The impurity sulfur embrittlement of Ni-based single alloys has long been wrapped in mystery. In this work, we report for the first time the sulfur-induced precipitation of brittle rhenium-rich & ouml; phase in a model Ni-based single alloy. The results showed that the addition of sulfur gave rise to the precipitation of rhenium-rich & ouml; phase with hexagonal close-packed (HCP) crystal structure in the dendritic cores. Besides, the enrichment of sulfur and nickel elements were detected in the & ouml; phase. The behavior of sulfur-induced precipitation of & ouml; phase was rationalized by the first-principles calculations, which indicates that there is a strong bonding interaction between sulfur and rhenium atoms. This work provides a novel perspective on the understanding of the sulfur embrittlement in Ni-based single alloys.

In the field of multi-principal element alloys (MPEAs), developing face-centered cubic (FCC) structured alloys with optimized strength-plasticity balance remains a critical challenge. This study systematically explores the impact of Mo content on the microstructure and mechanical properties of (CoNi)80-xCr10Fe10Mox alloys. With increasing Mo content, the alloy changes from a single phase to a dual phase structure. At the same time, the stacking fault energy decreases from 20.25 mJ/m2 of C-Mo5 to 19.62 mJ/m2 of C-Mo13. After thermomechanical processing, mu phase precipitates along grain boundaries in A-Mo13, which enhances the tensile strength of the alloy. Compared with the single-phase A-Mo9 alloy, the tensile strength of the A-Mo13 alloy increases by 300 MPa, with only an 8% loss in plasticity. The combined strengthening mechanisms primarily include grain refinement and second phase strengthening. Through the synergistic effects of thermo-mechanical treatment and mu -phase precipitation, the FCC grains were refined from 150 mu m to 7.4 mu m. Simultaneously, as a hard second phase, the Mo-rich phase impedes dislocation movement and alters the crack propagation path, thus improving the tensile strength and maintaining good plasticity. The findings of this study provide valuable theoretical and practical insights for the strengthening of FCC MPEAs.

To overcome the inherent strength-ductility trade-off in additively manufactured CoCrFeNi high-entropy alloys (HEAs), this study presents an innovative in-situ reactive alloying strategy. Using laser-directed energy deposition (L-DED) in ambient atmosphere, we fabricated (CoCrFeNi)94Al4Ti2 HEAs, where the intentional addition of Al and Ti facilitated the formation of nanoscale Al2O3/TiN core-shell structures through reactive synthesis with atmospheric elements. Microstructural characterization revealed a uniform dispersion of these core-shell nano-particles within a refined face-centered cubic (FCC) matrix. Advanced transmission electron microscopy (TEM) analysis identified the unique architecture of these reinforcing phases, consisting of Al2O3 cores enveloped by TiN shells, with an average particle size of 200 nm. These nanostructures effectively promoted grain refinement and increased the population of high-angle grain boundaries. The resulting material exhibited exceptional mechanical properties, achieving an optimal combination of ultimate tensile strength (681.5 MPa) and elongation (44.6 %), along with remarkable work-hardening capability. The strengthening mechanism is attributed to a synergistic effect: the Al/Ti addition simultaneously promotes the formation of oxide/nitride nanoparticles while enhancing matrix dislocation density. This approach successfully circumvents the conventional limitations of second-phase strengthening, demonstrating a novel pathway for designing high-performance HEAs with balanced mechanical properties through in-situ reactive additive manufacturing.

Understanding the plastic behavior of crack tips is crucial for improving the fracture toughness of nanometals. Although many studies are carried out, most previous studies focus on pure metals, and how the crack tip accommodates the plastic deformation of highly concentrated solid‐solution alloys is unclear owing to a lack of direct atomic‐scale evidence. In this study, the atomic‐scale plastic behavior of the crack tip in face‐centered cubic (FCC) AuCu alloy nanocrystals is observed in situ, which provides direct evidence that plastic deformation is governed by the generation of deformation twins and hexagonal close‐packed (HCP) 2H and 4H phases, recurrence of reversible FCC‐HCP phase transitions, and detwinning, which are rarely observed in pure metals. This unusual behavior originates from the inherent chemical inhomogeneity of the AuCu alloy, which inhibits twin thickening via partial dislocations on the adjacent plane, instead of random generation of deformation twins, phase transitions, and reversible processes. This naturally implies a similar behavior at the crack tip in other highly concentrated solid‐solution alloys, including high‐medium‐entropy alloys, providing important insights that greatly improve the understanding of the fracture toughness of metallic materials.

Nanosized metals usually exhibit ultrahigh strength but suffer from low homogeneous plasticity. The origin of a strength–ductility trade-off has been well studied for pure metals, but not for random solid solution (RSS) alloys. How RSS alloys accommodate plasticity and whether they can achieve synergy between high strength and superplasticity has remained unresolved. Here, we show that face-centered cubic (FCC) RSS AuCu alloy nanowires (NWs) exhibit superplasticity of ~260% and ultrahigh strength of ~6 GPa, overcoming the trade-off between strength and ductility. These excellent properties originate from profuse hexagonal close-packed (HCP) phase generation (2H and 4H phases), recurrence of reversible FCC-HCP phase transition, and zigzag-like nanotwin generation, which has rarely been reported before. Such a mechanism stems from the inherent chemical inhomogeneity, which leads to widely distributed and overlapping energy barriers for the concurrent activation of multiple plasticity mechanisms. This naturally implies a similar deformation behavior for other highly concentrated solid-solution alloys with multiple principal elements, such as high/medium-entropy alloys. Our findings shed light on the effect of chemical inhomogeneity on the plastic deformation mechanism of solid-solution alloys.

Permanent structural changes in pure metals that are caused by plastic activity are normally irreparable after unloading. Because of the lack of experimental evidence, it is unclear whether the plastic activity can be repaired as the size of the pure metals decreases to several nanometers; it is also unclear how the metals accommodate the plastic deformation. In this study, the in situ atomic-scale loading and unloading of ∼2 nm Ag nanocrystals was investigated, and three modes of plastic deformation were observed: (i) the phase transition from the face-centered cubic (fcc) phase to the hexagonal close-packed (hcp) phase, (ii) stacking faults, and (iii) deformation twin nucleation. We show that all three modes resulted in structural changes that were reparable, and their generation and restoration during loading and unloading were observed in situ. We discovered that the deformation modes of nanosized metals can be predicted from the ratio of the energy barriers of the fcc-hcp phase transition (ΔγH) and the deformation twin nucleation (ΔγT), which differ from those of the theoretical modes of relatively large-sized metals. The proposed ΔγH/ΔγT criterion provides insights into the deformation mechanism of nanometals.

The mechanical stability of metallic nanomaterials has been intensively studied due to their unique structures and promising applications. Although extensive investigations have been carried out on the deformation behaviors of metallic nanomaterials, the atomic-scale deformation mechanism of metallic nanomaterials with unconventional hexagonal structures remains unclear because of the lack of direct experimental observation. Here, we conduct an atomic-resolution in situ tensile-straining transmission electron microscopy investigation on the deformation mechanism of gold nanoribbons with the 4H (hexagonal) phase. Our results reveal that plastic deformation in the 4H gold nanoribbons comprises three stages, in which both full and partial dislocations are involved. At the early deformation stage, plastic deformation is governed by full dislocation activities. Partial dislocations are subsequently activated in regions that have undergone full dislocation gliding, leading to phase transformation from the 4H phase to the face-centered cubic (FCC) phase. At the last stage of the deformation process, the volume fraction of the FCC phase increases, and full dislocation activities in the FCC regions also play an important role.

Nanocrystalline metals often display a high strength up to the gigapascal level, yet they suffer from poor plasticity. Previous studies have shown that the development of hetero-sized grains can efficiently overcome the strength-ductility trade-off of nanocrystalline metals. However, whether this strategy can lead to the fabrication of nanocrystalline nanowires exhibiting both high strength and superplasticity is unclear, similar to the atomistic deformation mechanism. In this paper, we show that ultra-small nanocrystalline Au nanowires comprising grains in both the Hall–Petch and inverse Hall–Petch grain-size regions can exhibit extremely high uniform elongation（236%） and high strength（2.34 gigapascals） at room temperature. In situ atomic-scale observations revealed that the plastic deformation underwent two stages.In the first stage, the super-elongation ability originated from the intergrain plasticity of small grains via mechanisms such as grain boundary migration and grain rotation. This intergrain plasticity caused the grains in the heterogeneous-structured nanowires to grow very large. In the second stage, the superelongation ability originated from intragrain plasticity accompanied by the diffusion of surface atoms. Our results show that the hetero-grain-sized nanocrystalline nanowires, comprising grains with sizes both in the strongest Hall–Petch effect region and the inverse Hall–Petch effect region, were simultaneously ultrastrong and ductile. They displayed neither a strength-ductility trade-off nor plastic instability.

Incoherent-twin boundaries (ITBs) can significantly affect the mechanical properties exhibited bymetals. Although numerous studies have been conducted to date, the atomic structures of such ITBs remain unclear, owing to difficulties in imaging their structure. In this study, high-angle annular dark-field imaging was used to reveal the atomic structure of the ITBs present in Pt. We discovered that both the twin thickness and the dislocation-ITB interaction can affect the ITB phase structure. In thin twins, the {111} planes between the ITB remain flat without any obvious displacement along the < 111 > direction, whereas in thicker twins, the {111} planes between the ITB exhibit clear displacement along the < 111 > direction, with this displacement increasing as the twin thickness increases. The ITBs frequently absorb full dislocations, which leads to the formation of dislocation-misaligned ITBs. This twin- thickness effect and dislocation-ITB interaction, which resulted ITB-phase variation, has rarely been reported.

Understanding the deformation behavior of crack tips in metals is of great significance for improving fracture toughness. However, how crack tips in nanosized metallic alloys behave under loading is unclear, because most previous studies focused on pure metals. In this study, the atomic-scale deformation behavior of the crack tip in AuAg alloy nanocrystals was observed in situ. We revealed that the deformation mechanism near the crack tip depended on the distance from the tip. For the 'near region' close to the crack tip, plastic deformation was governed by partial dislocations, twinning, and their interactions. For the 'far region', further than -15 nm from the crack tip, full dislocations dominated, and their interactions resulted in Lomer-dislocation (LD) lock formation and destruction. We uncovered that the combination of blunting dislocation-twin interactions, twin-twin intersections, and formation and destruction of LD locks, as a previously unrecognized fracture toughness improvement mechanism in metals.

The formation of metallic glass through melt processing proves to be the most challenging for pure monatomic face-centered cubic (FCC) metals. Though it has long been conjectured that amorphous monatomic metals can be generated through deforming a solid at room temperature, there is rarely direct evidence to prove that is indeed the case. In this study, mechanical loading was applied to nanometer-sized crystals inside an aberration-corrected transmission electron microscopy, and atomic-scale in situ evidence is provided of strain-induced amorphization in Pt and Ni near room temperature. The loading was applied in such a way that the stress state is complicated, and the strain distribution is non-uniform, restricting dislocation activities in accommodating the imposed strain. The local lattice distortion is then rendered so large and the associated strain energy is so high that the crystal collapses into the amorphous state. As such, even elemental FCC metals can be forced to become amorphous.

Metals usually have three crystal structures: face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal-close packed (hcp) structures. Typically, metals exhibit only one of these structures at room temperature. Mechanical processing can cause phase transition in metals, however, metals that exhibit all the three crystal structures have rarely been approached, even when hydrostatic pressure or shock conditions are applied. Here, through in situ observation of the atomic-scale bending and tensile process of ∼5  nm-sized Ag nanowires (NWs), we show that bending is an effective method to facilitate fcc-structured Ag to access all the above-mentioned structures. The process of transitioning the fcc structure into a bcc structure, then into an hcp structure, and finally into a re-oriented fcc structure under bending has been witnessed in its entirety. This re-oriented fcc structure is twin-related to the matrix, which leads to twin nucleation without the need for partial dislocation activities. The results of this study advance our understanding of the deformation mechanism of small-sized fcc metals.

本文利用原位实验技术,在透射电镜中对AuAg纳米晶薄膜裂纹扩展以及裂纹前端塑性机制进行了原位原子尺度观察.原位观察发现,在初期,裂纹在晶界处形核并沿着晶界扩展,在裂纹前端晶粒内部有形变孪晶的形核与长大.随着应变的增加,逐渐转为穿晶断裂,此时晶粒内的生长孪晶及变形孪晶均展现出良好的塑性变形能力.本实验对人们理解面心立方合金纳米晶材料的裂纹扩展机制具有一定借鉴意义.

Detwinning is an important plastic deformation mechanism that can significantly affect the mechanical properties of twin-structured metals. Although many detwinning mechanisms have been proposed for pure metals, it is unclear whether such a deformation model is valid for nanocrystalline alloys because of the lack of direct evidence. Here, the atomicscale detwinning deformation process of a nanocrystalline AuAg alloy with an average grain size of ∼15 nm was investigated in situ. The results show that there are three types of detwinning mechanisms in nanocrystalline AuAg alloys. The first type of detwinning results from grain boundary migration. The second type of detwinning occurs through combined layer-by-layer thinning and incoherent twin boundary migration. The last one occurs through incoherent twin boundary migration, which results from the collective motion of partial dislocations in an array.

Incoherent twin boundaries (ITBs) can significantly affect the mechanical properties of twin-structured metals. However, most previous studies have focused on the deformation mechanism of the coherent twin boundary (CTB), and metals with ITB-accommodated plasticity still require further investigation. In this study, deformation mechanisms of FCC-structured nanocrystal metals with ITBs were investigated using molecular dynamic (MD) simulations. We revealed that three deformation mechanisms occur in metals with ITBs. The first type of deformation was observed in Au, where the plasticity is governed by partial dislocation intersections with CTBs or reactions with each other to form Lomer–Cottrell (L–C) locks. In the second type, found in Al, the deformation is governed by reversible ITB migration. The third type of deformation, in Ni and Cu, is governed by partial dislocations emitted from the ITB or the tips of the stacking faults (SFs). The observed L–C lock formation, as well as the reversible ITB migration and partial dislocation emission from the tips of SFs, have rarely been reported before.

Metallic nanowires usually exhibit ultrahigh strength but suffered low ductility. Previous studies on pure metals suggested this strength-ductility trade-off results from limiting the dislocation activities. However, it is unclear whether such deformation model is valid for a solid solution alloy as well. Here, for the first time, the atomic-scale deformation process of AuCu nanowires with size of-16 nm was investigated in situ. The results show the NWs exhibit superplasticity (-185%) and high strengths (-2.98 GPa) at room temperature. It was discovered that superplasticity originates from continuous full dislocation nucleation and disappearance, as well as dislocation dipole formation and annihilation etc., which differ from the previous studies in pure metals. The observed full dislocation activities, also different from the ones in the previous studies, suggested that, as the size of the metals is below-100 nm, their deformation should be governed by partial dislocation and twinning. (c) 2021 Elsevier Ltd. All rights reserved.

Negative magnetoresistance (MR) is not only of great fundamental interest for condensed matter physics and materials science, but also important for practical applications, especially magnetic data storage and sensors. However, the microscopic origin of negative MR is still elusive and the nature of the negative MR in magnetic topological insulators has still not been completely elucidated. Here, we report magnetotransport studies on Cr doped (Bi1-x Sb x )2Te3 topological insulator thin films grown by magnetron sputtering. At the temperature of 2 K, a giant negative MR reaching 61% is observed at H = 2 T. We show that the negative MR is closely related to the position of the Fermi level, and it reaches the maximum when the Fermi level is gated near the charge neutral point. We attribute these results to the Coulomb potential due to the random composition fluctuations in Cr doped (Bi1-x Sb x )2Te3. Our results provide a deeper insight into the mechanism of negative MR, and are helpful to realize the quantum anomalous Hall effect in the sputtered Cr-(Bi1-x Sb x )2Te3 thin-film systems by tuning the Fermi level and reducing disorder effects.

Twin-thickness-controlled plastic deformation mechanisms are well understood for submicron-sized twin-structural polycrystalline metals. However, for twin-structural nanocrystalline metals where both the grain size and twin thickness reach the nanometre scale, how these metals accommodate plastic deformation remains unclear. Here, we report an integrated grain size and twin thickness effect on the deformation mode of twin-structural nanocrystalline platinum. Above a ∼10 nm grain size, there is a critical value of twin thickness at which the full dislocation intersecting with the twin plane switches to a deformation mode that results in a partial dislocation parallel to the twin planes. This critical twin thickness value varies from ∼6 to 10 nm and is grain size-dependent. For grain sizes between ∼10 to 6 nm, only partial dislocation parallel to twin planes is observed. When the grain size falls below 6 nm, the plasticity switches to grain boundary-mediated plasticity, in contrast with previous studies, suggesting that the plasticity in twin-structural nanocrystalline metals is governed by partial dislocation activities.