ACTA Vol. 199，15 Oct. 2020, P311-325

Increasing magnetically favorable <100>//ND texture components is a key challenge in the preparation of high-efficiency non-oriented electrical steels. In this study, an Fe-1.3 wt% Si steel with strong Cube ({100}<001>) and Goss ({110}<001>) texture was successfully produced by novel twin-roll strip casting, cold rolling and annealing process. The microstructure and texture of the material was characterized by optical microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The origin and formation mechanism of texture are described from the perspective of deformation and recrystallization behavior of specifically oriented grains. It was observed that initial Cube rotated toward {013}<031>-{110}<110> besides rotation toward {001}<130>-{001}<120> during cold rolling. In addition, new Cube deformation bands were developed in the deformed {115}<051>-{115}<161> grains. Cube components were partly retained as large block, small band structure and crystallite after heavy cold rolling. The Cube deformation structures served as nucleation sites of new Cube grains. The shear band within {114}<110>, {112}<110> and {111}<112> matrix also provided some Cube nuclei. Morphology change from near bar-shaped to equiaxed occurred during the growth of Cube and Goss grains. The formation of recrystallization texture is attributed to the oriented nucleation mechanism, and the orientation pinning and size effects that impacted the intensity of texture component. The low thickness of strip and coarse solidification microstructure with strong {100} texture are the decisive factors to obtain strong Cube and Goss texture in strip-cast non-oriented electrical steel.

ACTA Vol. 199, 15 Oct. 2020, P326-339

Fusion-based additive manufacturing techniques such as selective laser melting, electron beam freeform fabrication cause solidification problems such as grain coarseness and high porosity. As a new technique, cold spraying (CS) can overcome such melting-induced drawbacks. In this study, a material model using dislocation dynamics was developed specifically for the CS process for describing the following five nanosecond-scale physical phenomena: strain hardening, normal-range strain rate hardening, ultra-high strain rate hardening, thermal softening and grain size evolution. A single Cu microparticle impact test was conducted, and a good agreement between experimental and model-predicted microparticle deformations was observed, indicating high model accuracy. The corresponding finite element model was established, and the individual effects of the above phenomena were discussed in detail to show that material deformation is mainly controlled by ultra-high strain rate hardening while jetting is controlled by thermal softening. Additionally, both simulated and actual grain size distributions indicated that grain refinement occurs only near the microparticle-substrate interface (mainly at the interface edge). Thus, the newly developed model could accurately reproduce the dynamic deformation behaviors of impacting particles and correctly predict grain refinement (particularly due to dynamic recrystallization).

ACTA Vol. 199, 15 Oct. 2020, P340-351

英文摘要

ACTA Vol. 199, 15 Oct. 2020, P352-369

As the numbers of medium- to high-entropy alloys being studied and impressive structural properties they exhibit increase rapidly, questions regarding the role played by their complex chemical fluctuations rise concomitantly. Here, using a combination of large-scale molecular dynamics (MD), a hybrid MD and Monte-Carlo simulation method, and crystal defect analysis, we investigate the role lattice distortion (LD) and chemical short-range order (CSRO) play in the nucleation and evolution of dislocations and nanotwins with straining in single crystal and nanocrystalline CoCrNi, a medium entropy alloy (MEA). LD and CSRO effects are elucidated by comparisons with responses from a hypothetical pure A-atom alloy, which bears the same bulk properties of the nominal MEA but no LD and no CSRO. The analysis reveals that yield strengths are determined by the strain to nucleate Shockley partial dislocations, and LD lowers this strain, while higher degrees of CSRO increase it. We show that while these partials prefer to nucleate in the CoCr clusters, regardless of their size, they find it increasingly difficult to propagate away from these sites as the level of CSRO increases. After yield, nanotwin nucleation occurs via reactions of mobile Shockley partials and is promoted in MEAs, due to the enhanced glide resistance resulting from LD and CSRO.

ACTA Vol. 199, 15 Oct. 2020, P370-382

Accurate indexing of EBSD patterns presents a challenging problem. We propose a new convolutional neural network (EBSD-CNN) to realize real-time indexing of EBSD patterns; we implement a disorientation loss function to adapt a standard CNN model for crystallographic orientation indexing. The indexing accuracy, rate, and robustness against noise are evaluated using both simulated and experimental data, and compared with other indexing methods (Hough-based indexing, dictionary indexing, and spherical indexing). The results suggest that a CNN can provide an alternative to commercial Hough-transform-based indexing with comparative accuracy and rate. We obtain insight into the network functionality by visualization of selected filters.

ACTA Vol. 199, 15 Oct. 2020, P383-396

In the present work, the influence of an axial magnetic field on the structure of directionally solidified Ni-Mn-Ga alloys is investigated experimentally. It has been found that a radial graded distribution of the gamma (γ) phase and grain fragmentation of the martensite/γ phase can occur under a magnetic field. Indeed, the structures, including a single martensite zone and martensite/γ phase cellular and eutectic mixed zones, are seen under a weak magnetic field (B ≤ 2 T). Under a strong magnetic field (B ≥ 4 T), the grain fragmentation of the martensite/γ phase occurs. Moreover, the application of a strong magnetic field induces a structural distortion in the martensite and dislocation rearrangement inside the γ phase. On the basis of the above investigation, a unique sample that includes columnar and equiaxed grains was successfully prepared using magnetic-field-assisted directional solidification. Subsequently, the mechanical behavior and detwinning evolution of the Ni-Mn-Ga specimen under compressive loading were studied. A general criterion is proposed to elucidate the compressive-stress-induced detwinning evolution based on the Schmid factor and deformation gradient tensor. The γ phase significantly enhanced the mechanical behavior of both the columnar and equiaxed grain samples. In particular, the random distribution of the γ phase was more prone to hindering fracture, even resulting in a larger compression strain in the polycrystalline Ni-Mn-Ga sample. This work highlights the effect of a magnetic field on the microstructure and also provides an in-depth understanding of the detwinning mechanism in Ni-Mn-Ga alloys containing the γ phase.

ACTA Vol. 199, 15 Oct. 2020, P397-412

Plate-lattices are an emerging category of mechanical metamaterials with exceptional mechanical performance. In this paper, a family of half-open-cell plate-lattices is innovated with exceptional mechanical properties and additive manufacturability. The elastoplastic properties and large strain response of the novel plate-lattices are investigated both numerically and experimentally. Design maps for tailoring the anisotropic index reveal that elastically isotropic plate-lattices can be obtained for a wide range of relative densities. Numerical results reveal that the isotropic plate-lattices exhibit significantly higher elastic properties than other competing topologies such as conventional truss-lattices and isotropic smooth shell-lattices, and their bulk modulus can attain the Hashin-Shtrikman upper bound for all relative densities. Large strain simulations demonstrate the remarkable energy absorption capacity of the novel plate-lattices. The numerical findings are confirmed through the compression experiments on the anisotropic and isotropic stainless steel 316 L specimens manufactured by selective laser melting. This work proposes a novel type of plate-lattices with both exceptional mechanical performance and good additive manufacturability, which opens a new channel for the design of lightweight mechanical metamaterials.

ACTA Vol. 199, 15 Oct. 2020, P413-424

In this work, instrumented nanoindentation was employed to investigate the effect of interstitial oxygen or nitrogen addition on the incipient plasticity and dislocation nucleation in a body-centered cubic NbTiZrHf high-entropy alloy (HEA) at loading rates of 10–1000 µN/s. We conducted quantitative statistical analysis and density functional theory (DFT) calculations to identify the role of interstitial oxygen/nitrogen during the onset of plasticity. Synchrotron X-ray diffraction and transmission electron microscopy were also performed to confirm that the oxygen/nitrogen atoms were indeed present as interstitial solutes. These interstitial solutes could increase the critical shear stress required to initiate plasticity, and nitrogen yielded a larger hardening effect than oxygen. The activation volumes were evaluated to be about 2–3 atomic volumes, indicating cooperative migration of multiple atoms during the dislocation nucleation, and neither oxygen nor nitrogen appeared to significantly affect this activation process. Hardness tests were also carried out and the result demonstrated that the enhancement of the critical shear stress for incipient plasticity was not caused by the traditional solid-solution strengthening mechanism. DFT calculations revealed that oxygen/nitrogen interstitials induced local charge transfer and improved the lattice cohesion, which was probably responsible for the enhanced pop-in load/stress in the current interstitially alloyed HEAs.

ACTA Vol. 199, 15 Oct. 2020, P453-468

In hypothetical accidental conditions, zirconium-based nuclear fuel claddings can absorb high hydrogen contents (up to several thousand wppm) and be exposed to high temperatures (β_Zrphase temperature range) before being cooled. This paper thoroughly investigates the microstructural and microchemical evolutions that take place in such conditions. Two zirconium-based alloys and unalloyed zirconium were pre-charged with hydrogen at various contents up to 3300 wppm and heat-treated at 1000 or 850 °C. Neutron diffraction analyses were performed in situ upon slow cooling from 700 °C and at room temperature. In the materials containing 3300 wppm of hydrogen, β_Zr progressively transforms into α_Zr during slow cooling then extensively transforms into α_Zr and δ_ZrH2-x hydrides precipitate via a eutectoid reaction. Thermodynamic predictions at equilibrium are in good agreement with the experimental results. However, depending on the cooling scenario and the average hydrogen content, the precipitation of γ_ZrHhydrides, potentially metastable, is evidenced below 350 °C and a significant amount of hydrogen can remain in solid solution in α_Zr. These metallurgical evolutions and the evolution of the different phase lattice parameters are strongly influenced by the partitioning of oxygen and hydrogen (revealed by electron probe and elastic recoil detection microanalyses) that occurs during the β_Zr to α_Zr transformation and hydride precipitation.

ACTA Vol. 199, 15 Oct. 2020, P480-494

We present a comprehensive experimental campaign of high-velocity microparticle impacts with different combinations of particle and substrate materials to identify possible deformation regimes. Based on experimental observations of the impact sites, we identify three typical modes of behavior, namely, splatting, co-deformation, and penetration. We develop a theoretical framework to predict the operative regime for a given particle/substrate combination, ranging from splatting and penetration at two extremes of a spectrum, and co-deformation in the center. We propose an impact ratio based on the materials’ properties, which can successfully quantify the spectrum. Co-deformation is expected when the ratio is around unity, while much larger or smaller ratios give rise to penetration and splatting, respectively.

ACTA Vol. 199, 15 Oct. 2020, P504-513

Many of the materials properties of high-entropy alloys (HEAs), like increased hardness, reduced thermal and electrical conductivity, and interesting hydrogen storage properties, are proposed to be related to local lattice distortions of the crystal structure due to the significant size differences between the elements of the alloy. However, direct evidence of this effect is very limited in the literature, and it therefore remains a hypothesis. This work presents a detailed assessment of the local lattice distortion in three body-centered cubic (bcc) HEAs TiVNb, TiVZrNb and TiVZrNbHf with varying atomic size differences using total scattering measurements and Reverse Monte Carlo structure modelling. The analysis indicates that the amount of local lattice distortion in the alloys increases with the elemental size difference in the alloy. The amount of lattice distortion is relieved when dideuterides with CaF₂-type structures () are formed from the bcc () HEAs. Analyses of the local environments around the deuterium atoms reveal an interesting correlation between the valence-electron concentration (VEC) of the nearest-neighbour metals and the stability of tetrahedral interstices with respect to deuterium occupation. Moreover, there is a tendency towards Ti/Nb short-range order in TiVNbD_5.7 where the mixing entropy is lowest. In TiVZrNbHfD₁₀, about 6% of the deuterium atoms are displaced from the tetrahedral interstices with smaller volumes to octahedral interstices.

ACTA Vol. 199, 15 Oct. 2020, P561-577

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in ‘soft’ grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring ‘hard’ grain that is poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was performed to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shifts from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. The critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slip modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τc^basal =252MPa and τc^prism =154MPa) and activation energy (ΔF^basal = 10.5×10⁻²⁰ J = 0.65 eV and ΔF^prism = 9.0×10⁻²⁰ J = 0.56 eV). The prism slip parameters correspond to a stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate has a significant effect on stress redistribution to ‘hard’ grain orientations during cold dwell fatigue.

ACTA Vol. 199, 15 Oct. 2020, P578-592

The multiaxial large deformation and ductile fracture behavior of laser powder bed fusion (L-PBF) additively manufactured austenitic 316L stainless steel was experimentally measured. Data from tests in two orientations, under five dissimilar stress states (shear, combined shear/tension loading states, plane strain tension, and uniaxial tension) were used to calibrate and validate anisotropic plasticity and fracture models, with different specimen geometries used to probe plasticity versus fracture. Shear softening, hypothesized to be due to shear band formation in the material due to high initial dislocation density and sub-micron cellular structures, was observed in shear dominated tests, and modeled through the adoption of a shear damage criterion in an anisotropic plasticity model. Using a combined experimental and computational approach, isotropic and anisotropic Hosford-Coulomb and modified Mohr-Coulomb ductile fracture models were calibrated for both sample orientations. The calibrated anisotropic Hosford-Coulomb fracture model best captures the stress state dependent and anisotropic failure behavior of L-PBF 316L.

ACTA Vol. 199, 15 Oct. 2020, P593-601

Manipulation of the atomic-scale structure of two-dimensional (2D) interfaces have been shown to provide nanocomposites with enhanced strength, deformability, and radiation damage resistance. In comparison with 2D interfaces, here we investigate the mechanical response of nanocomposites containing three-dimensional (3D) Cu/Nb interfaces consisting of a chemical/structural gradient separating pure Cu and Nb layers, through which the lattice mismatch between face-centered cubic Cu and body-centered cubic Nb is accommodated over a distance of several nanometers. It is demonstrated that 3D interfaces increase the yield and flow strength by 50% and 22%, respectively, over composites containing 2D interfaces at similar layer thicknesses. After 14% compressive strain, the onset of shear banding results in co-deformation of both Cu and Nb phases within and outside of the shear band. We conclude with a discussion of the role of interface structure in shear band formation and growth in 3D Cu/Nb.

ACTA Vol. 199, 15 Oct. 2020, P602-612

High-entropy alloys containing multi-principal-element systems significantly expand the potential alloy design space, and offer the possibility of overcoming the strength-ductility trade-off in metallurgical research. However, the gain in ultra-high strength through traditional grain refinement and precipitation-strengthening mechanisms inevitably leads to a drastic loss of ductility. Here, we report on the design and fabrication of heterogeneous-lamella structured, aged bulk high-entropy alloy, which attains gigapascal tensile strength while retaining excellent ductility (UTS ~1.4 GPa, elongation ~30%; UTS ~1.7 GPa, elongation ~10%). Our work shows that the improved strength-ductility synergy arises due to various complementary strengthening mechanisms, including solid-solution, interfaces, precipitation and martensitic transformation, which influence the hardening and deformation processes at different strain levels. In particular, the hetero-deformation that is associated with the formation of microbands as well as the stress-induced martensite promotes additional hardening and hence high ductility. The strategy described here, that is leveraging the concept of heterogeneous microstructure design, provides a practical and novel method for fabricating high-performance structural materials.

ACTA Vol. 199, 15 Oct. 2020, P613-632

Martensite is a key constituent in advanced high strength steels and plays an important role in providing the high strength. While the strength of martensite has been extensively studied in the past, its low elastic limit and extremely high strain hardening rate remain a puzzle for the steel community. Composite models proposed recently can successfully reproduce these features as result of gradual yielding of microstructural constituents with either variations in intrinsic yield strengths or transformation induced residual stresses. Although these composite models can explain certain observations associated with the deformation of as-quenched martensite, neither can self-consistently describe all the key characteristics in the tension-compression behaviour of as-quenched martensite. Attempts to extend these composite models to tempered martensite have been limited. In this contribution, we conduct a systematic experimental study on the strain hardening of as-quenched and tempered martensite with mechanical testing (e.g. monotonic tension and tension-compression) and interrupted X-ray diffraction. It is shown that the high strain hardening rate, large Bauschinger effect and diffraction line narrowing found in as-quenched martensite during straining can be sustained in tempered martensite tempered up to 400 °C. These phenomena can be understood by considering martensite as a multi-constituent composite having both variations in intrinsic yield strengths and relaxation of transformation induced residual stresses during straining.

ACTA Vol. 199, 15 Oct. 2020, P649-668

Atomic-resolution scanning transmission electron microscopy is used to study stacking faults and their configurations formed in cold deformed samples of a face-centered cubic (FCC) Fe₄₂Mn₃₈Co₁₀Cr₁₀ (at.%) high-entropy alloy (HEA). It is found that the deformed microstructures contain at least four types of intrinsic stacking faults and two types of extrinsic stacking faults that are bounded by different partial dislocations, including Shockley, Frank and unusual 1/6<411> partials. Additionally, seventeen types of stacking fault configurations are also found, including the well-known Lomer-Cottrell lock, Hirth lock and faulted dipole, the rarely reported intrinsic-extrinsic fault bend, and thirteen hitherto unreported configurations containing different stacking faults and partial dislocations. Among these seventeen configurations, fifteen of them are constructed by conjugate stacking faults connecting at their edges, with a stair-rod partial belonging to 1/6<110>, 1/3<110> or 1/3<100> lying at every joint point of the stacking faults; and the rest two configurations have two intersecting stacking faults but contain different stair-rod dipoles at the intersection. Fourteen of these configurations are all comprised exclusively of intrinsic stacking faults, while three of them consist of a mixture of intrinsic and extrinsic stacking faults. Moreover, three out of the seventeen configurations contain surprisingly a Frank partial at one edge, in contrast to previously reported configurations that are all bounded exclusively by Shockley partials. Based on Burgers vector analysis, the formation mechanisms of the six types of stacking faults and the seventeen types of configurations are proposed and discussed.