Origin identification and regulation of BCC precipitation in a CoCrFeNi high entropy alloy

The issue of whether CoCrFeNi is a single-phase or dual-phase high-entropy alloy (HEA) has long been in dispute. In this study, CoCrFeNi has been verified to be a dual-phase HEA composed of dendritic phases with an FCC structure and inter-dendritic phases with a BCC structure. The BCC phases within inter-dendritic regions are identified as Cr3O oxides which form inevitably under the arc-melting process. Undercooling treatment under a strong magnetic field can regulate the composition and precipitation of the BCC phase, refine the grain size, and enhance the solid solubility, consequently, the mechanical performances of CoCrFeNi HEA are effectively enhanced. GRAPHICAL ABSTRACT


Introduction
High entropy alloys (HEAs), developed based on the idea of mixing multiple elements, have aroused tremendous interest because of their unique properties, such as high strength-ductility combination and remarkable work-hardening rate [1,2].The equiatomic CoCrFeNi HEA has attracted wide attention thanks to its great ductility [3,4].Unfortunately, due to the low solid-solution strengthening effect, the strength of CoCrFeNi alloy is constrained.Therefore, strengthening CoCrFeNi HEAs becomes a non-trivial problem.To address this issue, it is important to recognise the microstructure characteristics of CoCrFeNi HEA.Hitherto, there still exist some controversies about the original microstructure of CoCrFeNi HEA.Many researchers believe that the CoCrFeNi HEA is composed of a single FCC dendrite phase which shows ahigher stability than other phases or intermetallic in the as-cast alloy [5,6].Nevertheless, it has to be admitted that a few precipitation phases do exist within the interdendrites.Some researches show that the phase constitution in the as-cast CoCrFeNi HEA might not be a single FCC structure [7][8][9].For instance, U. Dahlborg et al. [7] proved that the as-cast CoCrFeNi HEA is not singlephase by high-energy X-ray measurement, and there are at least FCC and BCC phases in dendrites and interdendrites, respectively.In addition, Li et al. [8,9] found that precipitation phases within the inter-dendrite in the as-cast CoCrFeNi HEA were Cr-rich BCC phases.Other phases like the orthorhombic Cr 7 C 3 phase or CrO phase could form during spark plasma sintering [10], which is ascribed to the oxidisation and carbonisation of the Cr element.Therefore, the above results show that the phase constitution in CoCrFeNi HEA is complicated.It is the complex microstructure that provides new opportunities for achieving excellent mechanical properties.
The undercooling solidification forms BCC phases in the CoCrFeNi HEA and realises an improvement in strength [8,9,11].Li et al. [8] obtained refined FCC grains and BCC precipitates in the CoCrFeNi HEA by an undercooling treatment, which substantially increased the strength compared to the as-cast alloy.With a high undercooling of up to 300 K, the compressive yield strength was increased by 400 MPa [9].Besides, other treatment technologies such as mechanical alloying can also cause the formation of the BCC phase [12].Therefore, the phase constitution of CoCrFeNi HEA could be tailored effectively.Up to now, it was still ambiguous about the constitution and its formation mechanism of the BCC phase in CoCrFeNi HEA.For tailoring the microstructures and properties, the strong magnetic field applied during material processing is effective for refining grain and controlling precipitation phases [13,14].However, seldom research is focused on the effect of a magnetic field on the undercooling CoCrFeNi HEAs, and the mechanism for optimising undercooling microstructures and properties under the magnetic field should also be clarified in depth.
In the present work, by conventional arc-melting technology under a high vacuum and pure argon atmosphere, the BCC precipitates were obtained in FCC-type CoCr-FeNi HEA.The crystallographic information, composition and formation mechanism of the BCC phase were studied in detail.In addition, undercooling treatment under a strong magnetic field was used to further regulate the precipitation characteristic of the BCC phase and optimise the grain size and solid-solubility of the alloy matrix.Consequently, the tensile strength, compressive strength, and hardness of the CoCrFeNi HEA are increased simultaneously.These results can provide a deeper understanding of the formation mechanisms of the BCC phase and optimise the serving performances of CoCrFeNi HEA.

Material and methods
CoCrFeNi master ingots of about 50 g were prepared by the arc-melting a mixture of constituent elements with purity higher than 99.999 wt.% in a Ti-gettered and high-purity argon atmosphere.The ingots were inverted and re-melted four times to ensure composition homogeneity.The final mass loss was guaranteed within 0.3 wt.%.One of the ingots was analysed by X-ray Diffraction (XRD, Bruker-D8 Advance), Scanning Electron Microscope (SEM, JSM-7200 F) and attached Electron Back-scattering Diffraction fittings (EBSD, EDAX Velocity Super) to show the as-cast microstructure.The phase structure and phase composition of the interdendritic BCC precipitation were further characterised using Transmission Electron Microscopy (TEM, FEI Talos F200 X), Electro Probe Micro-Analysis (EPMA, EPMA-1720 H).TEM lamella was cut from the prepolished surface using a dual-beam-focussed ion beam (FIB) workstation (Helios 600, FEI).Since the Cr content is extremely easy to be oxidised even melted under a pure Argon atmosphere with negligible Oxygen content in the furnace chamber, the Oxygen nitrogen hydrogen analyser (ONHA, ONH836-HMC) is used to quantitatively measure the oxygen content.
Other ingots were used to conduct the undercooling experiments without and with a 5 T magnetic field.The candidates with a mass of 30 g for undercooling experiments were machined from the master ingots, and they were put and covered with B 2 O 3 flux in a high-quality quartz tube.The tube containing the samples was placed in a uniform heating region of a high-temperature resistance heating furnace, which was also inserted in the bore of the high static magnetic field facility.A detailed description of the apparatus is available in Ref. [15].Each sample was cyclically heated and cooled down several times until a desired undercooling ( T ≈ 300 K) was achieved.The thermal history for each sample was kept consistent, i.e. the overheating temperature and holding time were 1700K and 10 min, respectively.The static magnetic fields of 0 and 5 T were then applied on the subsequent cycle.Because the boron element has a higher affinity with oxygen, the glass fluxing could act as the deoxidant for further homogenising and purifying melt physically and chemically.Then the ONHA was conducted on the undercooled sample with/without a magnetic field.The oxygen content is higher by the conventional arc-melting process and then decreases dramatically (as seen in Figure S1 in Supplementary materials).The undercooled samples were water-quenched at about 1000 K and analysed using similar techniques as the ascast sample, except the TEM foils were twin jet electropolished with 10% (vol.)HClO 4 in an alcohol solution at −30 °C.Tension and compression tests were performed on the mechanical testing machine (INSTRON 3382) with a strain rate of 10 −3 s −1 .The nano-hardness of phases was explored by nano-indentation (Hysitron TI 980 system) with a Berkovich tip, and the micro-hardness was obtained by the microhardness tester (MH-5L).

Results and discussions
Figure 1 shows the microstructure characterisation of the as-cast CoCrFeNi HEA.Well-developed dendrites and apparent inter-dendrite precipitates are observed.
The XRD pattern in Figure 1(b) reveals that the alloy consists of FCC and BCC phases rather than a single FCC phase.Further closer observation and EBSD analysis (Figure 1(c)) indicate that inter-dendrites precipitates are concentrated along the boundary in the network and exhibit a BCC structure without preferred orientations.TEM results shown in Figure 1(d) further confirm the presence of the BCC phase with an average lattice size of about 0.49 nm.The chemical distribution by EPMA analysis in Figure 1(e) reveals that the BCC phase is enriched with Cr and O, while the FCC phase is enriched with Fe, Co and Ni, which agrees with the previous work [9,16].The content ratio of Cr and O is near 3:1 within the BCC phase, indicating that the BCC phase should be recognised as a chromium oxide with a low content of oxygen.The XPS also proved the existence of the peaks of chromium oxides (See Figure S2 in Supplementary Materials).Furthermore, the BCC phase is identified as Cr 3 O with a lattice parameter of 0.45 nm [17], which is consistent with the results in Figure 1(d, e1 and e2).The increase in the lattice constant should be attributed to sluggish diffusion-induced lattice distortion [8].Compared with other chromium oxides, e.g.CrO 2 , CrO 3 , Cr 2 O 3 , etc., Cr 3 O usually forms in a low-oxygen environment.Nils [17] found that Cr 3 O could form when pure Cr was melted under a high-purity argon atmosphere even Ti and Zr getters were melted first to absorb the rest oxygen.This is ascribed to the fact that Cr has higher oxygen affinity than the other metal elements in the CoCrFeNi alloy.On the other hand, the appearance of Cr-rich and (Co, Fe, Ni)-rich regions in the CoCr-FeNi HEA is a general phenomenon, which is due to the highest migration energy barrier caused by the largest effective atomic radius of Cr.In addition, according to the alloying effect, Cr is a BCC stabiliser [10].Therefore, it is postulated that the emergence of the BCC phase, specifically identified as Cr 3 O, is inevitable in the CoCrFeNi HEA melted under a high purity argon atmosphere.This conclusion may be extended to the Cr-contained alloy satisfied with two premises.(1) Cr has the highest oxygen affinity among the alloying elements.(2) Formation of the Cr-rich regions.
Figure 2 shows the microstructure characterisation of the undercooled CoCrFeNi HEAs with T = 300 K, approximately, under 0 and 5 T magnetic fields.After undercooling treatment, well-developed FCC dendrites disappear and give place to the equiaxed grains with a mean size of 24.3 μm.Rod-like rather than network precipitates were distributed along the grain boundary.The volume fraction of the precipitates reduces from 9.4% (as-cast) to 4.1%.Plenty of twins are observed in the equiaxed grains.EBSD analysis indicates that the equiaxed grains and precipitates are FCC and BCC structures, respectively (Figure 2(b)).TEM characterisation in Figure 2(c) shows a BCC structure, with a lattice constant of 0.49 nm.To verify the composition of BCC precipitates, the EPMA analysis is also performed.Figure 2(d) shows that the BCC precipitates are rich in Cr, while the FCC equiaxed grains are rich in Fe, Co and Ni.It suggests that the oxygen content decreases dramatically whereas the content of Fe, Co and Ni rises within the BCC phase, as shown in Figure 2(d1).Further observations (Figure 2(d2 and d3)) indicate that the BCC precipitates still contain a little oxygen while the total amount could be neglected.The XPS results only show the characteristic peaks of the Cr element (See Figure S2 in the Supplementary Materials).Combined with the TEM, EPMA and XPS results, the BCC precipitates are Cr-rich phases, which resemble the metastable BCC structure of the Cr element [18].With the application of a 5 T magnetic field, there is no notable difference in morphology except that the equiaxed grains are further refined to a mean size of 20.1 μm and the volume fraction of rodlike precipitates distributed along the boundary is slightly increased.Grain refinement mainly lies in two aspects: the remelting effect of dendrites and the recrystallisation induced by the internal stress caused by undercooling solidification [19].In the present work, the T achieved with/without a magnetic field is enforced to be consistent.Therefore, further grain refinement should result from the difference in the degree of recrystallisation controlled by the magnetic field.As shown in Figure 3, the processing level of recrystallisation is higher with that applied a 5 T magnetic field since the internal stress is almost completely released (Figure 3(b1)) and the corresponding Grain reference orientation deviation (GROD) value is only 0.6°(Figure 3(b2)).
During the rapid solidification stage of the undercooled alloy melt, grain refinement always occurs with the aid of the recrystallisation process induced by the accumulated internal stress.When applying an external magnetic field, the additional force also could aggravate the internal stress and affect the recrystallisation process.That is, the magnetic field could yield forced convection that acted on the melt and solid phase, respectively.As a result, except for the accumulated stress during rapid solidification, the driving force for crystallisation is further improved (e.g.reaching 10 8 N/m 3 order of magnitude at 20 T magnetic field [20]).On this condition, the intensive convection motivated by the strong magnetic field could contribute to the propagations of dislocations, which could provide more vacancies for the approaching atoms to ease their locating [21].In addition, the excessive dislocations would also be exhausted to form more substructures with small angle grain boundaries and annealing twins [22], releasing the internal stress, which was verified by EBSD observations in Figure 3. Consequently, it can be inferred that the undercooling treatment under a strong magnetic field can facilitate the recrystallisation, resulting in further grain refinement.
The comparison of mechanical properties of the ascast CoCrFeNi HEA, undercooled CoCrFeNi HEA with 0 and 5 T magnetic fields is shown in Figure 4.It shows that the tensile yield strength (σ y ) and ultimate tensile   be noted that the ultimate compressive strength and elongation are difficult to obtain due to the outstanding ductility of the CoCrFeNi alloy.The σ y can be generally calculated from the contribution of different strengthening mechanisms during plastic deformation, such as solid solution strengthening (σ s ), dislocation strengthening (σ d ), precipitation strengthening (σ p ) and grain boundary strengthening (σ g ) [23]: where σ 0 is the intrinsic strength and is a basic parameter related to the alloy.For the as-cast and undercooled samples, the σ d can be ignored since the grains are nearly fully recrystallised.In addition, because BCC phases are mainly distributed along the grain boundaries, the contribution of σ p to yield strength can be ignored (seeing the Supplementary Materials).Therefore, the significant increase in yield strength in the undercooled samples was mainly related to the increase in σ g and σ s .The Hall-Petch equation is used to estimate the effect of grain size difference on the yield strength [24]: where k is the Hall-Petch coefficient and d is the average grain size.Here, the value of k is 226 MPa μm 1/2 [25].
The average grain diameter for the as-cast and undercooled samples are measured by EBSD, and the values are 104.3 and 24.3 μm, respectively.So, for the undercooled samples, the strengthening contributed by the grain refinement is 23.7 MPa.Moreover, the difference in solid solution strengthening ( σ s ) between the ascast and undercooled samples can be estimated using Equation (4): Therefore, the σ s = (260.8-193.6)MPa -23.7 MPa = 43.5 MPa.The application of the 5 T magnetic field makes little contribution to the improvement of the tensile strengths because the undercooled samples with/without a magnetic field have similar microstructures.Figure 4(c) shows the nano-indentation curves of the constituent phases, and the corresponding nano-hardness and Vicker hardness are shown in Figure 4(d).The nanohardness and the Vicker hardness of the FCC and BCC phases increased substantially for the undercooled samples which should be related to the higher solution-strengthening effect caused by the undercooling solidification.The application of a magnetic field can aggravate this phenomenon resulting in a further increase in hardness which can also be proved by the EPMA results (seeFigure 2(h3)).Besides, with the application of a 5 T magnetic field of the undercooled sample, the grain size of the FCC phase becomes finer, resulting in an improvement in σ g which can contribute to a higher σ y .Similar to the σ y , the improvement of σ cy results from the higher solid solubility and refined grain size.In addition, the BCC phase with a higher hardness for the undercooled sample with a 5 T magnetic field could play an important role in improving crack propagation resistance in the latter half of the compression deformation resulting in an excellent compression strength and work-hardening capacity.

Conclusions
The BCC phase in CoCrFeNi HEA fabricated by the conventional arc-melting technology is Cr 3 O oxide with a lattice parameter of 0.49 nm, which forms in a low oxygen environment during solidification.Compared to the as-cast CoCrFeNi HEA, undercooling treatment effectively eliminates the FCC dendritic structure, replacing it with equiaxed grains.The oxygen content decreases and BCC phases transit into Cr-rich phases.The application of a magnetic field during the undercooling serves to further refine the grain size and enhance the solid solubility of the matrix and optimise the BCC phase morphology from a network to a rod-like structure.As a result, the hardness, tensile and compressive properties of the CoCrFeNi HEA are systematically increased.Therefore, the undercooling treatment under a strong magnetic field has a promising application in optimising the mechanical properties of HEAs.