MicrostructuralevaluationandnanohardnessofanAlCoCu

International Journal of Minerals, Metallurgy and Materials Volume 26, Number 5, May 2019, Page 634https:///10.1007/s12613-019-1771-3Corresponding author: C.D. Gómez-Esparza E-mail: cynthia.gomez@.mx© University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2019Microstructural evaluation and nanohardness of an AlCoCuCrFeNiTihigh-entropy alloyC.D. Gómez-Esparza1), R. Peréz-Bustamante2), J.M. Alvarado-Orozco3), J. Muñoz-Saldaña4),R. Martínez-Sánchez1), J.M. Olivares-Ramírez5), and A. Duarte-Moller1,6)1) Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes No. 120, Chihuahua 31136, Chihuahua,México2) CONACYT-Corporación Mexicana de Investigación en Materiales S.A. de C.V. (COMIMSA), Ciencia Y Tecnología 790, Fracc. Saltillo 400, Saltillo 25290, Coahuila,México3) Centro de Ingeniería y Desarrollo Industrial, Querétaro Campus, Querétaro 76130, Qro., México4) Centro de Investigación y de Estudios Avanzados del IPN, Unidad Querétaro, Libramiento Norponiente No. 2000, Fracc. Real de Juriquilla, Querétaro, Qro., 76230,México5) Universidad Tecnológica de San Juan del Río, San Juan del Río 76800, Querétaro, México6) Escuela de Ingeniería Civil, Industrial y Mecánica, Universidad De La Salle Bajío, León 37150, Guanajuato., México(Received: 6 July 2018; revised: 1 December 2018; accepted: 2 December 2018)Abstract: An AlCoCuCrFeNiTi high-entropy alloy (HEA) was prepared by mechanical alloying and sintering to study the effect of Ti addi-tion to the widely studied AlCoCuCrFeNi system. The structural and microstructural characteristics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The formation of four micrometric phases was detected: a Cu-rich phase with a face-centered cubic (fcc) structure, a body-centered cubic (bcc) solid solution with Cu-rich plate-like preci-pitates (fcc), an ordered bcc phase, and a tetragonal structure. The XRD patterns corroborate the presence of a mixture of bcc-, fcc-, and te-tragonal-structured phases. The Vickers hardness of the alloy under study was more than twice that of the AlCoCuCrFeNi alloy. Nanoinden-tation tests were performed to evaluate the mechanical response of the individual phases to elucidate the relationship between chemical composition, crystal structure, and mechanical performance of the multiphase microstructure of the AlCoCuCrFeNiTi HEA.Keywords: high-entropy alloys; mechanical alloying; microstructure; nanoindentation1. IntroductionConventional alloys consist of one or two elements as ma-jor constituents with small amounts of other elements, which promotes the improvement of properties. High-entropy alloys (HEAs), defined by Yeh et al. [1] as those formed by at least five main elements, may exhibit characteristics superior to those of conventional alloys, such as solid solution forma-tion, nanometer microstructures, good thermal stability, high hardness, and good mechanical properties [2]. The solid so-lutions containing more than five major elements tend to have structures such as face-centered cubic (fcc), body-centered cubic (bcc), and bcc + fcc. They are more stable than the many conventional intermetallic structures that can be formed according to the predictions of binary phase dia-grams reported in the specialized literature. The structure and phases formed in the HEAs depend on the number of alloying elements and their nature (atomic radii, crystalline structure, electronegativity, melting point), as well as their synthesis process. HEAs have been mainly processed by liquid routes; however, interest in the use of powder metal-lurgy to synthesize these advanced materials has increased. Mechanical alloying has been used to produce nanocrystal-line solid-solution phases under the concept of HEAs [3–4].The AlCoCrCuFeNi system has been the most studied HEA in terms of microstructure and mechanical properties.C.D. Gómez-Esparza et al., Microstructural evaluation and nanohardness of an AlCoCuCrFeNiTi high-entropy alloy 635In principle, most of the investigated systems contain Cu, which improves plasticity. Ni, Co, and Fe are of interest in engineering applications because of their excellent mechan-ical performance, high-temperature mechanical resistance, and resistance to corrosion. They are constituents of most of the reported HEAs. Although Al is a low-melting and very ductile element, its effect has been extensively studied in HEAs. It tends to enhance the mechanical properties of HEAs because it is a bcc phase former. Cr is a high-melting-point element that improves mechanical strength and corrosion re-sistance. Ti is a high-melting element with an atomic radius similar to that of Al. Some authors have attributed the en-hancement of mechanical strength and ductility of HEAs to Ti content [5–6]. Although the effect of Ti in AlCoCrCuFeNi alloy has been investigated, the relationship between alloying elements and the crystalline structure of the formed phases in an equiatomic AlCoCrCuFeNiTi alloy is not yet clear.On the other hand, knowledge of local mechanical prop-erties in multiphase materials in micrometric scale, such as HEAs, can represent a great advance in understanding and predicting their bulk properties. Low-load indentation, or nanoindentation, using the Oliver–Pharr method is a very versatile technique that enables mechanical properties to be measured at small scale (micro and submicrometric) [7]. In past years, investigations related to nanoindentation tests of HEAs have been conducted to obtain more information about specific properties of individual phases [8–9]. Be-cause of the nature of alloying elements, equiatomic Al-CoCrCuFeNiTi alloy is expected to possess a multiphase microstructure. Hence, the aim of this work is to study an equiatomic AlCoCrCuFeNiTi HEA produced by mechanical alloying and conventional sintering to understand the effect of Ti on the chemical distribution and crystalline structure of the resulting phases. In addition, the evaluation of hardness and reduced elastic modulus of its individual phases through nanoindentation tests could establish a relationship between chemical composition, crystalline structure, and mechanical properties of AlCoCrCuFeNiTi-type HEAs.2. Experimental2.1. MaterialsElemental powders of Al, Co, Cr, Cu, Fe, Ni, and Ti with a purity level greater than 99% were used as raw materials. These powders were mixed in equiatomic composition to form a high-entropy AlCoCrCuFeNiTi alloy. The milling process was performed in a high-energy mill (SPEX-8000M) under an argon atmosphere to avoid excessive oxidation of the powders. A hardened steel vial and hardened steel balls were used to mill the samples and were pre-coated by mil-ling an equiatomic NiCoAl powder mixture for 10 h under an Ar atmosphere to avoid iron contamination from the vial and grinding media. Methanol was used as a process control agent to balance welding and fracturing processes during milling. The milling time was 10 h, and the ball-to-powder weight ratio was 5:1. After milling, the alloyed powders were cold compacted at a pressure of 1.5 GPa, sintered at 1200°C under vacuum for 3 h, and cooled in the furnace to room temperature.2.2. Microstructural characterizationThe sintered alloy was microstructurally characterized by field-emission scanning electron microscopy (FESEM; JEOL JSM-7401). The microscope was equipped with an X-ray scattering spectrometer. In order to characterize the phase composition, a sintered sample was prepared using a focused ion beam system (JEM-9320FIB) equipped with an Omniprobe 200 nanomanipulator system and then analyzed by transmission electron microscopy (TEM; JEOL JEM2200F). X-ray diffraction (XRD) analysis was performed on a Pana-lytical X'Pert PRO diffractometer equipped with a Cu Kαradiation source (λ = 0.15406 nm).2.3. Mechanical testingThe mechanical behavior of the alloy was evaluated by Vickers microhardness tests on a Future-Tech Corp. tester model FM-7 using a load of 300 g and a dwell time of 15 s. The nanoindentation tests were carried out on a Ubi1 nano-mechanical tester, Hysitron, USA, using a Berkovich trian-gular pyramid diamond tip.3. Results and discussion3.1. Microstructural characterization3.1.1. Transmission electron microscopyBecause of the complex chemical interaction among the different HEA elements during sintering, TEM analyses were carried out on sintered samples to study the micro-structural details and compositional analysis. The equili-brium microstructure of the AlCoCrCuFeNiTi alloy synthe-sized by mechanical alloying and sintering and the respec-tive chemical composition of their phases, as determined by energy-dispersive X-ray spectroscopy, are shown in Fig. 1. Four micrometric homogeneously distributed phases were observed (labeled as A, B, C, and D). The dark phase de-picted as A corresponds to the high-Fe matrix with low con-tents of Al and Ti and with rich-Cu plate-like nanoprecipi-tates well dispersed in it. The formation of nanoprecipitates636 Int. J. Miner. Metall. Mater ., Vol. 26, No. 5, May 2019in HEAs due to high mixing entropy has been reported [10]. In addition, the Cu segregation is related to the difference in electronegativities and enthalpies of formation with the rest of the elements present in the alloy under study. According to the binary phase diagrams, Cu has a very limited solid solubility with Fe, Co, and Cr, which explains the precipita-tion of Cu in a matrix with large Cr and Fe contents.In the zone denoted as B, a high content of Ni, Co, and Ti, an intermediate Al content, and low concentrations of Cu and Cr were detected. The phase C shows a high content of Cr, Fe, and Co and a very low concentration of Al and Ni. Also, the zone marked as D shows a Cu-rich crystallized phase. Cu tends to segregate at grain boundaries in HEAs synthesized via liquid routes [11]. Cu has a high positive mixing enthalpy with Co, Cr, and Fe, whereas it has a lowerpositive enthalpy with Ni and Al [12].Fig. 1. Z-contrast image by scanning transmission electron microscopy (STEM) of AlCoCrCuFeNiTi alloy, and chemical composi-tion of individual phases by EDS.To show the effect of Ti in the AlCoCrCuFeNiTi HEA, TEM micrographs of the microstructures of AlCoCuFeNi and AlCoCrCuFeNi alloys, previously reported and synthe-sized under the same conditions [13], are displayed together with a micrograph of the AlCoCrCuFeNiTi HEA alloy in Fig. 2 for comparative purposes. The AlCoCuFeNi alloy (Fig. 2(a)) has two main phases. The first phase is composed of a high-Fe-content matrix and Cu-rich nanometric preci-pitates in a plate-shape arrangement disposed in perpendi-cular and parallel angles, forming a basket-like microstruc-ture. The second phase has a high content of Fe, Co, and Ni, approximately 28at% each. The microstructure of the Al-CoCrCuFeNi alloy (Fig. 2(b)) is very similar to the AlCo-CuFeNi alloy. The effect of Cr addition is mainly evidenced by the formation of a third phase with high Cr content (˃80at%). In comparison with the previous alloys, the Al-CoCrCuFeNiTi alloy (Fig. 2(c)) has a similar phase with a high-Fe-content matrix with immersed Cu-rich plate-like precipitates (phase A). The spacing and size of the plate-like precipitates increase as more alloying elements are added (Cr and Ti). The phase identified as B has a high content of Ni, Co, Ti, and Al; in comparison with the previous alloys, the Fe content decreases, whereas substantial amounts of Al and Ti are detected. Phase C also has a high Cr content with a substantial increase in Fe and Co amounts. Phase D is a Cu-rich solid solution that was not observed in the previousFig. 2. TEM micrographs of the sintered alloys to compare the microstructural evolution as a function of chemical composition: (a) AlCoCuFeNi; (b) AlCoCrCuFeNi; (c) AlCoCrCuFeNiTi.C.D. Gómez-Esparza et al., Microstructural evaluation and nanohardness of an AlCoCuCrFeNiTi high-entropy alloy637alloys. The enthalpy of mixing values for the pairs Ti–Ni and Ti–Al are highly negative [14], which promotes the sol-id solubility of Ti in enriched Al and/or Ni phases. Hence, the results suggest that the complex microstructure observed is mainly due to the limited solid solubility of Cu with the other alloying elements.Figs. 3 and 4 show TEM images and selected-area elec-tron diffraction (SAED) patterns of the crystal-structure phases in the AlCoCrCuFeNiTi alloy. On the basis of the collected SAED pattern of phase A (Fig. 3), the matrix ([–111] zone axis) has a bcc structure, whereas the plate-like precipitates ([011] zone axis) have a fcc structure. A Z-contrast STEM micrograph (Fig. 4(a)) correlates the B, C, and D phas-es (Figs. 4(b)–4(d)) with their respective dot patterns.Fig. 3. TEM micrograph of phase A in the AlCoCrCuFeNiTi alloy (a), and SAED patterns of the matrix (b) and plate-like precipi-tates (c).Fig. 4. TEM micrograph (a) and SAED patterns of phase B (b), phase C (c) and phase D (d) in the AlCoCrCuFeNiTi alloy.Phases B and D were observed along the [200] and [321] zone axes, having a bcc and an fcc crystal structure, respec-tively. Phase C, which was observed along the [321] zone axis, is consistent with a tetragonal structure. Notably, both fcc phases, which correspond to plate-like precipitates of phase A and phase D, have a similar chemical composition; however, a Cu content greater than 70at% was detected. Phase B has a high Al content (an fcc-structured element) and crystallizes with a bcc structure, corroborating the as-sumption that Al is a bcc former [15].The precipitation of nanocrystalline phases in the HEA synthesized via a liquid route and subsequent thermal treat-ment is considered an important factor affecting the me-chanical properties of the alloys [16]. In this study, the alloy638 Int. J. Miner. Metall. Mater., Vol. 26, No. 5, May 2019AlCoCrCuFeNiTi processed by mechanical alloying gave rise to the formation of nanocrystalline phases without the need for a subsequent thermal treatment. These phases re-mained on the nanometer scale even after the conventional high-temperature sintering process.3.1.2. X-ray diffractionThe diffraction pattern of the sintered AlCoCrCuFeNiTi alloy is presented in Fig. 5. Two fcc-structured phases, two bcc-structured phases, one of which is ordered, and a fifth phase that does not correspond to a cubic structure are present. The lattice parameters of the phases were calculated from their peak positions. The lattice parameter of the two fcc phases (0.362 and 0.360 nm) is similar to that of pure Cu. These phases correspond to phase D and the plate-like precipitates of phase A. The ordered bcc phase (B2) [17] has a lattice parameter of 0.292 nm, similar to that reported for an Al11.1(TiVCrMnFeCoNiCu)88.9 alloy (a bcc = 0.293 nm) [10]. In a previous AlCoCrCuFeNi alloy, an ordered bcc solid solution was not detected [13]. Howev-er, Chen et al. [18] reported that ordering of a bcc phase in an Al0.5CoCrCuFeNiTi1.4 alloy occurs as a function of Ti content. Their finding corroborates that phase B, which has a bcc structure and contains a substantial amount of Ti (21at%), possesses a bcc-B2 phase. The solid solutions are formed by solute atoms of different elements. In HEAs, each atom is surrounded by different types of atoms with different atomic sizes depending on the chemical composi-tion of the alloy. Therefore, the crystalline structure suffers distortion, causing its lattice parameter to differ from that previously reported for a similar solid solution.Fig. 5. XRD pattern of AlCoCrCuFeNiTi alloy.The lattice parameter of the second bcc phase is 0.304 nm, which corresponds to the matrix of phase A. The fifth phase presents diffraction peaks at 46°, 46.98°, and 48.18° 2θ. Some authors have reported this phase as γ-NiCoCr [15] or a CoCr tetragonal phase [15,19], and the peak at 36.2° 2θin Fig. 1 has been attributed to a Ti2Ni-type phase [19]. The XRD peak locations were verified with the Inorganic Crys-tal Structure Database (ICSD), and the diffraction peaks were found to correspond to a structure similar to σ-CoCr (ICSD card No. 102316), differing slightly from the peaks of the isolated CoCr system [20]. The diffraction peaks at 34.09°, 36.73°, 40.42°, 56.69°, 73.00°, and 78.16° 2θ (not identified in the spectrum) correspond to aluminum oxide. HEAs have a tendency to form random solid solutions in-stead of intermetallic or complex phases, but some com-pounds with a high enthalpy of formation, such as oxides, carbides, and nitrides, will be formed [21]. The formation of aluminum oxide in sintered HEAs was reported in our pre-vious work [22]. The XRD results related to the formation of solid-solution phases corroborate the results obtained from SAED–TEM patterns.3.2. Mechanical testing3.2.1. Vickers microhardnessThe microstructure of HEA processed by a liquid route exhibits the peculiarity of segregation, which is a characte-ristic of this processing technique [23]. Although the alloy under study has a multiphase microstructure, the use of me-chanical alloying produces a homogeneous distribution of phases in the bulk material, which may exceed the distribu-tion achieved by the liquid route. The sintered AlCoCrCu-FeNiTi alloy was subjected to Vickers microhardness tests, and a value of HV 602 ± 44 was obtained. The microhard-ness values reported for the previous alloys AlCoCuFeNi and AlCoCrCuFeNi synthesized under the same conditions were HV 261 ± 31 and HV 269 ± 39, respectively [13]. Notably, the addition of Ti promoted an increase of hardness to more than twice that of its predecessors. A similar beha-vior was reported by Chen et al. [18], where the Al0.5CoCrCuFeNiTi x system exhibited an increase in me-chanical response as a function of the Ti content. Specifical-ly, the Al0.5CoCrCuFeNiTi x alloy exhibited a hardness value of HV 650.The atomic radii of the constituent elements of the stu-died alloy differ from each other. Al and Ti have the largest atomic radii of 0.143 and 0.146 nm, respectively. The ob-tained microhardness values can be attributed to the effect of solid-solution hardening, which increases the lattice distor-tion and avoids interplanar displacement and dislocations. Al has been reported to promote the formation of a bcc phase that is responsible for the enhancement of the me-chanical properties of AlCoCuCrFeNi-type alloys [24]. In the studied AlCoCrCuFeNiTi alloy, the formation of a bcc phase with high Al content is evident (phase B). However, phase precipitation of Cu at the nanoscale (plate-like preci-pitates in phase A) favors hardening of the alloy. These har-C.D. Gómez-Esparza et al., Microstructural evaluation and nanohardness of an AlCoCuCrFeNiTi high-entropy alloy 639dening mechanisms also contribute to the observed increase in hardness.The multiphase microstructure in advanced metallic ma-terials is responsible for their superior mechanical properties. The evaluation of local mechanical properties in multiphase HEAs can support future experimental chemical designs, giving deeper evidence of the relationship between chemical composition, crystal structure, and mechanical properties. Hence, nanoindentation tests were used to determine the lo-cal mechanical behavior of the sintered AlCoCrCuFeNiTi alloy.3.2.2. NanoindentationThe mechanical properties of individual phases in the AlCoCrCuFeNiTi alloy were evaluated by the nanoindenta-tion technique. Notably, the evaluated mechanical properties of the individual phases of the alloy were supported by mi-croscopy techniques. Nanoindentation impression images taken by in situ scanning probe microscopy (SPM) were compared with SEM micrographs collected in secondary electron (SE) mode (where the phases had already been pre-viously identified). An example is displayed in Fig. 6. The SEM–SE image (Fig. 6(a)) shows the morphology of the phases, whereas the SPM image (Fig. 6(b)) shows the na-noindentation impressions. At least 10 nanoindentation mea-surements were conducted for each phase. For practical pur-poses, the graph in Fig. 6(c) shows a single load–displacement curve for each tested phase. The chosen curves are repre-sentative of the individual phase response. The average val-ues of nanohardness and reduced elastic modulus are sum-marized in Table 1. The tetragonal phase exhibited the high-est nanohardness value (~19 GP), followed by the bcc-B2 phase (~17 GP). However, the reduced elastic modulus (E r ) value for the tetragonal phase was only 9% higher than that of the bcc-B2 phase. The tetragonal phase influences the hardening of the bulk alloy without sacrificing the ductility of the material. Even phase A (bcc matrix with fcc plate-like precipitates) reached only half the hardness of the bcc-B2 phase; its E r value was only 5% lower. Phase D (high Cu content and fcc structure) exhibited the worst mechanical properties among the phases. We assumed that the basket-like arrangement of Cu precipitates has a hardening effect on phase A, as evidenced by the slight reduction of modulus in comparison with those of the bcc-B2 and tetra-gonal phases.Fig. 6. SEM-SE micrograph (a), SPM micrograph of nanoindentation impressions (b) and representative load-displacement curves (c) for the four identified phases in the AlCoCrCuFeNiTi alloy.Table 1. Average and standard deviation (SD) of nanohardness and reduced modulus obtained from nanoindentation tests (1000 mN of load).Phase Crystal structure Nanohardness / GPa E r / GPaAverage SD Average SDAbcc matrix + fcc2 plate-like precipitates8.720.7433718B bcc-B2 17.25 0.75 356 20C Tetragonal 19.24 0.71 389 16D fcc1 6.48 0.65 242 25The experimental evidence suggests that, in addition to the entropy effect, other factors also influence the formation of phases in the HEA. In terms of relevance, a well-known principle in metallurgy describes the formation of phases: the Hume–Rothery rules on the formation of binary solid solutions. These rules establish that the high solubility of one element in another depends directly on a small differ-ence in their electronegativity, atomic radius, crystal struc-640 Int. J. Miner. Metall. Mater., Vol. 26, No. 5, May 2019ture, valence [25], and their melting temperature [26]. Even though there are different solubility levels of some elements in others, the formation of solid solutions instead of inter-metallic compounds is evident. One of the hardening me-chanisms of HEAs is solid-solution hardening [27]; atoms with larger sizes, such as Ti and Al, induce lattice distortion. In addition, high-melting-point elements exhibit less diffu-sion; the high-melting points are related to the increase in the modulus of elasticity. Tong et al. [28] reported a nano-sized spinodal microstructure in an AlCoCrCuFeNi alloy produced by a liquid route. They attributed the increase in hardness to the hardening effect of nanocomposite forma-tion. The Cu-rich plate-like nanoprecipitates are expected to enhance the mechanical properties of HEAs; therefore, some authors have adjudged this phenomenon as the cause of the hardening. However, in the present study, the Cu-rich phases were demonstrated to be those with the poorest me-chanical properties, even when they exist as precipitates with nanoscale size and spacing. However, previously re-ported nanoindentation results for AlCoCuFeNi and Al-CoCrCuFeNi alloys [13] show that Ti addition increases the matrix hardness of phases with Cu-rich nanoprecipitates. The phases with Cu-rich plate-like precipitates in the Al-CoCuFeNi and AlCoCrCuFeNi alloys exhibit a lower hard-ness and lower E r value compared with those reported for the similar phase in the studied AlCoCrCuFeNiTi alloy, which has a matrix Ti content of ~2at%. The solid-solution hardening effect is greater than the precipitation hardening effect. In addition, the phase sizes in the AlCoCrCuFeNiTi alloy are smaller than those observed in the AlCoCuFeNi and AlCoCrCuFeNi alloys (Fig. 2). In materials with mi-crometric-scale phase sizes, the volumetric fraction of grain boundaries increases with decreasing phase size. Grain boundaries act as obstacles to the movement of dislocations.4. ConclusionsAn AlCoCrCuFeNiTi HEA was synthesized by mechan-ical alloying and conventional sintering. The sintered alloy exhibits a multiphase microstructure composed of four phases with fcc, bcc, and tetragonal structures. According to TEM analyses, Cu promotes the formation of fcc phases, whereas Al and Ti are bcc formers. The tetragonal phase possesses high Cr, Fe, and Co contents. The elastic modulus and hardness for individual phases were successfully eva-luated by nanoindentation tests. The tetragonal phase exhi-bited the best mechanical properties, with average hardness and modulus values of 19.24 and 389 GPa, respectively, followed by the bcc-B2 phase, with average hardness and modulus values of 17.25 and 356 GPa, respectively. Ac-cording to the modulus values, the tetragonal phase achieved the highest hardening without substantially in-fluencing the ductility of the material. The Cu-rich phase with an fcc structure exhibited the worst mechanical proper-ties among the investigated phases. AcknowledgementsThe authors would like to thank W. Antunez-Flores and C.E. Ornelas-Gutiérrez for their technical assistance. References[1] J.W. Yeh, S.K. Chen, J.Y. Gan, S.J. Lin, T.S. Chin, T.T.Shun, C.H. Tsau, and S.Y. Chang, Formation of simple crys-tal structures in Cu–Co–Ni–Cr–Al–Fe–Ti–V alloys with mul-tiprincipal metallic elements, Metall. Mater. Trans. A,35(2004), p. 2533.[2] S.T. Chen, W.Y. Tang, Y.F. Kuo, S.Y. Chen, C.H. Tsau, T.T.Shun, and J.W. Yeh, Microstructure and properties of age-hardenable Al x CrFe1.5MnNi0.5 alloys, Mater. Sci. Eng. A,527(2010), No. 21-22, p. 5818.[3] R. Sriharitha, B.S. Murty, and R.S. Kottada, Phase formationin mechanically alloyed Al x CoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys, Intermetallics,32(2013), p. 119. [4] N.T.B.N. Koundinya, C.S. Babu, K. Sivaprasad, P. Susila,N.K. Babu, and J. Baburao, Phase evolution and thermal analysis of nanocrystalline AlCrCuFeNiZn high entropy alloyproduced by mechanical alloying, J. Mater. Eng. Perform.,22(2013), No. 10, p. 3077.[5] Y.F. Wang, S.G. Ma, X.H. Chen, J.Y. Shi, Y. Zhang, andJ.W. Qiao, Optimizing mechanical properties of AlCoCrFe-NiTi x high-entropy alloys by tailoring microstructures, ActaMetall. Sin.-Engl., 26(2013), No. 3, p. 277.[6] P. Cui, Y.M. Ma, L.J. Zhang, M.D. Zhang, J.T. Fan, W.Q.Dong, P.F. Yu, G. Li, and R.P. Liu, Effect of Ti on micro-structures and mechanical properties of high entropy alloys based on CoFeMnNi system, Mater. Sci. Eng. A,737(2018),No. 8, p. 198.[7] W.Y. Yan, C.L. Pun, and G.P. Simon, Conditions of applyingOliver–Pharr method to the nanoindentation of particles in composites, Compos. Sci. Technol., 72(2012), No. 10, p.1147.[8] C.S. Babu, N.T.B.N. Koundinya, K. Sivaprasad, and J.A.Szpunar, Thermal analysis and nanoindentaion studies on nanocrystalline AlCrNiFeZn high entropy alloy, ProcediaMater. Sci.,6(2014), p. 641.[9] V. Maier-Kiener, B. Schuh, E.P. George, H. Clemens, and A.Hohenwarter, Nanoindentation testing as a powerful screen-ing tool for assessing phase stability of nanocrystalline high-entropy alloys, Mater. Des.,115(2017), p. 479.[10] C.M. Lin and H.L. Tsai, Evolution of microstructure, hard-。