The contradiction between strength and ductility limits the application of high-entropy alloys (HEAs). To simultaneously improve the strength and ductility of HEAs, the cryogenic treatment was proposed and applied in this paper. The Al0.6CrFe2Ni2 HEA with dual-phase structure was selected as the experimental material for cryogenic treatment. The microstructure and mechanical properties of the HEA in an as-cast and cryogenically treated state were analyzed in detail. The results showed that the grain size of equiaxed crystal in the alloy decreased continuously by prolonging the cryogenic treatment time, and the average value was 44.6 μm for the cryogenically treated HEA at the time of 48 h, which was 46.5% lower than that of the as-cast alloy. The number and size of ordered body-centered cubic (B2) spherical nanophases embedded in the body-centered cubic (BCC) structured inter-dendritic region, however, increased continuously by extending the cryogenic treatment time. The cryogenic treatment also made more slip systems activate, cross-slip occurred in the alloy, and a large number of stacking faults were found in the transmission electron microscopy (TEM) microstructure for the alloy that underwent a long time in cryogenic treatment. The yield strength of the Al0.6CrFe2Ni2 HEA was gradually increased with the increase in cryogenic treatment time, and the maximum yield strength of the 48 h cryogenically treated alloy was 390 MPa, which was 39.3% higher than that of the as-cast. This increase in mechanical properties after cryogenic treatment was attributed to the refinement of grains and the large precipitation of nanophases, as well as the appearance of cross-slips and stacking faults caused by cryogenic treatment.
Al 0.4 CoCrFe 2 Ni 2 high-entropy alloys with different additions of TiO 2 nanoceramic particles (0, 1.25vol.%, 2.5vol.%, 3.75vol.% and 5vol.%, respectively) were prepared by using the vacuum arc melting method. The effects of TiO 2 addition on the crystal structure, microstructures and mechanical properties of the alloy were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile testing. The microstructure analysis shows that the TiO 2 nano-ceramic particles added in the alloy are decomposed, and a small amount of Al 2 O 3 and a great number of intermetallic compounds (γ' phases) with simple cube structure are formed. The γ' phases are enriched at inter-dendrite, which increases the resistance of dislocation movement during the deformation of the alloy, thus balancing the problem of high plasticity and low strength of the alloy. When the addition of TiO 2 is 2.5vol.%, the strength of the high-entropy alloy reaches the maximum of 489 MPa, which is 11.1% higher than the matrix alloy composed of single FCC phase.
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