Materials and Design (2017),
DOI: 10.1016/j.matdes.2017.07.054
Enhanced strength and ductility of bulk CoCrFeMnNi high entropy alloy having fully recrystallized ultrafine-grained structure S.J. Sun a, b, Y.Z. Tian a * H.R. Lin a, b, X.G. Dong c, Y.H. Wang d, Z.J. Zhang a, Z.F. Zhang a, b * a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
b
School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China c d
School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China
National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, China
Abstract A high efficient magnetic levitation melting technique was reported for fabricating bulk equiatomic CoCrFeMnNi high-entropy alloy (HEA) ingot with a diameter of 110 mm. The bulk ingot can be either forged or rolled. In particular, fully recrystallized ultrafinegrained (UFG) HEA with a minimum grain size of 503±181 nm was successfully obtained through a simple cold rolling and annealing process. The tensile properties of the HEA were studied by changing the grain size from the UFG regime to the coarsegrained regime. The UFG HEA exhibited a good balance of strength and ductility due to the low stacking fault energy. The linear Hall-Petch relationship was well fitted when the grain sizes range from 503 nm to 88.9 μm.
Keywords: High-entropy alloy (HEA); Ultrafine grain; Hall-Petch relationship; Strength; Ductility * Corresponding author.
[email protected] (Y.Z. Tian),
[email protected] (Z.F. Zhang).
1. Introduction HEAs are a new type of equiatomic or near equiatomic multicomponent alloys, which are different from the conventional concept of developing alloys with one main component [1-3]. In the last several years, HEAs have been extensively studied since they are promising candidates for structural applications due to their attractive properties, such as high strength [4], good thermal stability and high temperature strength [5], and exceptional corrosion resistance [6]. 1
Materials and Design (2017),
DOI: 10.1016/j.matdes.2017.07.054
Among various HEAs, the equiatomic CoCrFeMnNi alloy was not only one of the earliest reported HEAs but also one of the extensively studied HEAs [1,7-12]. Cantor et al. [1] firstly reported this HEA, which was a face-centered cubic (FCC) solid solution with dendritic microstructure. Since then this HEA has been employed as a model material to study the fundamental deformation mechanisms. Otto et al. [7] studied the mechanical properties of the HEAs with different grain sizes at different temperatures. It was found that the coarse-grained (CG; grain size d: d > 1 μm) HEAs exhibited low yield strengths at room temperature, but unprecedented comprehensive mechanical properties at cryogenic temperature. Gludovatz et al. [8] investigated the fracture toughness of the CG HEAs at room and cryogenic temperature, and found that the extremely high fracture toughness values exceeded most other materials, especially at cryogenic temperature. This was attributed to the intervention of deformation twinning at cryogenic temperature, which could not be frequently observed at room temperature [13]. Furthermore, some researchers studied the mechanical properties of nanocrystalline (NC; grain size d: d < 100 nm) HEAs prepared by severe plastic deformation (SPD) techniques [12,14]. Schuh et al. [12] reported the NC HEA with exceptional strengths prepared by high-pressure torsion (HPT) and low-temperature annealing, but the ductility was relatively low since nanostructured multiphase microstructures formed during the annealing process. Recent results reveal that alloys with recrystallized ultrafine-grained (UFG; grain size d: 100 nm < d < 1 μm) structure can obtain excellent balance of strength and ductility [15-17]. In addition, the recrystallized UFG alloys can also possess some exceptionally high fatigue properties, which indicates that they could be used as industrial materials [18]. However, very limited study was reported about the HEAs with recrystallized UFG structure [14]. In this study, bulk UFG CoCrFeMnNi HEA will be carefully fabricated, and the mechanical properties will be investigated by considering the grain size effect. 2. Experimental procedures It is well known that HEAs are normally fabricated by the arc melting technique, and the HEAs are melted for 4-5 times in order to ensure chemical homogeneity [7-9]. In the present study, a new energy-saving and high efficient magnetic levitation melting technique was introduced. A mixture of pure metals (purity >99.7 wt.%) of the equiatomic CoCrFeMnNi HEAs were melted and solidified in a magnetic levitation melting furnace with a high-purity nitrogen atmosphere. The ingot has a diameter of 110 mm and height of 90 mm. Note that the ingot was melted for one time and promising elemental homogeneity was achieved. The cylindrical ingot was solution treated at 1100 °C for 2 h and subsequently hot forged at 1000 °C to a rod with a final diameter of 30 mm. The rod was further cold rolled to sheets with final thickness of 1 mm. In order to obtain fully recrystallized specimens with different grain sizes, the cold-rolled sheets were annealed at various temperatures ranging from 650 °C to 1100 °C for 30 minutes to give grain sizes ranging from 503 nm to 35.1 m. In addition, the specimens were cut from the forged rod and annealed at 1100 °C and 1200 °C for 60 minutes to produce coarse grain sizes of 62 m and 88.9 m. Tensile specimens with a gauge length of 10 mm, a width of 4 mm and a thickness 2
Materials and Design (2017),
DOI: 10.1016/j.matdes.2017.07.054
of 1 mm were cut from annealed rods and sheets by electrical discharge machine. Tensile tests were carried out at an initial strain rate of 10-3 s-1 using an Instron 5982 testing machine at room temperature. Microstructural characterizations were conducted by a LEO SUPRA 35 field emission scanning electron microscopy (FE-SEM) equipped with a backscattered electron (BSE) detector and an electron backscattering diffraction (EBSD) system at an accelerating voltage of 20 kV. During the EBSD measurement, different step sizes ranging from 40 nm to 2.5 μm were used to characterize the recrystallized microstructures of the specimens with different grain sizes, respectively. The clean-ups of the EBSD data were performed to diminish the point of zero solutions. The grain size was measured by a linear intercept method, and all the high-angle grain boundaries (HAGBs) including twin boundaries (TBs) were counted. The EDX measurements of the composition were carried out using a JSM 6510 SEM equipped with an Oxford Instruments X-act detector. Transmission electron microscopy (TEM) characterization was conducted using an FEI Tecnai F20 operated at 200 kV. TEM foils were prepared using twin-jet electropolished method by Tenupole-5 in a solution of 70% methanol, 20% glycerine and 10% perchloric acid with a voltage of 20 V at -20 °C. 3. Results and discussion 3.1 Microstructures
Fig.1. (a) The ingot of the CoCrFeMnNi HEA after removing the surface by the lathe machining process; (b) the EDX spectroscopy and (c) SEM image of the as-cast specimen. EDX map of each element was also included by scanning the same area as (c).
It has been reported that HEA ingot can be prepared by magnetic levitation melting technique [19], but the ingot was much larger in this work. The cylindrical ingot of the HEA with the dimension of ~Ф100×75 mm3 after the lathe machining process is shown in Fig. 1a. The composition along with the EDX spectroscopy results of the ingot reveal 3
Materials and Design (2017),
DOI: 10.1016/j.matdes.2017.07.054
that the HEA is approximately equiatomic, as shown in Fig. 1b. Typical microstructure is shown in Fig. 1c, indicating that the HEA is a single-phase solid solution. EDX maps for the five principal elements revealed excellent elemental homogeneity, which was achieved after only one-time melting by the magnetic levitation melting technique.
Fig. 2. EBSD results of the UFG CoCrFeMnNi HEA with mean grain size of 503±181 nm: (a) inverse pole figure map; (b) band contrast and grain boundary map; the distributions of (c) grain size and (d) misorientation angle. The green, silver and red lines in (b) are related to low-angle grain boundaries, high-angle grain boundaries and twin boundaries, respectively.
HEA specimens with fully recrystallized microstructures were fabricated by cold rolling and annealing. Fig. 2a shows the typical inverse pole figure (IPF) map of the UFG HEA and the maximum pole density is 2.4 (not shown here), which indicates that a fully recrystallized microstructure with randomly oriented grains has been obtained. Fig. 2b shows the band contrast (BC) and grain boundary (GB) map of the HEA. The green, silver and red lines are related to low-angle grain boundaries (LAGBs), highangle grain boundaries (HAGBs) and twin boundaries (TBs), respectively. The mean grain size was measured as 503±181 nm by a linear intercept method after countering the HAGBs including the TBs, and it is defined as UFG HEA in this work. Fig. 2c and 2d display the distribution of grain size and misorientation angle. The recrystallized grains distributed in a narrow range, indicating that homogeneous microstructure was obtained. Fig. 2d exhibits a very strong peak at ~60°, which demonstrates that profuse 4
Materials and Design (2017),
DOI: 10.1016/j.matdes.2017.07.054
annealing twins formed during the recrystallization and grain growth. In addition, a rather large fraction (97%) of the HAGBs was found. The above results indicate that the present UFG HEA exhibits homogeneous microstructure, fine grain size, high fraction of HAGBs, which may be critical for superior mechanical properties. 3.2 Mechanical properties Fig. 3a shows the tensile engineering stress-strain curves of the CoCrFeMnNi HEA specimens with different grain sizes. The UFG HEA with a mean grain size of 503 nm shows an unprecedented high yield strength (YS, 0.2% proof stress) of 888 MPa and a high ultimate tensile strength (UTS) of 984 MPa as well as a good uniform elongation (UE) of 21%. With increasing the grain size, the YS decreases but the UE increases, exhibiting the trade-off relationship. Furthermore, the Hall-Petch relationship was fitted between the YS and the inverse square root of grain size, as shown in Fig. 3b. It is found that all the data fit well when the grain size falls in the range of 503 nm < d < 88.9 m, and the Hall-Petch equation can be expressed as σYS=194+490d-1/2. Note that the unit of the strength is megapascal and the unit of the grain size is micrometer. The slope value of 490 MPa·μm1/2 and friction stress value of 194 MPa are similar to the values of 494 MPa·μm1/2 and 125 MPa reported in the previous work [7], but the present study has revealed the Hall-Petch dependence in the alloy in a much wider range of grain sizes from the UFG regime to the CG regime. Very recently, Yoshida et al. [20] reported the Hall-Petch relationship in CoCrNi medium entropy alloy, and the friction stress is 218 MPa and the slope is 265 MPa·μm1/2. The friction stress is comparable with the present CoCrFeMnNi HEA, which can be attributed to the significant lattice distortion in both alloys; meanwhile, the much higher Hall-Petch slope for the CoCrFeMnNi HEA might be related to the dislocation glide mode which was determined by the low stacking fault energy as confirmed in Cu-Al alloys [21].
Fig. 3. (a) Tensile engineering stress-strain curves of the fully recrystallized CoCrFeMnNi HEA with different grain sizes. (b) Plots of yield strength against inverse square root of grain sizes for the CoCrFeMnNi HEA.
It is intriguing that the UFG HEA with a mean grain size of 503 nm can possess a 5
Materials and Design (2017),
DOI: 10.1016/j.matdes.2017.07.054
high UE of 21% along with a high YS of 888 MPa, as shown in Fig. 3a, indicating that the UFG HEA shows excellent strain-hardening capability. It is well known that the UE is governed by the strain-hardening rate (Θ= dσT/dεT, where σT and εT are true stress and true strain, respectively). According to Considére criterion, when Θ
