The present work verifies the existence of chemical short-range orders (CSROs) in high-entropy alloys (HEAs) using a cluster-plus-glue-atom model. Two HEAs with a composition of Al2M14 (M = Ni4Co4Fe3Cr3 or Ni1Co1Fe2Cr1) are designed, which is derived from the cluster model of [Al-M-12](M2Al1) in face-centered-cubic structure or [Al-M-14]Al-1 in body-centered-cubic structure. It is found that the calculated pair-distribution functions (PDFs) at short inter-atomic distances by these cluster models can describe the neutron PDFs better than the ones by the average crystal structures. It is due to the different CSROs characterized by cluster units that induce the structural transition of HEAs. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The single-phase face-centered-cubic Al7Co24Cr21Fe24Ni24 (in atomic percent, at%) high entropy alloy lacks sufficient hardness and strength at room temperature, which restricts its structural application in the future. To strengthen this material, the (Al7Co24Cr21Fe24Ni24)(100-x)Cr-x (x = 0, 11, 20, and 26) high entropy alloys were prepared and investigated. We found that with increasing the Cr content, the volume fraction of the BCC phase enriched with Al, Fe, and Cr elements increased gradually. In the BCC-contained HEAs, ordered B2 particles were always dispersed in the BCC solid solution, which resulted in a dramatic enhancement of comprehensive mechanical properties. Nanoindentation measurements showed that both the nanohardness and the elastic modulus values of the BCC phase are much greater than that of the FCC phase. The room-temperature compression test showed that the yield strength significantly increased from 249 MPa to 1649 MPa with the slight sacrifice of the ductility, along with increasing the Cr content. More importantly, the (Al7Co24Cr21Fe24Ni24)(74)Cr-26 alloy exhibits outstanding compression behavior, with the high yield strength (1649 MPa) and fracture strength (2830 MPa) as well as good compressive plastic strain of 24.9%. Furthermore, to facilitate the optimization of the comprehensive properties of alloys, the close correlation between the microstructures and mechanical properties was also discussed.

Limitations of strength and formability are the major obstacles to the industrial application of magnesium alloys. Here, we demonstrate, by producing the duplex phases and fine intermetallic particles in composition-optimized superlight Mg-Li-Al alloys, a unique approach to simultaneously improve the comprehensive mechanical properties (a strength-ductility balance). The phase components and microstructures, including the size, morphology, and distribution of precipitated-intermetallic particles can be optimized by tuning the Li content, which strongly influences the work-hardening behavior and tension-compression yield asymmetry. (c) 2017 Elsevier B.V. All rights reserved.

Long-term stability of high entropy alloys (HEAs) is a critical consideration for the design and practical application of HEAs. It has long been assumed that many HEAs are a kinetically-stabilized metastable structure, and recent experiments have confirmed this hypothesis by observing HEA decomposition after long-term equilibration. In the present work, we demonstrate the use of the CALculation of PHAse Diagrams (CALPHAD) approach to predict HEA stability and processing parameters, comparing experimental long-term annealing observations to CALPHAD phase diagrams from a commercially-available HEA database. We find good agreement between single-and multi-phase predictions and experiments. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The AlN/Y nanocomposite and nanomultilayered films with different Y contents were fabricated by the magnetron sputtering technique. The microstructures and mechanical properties of the AlN/Y nanocomposite and nanomultilayered films were characterized and measured, respectively. The AlN/ Y nanocomposite film is composed of equiaxed AlN nanocrystallites encapsulated by the Y interfaces. When the Y:Al ratio is 2:23, Y interfaces can exist as the crystallized state and keep the epitaxial growth with the adjacent AlN nanocrystallites. Accordingly, the crystallization degree and mechanical properties are improved. The AlN/ Y nanomultilayered film consists of the evident multilayered structure with distinct interfaces. When the Y-layer thickness is no more than 0.7 nm, Y layers are inclined to grow epitaxially with the adjacent AlN layers, leading to the improvement of crystallization degrees and mechanical properties. The interfacial evolutions and mechanical properties variations between AlN/ Y nanocomposite and nanomultilayered films have the common feature as the Y content grows. It has been experimentally and theoretically verified that the AlN/ Y nanocomposite and nanomultilayered films have the same coherent-interface strengthening mechanism. (C) 2017 Elsevier B.V. All rights reserved.

Nanoindentation tests were performed to investigate the nano-scale plastic deformation in the Al0.5CoCrCuFeNi high entropy alloys at room temperature (RT) and 200 degrees C, respectively. Serrated plastic flow, manifested as discrete bursts of plasticity on the load-displacement curves, was observed for both temperatures during the loading period, and its behavior and dependence on the temperature was analyzed from both the experimental and theoretical perspectives. The application of a mean-field theory indicated that the displacement bursts exhibited a temperature-dependent power-law distribution, and the universal exponents, K and A, were computed to be 1.5 and 0.04, respectively. With the use of the computed universal exponents, a critical annealing temperature for the slip-avalanche size distribution was estimated to be 1120 degrees C. Creep occurred during the nanoindentation holding period and exhibited very large stress exponent, implying that the dislocation glide-climb is the dominant mechanism. The creep simulations with a two-layer viscoplastic model further revealed that the deformation at a higher temperature (e.g., 200 degrees C) featured a greater and faster-growing plastic zone underneath the indenter, implying more pronounced dislocation activities. (C) 2018 Elsevier B.V. All rights reserved.