Materials and Design 88 (2015) 1057–1062

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Materials and Design

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Microstructural features and tensile behaviors of the Al0.5CrCuFeNi2high-entropy alloys by cold rolling and subsequent annealing

S.G. Ma a,⁎, J.W. Qiao a,b, Z.H. Wang a, H.J. Yang c, Y. Zhang d,⁎a Shanxi Key Laboratory of Material Strength & Structural Impact, Institute of Applied Mechanics and Biomedical Engineering, Taiyuan University of Technology, Taiyuan 030024, Chinab Laboratory of Applied Physics and Mechanics of Advanced Materials, College of Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, Chinac Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, Chinad State key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China

⁎ Corresponding authors.E-mail addresses: mashengguo@tyut.edu.cn (S.G. Ma)

(Y. Zhang).

http://dx.doi.org/10.1016/j.matdes.2015.09.0920264-1275/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 March 2015Received in revised form 15 September 2015Accepted 16 September 2015Available online 24 September 2015

Keywords:High-entropy alloyCold rollingMicrostructureTensile behavior

TheAl0.5CrCuFeNi2HEAswith three different statuses (as-cast, -rolled, and -annealed) are prepared to investigatetheir microstructure evolutions and tensile behaviors. The as-cast sample exhibits simple solid-solution dendritestructure, imperfect lattice configurations (dislocation lines and stacking faults), andmoderate tensile properties.Relying on the significantly-enhanced dislocation density, the as-rolled alloy demonstrates a rapidly-increasedtensile strength, but that has come at the expense of the tensile elongation. Annealing treatments reveal thatthe temperature effect evidently influences the tensile strength of the annealed samples, indicating the recoveryof the dislocations with the temperature. Annealing twinning crystals and subgrains induced by the aforemen-tioned lattice defects are embedded on the recrystallized grains, suggesting that multiple deformation mecha-nisms are possible upon sample loading. The annealed alloys mainly exhibit intergranular-dominated fracturefeatures, which yields a limited modification of the tensile plasticity as compared with the as-rolled alloy.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, high-entropy alloys (HEAs) or multiprincipal componentalloys [1–3] have generated people's great interest in the materialfield. Firstly, a novel alloy-design strategy has broken up conventionalmetallurgical theories, where all the components are composed of fiveor more elements in equimolar or near-equimolar ratios [1]. Secondly,simple solid-solution structures rather than complicated intermetalliccompounds are dominant in the phase formation of HEAs, which arecharacterized in forms of face-centered cubic (FCC), body-centeredcubic (BCC), or a mixed FCC plus BCC structure [1–10]. Thirdly, a seriesof unique properties have been found, including potential low/room/high-temperature properties [4–7], good wear and fatigue resistance[11–12], good thermal stability [7], high fracture toughness [4] and oth-erwise, which leads to the HEAs as a huge potential for the future engi-neering materials.

Among the investigated HEA systems, the AlxCoCrCuFeNi is a classicsystem, whose microstructures and properties have been widelystudied [1,3,8,12–14]. Yeh et al. [1] found that the AlxCoCrCuFeNi sys-tem exhibits simple crystal structures and extraordinary properties,for instance, extended solid solutions, multi-strengthening mecha-nisms, excellent resistance to anneal softening, good candidates for

, drzhangy@ustb.edu.cn

high-temperature applications, and quite high tolerance to impurity el-ements. Sing et al. [8] reported that the solidification behavior of theAlCoCrCuFeNi is a kinetic process, where obvious phase segregationswere observed in the casting condition while simple polycrystallinestructures were obtained for the splat quenching condition. Wanget al. [13]first investigated the tensile properties of the Al0.5CoCrCuFeNi,and found that it exhibited a tensile strength of at least 700MPa in com-binationwith a high ductility of 19%. Tsai et al. [14] first explored the de-formation and annealing behaviors of the Al0.5CoCrCuFeNi by hotforging and cold rolling, and found that the deformation alloy demon-strated a largework hardening capacity and excellent resistance to stat-ic anneal softening. Wu et al. [15] found that a family of FCC-structuredmulticomponent solid solution alloys shows a Hall–Petch dependencyof microhardness by cold rolling and subsequent annealing. In spite ofthis, it needs to be noted thatmost reports on themechanical propertiesof HEAs are still focused on the casting condition.

Until recently, a fresh Co-free AlxCrCuFeNi2 system has beeninnovatively introduced into the alloy design and development of theHEAs based on the cost consideration [16]. Guo et al. [16] reportedthat with increasing the Al content, a phase transformation from FCCto BCC was found among the AlxCrCuFeNi2 system. In addition, thisalloy system exhibits some interesting features, for example, anomaloussolidification behavior, sunflower-like morphology, slow diffusion ki-netics, and near-eutectic microstructure [17]. In our previous work,we have explored the phase stability, microstructural evolution, micro-hardness, and compression performance of the as-cast and -annealed

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AlxCrCuFeNi2 HEAs [18]. It was found that the Al0.5CrCuFeNi2 HEA ex-hibited a moderate alloy strength combined with a good malleability.Therefore, in order to further optimize its mechanical property, themethodology of cold rolling and subsequent annealing is applied tothe Al0.5CrCuFeNi2 alloy. Additionally, tensile behavior, strengtheningmechanism, recovery and recrystallization process, and fracture featureof the present alloys are discussed in detail.

2. Experimental procedures

Button ingots with a nominal composition of Al0.5CrCuFeNi2 (in amolar ratio) are prepared by arc-melting pure elements with a purityhigher than 99.95 wt.% under a high-purity argon atmosphere on awater-cooled Cu hearth. The synthesized alloys are remelted at leastfour times in order to obtain chemical homogeneity. Flat plates with athickness of 2 mm are extracted from the molten alloy that are furthercold rolled to 1 mm in thickness (a reduction of 50%). Annealing treat-ments are conducted upon the cold-rolled products at the temperatureof 1173 K–1373 K. Tensile samples are artificially machined into a dumb-bell geometry, which have a nominal gage diameter of 3 mm and gagelength of 20mm. Tensile tests are performed on a CMT 4105 tensile test-ing machine equipped with extensometers for the strain measurementsupon a nominal strain rate of 4 × 10−4 s−1 at room temperature. Atleast three samples for each condition are employed for the reproducibil-ity of the data. The crystal structures of the alloys are identified by X-raydiffraction (XRD) using a PHILIPS APD-10 diffractometer with Cu-Kα ra-diation (the applied voltage is 50 kV, the current is 50 mA, and the 2θ-scanning rate is 10°/min). Microstructure observations of the cross sec-tions are carried out by themetallographicmicroscope and scanning elec-tron microscope (EISS SUPRA55 SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The fine structures are analyzedusing transmission electron microscopy (Hitachi 800 TEM).

3. Results

Fig. 1(a) and (b) shows the XRD patterns of the Al0.5CrCuFeNi2 HEAsin both as-cast and cold-rolled statuses, respectively. It is suggested thatthe crystal structures of the alloys are mainly composed of FCC solid-solution phase regardless of which status. In addition, the reflectionpeak width of the deformed specimen is slightly higher than that ofthe as-cast sample, and a similar report was obtained in the cold rollingof Al0.5CoCrCuFeNi [19]. The reason is not easy to be clarified as a varietyof factors can contribute to this feature, such as instrumental effects,crystalline size, lattice strain, and microstructural defects[20] Among

Fig 1. The XRD patterns of the Al0.5CrCuFeNi2 high-entropy alloys: (a) cold rolling; (b) ascast.

these factors, the dendrite fragmentation and dislocation density pro-duced by severe plastic deformation may be two important reasonsfor the peak broadening, which will be stated in the following sections.

Fig. 2(a) and (b) shows the microstructural features of theAl0.5CrCuFeNi2 HEAs in both as-cast and cold-rolled statuses, respective-ly. Fig. 2(a) exhibits a typical as-cast dendrites structure, wherein thewhite region refers to as the dendrite matrix and the gray region de-notes the remaining interdendrites. One can see that the primary den-drites approximately have an arm-width of 10 μm, which are inclined90° to the secondary dendrites of 5 μm in width. Each dendrite tree issurrounded by the grain boundary, as labeled by the red arrows, thusconstituting a dendrite network. This feature is probably due to the con-stitutional supercooling effect produced by rapid solidification, whichoriginates from compositional changes and results in cooling a liquidbelow the freezing point ahead of solid/liquid interface during solidifi-cation [21]. Actually, the EDS results indeed reveal that obvious compo-sitional segregation can be present in the as-cast sample. The dendritesmatrixes are enriched with Cr and Fe and depleted in Al and Cu, whileopposite elemental distributions are present in the interdendrites, andNi is relatively equivalent in both regions. This feature is easy to be un-derstandable because the melting point of Cr and Fe is higher than thatof Al and Cu so Cr and Fe tend to solidify ahead of Al and Cu.Fig. 2(b) displays the as-rolled microstructure of the present HEA. It isnoted that severely-deformed microstructures along the cold-rolled di-rection can be observed, where the primary dendrites are distinctlydistorted and elongated, accompanying with the secondary dendritesfragmented and dispersed in the interdendrites.

Fig. 3(a) and (b) shows the true tensile stress–strain curves of theAl0.5CrCuFeNi2 HEAs in both as-cast and cold-rolled statuses, respective-ly. The corresponding engineering stress-strain curve of Fig. 3(a) wasalso observed in the literature [18]. It can be seen that the as-cast sample

Fig 2. Microstructures of the Al0.5CrCuFeNi2 high-entropy alloys: (a) as cast; (b) coldrolling. (For interpretation of the references to color in this figure, the reader is referredto the web version of this article.)

Fig 3. The true tensile stress–strain curves and TEM analyses of the Al0.5CrCuFe Ni2 high-entropy alloys for as-cast (b), (c) and (d), and cold-rolled (a) and (e) statuses, respectively. (Forinterpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig 4. The tensile fractographs of the Al0.5CrCuFeNi2 high-entropy alloys: (a) as cast;(b) cold rolling. (For interpretation of the references to color in this figure, the reader is re-ferred to the web version of this article.)

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exhibits a 0.2% yielding strength of ~402 MPa and tensile elongation of~17.5%, while the cold-rolled specimen demonstrates a large yieldingstrength of ~1132 MPa but a very limited tensile elongation of ~1.6%.In other words, after cold rolling, the yielding strength rapidly increasesby 182%while the tensile elongation sharply decreases by 91%. This fea-ture can be associated with the dislocation behaviors for both cases.Fig. 3(c) and (d) presents the bright-field TEM patterns of the as-castsample. Specifically, as shown in Fig. 3(c), individual dislocation lines,emphasized in red arrows, are clearly observed on the alloy surface, sug-gesting that imperfect lattice configurations still prevail in the composi-tionally complex alloys. Fig. 3(d) reveals that stacking faults, as a planardefect (labeled in blue arrows), are formed in the close-packed struc-ture, indicating that this HEA may have low stacking fault energy.Fig. 3(e) shows the dislocation pattern of the cold-rolled alloy. Notethat an enhanced dislocation density in the form of dislocation tangleis obtained due to the formation of new dislocations and dislocationmultiplication generated by the effects of strain hardening [22].

Fig. 4(a) and (b) displays the tensile fratographs of theAl0.5CrCuFeNi2HEAs in both as-cast and cold-rolled statuses, respective-ly. As shown in Fig. 4(a), an obvious necking behavior occurs in the as-cast sample, which is in accordance with the good tensile ductility asseen in Fig. 3(b). In particular, two insets in Fig. 4(a), marked by insets1 and 2, further reveal the side and positive views of the fracture surface,respectively. Inset 1, an enlarged view of the rectangle region, suggeststhat multiple slip bands are formed, which can be used to slow downthe formation of critical crack size and accommodate the increasingplastic-strain energy during tensile loading [23]. Inset 2 reveals that aductile fracture mode, characterized by a large amount of dimpleswith awidth of 1–2 μm, is obtained that is possibly ascribed to the com-plex stress state composed of normal and shear stress [24]. Next, as ob-served in Fig. 4(b), a shear fracture feature occurs in the cold-rolledalloy, and the fracture surface has an angle of ~45° with the stress axisas measured in inset 1. A small amount of shear bands marked by thered arrows on the side view, together with a relatively-flat fracture sur-face and insignificant dimples as seen in inset 2 [18], well correspondwith the tensile response as shown in Fig. 3(a).

Fig. 5 demonstrates the optical patterns of the as-annealed samples.Fig. 5(a) shows the 1173 K-annealed alloy structure. Note that the spec-imen still remains the strain hardened microstructure but the crystallo-graphic orientation tends to be diverse, indicating that recovery takes

Fig 5. The optical micrographs of the Al0.5CrCuFeNi2 high-entropy alloys at different annealing conditions.

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place at the temperature in spite of a relatively high T/Tm (Tm is themelting temperature and about 1515 K [25], and thus T/Tm is ~0.7 Tm)and an annealing time of 2 h. As the temperature increases to 1273 K(0.8 Tm), as shown in Fig. 5(b), the strain hardened microstructure be-gins to be consumed and the nuclei of recrystallization grains start todevelop along the boundary between dendrites and interdendrites,which reveals that the next stage of recrystallization has in fact oc-curred. Subsequently, at 1373 K (0.9 Tm), based on the drive pressurefrom the stored energy associated with the dislocations produced dur-ing prior straining, the recrystallization process is completed as seenin Fig. 5(c) and the recrystallized grain size is 65± 5 μm. It is commonlyaccepted that recrystallization takes place at the temperature of about0.5 Tm, and the temperature at which recrystallization can be accom-plished in 1 h is generally referred to as the recrystallization tempera-ture. [26] Hence, it is suggested that the current HEA exhibits a veryhigh recrystallization temperature of about 0.8–0.9 Tm, indicating itshigh thermal stability for potential applications at high temperatures[27], possibly due to the intrinsicmaterial factors such as thedistinct lat-tice strain, unique sluggish diffusion, and cooperativemotions of severalatoms [1,27–28]. In addition, as emphasized by the red arrows, similarto the previously-reported CoCrFeNiAl0.3 and CoCrFeNiMn HEAs [15,29], annealing twins with an average size of 5–10 μm are formed thatare closely correlated with the stacking faults (shown in Fig. 3(d)) andthe low stacking fault energy for the present system. Significant disloca-tion density, as shown in Fig. 3(e), also induces a large volume fractionof subgrains embedded in the grain matrix during annealing, and themean subgrain size is determined to be about 1 μm. With an extendedtime of 20 h at the temperature, another stage of grain growth beginsto develop on the consideration of reducing the boundary energy, asshown in Fig. 5(d), where twins and subgrains are also consumed bymeans of the long-range migration of the boundaries and the finalgrain size is 78 ± 5 μm.

4. Discussion

It is generally thought that for a plastically cold-rolledmetal, the dis-location density ρ can be up to 1012 dislocations per cm2, and the flowstress τ is approximately proportional to the square root of dislocationdensity [23]:

τ ¼ τ0 þαGbffiffiffi

ρp ð1Þ

whereα is a constant of about 0.3–0.6, τ0 is the initial stress necessary tomove a dislocation in the absence of other dislocations, G is the shearmodulus, and b is the Burgers vector. For the current system, the dislo-cation density by cold rolling is significantly enhanced (shown inFig. 3(e)) compared to the as-cast condition. As a result, multiple inter-actions between dislocations ultimately increases the resistance to fur-ther dislocation motion as deformation progresses and leads to thesignificant alloy strengthening as described in Fig. 3(a). In addition, anextended solid solution strengthening cannot be ignored especially forthe HEAs with massive solid solutions. Generally, the solid solutionstrengthening can be simply expressed as [30]:

Δσ ∝ cn ð2Þ

where Δσ represents the strengthening effect, c is the concentration ofthe solute atom, and n is a constant of about 0.5. As in HEAs, each atomcan be considered as a solute atom, and hence it is reasonable to expecta higher c value and thus a greater Δσ effect.

In summary, Fig. 6(a) plots the 0.2% yielding strength, σy, ultimatetensile strength (UTS), σu, and elongation, εP, as a function of alloy sta-tuses. Note that compared with the cold-rolled alloy, the σy of theannealed samples evidently decreases from 1132 to 430 MPa and theσu reduces from 1196 to 585 MPa, while the εP has an improvement

Fig 6. The tensile mechanical parameters as a function of alloy statuses (a) and 2 h-annealed tensile fratographs at the temperatures of 1173–1373 K (b)–(d) of the Al0.5CrCuFeNi2 high-entropy alloys.

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from 1.6 to 6.8%, with an increase in temperature. In particular, the ten-sile strength almost declines in a linear way with the temperature, indi-cating that the temperature factor significantly influences themechanical behavior as well as the phase stability of the present HEAs[18,25]. Another important factor may be ascribed to the stress relaxa-tion and dislocation annihilation induced by the recovery and recrystal-lization process [26]. Moreover, fracture observations of the as-annealed samples at the temperature of 1173–1373 K, as shown inFig. 6(b)–(d), help to understand the tensile response as plotted inFig. 6(a). Fig. 6(b) shows a relatively-flat fracture surface, and only anenlarged view reveals a dimple-like morphology, well correspondingto the poor tensile elongation of the 1173 K-annealed alloy. Anintergranular-dominated fracture feature, in the form of cracks initiat-ing from the boundary [22,31], can be observed in Fig. 6(c), indicatingthat the 1273 K-annealed alloy mainly experiences brittle fracture.The 1373 K-annealed sample, shown in Fig. 6(d), exhibits a mixedtransgranular and intergranular fracture mode, which can be responsi-ble for a limited modification of the tensile elongation when comparedwith the as-rolled alloy. Recently, Sistla et al. [32] found that better me-chanical properties could be attained by optimizing the Al/Ni elementratio and proper heat treatments in the AlxCoCrFeNi2 − x HEAs. In addi-tion, it should be noted that the prevalence of subgrains and twins dur-ing recrystallization (shown in Fig. 5(c)) also probably affects the plasticflows of the annealed alloys, such as yieldingmore dislocation accumu-lations at the boundaries of twins and subgrains, as observed during re-covery of cold-rolled aluminum [33]. Fang et al. [34] also reported thatthe formation of nanoscale deformation twins by mechanical alloyingand spark plasma sintering evidently improved the compressivestrength and Vickers hardness of the Al0.5CrFeNiCo0.3C0.2 alloy. Wuet al. very recently found that the promptness of deformation twinningplays an important role in the promising strength and ductility of thecarbon-containing CoCrFeNiMn and another Co-free FeNiMnCr18 alloysespecially at cryogenic temperature [35–36].

5. Conclusions

In conclusion, microstructural features and tensile behaviors of theAl0.5CrCuFeNi2 HEAs with three different statuses are summarized asfollows:

(1) The as-cast sample exhibits simple solid-solution structure,equiaxed dendritesmorphology, and imperfect lattice configura-tions (dislocations and stacking faults) possibly due to the limit-ed time scale of rapid solidification and the sluggish diffusion

effect of HEAs.(2) Significant dendrites orientation and enhanced dislocation den-

sity strongly contributes to the rapid increment of the tensilestrength but also causes a sharp degradation of the tensile elon-gation in the cold-rolled alloy.

(3) The current HEA exhibits a high recrystallization temperature ofabout 0.8–0.9 Tm and insignificant grain growth, indicating itspotential applications at high temperatures. Twins as well as alarge number of subgrains are formed in the annealed alloys, sug-gestingmultiple deformationmechanisms on the tensile proper-ties. The intergranular-dominated fracture mode, possiblytogether with the prevalence of subgrains in the annealed sam-ples, induces a lower tensile plasticity than the as-cast alloy.

Acknowledgments

S.G. Ma would like to acknowledge the financial support of the Na-tional Natural Science Foundation of China (no. 51501123), the YouthNatural Science Foundation of Shanxi (no. 2015021006), and Scientificand Technological Innovation Programs ofHigher Education Institutionsin Shanxi (no. 2015127). J.W. Qiaowould like to acknowledge the finan-cial support of the National Natural Science Foundation of China (no.51371122), the Program for the Innovative Talents of Higher LearningInstitutions of Shanxi (2013), and the Youth Natural Science Foundationof Shanxi (no. 2015021005). Z.H. Wang would like to acknowledge thefinancial support of the National Natural Science Foundation of China(no. 11390362), the Top Young Academic Leaders of Shanxi (2013),and the Outstanding Innovative Teams of Higher Learning Institutionsof Shanxi (2011). H.J. Yangwould like to acknowledge the financial sup-port of the National Natural Science Foundation of China (no.51401141).

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