Kuznetsov-2013-MSF-v735-p146(Superplasticity-AlCoCuFeNI-HEA)

Superplasticity of AlCoCrCuFeNi High Entropy AlloyA. V. Kuznetsov1, a, D.G. Shaysultanov1, b, N.D. Stepanov1, c,G.A. Salishchev 1, d, O.N. Senkov2, e1 Laboratory of Bulk Nanostructured Materials, Belgorod State University, 85 Pobeda St., Belgorod308015, Russia2 UES, Inc., 4401 Dayton-Xenia Rd., Dayton, OH 45432, USAa kuznetsov@.ru,b shaysultanov@.ru,c stepanov@.ru,d salishchev@.ru,e oleg.senkov@Keywords: High entropy alloy, multi-directional forging, microstructure, superplasticity. Abstract.An AlCoCrCuFeNi high entropy alloy was hot worked by multi-directional isothermal forging at 950°C to produce an equiaxed fine-grain structure with the average grain/particle size of ~1.5 µm. The forged alloy exhibited superplastic behavior in the temperature range of 800-1000°C. For example, during deformation at a strain rate of 10-3s-1, tensile ductility increased from 400% to 860% when the temperature increased from 800°C to 1000°C. An increase in strain rate from 10-4 to 10-2 s-1 at T = 1000°C did not affect tensile ductility and elongation to failure was about 800%. The strain rate sensitivity of the flow stress was rather high, m = 0.6, which is typical to the superplastic behavior. The equiaxed morphology of grains and particles retained after the superplastic deformation, although some grain/particle growth was observed.Introduction.A new class of promising structural metallic materials, so called high entropy alloys (HEAs), has been developed during last decade [1-9]. These alloys typically contain from 5 to 13 principal elements in approximately equimolar concentrations [1]. A high entropy of formation of disordered solid solutions slows down formation of intermetallic phases in these alloys, and the alloys are thought to mainly consist of disordered solid solutions [1,4,5].Some of the properties expected from this class of alloys, such as high hardness, high strength, high wear and creep resistance, high temperature stability, and low thermal conductivity make them attractive for use in different applications [1,6,7]. Unfortunately, low ductility in the as-cast condition limits formability of many HEAs. One of the methods to improve formability is grain refinement [10]. Conventional grain refinement methods, which include cold rolling with subsequent recrystallization annealing, cannot be applied to HEAs with low ductility. One of the advanced methods of refining cast microstructure and improving properties is multi-directional forging [11,12]. Recently this method was successfully applied to refine the grain structure of a cast AlCoCrCuFeNi alloy [11].In this work, microstructure and elevated temperature tensile properties of the hot forged high entropy AlCoCrCuFeNi alloy are reported. The effects of temperature and strain rate on tensile ductility, flow stress and strain rate sensitivity were studied. The AlCoCrCuFeNi alloy was chosen as a material of investigation because it has the most extensive database among all other reported HEAs [1, 13-16].Materials and methods.An ingot of the AlCoCrCuFeNi alloy, with the composition shown in Table 1, was produced by induction melting of pure elements. The ingot was then electro-slag re-melted and cast into a water-cooled copper mold. Melting and casting was performed under an inert atmosphere of high-purity argon.Table 1. Chemical composition of the studied alloy.Al Cr Cu Ni Fe Co At. % 16.16±0.63 15.86±0.05 17.42±0.01 16.65±0.23 15.96±0.15 17.07±0.19 Wt. % 8.20±0.40 15.65±0.05 20.95±0.05 18.55±0.25 17.80±0.10 18.10±0.20The cast ingot was trimmed by cutting out about 10 mm thick pieces from the top and bottom of the ingot, and a blanket of 40 mm in diameter and 35 mm in length was machined from the remaining part of the ingot. The blank was homogenized at 960°C for 50 hours and hot forged in three orthogonal directions (so called “a-b-c forging” [9,10]) at 950°C. Forging was performed using a DEVR 4000 hydraulic press (the maximum force of 0.4 MN) equipped with isothermal stamping tool. The ram speed was 1mm/s and the total true strain achieved during forging was ~ 1000%. Uniaxial tensile tests were conducted using an Instron 5882 testing machine equipped with a furnace for heating up to 1200°C. Tensile specimens were heated to a testing temperature with a heating rate of ~15°/min, equilibrated at the temperature for ~15 minutes and tension tested with a crosshead speed of 0.016 mm/s, which corresponded to the initial strain rate of 10-3 s-1. The gauge length of the specimens was 16 mm and the gauge cross-section was 3×1.5 mm2. Several specimens were tested at different strain rates in the range of 10-5- 10-1 s-1. The microstructures of polished cross-sections of the deformed and non-deformed specimens were studied using scanning electron microscopes (SEM) Quanta 200 3D and Quanta 600 equipped with backscattered electron detectors,energy- dispersive spectrometers (EDS) and electronbackscattereddiffraction(EBSD)detectors. Standard linear interception method was used to determine size of grains/particles.Results and discussion.Microstructure after multi-directional isothermal deformation.The microstructuresof the alloy in the as-cast condition and after multi-directional isothermal forging at 950°C are shown in Figs. 1a and 1b, respectively. In as-cast condition, the alloy exhibits typical dendritic structure (fig. 1a) with the average dendrite size of about 50 µm. According to the EBSD analysis, the cast microstructure consists of the matrix with the BCC crystal structure (dark areas, the volume fraction is about 53%) and elongated particles with the FCC crystal structure (light areas, the volume fraction is about 47%). The FCC particles located at grain boundaries (i.e. interdendrite areas) are larger and brighter than the grey particles located inside the BCC matrix grains (i.e. inside the dendrites).a bFigure 1 – Microstructure of AlCoCrCuFeNi alloy in as-cast condition (а) and after multi-directional isothermal forging at 950°С (b).After multi-directional isothermal forging, the alloy has a fully recrystallized duplex structure (Fig 1b). Fine nearly equiaxed FCC-type particles are homogeneously distributed and mixed with the refined equiaxed grains of the BCC matrix. The volume fraction of the BCC matrixis ~60%, and the volume fraction of FCC particles is ~40%. The microstructure is characterized by the lognormal size distribution of grains/particles. The average grain/particle size is determined to be 1.5±0.9 µm. The high value of the standard deviation (s=0.9 µm) is due to large variations of the grain/particle sizes (from 0.2 to 6.1 µm) in the microstructure.Mechanical properties after multi-directional isothermal forging.The stress-strain curves obtained during tensile testing are shown in Fig. 2: at different temperatures and a constant strain rate έ = 10-3s-1(Fig. 2a) and at different strain rates and a constant temperature 1000°C (Fig. 2b). The values of the yield strength, σ0.2, ultimate tensileFigure 2 – Tensile stress-strain curves of the hot forged AlCoCrCuFeNi alloy obtained at (a) different testing temperatures (έ=10-3 c-1) and (b) different strain rates (Т=1000°С).Table 2. Tensile properties of the hot forged AlCoCrCuFeNi alloyT, °C 700 800 900 1000έ, s-110-310-310-310-410-310-210-1σ0.2, MPa 63 22 14 1 4 15.5 57σu, MPa 91 26 18 1.3 5 16 67 δ,% 63 604 405 753 864 858 442m - - - 0.59 0.51 0.62 - During tension testing at 700°C, plastic yielding occurs at 63 MPa and then the flow stress continuously increases, reaches a maximum value, σu = 91 MPa, at the strain of 10% and gradually decreases with further deformation due to strain localization and necking. At higher temperatures, from 800°C up to 1000°C, the stress-strain curves appear to be typical for superplastic behavior, with a prolonged steady-state flow stage. An increase in the testing temperature results in a significant increase in elongation and a decrease in flow stress. For example, at 700°C failure occurs at δ=63%, while at 1000°С δ reaches 864%. Both σ0.2 and σu decrease respectively from 63 MPa to 4 MPa and from 91 MPa to 5 MPa when the temperature increases from700°C to 1000°C (Fig. 2a and Table 2). Elongation to failure remains almost constant at strain rates in the range of 10-4-10-2 s-1 during testing at 1000°C. However, the flow stress increases with an increase in the strain rate: σ0.2 increases from 1 to 15.5 MPa, and σu increases from 1.3 to 16 MPa respectively. The plastic flow at these strain rates is characterized by very high strain rate sensitivity values, m = 0.51-0.62. With a further increase in strain rate from 10-2 to 10-1 s-1, elongation decreases to 442%, σ0.2 increases to 57 MPa, and σu increases to 67 MPa (Table 2). It should be noted that even at such a high strain rate the alloy still shows superplastic behavior. No signs of necking were found on tested specimens despite very high elongation values (Fig. 3).Figure 3 – Photographs of a non-deformed sample and tensile samples after deformation at 1000°Cand different strain rates.Evolution of microstructure during tensile testing.After tensile testing of the forged alloy at έ=10-3 s -1 in the temperature range of 700°С-1000°С, a noticeable increase in the grain/particle size occurs, especially at T = 900°C and 1000°C, however, the equiaxed morphology of grains and particles retains (see Fig. 4 and Table 3). For example, the average grain/particle size in the head (non-deformed part) of the samples increases from 1.5 µm (after forging) to 2.5 µm and 2.8 µm after tensile testing at 900°C and 1000°C, respectively. The grain/particle size in the heavily deformed region is slightly smaller (~2.2 µm at 900°C and 2.6 µm at 1000°C) than in the non-deformed region, which is probably an evidence of deformation-induced particle refinement.a bс dFigure 4 – Microstructure of the AlCoCrCuFeNi alloy after tensile testing tofracture at Т=1000°C and different strain rates: (a,b) 10-4 s -1; (c,d) 10-1 s -1; (a, c) non-deformed (sample head) and (b, d)deformed regions.A decrease in the strain rate from 10-1 to 10-4 s-1 at T = 1000°С leads to a more pronouncedgrain/particle growth in the non-deformed region, which seems to indicate static recrystallization.For example, after deformation at έ=10-1 s-1 the average grain/particle size is 2.2 µm, while afterdeformation at έ=10-4 s-1 this value increases to 4.4 µm. (Table 3, Fig. 4). In the deformed regionthe microstructure is refined, apparently due to dynamic recrystallization, and the average size of the structural elements is 1.4 µm and 2.7 µm at ε = 10-1and 10-4s-1, respectively. Intergranular porosity forms during deformation (Table 4). The volume fraction of pores is ~10.5%, 12.5% and7.9% after tension testing with έ=10-3 s-1 at 800ºC, 900°C, and 1000ºC, respectively. Pores almostdo not form during tensile testing at 1000°C with έ=10-4s-1(their volume fraction is ~0.1%).However, an increase in the strain rate to 10-1 s-1 increases the volume fraction of pores to ~12.9%.Table 3. The average grain/particle size and standard deviation (µm) in the multi-directionally forged AlCoCrCuFeNi alloy after tensile testing to fracture at given temperatures and strain rates. T, °C 700 800 900 1000έ, s-110-310-310-310-410-310-210-1 Porosity, (%) 10.5 10.8 12.5 0.1 7.9 11.4 12.9 Sample head 1.6±0.9 1.9±1.0 2.5±1.3 4.4±2.0 2.8±1.3 2.1±1.0 2.2±1.4 Gauge-neck 1.5±0.8 1.6±0.9 2.2±1.1 2.7±1.4 2.6±1.4 1.9±0.9 1.4±0.7 Table 4. Volume fraction of pores in the multi-directionally forged AlCoCrCuFeNi alloy after tensile testing to fracture at given temperatures and strain rates..T, °C 700 800 900 1000έ, s-110-310-310-310-410-310-210-1Porosity, (%) 10.5 10.8 12.5 0.1 7.9 11.4 12.9DiscussionApplication of multi-directional isothermal hot forging promoted high deformability of the AlCoCrCuFeNi high entropy alloy, which had low ductility in as-cast condition. A fine-grained duplex structure consisting of a mixture of BCC and FCC particles was formed after hot forging due to fragmentation of the initial dendritic structure and dynamic recrystallization. After hot forging, the average grain/particle size was ~1.5 µm and the volume fractions of the BCC and FCC phases were 60 and 40% respectively. The hot deformation was found to be an effective way to achieve high superplastic properties at the temperature range of 800-1000°С and the strain rate range of 10-4-10-1 s-1.Multistep isothermal forging significantly increases density of grain and interphase boundaries, which are known to become softer than the grain interiors at elevated temperatures [17].A noticeable increase in the tensile ductility and a decrease in the flow stress at temperatures higher than 700°C are likely related to diffusion-controlled grain boundary sliding (GBS). The GBS is more developed in a finer-grained microstructure. Dynamic recovery is also enhanced in fine-grained materials due to annihilation of dislocations at moving grain boundaries. Moreover, the plastic deformation controlled by GBS is characterized by a high strain rate sensitivity of the flow stress, which impedes necking and promotes superplastic deformation [16]. Prolonged stages of steady-state flow, observed during testing in the temperature range of 800-1000°С, are result of the superplastic behavior. In spite of very high plastic strains, grains and particles remained equiaxed and their growth is limited in the deformed region. It is likely that the extensive GBS accommodated by dynamic recovery and dynamic recrystallization [18] is responsible for equiaxed morphology of grains and particles after superplastic deformation of the HEA. The GBS also causesaccumulation of inter-granular porosity. It is interesting to note a considerable decrease in the volume fraction of pores, from 12.9% to 0.1%, with a decrease in the strain rate from 10-1 s-1 to 10-4 s-1during deformation at 1000ºC. Such behavior indicates that the rates of dislocation- and diffusion-driven recovery processes are not yet sufficient to accommodate the GBS at high strain rates, even at 1000ºC, but these recovery processes become efficient at low strain rates. This result seems to be in good agreement with the known fact that the high entropy alloys have lower diffusivity of the alloying elements relative to conventional alloys [6,7,9,19]. However, a detailed analysis of the mechanism of superplastic deformation of the AlCoCrCuFeNi high entropy alloy requires additional investigations.Conclusions.1. A fine equiaxed duplex structure with an average grain/particle size of ~1.5 µm was formed in the AlCoCrCuFeNi high entropy alloy after hot multi-directional forging.2. The forged alloy exhibited superplastic behavior in the temperature range of 800-1000°C and at έ=10-3s-1. Elongation to failure approached 604% at 800°C, decreased to 405% at 900°C and increased again to 860% at 1000°C. The absence of necking was an indirect indication of high strain rate sensitivity of the flow stress over the mentioned temperature range.3. The superplastic behavior was also observed over a wide strain rate range at 1000ºC. At the strain rates from 10-4 to 10-2 s-1 the elongation to failure was in the range of 753-864%, while the ultimate tensile stress increased from 1.3 MPa (at έ=10-4s-1) to 16 MPa (at έ=10-2s-1). The strain rate sensitivity m was in the range of 0.51-0.62. An increase in the strain rate to 10-1 s-1 led to a decrease in the elongation to 442% and an increase in the tensile stress to 67 MPa. Acknowledgements.This work was supported by Russian Ministry of Science and Education under grant №02.740.11.0510.References[1] J.-W.Yeh, S.-K. Chen, S.-J. Lin, J.-Y.Gan, et al.: Adv. Eng. Mater. Vol. 6 (5) (2004), p. 299[2] J.-W. Yeh: Ann. Chim: Sci. Mater. Vol. 31 (2006), p 633[3] J.-W. Yeh, Y.-L. Chen, S.-J. Lin, S.-K. Chen: Mater. Sci. Forum Vol. 560 (2007), p. 1[4] O.N. 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