查看更多>>摘要：? 2022 Acta Materialia Inc.Large thermal hysteresis (Thys) is favorable in shape memory alloys (SMAs) because it renders wide service and storage temperature range of the SMA-made parts. In this letter, we show that large intrinsic Thys up to 100 K is achieved in a low Nb content (2 at.%), β-Nb-free and single-phase NiTiNb SMA fabricated by casting, forging, wire-drawing and annealing. Such a large Thys is comparable to that of the widely used dual-phase NiTiNb SMAs in which high Nb content and β-Nb are required. By exploiting the grain-size-dependence of Thys in our single-phase NiTiNb SMA and that in an equiatomic NiTi SMA, we attribute the large Thys to the enhanced kinetic resistance of transformation by the small grains. The connection between the Thys and the kinetic resistance is demonstrated by a modified dislocation-based kinetic model.
查看更多>>摘要：? 2022 Acta Materialia Inc.As the reverse process of relaxation, rejuvenation is the structural excitation process that can bring metallic glasses (MGs) to a higher energy state and usually increases their free volume. Here, using a dilution procedure conducted by randomly removing atoms from the modeled glass matrix, the degree of rejuvenation can be systematically controlled and the maximum rejuvenation threshold of MGs is identified. The structural relaxation is activated during the rejuvenation process and the dynamic balance between free volume creation and annihilation defines the rejuvenation ability of MGs. The highest degree of rejuvenation correlates to the flow strain of the materials and the structure is similar to that found in shear bands.
查看更多>>摘要：? 2022 Acta Materialia Inc.The outstanding thermal stability of coherent γ′ phases at 1073 K in Ni46Cr23Fe23Al4Ti4 and Ni46Cr23Co23Al4Ti4 medium-entropy alloys were systematically investigated. The coarsening behaviors of coherent γ′ phase in the investigated alloys follow the classical Lifshitz-Slyozov-Wagner (LSW) model. The γ/γ′ lattice misfit plays an important role in the morphology evolution and coarsening behavior of γ′ phase. The lower γ/γ′ lattice misfit helps to maintain the spherical morphology of γ′ phase and leads to a lower γ/γ′ interfacial energy. The small γ/γ′ lattice misfit (0.087%) and interfacial energy (1.73 mJ/m2) in Ni46Cr23Fe23Al4Ti4 alloy result in the sluggish coarsening behavior of coherent γ′ phase. Reducing γ/γ′ lattice misfit and decreasing interfacial energy should be effective strategies for developing γ′-strengthened high/medium-entropy alloys for high-temperature application.
查看更多>>摘要：? 2022Phase transition of the η-MgZn2 Laves phase from C14 to C15 structure in peak aged Al-Zn-Mg-Cu alloy were studied by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and density functional theory (DFT) calculations. The formation of a well ordered C15 structural MgZn2 as independent precipitate is for the first time observed in Al-Zn-Mg-Cu alloy. The C15 structure is confirmed to be transformed from the C14 by the activation of synchroshear in every other glide plane, which involves the shear induced by two Schockley partial dislocations in the triple-layer of the Laves phase. Besides, the twin-like structure and five-fold symmetry twin are formed for composing C15 structure due to multiple synchroshear. These results expand the structural understanding of the η-MgZn2 precipitates in Al-Zn-Mg-Cu alloy.
查看更多>>摘要：? 2022 Acta Materialia Inc.In α-phase dominant Ti alloys, the α/α boundaries are the main interfaces in the microstructure in terms of both their distribution and total area. This work reports identification of unusual large zones of Category I triple-α-variant clusters in laser metal (powder) deposited (LMD) Ti-4Al-2V alloy. These clusters lead to a continuous distribution of the Type 2 α/α boundary ([112ˉ0]/60°) in the microstructure, which possesses lower boundary energy than the Types 3–6 α/α boundaries, where the distribution of Type 1 α/α boundary is negligible. The formation of these large zones of Category I clusters can be understood using the autocatalysis nucleation and subsequent chain reaction theory suggested by J W Christian based on self-accommodation. These experimental observations imply that it is perhaps plausible to produce low energy α-variant boundary configurations through exploiting the confluence of alloy composition and additive manufacturing conditions.
查看更多>>摘要：? 2022Hard β-Ta phase is generally produced during Ta film deposition. While in some cases, soft α-Ta phase is preferred to ensure deformation capability. In this work, templated growth strategy by introducing Cr buffer layer is proposed to tailor the phase composition in Ta films. A broad spectrum of β-Ta/α-Ta ratio from 90% to 0% can be continuously and precisely achieved when the buffer layer thickness is correspondingly controlled from 0 to ～90 nm. Hardness of the Ta films is well proportional to the β-Ta/α-Ta ratio. The underlying mechanism of phase tailoring is rationalized by demonstrating the thickness-dependent crystallization of buffer layer.
查看更多>>摘要：? 2022 Acta Materialia Inc.The method of Si substituting for Al was proposed to design a novel hot-rolled lightweight steel with ultra-high ultimate tensile strength (2046 MPa) and good total elongation after fracture (7.4%). The good ductility mainly originates from high screw dislocation fraction and transformation strain of retained austenite due to Si addition. This work breaks the impression of Si making steel brittle and deteriorate for many years and opens up a new route for alloy design through adjusting dislocation character and transformation strain.
查看更多>>摘要：? 2022 Acta Materialia Inc.Formation of nano-twinned domains associated with ferro-elastic transformation in 2 mol.% ZrO2 alloyed YTaO4 has been investigated using a correlative microscopy approach. Transmission Kikuchi diffraction reveals the presence of two phases, monoclinic (M) and tetragonal (T) in the material sintered at 1485 °C for 3 hr. The microstructure comprises of alternate nano-twinned domains with an average spacing of 154.07 ± 42.52 nm. Crystallographic symmetry based spontaneous strain analyses support the observation of single domain state. Ab-initio calculations further indicate the propensity of stabilizing both M and T phases at room temperature as a function of ZrO2 alloying, without any change in chemical composition, as is evidenced by atom probe tomography.
查看更多>>摘要：? 2022 Acta Materialia Inc.Silver nanodendrite has gained the interest of scholars because of its broad range of applications for its unique features. The growth of nanodendrites typically takes place at the epitaxial intra-dendrite interface between the single-crystal primary and secondary dendrites. In this paper, we reported for the first time a newly identified “non-epitaxial” intra-dendrite interface in silver nanodendrites. The silver nanodendrites were fabricated using a water drop electrodeposition method to induce electrochemical migration in pure water solution under applied bias. The non-epitaxial intra-dendrite interface featured a peculiar 3–5 nm helical nano-yarn structure which can support the growth of a high-volume secondary dendrite due to its interfacial strengthening characteristic. The volume or mass supported through the non-epitaxial intra-dendrite interface was 282 times greater than the one through the typical epitaxial one. This study is believed to give new understandings of the non-epitaxial dendrite growth via nano-yarn bundling and its strengthening mechanism.
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