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Uniaxial tension tests were performed on pure polycrystalline copper, iron, and titanium specimens with various applied constant (dc) current levels and at matching temperatures, i.e., zero current with temperature histories matched to the current tests. The experiments achieved uniform strain, current density, and temperature conditions along the specimen gage length for unambiguous interpretation of the test data. The results showed non-thermal current effects only with the titanium; 20% reduction in ultimate strength with respect to the strength from the matching temperature tests was observed as well as significant inhomogeneous grain growth. No discernable changes in microstructure were observed in specimens deformed at matching temperatures or with applied current but no deformation (matching temperatures). The electron-wind and local Joule heating mechanisms for electrically-assisted deformation (EAD) do not produce effects large enough to explain the observed titanium results. Dislocation scattering by thermal phonons and electrons associated with the radial and axial heat fluxes generated in the titanium tensile specimens with bulk Joule heating is suggested as a potential mechanism for the observed EAD effects. The experimental results and the possible link to thermal phonon/electron scattering suggests several new avenues of research for understanding EAD of metals.

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The (110) and (320) surfaces of the single-phase FeCrMnNiCo solid solution have been studied on two adjacent millimeter size grains using surface science and transmission electron microscopy (TEM) techniques. The structural and chemical evolutions of the high entropy alloy (HEA) surfaces have been determined for various sputtering conditions, annealing temperatures and durations. Up to 873 K, angle-resolved X-ray photoelectron spectroscopy measurements indicate a clear Mn and Ni surface co-segregation. We propose that the surface segregation of Mn is driven by its low surface energy. The attractive interaction between Mn and Ni promotes Ni segregation which accompanied the Mn diffusion to the surface. Regarding the structures investigated by low energy electron diffraction and scanning tunneling microscopy, the (320) surface presents a terraced morphology with an ordered structure consistent with a (1×1) termination. On the contrary, the (110) surface reveals an important degree of structural disorder and local reconstructions. Its highly anisotropic morphology resembles rows propagating along the [001] direction. Above 873 K, Mn desorption occurs while the Ni content keeps increasing linearly with the temperature. TEM analysis show no evidence for HEA decomposition into metallic or intermetallic phases even after repeated annealing and sputtering cycles. The above results set the upper temperature limit above which the surface stoichiometry departs from the quinary HEA concept. It also defines the temperature range for the use of FeCrMnNiCo based coating under high vacuum conditions and for aerospace applications.

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Mg–Zn–Y ternary alloys containing the long-period stacking ordered (LPSO) phase exhibit superior mechanical properties. This is believed to be originating from the LPSO phase acting as the strengthening phase. However, we first clarify that the mechanical properties of the matrix Mg solid solution in the Mg/LPSO two-phase alloy are significantly different from those of pure Mg. The yield stress of a Mg_99.2Zn_0.2Y_0.6 single crystal (matrix Mg solid solution) is almost the same as that of an LPSO single-phase alloy. This is ascribed to the formation of thin stacking-fault-like defects, named “LPSO nanoplate”. In Mg_99.2Zn_0.2Y_0.6, kink-band formation is induced in the same manner as that in the LPSO phase in deformation, resulting in high strength accompanied with increased ductility. Our results suggest that the strengthening mechanism of the Mg/LPSO two-phase alloy must be reconsidered depending on the microstructure. Furthermore, the results suggest that new ultrahigh-strength Mg alloys, which have much lower Zn and Y contents but the mechanical properties are comparable or superior than the present Mg/LPSO two-phase alloys, are expected to be developed via the appropriate control of LPSO nanoplate microstructures.

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向上滑動閱覽英文摘要

Single-crystal or coarse-grained nanoporous gold (NPG) prepared by dealloying is considered stable against sintering or densification during annealing. Herein, we report that this phenomenon only applies to high-density NPG with relative density (φ) exceeding a critical value of 0.31, which densifies very slightly during annealing. In low-density NPG with φ < 0.31, annealing induces excessive shrinkage that increases with decreasing φ. Sintering along grain boundaries may account for the small densification of high-density NPG, but it fails to explain the excessive shrinkage of low-density NPG. This densification may be linked to ligament pinch-off, which dominates the coarsening of NPG with φ below ∼0.30. However, ligament pinch-off does not directly lead to densification. Instead, we suggest that reattachment of dangling ligaments (generated by pinch-offs) is responsible for the large shrinkage in low-density NPG. This break-and-reconnect scenario is supported by quasi-in situ scanning electron microscopy observations and the evolution of scaled genus density for different type of NPGs during annealing. Our findings suggest that annealing-induced densification can be suppressed by increasing the relative density of NP metals to above 0.31, which is critical to many mechanical and functional explorations.

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向上滑動閱覽英文摘要

In this study, we investigate the effect of the heterogeneous micromechanical stress fields resulting from the grain-scale anisotropy on the redistribution of hydrogen using a diffusion coupled crystal plasticity model. A representative volume element with periodic boundary conditions was used to model a synthetic microstructure. The effect of tensile loading, initial hydrogen content and yield strength on the redistribution of lattice (C_L) and dislocation trapped (C_x) hydrogen was studied. It was found that the heterogeneous micromechanical stress fields resulted in the accumulation of both populations primarily at the grain boundaries. This shows that in addition to the well-known grain boundary trapping, the interplay of the heterogeneous micromechanical hydrostatic stresses and plastic strains contribute to the accumulation of hydrogen at the grain boundaries. Higher yield strength reduced the amount of C_x due to the resulting lower plastic deformation levels. On the other side, the resulting higher hydrostatic stresses increased the depletion of C_L from the compressive regions and its diffusion toward the tensile ones. These regions with increased CL are expected to be potential damage initiation zones. This aligns with the observations that high-strength steels are more susceptible to hydrogen embrittlement than those with lower-strength.

Vol. 209，1 May. 2021, 116796