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For the mechanical behaviors of the BMGs, Rezvan I et al. i report the deformation-mode-sensitive behavior under dynamic loading. This topical collection is to honor Professor Peter K. Liaw for his significant contributions to materials science and engineering and to The Minerals, Metals & Materials Society (TMS). Besides HEAs, Wang I et al. i report the formation and mechanical behavior of a body-centered-cubic medium-entropy alloys (MEAs). [Extracted from the article]

To understand the underlying strengthening mechanisms, thermal activation processes are investigated from stress-strain measurements with varying temperatures and strain rates for a family of equiatomic quinary, quaternary, ternary, and binary, face-center-cubic-structured, single phase solid-solution alloys, which are all subsystems of the FeNiCoCrMn high-entropy alloy. Our analysis suggests that the Labusch-type solution strengthening mechanism, rather than the lattice friction (or lattice resistance), governs the deformation behavior in equiatomic alloys. First, upon excluding the Hall-Petch effects, the activation volumes for these alloys are found to range from 10 to 1000 times the cubic power of Burgers vector, which are much larger than that required for kink pairs (i.e., the thermal activation process for the lattice resistance mechanism in body-center-cubic-structured metals). Second, the Labusch-type analysis for an N-element alloy is conducted by treating M-elements (M < N) as an effective medium and summing the strengthening contributions from the rest of N-M elements as individual solute species. For all equiatomic alloys investigated, a qualitative agreement exists between the measured strengthening effect and the Labusch strengthening factor from arbitrary M to N elements based on the lattice and modulus mismatches. Consequently, the Labusch strengthening factor provides a practical critique to understand and design such compositionally complex but structurally simple alloys. [ABSTRACT FROM AUTHOR]

There have been numerous efforts to develop creep-resistant materials strengthened by incoherent particles at high temperatures and stresses in response to future energy needs for steam turbines in thermal-power plants. However, the microstructural instability of the incoherent-particle-strengthened ferritic steels limits their application to temperatures below 900 K. Here, we report a novel ferritic alloy with the excellent creep resistance enhanced by coherent hierarchical precipitates, using the integrated experimental (transmission-electron microscopy/scanning-transmission-electron microscopy, in-situ neutron diffraction, and atom-probe tomography) and theoretical (crystal-plasticity finite-element modeling) approaches. This alloy is strengthened by nano-scaled L21-Ni2TiAl (Heusler phase)-based precipitates, which themselves contain coherent nano-scaled B2 zones. These coherent hierarchical precipitates are uniformly distributed within the Fe matrix. Our hierarchical structure material exhibits the superior creep resistance at 973 K in terms of the minimal creep rate, which is four orders of magnitude lower than that of conventional ferritic steels. These results provide a new alloy-design strategy using the novel concept of hierarchical precipitates and the fundamental science for developing creep-resistant ferritic alloys. The present research will broaden the applications of ferritic alloys to higher temperatures. [ABSTRACT FROM AUTHOR]

The article discusses a symposium on Bulk Metallic Glasses (BMG) at 2013 TMS Annual Meeting and Exhibition in San Antonio, TX on March 3-7, 2013. Topics include alloy development and application, mechanical behavior, microstructure characterization. The symposium featured several officials including Mary Lynn Brown, David E. Laughlin, and Dora Moscatello.

[Display omitted] Stacking fault energy (SFE) in fcc materials is a fundamental property that is closely related to Shockley partial dislocations and deformation twinning, the latter of which is potentially responsible for some of the exceptional mechanical properties observed in Ni-based high/medium-entropy alloys. In this study, the SFEs and twinning energies over a wide range of compositions are systematically determined in model Ni-based binary alloys using both first-principles density functional theory (DFT) and atomistic simulations with interatomic potentials. Particularly, different compositional dependences of SFEs are observed in the selected binary alloys (Ni-Cu, Ni-Co, and Ni-Fe) from DFT calculations. We find that SFEs of Ni-Co follow a linear-mixing rule while Ni-Cu and Ni-Fe systems exhibit non-linear compositional dependences, especially in the concentrated region towards the center of the phase diagram. Analyses of the magnetic structures help clarify the origins of the non-linear dependences. The fidelity of existing interatomic potentials for these alloys is evaluated against DFT. Using the interatomic potentials with the overall highest fidelity, the SFE calculations are extended to Cantor-related ternary alloys (Ni-Co-Cr and Ni-Co-Fe) and the spatial features of the fault energies in atomistic simulations are also discussed. These results provide the basis for a systematic understanding of the compositional effects on the SFEs and twinning energies, which could be useful for a systematic tuning of mechanical properties in non-equimolar alloys, thus leading to a broad field in alloy design. [ABSTRACT FROM AUTHOR]

Fatigue properties are essential for structural materials in industrial applications. High-entropy alloys are promising fatigue-resistant alloys with high strengths and good ductilities. In this study, the B2 precipitate significantly increased the yield strength of the Al 0.3 CoCrFeNi high-entropy alloy but at the same time decreased the low-cycle fatigue life. A detailed investigation of the underlying mechanisms will provide critical insights into the precipitate effect on the fatigue performance of high-entropy alloys. Here, the Al 0.3 CoCrFeNi alloys with single face-centered-cubic (FCC) phase and with FCC + B2 phases are used for the investigation of the low-cycle fatigue behavior by combining in situ neutron diffraction measurements and elastic-viscoplastic self-consistent modeling. The simulation results indicate that the B2 precipitates highly increased the back stresses during cyclic loading by facilitating the kinematic hardening in the matrix, while the effect of dynamic recovery is relatively weak. The increase in back stresses provides more fatigue-crack-initiation sites and thus the damage accumulation, which will further lead to a shorter fatigue life. [Display omitted] • The B2 precipitates increased the yield strength and decrease the LCF life in the Al 0.3 CoCrFeNi HEA. • Lattice strain and full-width half-maximum results were obtained from in situ neutron diffraction. • Diffraction experiments and crystal plasticity modeling reveal the role played by B2 precipitates. [ABSTRACT FROM AUTHOR]

A new single-phase face-centered-cubic (FCC) Co 9 Cr 7 Cu 36 Mn 25 Ni 23 [atomic percent, similar hereinafter] high-entropy alloy (HEA) was prepared by arc melting. A uniform distribution of nanometer-sized precipitates was achieved. The tensile yield strength, ultimate tensile strength, and elongation were 401 MPa, 700 MPa, and 36%, respectively. The energy-dispersive spectrometer results showed that the nano-precipitates were rich in Co and Cr elements. Moreover, the crystal-forming behavior and the nanoscale-precipitates-forming mechanism were revealed. The nano-precipitates phase forming process. Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract A fundamental understanding of the fatigue and fracture behavior of bulk metallic glasses (BMGs) and their composites is of critical significance for designing new BMG systems and developing new manufacturing and processing techniques so as to broaden the scope of applications of BMGs and their composites. However, the fatigue and fracture studies on BMGs are limited so far, compared to other mechanical properties. The present work reviews the fatigue and fracture behavior of BMGs and their composites, as well as that of metallic-glass films, ribbons, and wires. The grand challenge for the fatigue and fracture performance of BMGs is: What produces a large difference among the fatigue and fracture results of BMGs? According to the fatigue and fracture investigations of crystalline alloys including recently invented high entropy alloys, many factors could be involved, such as the composition, material quality, specimen geometry, chemical environment, surface condition, temperature, cyclic frequency, mean stress, and residual stress, etc. Based on this challenge, the present work will review and address the factors affecting the fatigue and fracture behavior of BMGs and their composites. Furthermore, the mechanisms of fatigue-crack initiation, propagation, and fracture of BMGs and their composites in different loading conditions and environments will be outlined, analyzed, and discussed. Future research directions of fatigue and fracture of BMGs and their composites are provided for reference. [ABSTRACT FROM AUTHOR]

• A novel high entropy alloy (HEA), Al 0.3 CoCrFeNi, is found to exhibit minimum creep strain rates that are several orders of magnitude lower than typical HEAs. • Atom probe tomography and other advanced characterization tools reveal the formation of coherent L1 2 precipitates in early stage of creep deformation, followed by B2 precipitates in later stages. • The enhanced creep resistance is quantitatively evaluated by various dislocation strengthening mechanisms. Improving creep resistance has commonly been achieved by the optimization of alloy design that results into strong solid-solution strengthening and/or coherent precipitates for dislocation blockage. High-entropy alloys (HEAs), despite their single-phase solid-solution nature, only exhibit creep properties that are comparable to precipitate-strengthened ferritic alloys. Moreover, many HEAs are found to be plagued with many incoherent second phases after long-term annealing, which reduces the lifetime and thus prohibits their usage at elevated temperatures. The present work demonstrates the extraordinary creep resistance of a non-equiatomic Al 0.3 CoCrFeNi HEA, in which the creep strain rate is found to be several orders of magnitude lower than the Cantor alloy and its subsets. Using a suite of characterization tools such as atom probe tomography (APT) and transmission electron microscopy (TEM), it was shown that a B2 precipitate phase that has been widely seen during annealing is suppressed during the early stage of the creep deformation. Currently, metastable and coherent L1 2 precipitates emerge and provide significant creep strengthening. This observation is rationalized by the coupling between the applied stress and the lattice mismatch. In the range of 973 ∼ 1033 K, the stress exponent and activation energy were determined to be 3–6.53 and 390–548.2 kJ·mol−1, respectively. The creep lifetime, on the other hand, is comparable to Cantor subset alloys because the precipitate free zone near the grain boundaries does not provide sufficient constraint for the grain boundary cavity growth. The present work provides a pathway to design novel HEAs with improved creep resistance. [Display omitted] [ABSTRACT FROM AUTHOR]

• The creep resistance is the highest in CoCrNi, followed by CoCrFeMnNi and then CoCrFeN alloys, while the Monkman-Grant relationship gives similar failure strain. • While the room and cryogenic temperature strength is governed by solid solution effects, the contrast of creep behavior in these subset alloys is primarily governed by the stacking fault energy. • The best performing CoCrNi has comparable creep resistance as commonly used precipitate/martensite-strengthened ferritic alloys. Long-term tensile creep experiments were performed at 973 K on a family of multicomponent equiatomic solid solution alloys with face-centered cubic crystal structures, including quinary CoCrFeMnNi alloy and quaternary CoCrFeNi alloy, together with our previous report on ternary CoCrNi alloy. Analyzing the steady-state and transient creep properties and characterizing the precipitate evolution, it is suggested that dislocation creep be the dominant deformation mechanism for all these alloys. Although CoCrNi shows the highest room-temperature strength and the lowest creep rate at 973 K, the creep lifetime data for all three alloys are similar and can be described by the Monkman-Grant relationship. [Display omitted] [ABSTRACT FROM AUTHOR]

Unlike many Mg alloys that exhibit limited ductility, duplex Mg–Li alloys possess enhanced ductility but the underlying mechanisms are unclear. Using real-time in situ neutron diffraction measurements, we show that the underlying deformation modes in this duplex microstructure include an early yield of body-centered-cubic (BCC) β-Li phase and the later elevated hardening in this phase at large macroscopic plastic strain, and the sequential activation of basal and pyramidal < a > slip systems in hexagonal-close-packed (HCP) α-Mg phase. The latter relieves the Mises constraint for deformation compatibility, and the successive yielding sequence promotes the overall work hardening rate, both of which are beneficial for ductility enhancement. No obvious twinning activities were found and correspondingly the hysteresis loops were symmetric upon a full loading cycle within ±1% applied strain. [ABSTRACT FROM AUTHOR]

Research in the field of high-entropy alloys has been surging since the 2010s. As widely acknowledged, research interests in this field are largely sparked by the enormous possibilities in compositions, microstructures, and properties. In the first decade of extensive investigations, while many avenues of research in this vibrant field are being continuously opened, a considerable portion of endeavors is directed toward two very fundamentally important aspects. One is the exploration of the huge compositional space and microstructures with the aid of thermodynamics and kinetics, while the other is the characterization and comprehension of their mechanical behavior from fundamental and practical perspectives. Each of these two aspects contains numerous, scattered, sometimes confusing or conflicting data and findings in the literature. Thanks to several excellent comprehensive reviews [1–3], the accomplishments and challenges in the first aspect are neatly presented to researchers in the field. Nonetheless, the second aspect, i.e. , the mechanical behavior of high-entropy alloys, is still largely afloat in abundant literature in a spite of some recent reviews on certain specialized topics [4–6]. In consideration of this insufficiency, the present review intends to offer a comprehensive, critical review of the mechanical behavior of high-entropy alloys and some closely related topics. [ABSTRACT FROM AUTHOR]

• Oxidation in thermal shocks was comparably analyzed to isothermal oxidation. • Cracks in thermal shocks were initiated by thermal stress and propagated following Paris's Law. • The cracking behavior during thermal shocks is oxidation-accelerated cracking. • The oxidation behavior during thermal shocks is stress-assisted and crack-accelerated oxidation. Oxidation and cracking behavior during thermal-shock experiments of a new Co-based alloy were analyzed in the present work. Comparing to the static conditions, the oxidation behavior during thermal shocks is stress-assisted and crack-accelerated. Cracks during the thermal shocks were initiated from the sample surfaces and their growth are driven by various stresses, including those induced by volume change, mismatch of coefficients of thermal expansion (CTEs), and temperature gradient between the matrix and the internal oxides. It is found that the crack propagation correlates to these stresses in the classic Paris' law. [ABSTRACT FROM AUTHOR]

Tensile creep behavior of the CoCrNi medium entropy alloy in a temperature range of 973–1073 K was investigated in this study. According to the fitted stress exponent and activation energy, dislocation climb and lattice diffusion are proposed to be the dominated creep deformation mechanism for this alloy. During creep tests, the plate-like Cr-rich σ phase forms along the grain boundaries of the originally single-phase face-centered-cubic (FCC) CoCrNi alloy. It is suggested that low stacking-fault energy and local chemical ordering be responsible for the superior creep resistance and lifetime as opposed to that of the CoCrFeMnNi alloy. Image 1 • CoCrNi alloys have comparable high-temperature properties to other commercial steels, but better than CoCrFeMnNi alloys. • Their superior creep properties derive from the solid solution strengthening and low stacking faculty energy. • During creep tests, the plate-like Cr-rich σ phase forms along the grain boundaries. [ABSTRACT FROM AUTHOR]