Search Results (3)

A (Fe, Cr)-free Co_39.2Ni_39.2Al_21.6 eutectic medium-entropy alloy (EMEA) was designed and fabricated to study the microstructure and its evolution during slow solidification under different intensities of high static magnetic field (0 T, 5 T and 10 T). It was found that the original microstructure was characterized by FCC/BCC mixed herringbone eutectics consisting of two types of lamellar structures: a curved and wormy anomalous eutectic in the fringe, and a straight and long regular eutectic in the center. Nano-sized L1₀-type martensite layers are also distributed on the BCC lamellar as the martensitic transformation product. The FCC and BCC phases were enriched in Co and Al elements, respectively, while Ni element was distributed homogenously in both phases. With increasing magnetic field intensity, the herringbone eutectic structures remained stable, without the formation of a primary phase, while the phase constitution and the orientation relationships in the eutectic structures remained unchanged, with no obvious magnetically induced alignments. However, the lamellar spacing of the regular lamellar eutectic decreased significantly from 3.3 μm (0 T) to 1.93 μm (10 T); by contrast, the volume fraction of the anomalous eutectics increased considerably from 28.35% (0 T) to 55.14% (10 T), and the assumption that the imposed convection and destabilization of lamellar eutectics is controlled by the magnetic field is discussed in depth. Our results show a great potential for tailoring microstructures and properties by applying a strong magnetic field during the solidification process of EMEAs. Full article

In this study, we establish a mathematical rule for accelerating the prediction of low-cost Co-free AlCr_aFe_bNi_c FCC/B2-structured eutectic medium entropy alloys (EMEAs). The mathematical formulas are c ≥ 1.0, 4.38a + 4.28b + 3.97c ≈ 20.55, and c − a ˃ 1.0. With this rule, we successfully predict the AlCr_1.18FeNi_2.8 and AlCrFe_1.46Ni_2.5 eutectic alloys and verify their eutectic morphology by experiments. Both the AlCr_1.18FeNi_2.8 and AlCrFe_1.46Ni_2.5 EHEAs exhibit excellent compressive mechanical properties, with yield stress higher than 500 MPa, compressive fracture strength higher than 2450 MPa, and fracture strain > 40%, which can be attributed to their unique lamellar microstructure. Moreover, both alloys exhibit good corrosion resistance in 3.5 wt.% NaCl solution. Among them, the AlCr_1.18FeNi_2.8 EHEA exhibits better corrosion resistance due to the higher content of the FCC phase. Full article

Alloy design of Cr-Co-Ni-Ta eutectic medium entropy alloys (EMEAs) was performed through a CALPHAD method coupled with experimental study, with the aim to attain high phase stability as well as excellent mechanical properties. Based on calculated pseudo-binary diagram, CrCoNiTax (x = 0.1, 0.3, 0.4, 0.5, 0.7) medium entropy alloys were investigated. Two phases, FCC solid solution and Laves phase, were identified in the alloys. With increasing Ta content, the volume fraction of hard and brittle Laves phase increased, microstructure changed from hypoeutectic (Ta0.1, Ta0.3) to eutectic (Ta0.4) and then to hypereutectic (Ta0.5, Ta0.7). The stability of phases was assessed by considering the thermodynamic parameter Ω and valence electron concentration (VEC). The eutectic phases become stable when 1.42 < Ω < 0.74 and 7.5 < VEC < 8.25. In addition, based on nanoindentation, the results indicated that solid solution strengthening in γ phase was significantly enhanced, eutectic phase in CrCoNiTa_0.4 EMEA was found to process the highest microhardness and elastic modulus. Finally, the hardness of alloys was positively correlated with the content of Ta and the plastic strain of alloys obviously decreased, while the compression strength firstly increased and then decreased. CrCoNiTa0.4 was the most promising alloy with the highest compression strength (2502 MPa) and high plastic strain (20.6%). Full article