Accepted Manuscript Effects of Ta Addition on the Microstructures and Mechanical Properties of CoCrFeNi High Entropy Alloy
Hui Jiang, Kaiming Han, Dongxu Qiao, Yiping Lu, Zhiqing Cao, Tingju Li PII:
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Materials Chemistry and Physics
09 March 2017
24 May 2017
29 May 2017
Please cite this article as: Hui Jiang, Kaiming Han, Dongxu Qiao, Yiping Lu, Zhiqing Cao, Tingju Li, Effects of Ta Addition on the Microstructures and Mechanical Properties of CoCrFeNi High Entropy Alloy, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.05.056
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ACCEPTED MANUSCRIPT Effects of Ta Addition on the Microstructures and Mechanical Properties of CoCrFeNi High Entropy Alloy Hui Jiang, Kaiming Han, Dongxu Qiao, Yiping Lu*, Zhiqing Cao, and Tingju Li Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, PR China *Corresponding author tel: +086 411 8470940; email address: [email protected]
Abstract In this study, CoCrFeNiTax (x value in molar ratio, x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.75) high entropy alloys were prepared to investigate the alloying effects of Ta on the microstructures and mechanical properties. With the addition of Ta element, the microstructure changed from an initial single FCC solid solution (x = 0) to a hypoeutectic microstructure (x = 0.1 - 0.3), then to a fully eutectic microstructure (x = 0.4) with a mixture of FCC and Co2Ta-type Laves phases, and finally to a hypereutectic microstructure (x = 0.5 - 0.75). The Ta-free (CoCrFeNi) alloy showed high ductility but low strength. With the addition of Ta, the yield strength and hardness of the CoCrFeNiTax alloys increased but decreased in ductility. Therein, the fully eutectic CoCrFeNiTa0.4 alloy displayed a high fracture strength of 2293 MPa with a compression plasticity of 22.6%. The regular/uniform ultrafine lamellae structure was the primary reason for the remarkable properties of the CoCrFeNiTa0.4 eutectic high entropy alloys. Keywords: High entropy alloy; Eutectic microstructure; Mechanical properties
ACCEPTED MANUSCRIPT 1. Introduction Within the last several years, a new alloy concept was proposed by Yeh and Cantor et al. called high entropy alloys (HEAs), or multi-principal element alloys [1, 2]. These alloys have unique physical, chemical, and mechanical properties and are a popular research topic in the metallic materials community [3-6]. HEAs have been thought to form simple crystal structures rather than complex intermetallic phases with their high configurational entropy. However, recent research has discovered that intermetallic compound phases form in certain HEAs, and many are not true singlephase alloys [5-7, 14, 15]. In previous studies, CoCrFeNi-based alloys were widely investigated because of their special properties and potential industrial application value . And many other alloy elements, such as Al, Mo, Ti, V, Nb, and Si, were added into CoCrFeNi alloy to investigate their alloying effects on the microstructure and properties of alloy [9, 1723]. In the present work, CoCrFeNiTax HEAs were designed and prepared. The Ta element was chosen based on the following two reasons: 1) Ta, with a high melting point, is often added to superalloys to enhance structure stability; 2) Eutectic high entropy alloys (EHEAs) demonstrate good liquidity, regular structure, and excellent mechanical properties [24, 27]. Based on binary diagrams, Ta-Ni, Ta-Co, Ta-Cr, and Ta-Fe systems exist in the eutectic structure [28-31]. As such, it is possible for CoCrFeNiTax HEAs to exist in a eutectic composition point. With that in mind, this paper conducted a systematic study on the alloying effects of Ta on the microstructure
ACCEPTED MANUSCRIPT evolution and mechanical properties of CoCrFeNiTax (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.75) alloys. Then, the relationship between the microstructures and mechanical properties of the alloys was discussed. 2. Experimental method In this study, novel HEAs of CoCrFeNiTax (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.75, denoted by Ta00, Ta01, Ta02, Ta03, Ta04, Ta05, and Ta075, respectively) were prepared by arc melting in an Ar atmosphere using pure elements of Co, Cr, Fe, and Ni (99.99 wt.%) and Ta (99.95 wt.%). The alloys were melted a minimum of five times to ensure chemical homogeneity. The microstructures and compositions of the alloys were investigated by scanning electron microscope (SEM, Zeiss Supra 55) equipped with an attached X-ray energy dispersive spectrometer (EDS). The volume fractions of the strengthening phase in the alloys were estimated using Image-Pro Plus (IPP) software. The high-temperature differential scanning calorimeter (DSC, Netzsch STA
measured at a heating/cooling rate of 10 K/min. Structure characterization was carried out using a Bruker D8 Focus X-ray diffractometer with Cu Kα radiation scanning from 20º to 100° at a scanning rate of 4°/min, and a TECNAI G20 transmission electron microscope (TEM) operated at 200 KV. The TEM samples were prepared by conventional techniques of cutting, mechanical grinding, polishing, and twinjet electro-polishing. Compressive tests were performed on the cylindrical specimens measuring 5 mm in diameter and 10 mm in length using a WPM machine at a strain rate of 1×10-3 s-1. The hardness values of the alloys were measured with a Vickers
ACCEPTED MANUSCRIPT hardness tester (MH-50) under a load of 500 g for 15 s. An average value of five different points on each sample was obtained. 3. Results and discussion 3.1 Crystal structures and microstructures XRD patterns of the CoCrFeNiTax alloys are shown in Fig. 1a. As can be seen, only FCC structured diffraction peaks appeared in the Ta-free CoCrFeNi alloy. With the addition of Ta element, a new reflection peak emerged, corresponding to the Co2Ta-type Laves phase with lattice parameters of a = b = 4.797Å and c = 7.817 Å, and C14 hexagonal structure, according to the JCPDS card number 15-0039. Similar experimental results were reported in Refs 8 and 22. With increases in the Ta content, more reflection peaks corresponding to the Laves phase were clearly observed and the intensity of the peaks also increased. The enlarged image of the (111)FCC peak is shown in Fig. 1b. With the addition of Ta, the peak of the FCC phase gradually shifted towards the lower 2θ angle, which indicated that the lattice parameter of the FCC phase increased slightly.
Fig. 1. (a) XRD patterns of the CoCrFeNiTax alloys; (b) Detailed scans for the peak of (111) of the FCC phase
ACCEPTED MANUSCRIPT It was deduced that the Ta element, with a larger atomic radius relative to the other composition elements, dissolved into the CoCrFeNi alloy, which caused an increase of the lattice constants. Figs. 2 and 3 show the SEM images of the CoCrFeNiTax alloys with different Ta contents. A single-phase structure consisting of coarse dendrite measuring hundreds of microns in width was observed in the Ta-free Ta00 alloy. XRD images showed that the Ta00 alloy consisted of a single FCC solid solution phase, as can be seen in Fig. 1a. In the Ta01 alloy, a few white regions (identified as Laves phase) were observed at the grain boundaries of the FCC phase, as shown in Fig. 2b. And the white regions exhibited lamellar morphology, as can be seen in the high-magnification image in Fig. 3a, which indicated that the Ta01 alloy was a hypoeutectic alloy with a small fraction of eutectic structure. The Ta02 and Ta03 alloys (as shown in Figs. 2c and d, and 3b and c) exhibited a typical hypoeutectic microstructure, wherein the primary phase was the FCC solid solution phase (marked by α), and the eutectic structure was a mixture of FCC phase and Laves phase. When the Ta content reached 0.4, a fully eutectic microstructure was observed. This fully eutectic alloy, Ta04, exhibited a colony structure with fine lamellar morphology inside the colony and coarse lamellar spacing near the colony boundary, as can be seen in Figs. 2e and 3d. A similar eutectic colony morphology has also been observed in other HEAs [32, 33]. And the formation mechanism of the eutectic colony structure has been previously investigated by other scholars [34, 35]. This formation of eutectic colony structure has been associated with the combined effects of the impurities rejected from the solidifying eutectic, the growth rate, and the
ACCEPTED MANUSCRIPT imposed thermal gradient at the solid/liquid interface, which produce a zone of constitutionally undercooled liquid ahead of the growing interface. Under these conditions of growth, the planar solid/liquid interface becomes unstable and transforms into a cellular interface. For the Ta04 eutectic alloy, the growth interface became cellular due to the two-phase instability that resulted from the higher cooling rate. For the hypereutectic Ta05 and Ta075 alloys, the primary phase was the Laves phase (marked by β in Figs. 2f and g), and a typical hypereutectic structure was observed. The chemical compositions of different regions in the prepared alloys are listed in Table 1. In the Ta00 alloy, all the elements were homogeneously distributed and the compositions of the grains were close to the nominal composition, which was consistent with the single FCC structure. For the CoCrFeNiTax (x > 0) alloys, it was seen that the FCC phase (α) was enriched with Cr, Fe, and Ni elements and a small amount of Ta element. When the Cr, Fe, and Ni elements were depleted, the Ta element was enriched in the eutectic region. It was noted that the Co was relatively uniform in both areas. The content of Ta in the FCC phase increased with additions of the element, which was consistent with the XRD results. The chemical composition of the Co2Ta-type Laves phase corresponded to the presence of stoichiometric (Co, Ni)2Ta. Previous results have also shown that formation enthalpies increase in the order of Cr > Fe > Co > Ni for Laves phases formed with Ta . Thus, the (Co, Ni)2Ta Laves phase formed preferentially in the CoCrFeNiTax alloys. In order to further research the microstructure, TEM was carried out for the
ACCEPTED MANUSCRIPT representative Ta04 alloy. Fig. 4 shows a typical bright-field TEM image and the corresponding electron diffraction (SAED) patterns obtained from the different regions. A typical eutectic morphology was observed, and the average interlamellar spacing was about 150-200 nm in the Ta0.4 alloy. The SAED patterns further confirmed that this alloy consisted of a FCC phase (grayish white phase) and a Co2Tatype Laves phase (black phase). Furthermore, a preferred crystallography relationship was found between the FCC and Laves lamellar, i.e., FCC//Laves. The DSC curve of the Ta04 alloy shown in Fig. 5 illustrates only one sharp exothermic and endothermic peak, which further evidenced the eutectic composition of the Ta0.4 alloy. A bulk ultrafine (nearly nanometer-sized)/regular lamellar structure can be obtained with a direct solidification method without the following process. The results are interesting and unusual, and the formation mechanism can be studied further.
Fig. 2. Microstructures of the CoCrFeNiTax HEAs: (a) x = 0; (b) x = 0.1; (c) x = 0.2; (d) x = 0.3; (e) x = 0.4; (f) x = 0.5; (g) x = 0.75
Fig. 3. High magnification microstructures of the CoCrFeNiTax HEAs: (a) x = 0.1; (b) x = 0.2; (c) x = 0.3; (d) x = 0.4; (e) x = 0.5; (f) x = 0.75
ACCEPTED MANUSCRIPT Table 1. Chemical composition of different regions in CoCrFeNiTax HEAs Alloys Ta00
Fig. 4. (a) TEM micrograph of the CoCrFeNiTa0.4 alloy; (b)-(d) SAED patterns corresponding to A, B, and C regions in (a)
Fig. 5. DSC curve of the CoCrFeNiTa0.4 alloy 3.2 Mechanical properties The compressive engineering stress-strain curves of the CoCrFeNiTax alloys are shown in Fig. 6. The detailed compressive and Vickers hardness values are listed in Table 2. It was noted that the Ta-free alloy (Ta00) presented the lowest Vickers hardness and yield strength (141 HV and 145 MPa, respectively) but demonstrated excellent ductility, which was compressed to 50% height reduction without fracture. Both the Ta01 and Ta02 alloys exhibited a behavior similar to the Ta00 alloy, but their Vickers hardness (180, 277 HV, respectively) and yield strength (267, 405 MPa, respectively) were obviously higher than those of the Ta00 alloy. With further increases of Ta content, the Vickers hardness and yield strength of CoCrFeNiTax alloys showed substantial increases, while the plastic strain showed significant decreases, which was attributed to high volume fraction of the hard/brittle Laves
ACCEPTED MANUSCRIPT phase. The eutectic Ta04 alloy displayed the highest fracture strength along with good plasticity. The fracture strength, yield strength, Vickers hardness, and plastic strain of the fully eutectic Ta04 alloy were 2293 MPa, 1316 MPa, 492 HV, and 22.6%, respectively, showing high potential for its application in structural materials.
Fig. 6. Compressive stress-strain curves of the CoCrFeNiTax (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75)
ACCEPTED MANUSCRIPT Table 2. Mechanical properties and volume fraction of Laves phase of CoCrFeNiTax (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75) alloys Volume Yield
Vickers fraction of
hardness Laves phase
The yield strength/hardness of alloys can be predicted if the strength of each phase is known. For the CoCrFeNiTax (x ≤ 0.5) alloys, which consisted of FCC and Laves phases, the yield strength and hardness levels were predicted from the rule of mixtures of the existing FCC and Laves phases. Since the FCC solid solution phase contained a number of Ta content, the strength of the FCC phase was determined by the contribution of solid solution strengthening for convenience in this paper. And the strengthening increment of the FCC solid solution strengthening was caused by the larger lattice strain of the phase. The strengthening increment due to the hard Laves phase was most likely related to the volume fraction of the Laves phase. Thus, the
ACCEPTED MANUSCRIPT effects of the lattice strain of the FCC phase and volume fraction of the Laves phase on the yield strength and hardness were investigated. The lattice strain was expressed as ε = ∆a/a0, where ∆a = |a − a0|; here, the crystal of the Ta00 alloy was regarded as the perfect crystal. And the lattice constants of the FCC solid solution phase, which were estimated from the (111) peak, were 3.5752, 3.5830, 3.5860, 3.5898, 3.5905, and 3.6009 Å in the CoCrFeNiTax alloys corresponding to x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, respectively. The trends of the lattice strain of the FCC phase corresponding to increases of Ta content are shown in Fig. 7a. It was observed that the lattice strain of the FCC phase increased as the Ta content increased. It was deduced that the lattice distortion energy increased significantly and the effects of FCC solid-solution strengthening were enhanced. Therefore, ∆σ = σTax − σTa00 and ∆H = HTax− HTa00 where σTax and σTa00 are the yield strength of the CoCrFeNiTax alloy and the CoCrFeNi alloy, respectively. HTax and HTa00 are the Vickers hardness of the CoCrFeNiTax alloy and the CoCrFeNi alloy, respectively. The experimental strength enhancement with the lattice strain of the FCC phase was plotted, as shown in Fig. 7b. As the lattice strain increased, the yield strength and hardness also increased. Particularly, for yield strength, a good liner relationship was observed in the CoCrFeNiTax (0.1 ≤ x < 0.5) alloy. However, the solid solution hardening was not the dominant mechanism due to the soft FCC structure and the limited solubility of the Ta element in the FCC phase. The volume fractions of the Laves phase in the CoCrFeNiTax alloys were estimated using Image-Pro Plus (IPP) software with SEM images; the results are
ACCEPTED MANUSCRIPT shown in Table 2 and Fig. 7a. As can be seen in Fig. 7c, with an increasing volume fraction of the Laves phase, the increment of yield strength and hardness increased significantly in the Ta00, Ta01, Ta02, Ta03, and Ta04 alloys and then increased slightly in the Ta05 alloy. For example, with a slightly increased volume fraction of the Laves phase from 34.7% in the Ta03 alloy to 37.5% in the Ta04 alloy, the yield strength and hardness increased sharply. However, the greatly increased volume fraction of the Laves phase from 37.5% in the eutectic Ta04 alloy to 55% in the hypereutectic Ta05 alloy resulted in an increase of yield strength and hardness. This indicated that the hardening of the eutectic Laves phase was higher than that of the primary Laves phase. The size of the Laves phase was critical to the mechanical properties of the alloy. In general, a fine microstructure results in a high yield strength (referred to as the Hall-Petch relation). Thus, in the CoCrFeNiTax alloys, the eutectic Laves phase with an ultra-fine lamellae microstructure had higher strength and hardness than that of the coarse primary dendrite Laves phase. Therefore, this ultrafine lamellar Laves phase contributed to the ultra-high strength of the Ta04 alloy. The high plasticity of the hypoeutectic alloys were ascribed to the ductile primary FCC solid solution phase. The present study showed that proper volume fraction of the ductile FCC primary phase and hard/brittle Laves phase resulted in a balanced strength and plasticity.
Fig. 7. (a) Measured lattice constant and lattice strains of FCC phase and calculated volume fraction of Laves phase with increasing Ta content; (b) and (c) Variation of the yield strength and hardness with the lattice strains of FCC phase and the volume fraction of the Laves phase, respectively 5. Conclusion In this paper, the microstructures and mechanical properties of CoCrFeNiTax (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.75) HEAs were investigated. According to the experimental results, the following conclusions were drawn: (1) With the addition of Ta element, the microstructure changed from an initial single FCC solid solution (x = 0) to a hypoeutectic microstructure (x = 0.1 - 0.3), to a fully eutectic microstructure (x = 0.4) with a mixture of FCC and Co2Ta-type Laves phases, and finally, to a hypereutectic microstructure (x = 0.5 - 0.75). (2) The bulk ultrafine (100-200 nm) lamellar microstructure of the Ta04 EHEA was obtained using the direct solidification method without post processing.
ACCEPTED MANUSCRIPT (3) The hypoeutectic Ta01 and Ta02 alloys exhibited excellent compressive ductility (εp > 50%), which was similar to that of CoCrFeNi alloy, but demonstrated higher Vickers hardness and yield strength than those of CoCrFeNi alloy. The fully eutectic CoCrFeNiTa0.4 alloy displayed excellent mechanical properties with a yield strength of 1316 MPa, fracture strength of 2293 MPa, and plastic strain of 22.6%. (4) As the Ta content increased in the CoCrFeNiTax alloys, the increased lattice strain of the FCC phase and volume fraction of the hard Laves phase resulted in increases of yield strength and hardness. Therefore, the regular/fine lamellar eutectic microstructure drastically enhanced the strength of the CoCrFeNiTax alloys.
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ACCEPTED MANUSCRIPT 1. The fully eutectic CoCrFeNiTa0.4 alloy displayed excellent mechanical properties. 2. The ultrafine lamellar microstructure was obtained using the direct solidification method. 3. The ultrafine lamellar microstructure contributed high strength and hardness.