Nanoobjects microscopy
Nano-scale precipitates and crystallites formed by ultrasonic impact treatment in AlCuCrCoNiFe high-entropy alloy
Mordyuk B.N., Volosevich P.Yu., Makarenko S.Yu.
G.V.Kurdyumov Institute for Metal Physics, NAS of Ukraine, 36 Academician Vernadsky Blvd., Kyiv 03142, Ukraine. E-mail:mordyuk@imp.kiev.ua

Last decades, the multi-based or so-called high-entropy (HEA) alloys attracted more attention of material scientists. Strength and plasticity of HEA of AlCuCrCoNiFe system are known to depend on their phase composition and the Al content [1], and the BCC phase usually has higher hardness and less plasticity than those of the FCC phase. Only a few works reported on strain-induced grain refinement in HEA by means of high-pressure torsion (HPT) [2, 3]. 
This report describes the microstructural changes in the surface layers of equiatomic AlCuCrCoNiFe alloy subjected to severe plastic deformation induced by ultrasonic impact treatment (UIT) at room temperature. 
The UIT processing of constrained cube-shaped samples of 5x5x5 mm3 was performed using ultrasound equipment with an operating frequency of ~ 21 kHz and a power of 0.4 kW [4]. The samples were placed on the anvil of superhard material (WC) of hardness HV ~ 12 GPa) much higher than those of both the initial (HV = 3.65-4.49 GPa) and deformed (HV = 6-6.5 GPa) samples. Analysis of microstructure in the UIT processed surface layers of HEA samples was performed using transmission electron microscopy (TEM), selective area electron diffraction (SAED) analysis and X-ray diffraction (XRD) studies.
According to TEM observations, as-cast AlCuCrCoNiFe HEA contains fcc and bcc phases. This is also confirmed by X-ray studies. The ordered bcc phase is often observed as intergranular filaments along the boundaries of fcc grains [2, 5]. Such microstructure dictates relatively high hardness and rather limited plasticity of the as-cast alloy. The UIT process did not change the phase composition but it resulted in a number of microstructural rearrangements.
TEM analysis shows that UIT did not change the phase composition but it resulted in a number of microstructural rearrangements. The grains of FCC and BCC phases underwent deformation of different extents. The FCC-grains contain deformation twins of 20–30 nm thick (Fig. 2a). It correlates to the data describing the texture and microstructure development the two-phase AlCuCrCoNiFe HEA reported in [9], which indicate on low stacking fault energy (SFE) of the FCC phase. Low SFE of metallic material is known to dictate a privilege of mechanical twinning and shear-banding above the dislocation activity, when possible deformation mechanisms are considered, especially regarding the very beginning stages of the strain induced microstructure formation. Additionally, the dislocation density is significantly increased (up to 7 109) indicating relatively high strain extent. A vast majority of FCC grains are transformed in the ultrafine substructure. it is confirmed by the appropriate SAED patterns containing azimuthal y dispersed spots from FCC phase. 
	
Fig. 1. TEM bright-field images (a, c) and SAED patterns (b, g) of the microstructure of the UIT processed surface layer of AlCuCrCoNiFe HEA.	
On the contrary, the diffraction spots from BCC phase on the SAED patterns are distinct and clear; and BCC grains remain relatively large (size of the BCC grains is about 1 µm). The UIT processing of the plate-like structures formed as a result of decomposition of solid solution leads to intensive precipitation of nanoscale globular particles (~ 10-30 nm) within grey plates. Moreover, the interplate regions (more bright ones) contain even smaller precipitates (~ 5 nm). 
It is shown that the UIT induced severe plastic deformation results in microstructural rearrangements mainly in the FCC phase, causing the appearance of the deformation twins, the increase in the dislocation density, and the formation of misoriented dislocation cell structure with cells size of 50-200 nm. Due to higher hardness of the plate-shaped BCC grains they underwent less deformation and precipitation of nanoscale (10-30 nm) precipitates.

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