Additively manufactured high-entropy alloys are of interest because of their unique combination of high yield strength and large ductility achieved with far-from-equilibrium crystalline phases and micro/nanostructure morphology. We report on the phase transformation and thermal equation of state of the eutectic high-entropy alloy (EHEA) Al18Co20Cr10Fe10Ni40W2, produced by laser powder-bed fusion (L-PBF). The EHEA was studied in a large-volume Paris–Edinburgh cell using energy-dispersive x-ray diffraction to a pressure of 5.5 GPa and a temperature of 1723 K. Static compression studies in diamond anvil cells using angle-dispersive x-ray diffraction extended the high-pressure structural data to 317 GPa at ambient temperature. The initial dual-phase nanolamellar face-centered cubic (FCC) and body-centered cubic (BCC) structure of Al18Co20Cr10Fe10Ni40W2 transforms into a single FCC phase under high pressure, with the BCC-to-FCC phase transformation completing at 9 ± 2 GPa. The FCC phase remained stable up to the highest pressure of 317 GPa. The measured thermal equation of state for the FCC phase of Al18Co20Cr10Fe10Ni40W2 is presented up to 5.5 GPa and 1473 K. We observed melting of the EHEA at 1698 ± 25 K at a pressure of 5.5 GPa, and the recrystallized sample shows an increased fraction of the CsCl-type (B2) phase at ambient conditions following release from the high-pressure high-temperature state. The BCC-to-FCC phase transition completion pressure is correlated with the nanolamellae thickness of the BCC layer in this diffusion-less transformation at ambient temperature.

High-entropy alloys (HEAs) show great promise for applications under extreme conditions due to their excellent mechanical properties and stability at high temperatures.1,2 One such class achieving simultaneously high strength and ductility under tensile loading are the eutectic high entropy alloys (EHEAs), named for their dual-phase microstructures comprising alternating body-centered cubic (BCC) and face-centered cubic (FCC) lamellae that share a common melting temperature.3 Recently, Ren et al. have further pushed the performance envelope of EHEAs with laser powder-bed fusion (L-PBF) additive manufacturing (AM), where rapid cooling rates (105–107 K/s) drastically change the microstructure of the alloys compared to their as-cast counterparts by reducing the lamellar thickness by an order of magnitude and suppressing the formation of ordered phases and precipitates.4,5 In their work on the EHEA AlCoCrFeNi2.1 (molar ratio), L-PBF AM processing produced an alloy with a tensile yield strength of 1.3 GPa and uniform elongation of 14%.4 Similarly, in their work on another EHEA Al18Co20Cr10Fe10Ni40W2 (atomic percentage), L-PBF AM produced an alloy with a tensile yield strength above 1.4 GPa and uniform elongation of 17%.5 Both AM EHEAs exhibit mechanical properties far surpassing the performance of their as-cast counterparts.4,5

In addition to studying their thermal stability and mechanical properties, there has been an increasing emphasis on exploring the structural stability of HEAs under the application of pressure.6,7 As Zhang et al. note in their review,7 phase changes are commonly observed in FCC (e.g., CoCrFeMnNi) and hexagonal close-packed (HCP) (e.g., HoDyYGdTb) HEAs under the application of pressure, but many BCC HEAs (e.g., Al2CoCrCuFeNi) are robust, showing no evidence of phase change up to the maximum pressures reached in earlier studies. A recent study showed that the dual-phase BCC-FCC EHEA AlCoCrFeNi2.1 produced by L-PBF AM deviates from this trend.8 By using a diamond anvil cell (DAC) to apply hydrostatic pressure, Pope et al. observed a gradual BCC-to-FCC phase transition beginning at 5.5 GPa and completing at 20 GPa at ambient temperature, with the FCC phase remaining stable up to the maximum pressure of 42 GPa.8 The transformation was found to be irreversible following the release of pressure.

In this paper, we take the AM EHEA Al18Co20Cr10Fe10Ni40W2 as a model system and focus on its high-pressure and high-temperature behavior. The alloy has a nanolamellar structure consisting of alternating FCC (average thickness ∼120 nm) and BCC (average thickness 43 nm) layers.5 Using synchrotron radiation, we study the BCC-to-FCC phase transition under the application of pressure and its thermal equation of state and melting temperature, as well as the stability of the FCC phase up to multi-megabar pressures.

The microstructure of the as-printed Al18Co20Cr10Fe10Ni40W2 EHEA sample was investigated by scanning electron microscopy (SEM) in an FEI Magellan 400 microscope. The sample was mechanically polished down to 1200 grit sandpaper, followed by 20 nm colloidal silica suspension polishing. Three separate high-pressure and high-temperature experiments were conducted at the Advance Photon Source, Argonne National Laboratory at HPCAT beamlines 16-BM-B and 16-ID-B to examine the phase transformation behavior of the AM EHEA. The first experiment utilizing energy dispersive x-ray diffraction (EDXRD) was conducted with the white x-ray source and Paris-Edinburgh press at beamline 16-BM-B. Data were collected with a diffraction angle 2θ = 5° and over an energy range of 5–120 keV up to a maximum pressure of 5.5 GPa and temperature of 1723 K (above the melting temperature). The second experiment employed angle dispersive x-ray diffraction (ADXRD) at beamline 16-ID-B using a diamond anvil cell (DAC) with a methanol–ethanol pressure medium and ruby pressure marker, with a primary goal to measure the pressure of the BCC-to-FCC phase transformation in this EHEA. This experiment was conducted to a maximum pressure of 31 GPa, and the sample was decompressed to examine the irreversibility of the phase transformation. The third experiment also employed ADXRD at beamline 16-ID-B to investigate the stability of the FCC phase to the ultrahigh pressure of 317 GPa. For this experiment, a 100 µm thick as-printed sample foil was stacked on a 50 µm thick tungsten foil pressure marker. The sample-marker combination was pre-indented to ∼40 µm thickness before beginning the experiment. Importantly, peak pressures during pre-indentation were sufficient to irreversibly transform the BCC phase in the sample, so our ultrahigh pressure experiment only captured the behavior of the FCC phase of the EHEA. Pressure was applied until diamond failure using a symmetric DAC with 25 µm culet beveled diamond anvils and a gas pressure controller. Synchrotron x-ray radiation with wavelength 0.424 59 Å was used in both experiments at 16-ID-B. The sample-to-detector distance of 313 mm was calibrated with a CeO2 sample. Diffraction patterns were integrated using Dioptas,9 peak fitting and background subtraction were conducted with GSAS-II,10 and equation of state (EoS) fitting was performed with EoSFit7.11 

Figure 1 shows the secondary electron micrograph of the as-printed EHEA Al18Co20Cr10Fe10Ni40W2 featured by microscale (10–20 µm) eutectic colonies that comprise alternating BCC and FCC nanolamellae with an average lamellar thickness of 43 ± 12 and 119 ± 33 nm, respectively. Such lamellar thicknesses of the as-printed sample are almost five times smaller than those of the as-cast counterpart.5 This distinct microstructure arises mainly from the non-equilibrium solidification process and high cooling rates of L-PBF AM that can largely suppress the kinetics of elemental partitioning during the coupled growth of the EHEA.

Figure 2 shows ADXRD spectra as a function of pressure for the first experiment at 16-ID-B. Upon increasing pressure, the initial BCC-FCC dual-phase mixture gradually transforms into a single FCC phase, as evidenced by the reduction in intensity of (110) diffraction peak of the BCC phase. A single-phase FCC diffraction pattern is achieved at 9 ± 2 GPa, and the FCC phase is found to be stable up to the highest pressure of 30.8 GPa in this experiment.

Figure 3 shows the results of our high-pressure high-temperature EDXRD study up to 5.5 GPa and 1723 K. In Fig. 3(a), we show heating results at 5.5 GPa, where the BCC-FCC dual-phase mixture is found to be stable between 300 and 1673 K. There is a change in the relative intensities of the (110) reflection of the BCC phase and the (200) reflection of the FCC phase, which could result from increased preferred orientation or grain growth at high temperatures. Upon further increasing the temperature to 1723 K, we observe that all crystalline diffraction peaks disappear, leaving a liquid diffraction pattern overlapping tungsten fluorescence lines, as shown in Fig. 3(b). Based on this observation, we constrain the melting temperature of L-PBF Al18Co20Cr10Fe10Ni40W2 to be 1698 ± 25 K at 5.5 GPa.

After melting, the sample was first cooled to ambient temperature to allow for recrystallization, and then, pressure was released. Figure 4 shows the EDXRD pattern of the recrystallized Al18Co20Cr10Fe10Ni40W2 sample at ambient pressure and ambient temperature. There is a mixture of FCC and BCC/B2 phases following recrystallization in Fig. 4; the additional (111) diffraction peak characteristic of the B2 phase is readily observable. The measured lattice parameters of the decompressed FCC and BCC/B2 phases were 3.591 and 2.865 Å, respectively.

Figure 5 shows the measured pressure–volume–temperature (PVT) data for the FCC phase of Al18Co20Cr10Fe10Ni40W2 up to 5.5 GPa and 1473 K. The thermal equation of state data was obtained by heating the sample at multiple pressures to 1473 K, making sure to stay below the melting temperature. GSAS-II was used to determine the unit cell volume through structural refinement of the XRD data. The data were fit to a PVT EoS comprising the third-order Birch–Murnaghan (B-M) isotherm, the thermal expansion model of Fei,12 and a linear relation for the change in isothermal bulk modulus with temperature at zero pressure.11 The B-M isotherm is
p=3K0f(1+2f)521+1.5(K04f,
where
f=0.5(V0/V)231,
p is the pressure, V is the volume, V0 is the ambient volume, K0 is the ambient isothermal bulk modulus, and K is its derivative with respect to pressure. The thermal expansion model used at zero pressure is
V=V0expα1(TTref)+12α2(T2Tref2),
where T is the temperature, Tref is the reference temperature of 300 K, and α1, α2 are the thermal expansion fitting parameters. The model parameters are given in Table I.

In Fig. 6, we show select ADXRD spectra and the two-dimensional detector image for Al18Co20Cr10Fe10Ni40W2 at the highest pressure of 317 GPa obtained in the second experiment at 16-ID-B. For this experiment, pre-indentation nearly completely and irreversibly transformed the BCC phase, leaving only a remnant of the BCC phase in the diffraction pattern at low pressures as a shoulder to the (111) FCC reflection at higher 2θ. The FCC phase is stable and its (111), (200), and (220) reflections are still observable at the highest pressure of 317 GPa. Lattice parameters for tungsten and the EHEA are computed using their (110) and (111) reflections, respectively, and used to mark Bragg positions in the figures. Pressure is computed using the Vinet EoS for tungsten calibrated by Mashimo et al.13 Under applied pressure, the (200) reflection of the EHEA shifts away from the expected Bragg position. Such shifts are expected in non-hydrostatic stress environments14 and are often used to estimate the yield stress of the sample when elastic constants are known or can be confidently extrapolated to high pressures.15,16 We fit the isotherm of the FCC phase to the third-order B-M equation with V0 = 11.53 ± 0.02 Å3/atom, K0 = 184 ± 3 GPa, and K′ = 4.38 ± 0.04, where the uncertainties are the estimated standard deviations of the least-squares fit. The lattice parameters for the tungsten pressure marker and the EHEA sample at 317 GPa are 2.738 and 3.008 Å, respectively. The measured volume compression for EHEA is V/V0 = 0.59 at a pressure of 317 GPa.

The relative volumes of the body-centered cubic (BCC) and the face-centered cubic (FCC) phases at ambient pressure are key indicators of their relative stability under high pressures. The FCC phase has a higher packing fraction of 74% compared to the BCC phase of 68%. The measured volume per atom by x-ray diffraction on the as-printed foil of Al18Co20Cr10Fe10Ni40W2 yields a value of 11.89 Å3/atom for BCC and 11.64 Å3/atom for the FCC phase. This implies that the FCC phase has a 2% higher density compared to the BCC phase for similar alloy composition. Thus, the application of high pressure will tend to favor high-density FCC phase under high pressure. Another key observation from our experiment is that the phase transformation completion pressure of Al18Co20Cr10Fe10Ni40W2 of 9 GPa is lower than the 20 GPa completion pressure reported for AlCoCrFeNi2.1. This is related to a reduced nanolamellar thickness of 43 nm for the BCC Al18Co20Cr10Fe10Ni40W2, as shown in Fig. 1, as compared to a nanolamellar thickness of 64 nm for BCC AlCoCrFeNi2.1. The reduced nanolamellar thickness in the BCC phase of the former alloy leads to a lower phase transformation completion pressure in this diffusion-less transformation at ambient temperature. This illustrates how nanolamellar structure differences can significantly influence phase behavior under pressure in additively manufactured eutectic high entropy alloys.

In summary, the eutectic high entropy alloy (EHEA) Al18Co20Cr10Fe10Ni40W2 synthesized by laser powder-bed fusion (L-PBF) was studied under extreme conditions using a combination of large-volume press and diamond anvil cell (DAC) experiments. The DAC experiments reveal that the dual-phase face-centered cubic (FCC) and body-centered cubic (BCC) structure of Al18Co20Cr10Fe10Ni40W2 is transformed into a single FCC phase under high pressure, with phase transformation completed at 9 ± 2 GPa. The observed transformation pressure of 9 GPa for Al18Co20Cr10Fe10Ni40W2 is lower than the 20 GPa transformation pressure reported for AlCoCrFeNi2.1 and is likely related to the lower BCC nanolamellae thickness in Al18Co20Cr10Fe10Ni40W2. The large-volume high-pressure high-temperature study to 5.5 GPa and 1723 K was used to obtain a thermal equation of state with a bulk modulus of 171 GPa and a zero-pressure temperature derivative of the bulk modulus of −0.055 GPa/K. Melting of the EHEA is observed at 1698 ± 25 K and 5.5 GPa, and the recrystallized sample shows the formation of a B2 (CsCl) phase in addition to the BCC phase. The ultrahigh-pressure studies with a diamond anvil cell show that the FCC phase of Al18Co20Cr10Fe10Ni40W2 is stable up to the highest pressure reached in this study of 317 GPa corresponding to a volume compression of V/V0 = 0.59. The stability of the FCC phase over a large pressure range combined with its high strength and ductility make EHEA an ideal candidate for applications in extreme environments.

This material was based upon work supported by the Department of Energy-National Nuclear Security Administration Center of Excellence CAMCSE (Award No. DE-NA0004154). Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations were supported by DOE-NNSA’s Office of Experimental Sciences. Wen Chen also acknowledged support from the National Science Foundation CAREER Award (No. DMR-2238204) for 3D printing and microstructural characterization of the EHEA in this work. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory –(Contract No. DE-AC02-06CH11357).

The authors have no conflicts to disclose.

Andrew D. Pope: Data curation (equal); Formal analysis (equal); Writing – original draft (equal); Writing – review & editing (equal). Seth Iwan: Methodology (equal). Matthew P. Clay: Formal analysis (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Jie Ren: Methodology (equal). Wuxian Yang: Methodology (equal). Wen Chen: Conceptualization (equal); Formal analysis (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Yogesh K. Vohra: Conceptualization (equal); Funding acquisition (lead); Project administration (lead); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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