Vol. 202 目录

ACTA

Twinning plays an important role in governing the balance between strength and ductility in hexagonalclose-packed (HCP) metals. Here, we report a combined experimental and theoretical study of twin nucleation from a single dislocation in HCP crystals. Specifically, high-resolution transmission electron microscopy has been used to identify {11-21} twin nuclei in HCP rhenium, providing evidence of their nucleation from a dislocation. The favorability of this dislocation-based nucleation mechanism is rationalized by an anisotropic elasticity model of dislocation dissociation, parametrized by density functional theory calculations, which suggests the conditions for disconnection nucleation and propagation, under which this {11-21} twinning mechanism is expected to be effective. The analysis serves to advance our understanding of the origin of the unique predominance of {11-21} twinning in rhenium, which correlates with the high strength and ductility featured by this metal. It also provides new insights into design strategies that may be effective in activating this twinning mode and enhancing the balance between strength and ductility in HCP alloys more broadly.

ACTA

In this work, we calculate the local slip resistances (LSRs) in equal-molar MoNbTi multi-principal element alloy via molecular static simulations. We consider dislocations of either screw or edge character gliding on four types of slip planes, {110}, {112}, {123}, and {134}, in either forward or backward sense of the 111 slip direction. As references, we also compute the Peierls stresses of the same dislocations in two natural reference metals, Mo and Nb, and a synthetic one, the mean-field, A-atom potential-based MoNbTi. Further, we compare the LSRs with the corresponding ideal shear strengths that do not account for the lattice distortions induced by dislocation cores. We show that for neither dislocation character is the LSR on the {110} plane the lowest in MoNbTi, in contrast to Mo and Nb. For edge dislocations, slip on the {134} plane is the easiest, but for the screw dislocations, it is the hardest. For screw dislocations, the {112} glide plane is the most favored, while for edge dislocations, it is the least favored. We also find that the screw-to-edge ratio in the slip resistance is reduced by one order of magnitude in MoNbTi compared to that of any pure reference metal for the same type of slip plane. These results suggest that, in contrast to pure body-centered cubic (BCC) metals, BCC MPEAs could deform by a multiplicity of slip modes due to the lower screw-to-edge ratios and the lower LSRs for edge dislocations on the three higher order planes.

ACTA

ACTA

ACTA

An ultra-low-strain deformation microstructure was revealed for the first time non-destructively in the bulk interior of an annealed laminated Ti-Al composite—in the “fully recrystallized” Al layer—by a synchrotron-based micro-diffraction technique, namely differential aperture X-ray microscopy (DAXM), through real space mapping with a very high angular resolution (0.01°). This ultra-low-strain deformation microstructure was found to result from the thermal stress, induced during cooling after annealing, due to the different coefficients of thermal expansion for the Ti and Al layers. The annealed sample was further tensile deformed to a strain of 1.66% and followed by in situ DAXM and analyzed by various misorientation mapping methods. The results pointed to the important effects of the initial microstructure and the interface constraint, as well as the grain size and crystal orientation, on the plastic deformation. A gradient in dislocation density from the layer interface to the center was found in the Al layer of the annealed sample, and this gradient increased slightly during tensile deformation. The variation of the dislocation density was further discussed based on the activation and interaction of dislocations in grains of different sizes and orientations during plastic deformation. The findings of this study provided valuable insights in understanding the constraint effect of the laminated metal composite and the design of novel composite materials.

ACTA

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In the cold-spray process, jetting is often considered a precursor to particle-substrate adhesion, but the conditions under which these phenomena occur remain unclear. This paper presents a systematic sitespecific study of single-particle impact across a wide range of impact velocities for Cu particles impacting a Cu substrate. A high degree of velocity control enables identification of new behaviors that emerge in distinct velocity ranges. At the lowest velocities, we observe a fully plastic impact regime with rebound of the incident particle. As velocity increases, substrate jetting emerges even while the particle still rebounds. At the critical adhesion velocity, vcr, jetting only occurs on the substrate, leading to poor metallurgical bonding. Bonding improves as velocity increases and the particles also begin to jet, but there is an apparent maximum in bonding extent at ~1.3 vcr. This is followed by a drop in metallurgical bonding due to the ‘backward’ jetting of the particle that is associated with peeling forces on the interface. At ~1.6 vcr and above, we observe hydrodynamic penetration of the particle and possible erosion, as indicated by petalling of the substrate and burial of the particle below the substrate impact plane. Taken together, this catalog of phenomena, correlated to specific velocities, points to an impact-structure-based approach to establishing processing windows for cold spray.

ACTA

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The size, distribution and chemical composition of grain boundary η-phase precipitates (GBPs) and microsegregation present in thick plate (140 mm) 7xxx Al alloys has been quantified across a range of length scales. To address the known limitations of individual characterisation methods, a number of crosscorrelated, high resolution techniques have been used, including atom probe tomography (APT). A newgeneration high-Zn alloy (AA7085) has been compared to a more established material, AA7050, in T7651 temper conditions. The results show that high angle grain boundaries in both alloys are dominated by quench-induced GBPs (Q-GBPs), covering up to ~40% of the area in AA7050. When viewed on brittle intergranular fracture surfaces and in 3D, the Q-GBPs appear much larger than previously reported and exhibit complex branched, dendritic-like morphologies. In AA7050, the Q-GBPs contain substantially higher levels of Cu (by 29 %) and Al (by 37%) and lower Zn (by 33 %) than AA7085. Classical modelling demonstrates that these differences result from different transformation pathways, with precipitates in the more quench sensitive AA7050 alloy nucleating at higher temperatures, which exaggerates the effect of alloy chemistry. In both alloys GB segregation was limited, with low levels of Zn detected relative to the matrix, but more Mg and less Cu in AA7050. It is, therefore, proposed that the higher Cu and Al, and lower Zn content, of the large Q-GBPs present in AA7050 is the main difference in GB microchemistry between the two materials and is the primary reason its GBs are less chemically active. The implications for the relative susceptibilities of the alloys to environmentally assisted cracking are briefly discussed.

ACTA

向上滑动阅览英文摘要

Based on the growing power of computational capabilities and algorithmic developments, with the help of data-driven and high-throughput calculations, a new paradigm accelerating materials discovery, design and optimization is emerging. Titanium (Ti) alloys have been chosen herein to highlight an integrated computational materials engineering case study with the aim of improving their strength and ductility. The electronic properties of elemental building blocks were derived from high-throughput first-principles calculations and presented in the form of the Mendeleev periodic table, including their electron work function(Ф), Fermi energy(EF), bonding charge density(ρ), and lattice distortion energy. The atomic and electronic insights of the composition–structure–property relationships were revealed by a data mining approach, addressing the key features/principles for the design strategies of advanced alloys. Guided by defect engineering, the deformation fault energy and dislocation width were treated as the dominating criteria in improving the ductility. The proposed yield strength model was utilized quantitatively to present the contributions of solid-solution strengthening and grain refinement hardening. Machine learning was used collaboratively with fundamental knowledge and feed back into a new training model, shown to be superior to the empirical molybdenum equivalence method. The results draw a conclusion that the integration of data mining and machine learning will not only generate plausible explanations and address new hypotheses, but also enable the design of strong and ductile Ti alloys in a more effiffifficient and cost-effective way.