work hardening rate consequently enhancing the strength and ductility. * Corresponding author. Shanghai Key Laboratory of Materials Laser Processing and Modication, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Fax: þ86 21 34203024. ** Corresponding author. Shanghai Key Laboratory of Materials Laser Processing and Modication, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail addresses: [email protected] (K. Feng), [email protected] (Z. Li). Peer review under responsibility of Editorial Board of International Journal of Lightweight Materials and Manufacture. Contents lists available at ScienceDirect International Journal of Lightweight Materials and Manufacture journal homepage: https://www.sciencedirect.com/journal/ international-journal-of-lightweight-materials-and-manufacture https://doi.org/10.1016/j.ijlmm.2018.02.001 2588-8404/© 2018 The Author. Publishing Services provided by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). International Journal of Lightweight Materials and Manufacture 1 (2018) 33e39

Transcript of Cryogenic deformation mechanism of CrMnFeCoNi high-entropy ... · cryogenic mechanical properties...

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lable at ScienceDirect

International Journal of Lightweight Materials and Manufacture 1 (2018) 33e39

Contents lists avai

International Journal of Lightweight Materials and Manufacture

journal homepage: https: / /www.sciencedirect .com/journal /internat ional- journal-of- l ightweight-mater ia ls-and-manufacture

Original Article

Cryogenic deformation mechanism of CrMnFeCoNi high-entropy alloyfabricated by laser additive manufacturing process

Zengcheng Qiu a, b, Chengwu Yao a, b, Kai Feng a, b, c, **, Zhuguo Li a, b, *, Paul K. Chu c

a Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University,Shanghai 200240, Chinab Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, Chinac Department of Physics, Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, SpecialAdministrative Region

a r t i c l e i n f o

Article history:Received 19 December 2017Received in revised form31 January 2018Accepted 1 February 2018Available online 8 February 2018

Keywords:High-entropy alloysLaser additive manufacturingCryogenicsDislocation densityDeformation twinning

* Corresponding author. Shanghai Key Laboratoryand Modification, School of Materials Science and EngUniversity, Shanghai 200240, China. Fax: þ86 21 342** Corresponding author. Shanghai Key Laboratoryand Modification, School of Materials Science and EngUniversity, Shanghai 200240, China.

E-mail addresses: [email protected] (K. Feng), liPeer review under responsibility of Editorial Boa

Lightweight Materials and Manufacture.

https://doi.org/10.1016/j.ijlmm.2018.02.0012588-8404/© 2018 The Author. Publishing Servicescreativecommons.org/licenses/by-nc-nd/4.0/).

a b s t r a c t

Well-formed equimolar CrMnFeCoNi high-entropy alloy (HEA) bulk samples with good tensile properties arefabricatedby laser additivemanufacturing (LAM)processing. To elucidate thedeformationmechanism, tensiletests are performed on at 77 K and 293 K and interrupted at different strains. Electron backscatter diffractionandX-ray diffraction indicate that the large initial dislocation density introduced by LAMprocessing increasesthe yield strength significantly and dislocation motion is the dominant deformation mechanism. In addition,deformation twinning is a large addition at large strain levels under cryogenic conditions. These two mech-anisms and their interaction produce the excellent mechanical properties of bulk HEA.© 2018 The Author. Publishing Services provided by Elsevier B.V. on behalf of KeAi. This is an open access

article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

High-entropy alloys (HEAs), a new type of alloys, was intro-duced by Yeh et al., in 2004 [1]. HEAs generally defined as alloyscomposed of 5 or more alloying elements in equiatomic or near-equiatomic ratios are unique in that they have simple solutionstructures (mostly FCC or BCC) instead of intermetallic phases dueto the high configurational entropies. Recent research reveals thatthe unique structure of HEAs may produce high strength [2] andhardness [3], outstanding wear resistance [4], nobler corrosionresistance [5], as well as excellent electrical and magnetic proper-ties [6]. Among the various kinds of HEAs, the equiatomicCrMnFeCoNi alloy, one of the most widely investigated since 2004

of Materials Laser Processingineering, Shanghai Jiao Tong03024.of Materials Laser Processingineering, Shanghai Jiao Tong

[email protected] (Z. Li).rd of International Journal of

provided by Elsevier B.V. on beha

[7], has attracted much attention in many aspects, including themicrostructure and phases [8e10], recrystallization [11], latticestrain [12], sluggish diffusion [13], oxidation [14] and corrosionproperties [5]. Lately, it's surprisingly found that CrMnFeCoNi alloyexhibits fascinating cryogenic mechanical properties [15,16]consequently spurring much research activity [17e19]. Itsstrength and ductility increase dramatically with decreasing tem-perature in addition to outstanding fracture toughness at roomtemperature that remains high even at 77 K. These exceptionalcryogenic mechanical properties are attributed to nanoscaledeformation twinning that occurs under cryogenic conditions atlarge strain scale besides planar-slip dislocation which is thefundamental deformation mechanism at room temperature. Inmost researches, deformation twinning has been observed at77 K at ~20% strain [16] but is absent at 273 K [15,16,19]. However,according to G. Laplanche's study [18], the critical strain for defor-mation twinning is ~7.4% at 77 K and ~25% at 293 K while thecritical tensile stress is ~720 MPa. Despite all that, the deformationmechanism transitioning from planar-slip dislocation activity todeformation twinning is fundamental to a good outstanding of themechanical properties. In fact, deformation twinning leads to acontinuous high work hardening rate consequently enhancing thestrength and ductility.

lf of KeAi. This is an open access article under the CC BY-NC-ND license (http://

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Arc melting and drop casting have been employed to produceCrMnFeCoNi HEA [16,18,20,21] but the size of the bulk ingot islimited to a rapid cooling rate and the ingot is often going throughheat treatment to homogenize themicrostructure and components.Moreover, the as-cast product has natural limitations like shrinkageand pores thereby requiring further processing to eliminate thedefects. Owing to the shortcomings of casting, laser additivemanufacturing (LAM), a flexible processing technique to produceproducts with a complex shape without casting mold has beenapplied to themanufacturing of HEAs. The big advantage is the highsolidification rate (104e106 K/s) [22] which improves the solidsolubility limit, ensures the formation of a simple solid solutionphase, and suppresses elemental segregation, resulting in a ho-mogeneous microstructure. Although laser processing technologyhas been applied to HEAs to fabricate bulk materials and coatings[23e25], there have been few studies on laser processing ofCrMnFeCoNi HEA. Ye et al. [5] prepared CrMnFeCoNi HEA coatingson 304 stainless steel by laser cladding and investigated thecorrosion behavior. Laser metal deposition technology wasemployed to the synthesize bulk CrMnFeCoNi HEA and thecompression properties were studied at room temperature [26]. Inspite of recent studies, the tensile properties and deformationmechanism of LAM CrMnFeCoNi HEA, especially under cryogenicconditions, have been rarely investigated. In this work, the LAMtechnology is adopted to fabricate bulk CrMnFeCoNi HEA and thetensile properties are determined at both room temperature andcryogenic temperature. The microstructure and cryogenic defor-mation mechanism are investigated systematically.

2. Material and methods

The bulk CrMnFeCoNi HEA samples with dimensions of70mm� 25 mm� 3mmwas produced on a LAM system equippedwith a 10 kW high power fiber laser unit (IPG YLS-10000). Thesurface-polished and cleaned Q235 low-carbon mild steel (Si: 0.37,C: 0.17, Mn: 0.08, S: 0.039, P: 0.036, Fe balance in mass percentage)sheet with dimensions of 150 mm � 150 mm � 20 mmwas chosenas the substrate. The pre-alloyed spherical equiatomic CrMnFeCoNipowder prepared by nitrogen gas atomization with size of ~50 mmwas employed as the depositing materials and fed into the moltenpool by a coaxial nozzle during the LAM process. After a series ofpreliminary experiments, the optimal LAM parameters (laser po-wer: 1.7 kW, scanning speed: 2 mm/s, powder feeding rate: 10 g/min) were determined. High purity argon gas was used as theshielding gas to protect the feeding powder and molten pool fromoxidation.

To evaluate the tensile properties and study the deformationmechanism at different temperature, specimens with a dog-boneshape with a 10 mm gauge length were machined from the bulkCrMnFeCoNi HEA with the longitudinal direction parallel to thelaser processing direction. The tensile specimens were polishedwith 1000# SiC abrasive paper resulting in a final gauge width of~3.8 mm and gauge thickness of ~1.0 mm. Tensile tests were per-formed on the MTS 370 machine at a constant tensile rate of0.2 mm/min at 77 K and 293 K, respectively. Each test was con-ducted at least 3 times and the results were quite consistent.Additional tensile tests were carried out to investigate the defor-mation mechanism andmicrostructure evolution at different strainlevels at 77 K. Three specimens were tested and interrupted atvarious strain levels (6%, 12%, 18%) and then cut into half for furthercharacterization.

The morphology and microstructure of the LAM HEA sampleswere characterized by Zeiss Axioplan 2 optical microscopy (OM)and field-emission scanning electron microscopy (NOVA, Nano-SEM230, FEI) after mechanical polishing and etching with aqua

regia. The degree of elemental segregation was studied by energydispersive X-ray spectroscopy (EDS, Aztec X-Max 80) and electronbackscattering diffraction (EBSD) was employed to reveal themicrostructure and deformation mechanism at different strainlevels of 0%, 6%, 12%, 18%, and 36% at 77 K. The longitudinal sectionsof the specimens were cut and prepared by standard mechanicalpolishing and vibration polishing. The analysis was conducted in anEBSD system (Aztec HKL Max) operated at 20 kV with a step size of0.1 mme10 mm. The data were analyzed using the HKL TechnologiesChannel 5 software. X-ray diffraction (XRD) was performed on theD8 ADVANCE DAVINCI (Bruker) to identify the phase and calculatethe lattice constant as well as dislocation density of the LAM HEAbefore and after deformation. The radiation source was Cu Ka

(wavelength ¼ 1.54 Å). The scanning rate was 1�/min and scanningrange was 40�e100�.

3. Results and discussion

The XRD pattern of the initial CrMnFeCoNi HEA in Fig. 1a showsthat the LAM HEA has a simple FCC crystalline structure withdiffraction peaks at 43.490�, 50.670�, 74.460�, 90.400�, 95.670�. Thelattice constant is calculated to be 3.598 Å using the NelsoneRileyextrapolation method [27]. The OM image in Fig. 1b reveals thecross-sectional microstructure of the initial CrMnFeCoNi HEA. TheLAM HEA has a typical dendritic structure without evident cracksand voids. The growth direction of the dendrite is perpendicular tothe laser scanning direction due to rapid directional solidification inthe manufacturing process. The elemental distribution is obtainedby EDS point analysis of the region of Fig. 1c and the elementalmaps shown in Fig. 1d shown that all the alloying elements areessentially homogeneously distributed with the exception of a fewprecipitates (marked by the arrow in Fig. 1c) identified as Mn-richand Cr-rich compounds that have been observed previously[9,28,29]. Gludovatz et al. [15] and Gali et al. [17] have observedMn-rich and Cr-rich particles in CrMnFeCoNi HEA but not in Mn-free CrFeCoNi alloy, suggesting the role of Mn in the formation.These Mn-rich and Cr-rich compounds are found from the voids ofthe fracture (Fig. 2b) and act as initiation sites that deteriorate thetensile properties.

Fig. 2a depicts the representative engineering stressestraincurves of the CrMnFeCoNi HEA at 77 K and 298 K. The yieldstrength sy and ultimate tensile strength su increase by about 60%and 65% to 564 MPa and 891 MPa, respectively, whereas theductility increases by 38% to 0.36 as the temperature decreasesfrom 298 K to 77 K. The tensile properties are enhanced withdecreasing temperature as consistent with the literature [15,16].The underlying mechanism is complex and multifaceted. Excellenttensile properties have been observed from casted CrMnFeCoNiHEAs [15e17,19] which have undergone forging or cold/hot rolling,homogenizing annealing, as well as recrystallization annealing inorder to eliminate defects, dissolve the components, reducesegregation, and strengthen the materials. As previously observed,the microstructure of LAM CrMnFeCoNi HEA is quite uniformcompared with the one prepared by traditional casting [8] becauseelemental segregation is suppressed due to the fast solidificationrate. In addition, rapid solidification and cooling may increase thedislocation density [30] resulting in higher strength. Here, theinitial CrMnFeCoNi without processing and annealing is tested toassess the tensile properties and evaluate the process performance.The yield strength is better than that of the CrMnFeCoNi HEAwith agrain size of 50 mmaccording to Otto [16] because of the large initialdislocation density. Nevertheless, on account of the highly orientedheat flux, coarse and oriented dendritic crystals are obtained.Compared to the micrometer-scale grains reported previously

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Fig. 1. Phase and microstructure of the CrMnFeCoNi HEA bulk sample. (a) XRD pattern of initial undeformed HEA and deformed HEA; (b) OM image and (c) EDS point analysis(ten points chosen randomly) of the HEA bulk; (d) EDS maps of the same area.

Fig. 2. Tensile properties and fracture morphology of the CrMnFeCoNi HEA bulk sample. (a) Typical engineering stressestrain curves of CoCrFeMnNi HEA tested at 77 K and293 K; (b) SEM image of the fracture surface of a sample tested at 77 K.

Z. Qiu et al. / International Journal of Lightweight Materials and Manufacture 1 (2018) 33e39 35

[15e17,19], the large grains undoubtedly undermine the tensileproperties accounting for the reduced strength.

The phase maps at different strain levels at 77 K in Fig. 3a reveala single FCC phase in agreement with XRD. The indexing rate is over99% and the unindexed points are detected as manganese andchromium compounds or pores as aforementioned. No phasetransformation is observed in each stage, even under heavy plasticdeformation conditions thereby verifying the high stability of theFCC phase. With increasing deformation, the density of the high-angle grain boundary decreases and the one of a low-angle grainboundary increases dramatically, indicating rising dislocationdensities. Plastic deformation is observed, especially appearance ofthe sub-grain boundary and severe fragmented microstructure inthe interior of the grains as shown in the Euler maps in Fig. 3b.However, in 0%ε, 6%ε, and12%ε, the twinned-grains cannot be found[twinning boundaries (

P3 ¼ 60�<111>) are depicted by yellow

lines in Fig. 3a]. When the strain is increased to 18%, obvious

deformation twinning appears (Fig. 3a) and the morphology of thetwinning at higher magnification is shown in Fig. 3c. The density oftwinning is limited while the distribution is very heterogeneous.When the strain reaches the largest value (~36%), the density oftwinning rises and twinning is more dispersed (Fig. 3d). Nonethe-less, the limited quantity and lack of twinning in most areas indi-cate that twinning is not the dominant deformation mechanismeven at a high strain level.

In addition to deformation twinning, dislocation is regarded asthe main deformation mechanism especially at low strain level[15,16]. The local misorientation map (Fig. 4a) is used to assess thedistribution and change of dislocation in the various stages ofdeformation. Because dislocations are usually defined as line de-fects causing relative displacement of the crystalline lattice, thedensity of dislocation can be revealed by the orientation differencesbetween neighboring points [31] by the EBSD local misorientationmodule. It is apparent that the density of dislocation increases

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Fig. 3. EBSD analysis of the CrMnFeCoNi HEA bulk sample at different strain levels at 77 K. (a) Phase map; (b) Euler map; (c) and (d) Enlarged Euer map of the area circled by thered rectangle in (b).

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rapidly and the uniformity gets better with increasing strain,indicating that dislocations play an important role in all the stagesof deformation. The high dislocation intensity can be deduced fromred and orange patches observed all around in 36%ε.

Fig. 4. Dislocation densities of the CrMnFeCoNi HEA bulk sample at different strain levelHEA at different strain levels at 77 K; (b) Dislocation densities of initially undeformed HEmisorientation map.

In order to determine the dislocation densities difference be-tween the initial CrMnFeCoNi HEA and the one in 36%ε, quantita-tive XRD is performed. The relationship between the dislocationdensity and full-width at half-maximum (FWHM) [32,33] is shownin the following:

s and interaction between dislocation and twinning. (a) Local misorientation map ofA and deformed HEA; (c) Morphology of deformed twinning and corresponding local

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rðhklÞ ¼b2ðhklÞ4:35b2

(1)

where rðhklÞ is the dislocation density of the different crystal faces,bðhklÞ is the FWHM of related diffraction peaks, and b represents forthe Burt's vector. The XRD pattern of the deformed (36%ε)CrMnFeCoNi HEA is shown in Fig. 1a. A simple FCC crystallinestructure with diffraction peaks at 43.685�, 50.653�, 74.603�,90.541� and 96.162� is identified corresponding to the EBSD resultthus indicating excellent phase stability under stress. The FWHM ofthe undeformed/deformed CrMnFeCoNi HEA are calculated usingMDI's Jade 6.5 (USA) software. rðhklÞ can be derived from bðhklÞ and band the comparative analysis of rðhklÞ is shown in Fig. 4b.

The dislocation densities of different crystal faces are twice orthree times larger after deformation confirming the important roleof dislocations during deformation. The dislocation density of theinitial CrMnFeCoNi HEA is substantial and bigger than that in theliterature [18,20]. This may be attributed to the high solidificationand cooling rates which are advantages of the LAM technology. Thedislocation densities inmost annealed polycrystallinemetals are 106

to 108 cm�2, whereas those in metals after severe cold deformationcan be as large as 1010 to 1012 cm�2. The large quantity of disloca-tions introduced by laser processing undoubtedly enhances the yieldstrength which is higher than that of CrMnFeCoNi HEA with asimilar grain size [16]. According to the previous reports [15,16,34],owing to the moderate stacking fault energy, the strong frictionstress caused by lattice distortion promotes the incipient planar slipand planar slip of the 1/2<110> type dislocations on {111} planesand is the basic deformation mechanism in CrMnFeCoNi HEA.Therefore, the high ultimate tensile strength stems from themassivedislocation interactions. Nevertheless, the effect of deformationtwinning at high strain levels is not negligible. It constitutes anadditional deformation mechanism and furthermore, the interac-tion between dislocations and twinning leads to the increase ofdislocation. As shown in Fig. 4c, there are many dislocations aroundthe twinning. Twinning can hinder the motion of dislocations,which results in continuous accumulation of dislocations, ensuring ahigh work hardening rate. As a result, good tensile properties areobserved under cryogenic conditions as a result of dislocations anddeformation twinning as well as their interactions.

4. Conclusions

In summary, an equimolar bulk CrMnFeCoNi HEA sample isproduced by LAM processing and its microstructure, tensile prop-erties at both room and cryogenic temperature, as well as thedeformation mechanism are studied. The bulk HEA samples has ahomogeneous FCC phase and identical dendritic structure. It hasexcellent room and cryogenic tensile properties and especiallyoutstanding yield strength. In the incipient stage of deformation,abundant dislocations introduced by LAM increase the yieldstrength. During deformation, dislocation glide is the dominantmechanism and deformation twinning appearing at large strainlevels under cryogenic conditions plays a significant role inimproving the ultimate strength and ductility. Impediment of thedislocation motion caused by twinning can strengthen the bulkHEA sample as well.

Acknowledgements

Financial support from the National Natural Science Foundationof China under grant number 51775338, “Chen Guang” projectShanghai Municipal Education Commission, Shanghai Education

Development Foundation (Grant Number 13CG07), “Chenxing”young scholar project of Shanghai Jiao Tong University (GrantNumber 14X100010017), Hong Kong Research Grants Council (RGC)General Research Funds (GRF) No. 11301215, and City University ofHong Kong Applied Research Grant (ARG) No. 9667122 isacknowledged.

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