High Entropy Alloy under Shock Compression: Optical-Pump X-Ray-Probe

Abstract

High entropy alloys (HEAs) are multi-principal-element alloys designed for tailorable mechanical performance and have been attracting significant engineering interest, yet their fundamental behaviour under extreme dynamic conditions, such as shock loading, remains unexplored. Here, we report laser-shock experiments on two different types of 1-micrometers-thick HEA microfilms, CuPdAgPtAu and CrFeCoNiCuMo, on 25-micrometers-thick black-Kapton ablator driven by a high intensity laser pulse (532 nm, 5 ns, 16 J, 0.5-mm diameter focal spot) and probed by an X-ray free electron laser (XFEL) pulse (12 keV, 7 fs). Time-resolved X-ray diffraction (XRD) shows the formation of a transient phase with a lattice compression up to 5.1% of the CuPdAgPtAu HEA along the (111) plane; this transient compressed phase existed for 0.3 ns. The impedance matching Hugoniot analysis estimated a shock pressure of 55 +/- 6 GPa in the HEA film, while Au- and Fe-based equations of state (EoS) modelling predict 80 GPa (0.8 MBar) at the free HEA surface. The free HEA surface reached maximum velocities of ~ 5 km/s as recorded from in situ monitoring with the velocity interferometry system for any reflector (VISAR) imaging. These initial HEA results show the suitability of HEA sample preparation and XFEL-based XRD characterisation under extreme shock loading, and are promising for experimental determination of the EoS of this emerging class of materials (beamtime proposal No.: 2024A8503 for a 6-hour preliminary experiment).

Figures (17)

• Figure 1: Setup used in the 2024A8503 SACLA beamtime; all capabilities are described in ref. BL3. (a) Geometry of experiment with an XRD flat-panel detector (FPD) at the bottom (transmission geometry), and two VISARs (velocity interferometer system for any reflector) at 532 nm wavelength. (b) Mounted samples after the experiment. The inset shows a $7\times 7$ array of 1-cm-diameter holes for samples. HEA samples of CuPdAgPtAu (Au-HEA) and CrFeCoNiCuMo (Fe-HEA) were deposited as $1~\mu$m-thick film on a $25~\mu$m-thick black Kapton ablator. Annealed samples are denoted "+T" and alumina-coated samples are shown with AlO$_x$ ($\sim 0.8~\mu$m film for better VISAR visualisation and reduced shock reflection). One hole was left without a sample for XFEL beam alignment. Samples of alumina film on black Kapton had gold patches sputtered at a corner for alignment.
• Figure 2: Optical images of (a) X-ray induced ablation on the Fe-HEA film on black Kapton and (b) damage from a $\sim 16$ J, 532 nm, 5 ns optical pulse through the Fe-HEA film on black Kapton (top-view from the HEA side). Insets in (b) show low-energy optical pulse damage on the top surface of the HEA film and an optical image of an approximately 250 $\mu$m diameter (FWHM) top-hat beam (half the $\sim$500 $\mu$m full beam diameter, courtesy of the BL3 beamline team). A dashed cross at the bottom image in (b) shows the orientation of the DOE, which was used to generate the top-hat pump beam.
• Figure 3: The structural evolution upon the application of shock wave probed by high-speed XFEL-based XRD structural analysis of CuPdAgPtAu HEA (Au-HEA) film: (a) geometry of interaction region, (b) azimuthally integrated $2\theta$-angular XRD images and (c) integrated profiles vs. $Q$ [$\text{\AA}^{-1}$]; see Fig. \ref{['f-AuSlice']} for the side-view microtome cross-section. Simulations using Crystal Maker/Diffract for 9-nm FCC structures are shown is this 9 nm the black lines in file "f-auxrd1" for different lattice sizes?. The surface of Au-HEA coating exhibits a random pattern of protrusions up to 150 nm in height (as observed by SEM and AFM). (d) The Pawley analysis of the lattice period evolution showing a split into two phases, which converge into a single and more disordered phase(see text for discussion).
• Figure 4: The structural evolution upon the application of pressure wave probed by high-speed XFEL-based XRD structural analysis of a $\sim 1~\mu$m CrFeCoNiCuMo HEA film: (a) azimuthally integrated $2\theta$-angular XRD images and (b) integrated XRD $Intensity~vs.~Q$ [$\text{\AA}^{-1}$] plotted with vertical offset. See Supplement for the bandwidth changes during pressure loading.
• Figure 5: Velocity vs. time from VISAR1 and VISAR2 data sets for Fe-HEA and Au-HEA samples. Each line corresponds to a single pulse of optical-pump pulse $\sim 16$ J; those lines are dots in Fig. \ref{['f-2vis']}. At the negative acceleration point in VISAR2, a $2\pi$ phase was added. This resulted in a good correspondence between pressure wave velocities reached at 9-9.5 ns $\sim 5$ km/s for both detectors. The shot number $\#1411270$ corresponded to approximately half of the interferogram (and partial circles of the XRD pattern), most probably due to the edge of film/sample. The $\sim$5 ns optical pump finishes at approximately 1 ns VISAR timer position; see Fig. \ref{['f-cond']}(a) for an oscillogram of the optical pump pulse. The initial acceleration reached $\sim 7.5\times 10^{12}$ m/s$^2$ for Au-HEA. Laser pulse jitter is $\sim 160$ ps; uncertainty in determination of the instant velocity was $\pm 0.25$ km/s.