Density Of Twin Boundaries Research Articles

Anisotropy in the tensile creep behaviors of a hot-rolled Mg-5.5wt%Y alloy sheet

In this work, the tensile creep anisotropy was investigated in a hot-rolled Mg-5.5 wt%Y alloy sheet. The loading directions were performed along rolling, transverse and normal directions (RD, TD and ND), respectively. The results showed that a poor creep resistance was obtained along ND due to serious stress concentration induced by high-density twin boundaries blocking dislocations. While a similarly excellent creep resistance was obtained along RD and TD due to pyramidal <c + a > slip contributes to the strain hardening during creep process. Meanwhile, this work found that an addition of 5.5 wt%Y could enhance the crack resistance of grain boundary over that of twin boundary. When twin boundaries were heavily generated to block dislocations during tensile creep, crack initiation occurred rapidly. It is suggested that both the density of twin boundaries and the Y content should be considered for improving the creep resistance of MgY based alloys.

A microstructure study of colloidal gold nanoparticles by X-ray diffraction line profile analysis

We present the colloidal synthesis of gold nanoparticles (Au NPs) using the chemical reduction of gold precursors by changing the reducing agent (Na3C6H5O7) volume. We aimed at achieving different particle sizes, morphologies, and defect structures. The modified process impacted the overall synthesis time and nanoparticles' microstructures. We used X-ray diffraction profile analysis (XPA) and transmission electron microscopy (TEM) to follow the nanoparticles’ size and lattice strain details. The NPs' size distributions agree well between the indirect XPA and TEM direct measures. Nanocrystal shapes change from triangular, octahedral, and decahedral to hexagonal plate-like; it shows how the NPs minimize their energy formation by creating defects and well-defined crystallographic directions, including crystal fusion. The combined effects of dislocations, twin boundaries, and surface tension add together to broaden the diffracted line profiles. In particular, the density of dislocation and twin boundaries increase toward the smaller NPs.

New perspectives on twinning events during strain-induced grain boundary migration (SIBM) in iteratively processed 316L stainless steel

Characterization of microstructure and grain boundary character distribution (GBCD) during iterative thermo-mechanical processing (TMP) of 316L stainless steel were done using electron backscatter diffraction (EBSD). Results from EBSD scans were analyzed in terms of the fraction of special boundaries, their deviation from ideal misorientation, connectivity of random high-angle grain boundaries, and grain size. In order to understand the efficacy of the iterative processing route leading to a well-connected twin boundary network, additional analysis was done in terms of twin-related domain statistics and triple junction distribution. Results from these analyses show that initial iteration results in a microstructure with insufficient twin boundary density, and a poor twin boundary network. Although the next iteration leads to a small increase in twin statistics, it also does not significantly improve the statistics of favorable grain boundaries and their network. However, the fourth iteration of the processing results in a large improvement in both the population and distribution of favorable grain boundaries. All the subsequent iterations were found to result in microstructural deterioration in terms of both population and network of these grain boundaries. Based on the analysis, role of underlying mechanisms such as strain-induced grain boundary migration and static recrystallization in the effective optimization of GBCD was analyzed. The present research gives important insights into these mechanisms and their control during TMP in order to achieve an engineered microstructure.

Irradiation-Induced Extremes Create Hierarchical Face-/Body-Centered-Cubic Phases in Nanostructured High Entropy Alloys.

A nanoscale hierarchical dual-phase structure is reported to form in a nanocrystalline NiFeCoCrCu high-entropy-alloy (HEA) film via ion irradiation. Under the extreme energy deposition and consequent thermal energy dissipation induced by energetic particles, a fundamentally new phenomenon is revealed, in which the original single-phase face-centered-cubic (FCC) structure partially transforms into alternating nanometer layers of a body-centered-cubic (BCC) structure. The orientation relationship follows the Nishiyama-Wasser-man relationship, that is, (011)BCC || ( 1¯1¯1)FCC and [100]BCC || [ 11¯0]FCC . Simulation results indicate that Cr, as a BCC stabilizing element, exhibits a tendency to segregate to the stacking faults (SFs). Furthermore, the high densities of SFs and twin boundaries in each nanocrystalline grain serve to accelerate the nucleation and growth of the BCC phase during irradiation. By adjusting the irradiation parameters, desired thicknesses of the FCC and BCC phases in the laminates can be achieved. This work demonstrates the controlled formation of an attractive dual-phase nanolaminate structure under ion irradiation and provides a strategy for designing new derivate structures of HEAs.

Molecular Beam Epitaxy of Two-Dimensional Vanadium-Molybdenum Diselenide Alloys.

Two-dimensional (2D) alloys represent a versatile platform that extends the properties of atomically thin transition-metal dichalcogenides. Here, using molecular beam epitaxy, we investigate the growth of 2D vanadium-molybdenum diselenide alloys, VxMo1-xSe2, on highly oriented pyrolytic graphite and unveil their structural, chemical, and electronic integrities via measurements by scanning tunneling microscopy/spectroscopy, synchrotron X-ray photoemission, and X-ray absorption spectroscopy (XAS). Essentially, we found a critical value of x = ∼0.44, below which phase separation occurs and above which a homogeneous metallic phase is favored. Another observation is an effective increase in the density of mirror twin boundaries of constituting MoSe2 in the low V concentration regime (x ≤ 0.05). Density functional theory calculations support our experimental results on the thermal stability of 2D VxMo1-xSe2 alloys and suggest an H phase of the homogeneous alloys with alternating parallel V and Mo strips randomly in-plane stacked. Element-specific XAS of the 2D alloys, which clearly indicates quenched atomic multiplets similar to the case of 2H-VSe2, provides strong evidence for the H phase of the 2D alloys. This work provides a comprehensive understanding of the thermal stability, chemical state, and electronic structure of 2D VxMo1-xSe2 alloys, useful for the future design of 2D electronic devices.

The Convolutional Multiple Whole Profile (CMWP) Fitting Method, a Global Optimization Procedure for Microstructure Determination

The analysis of line broadening in X-ray and neutron diffraction patterns using profile functions constructed on the basis of well-established physical principles and TEM observations of lattice defects has proven to be a powerful tool for characterizing microstructures in crystalline materials. These principles are applied in the convolutional multiple-whole-profile (CMWP) procedure to determine dislocation densities, crystallite size, stacking fault and twin boundary densities, and intergranular strains. The different lattice defect contributions to line broadening are separated by considering the hkl dependence of strain anisotropy, planar defect broadening and peak shifts, and the defect dependent profile shapes. The Levenberg–Marquardt (LM) peak fitting procedure can be used successfully to determine crystal defect types and densities as long as the diffraction patterns are relatively simple. However, in more complicated cases like hexagonal materials or multiple-phase patterns, using the LM procedure alone may cause uncertainties. Here, we extended the CMWP procedure by including a Monte Carlo statistical method where the LM and a Monte Carlo algorithm were combined in an alternating manner. The updated CMWP procedure eliminated uncertainties and provided global optimized parameters of the microstructure in good correlation with electron microscopy methods.

Analysis of internal stress build-up during deposition of nanocrystalline Ni thin films using transmission electron microscopy

Ni thin films sputter-deposited at room temperature with varying Ar pressures were investigated with automated crystal orientation mapping in a transmission electron microscope to uncover the mechanisms controlling the internal stress build-up recorded in-situ during deposition. Large grains were found to induce behaviour similar to a stress-free nucleation layer. The measurements of grain size in most of the Ni thin films are in agreement with the island coalescence model. Low internal stress was observed at low Ar pressure and was explained by the presence of large grains. Relaxation of high internal stress was also noticed at the highest Ar pressure, which was attributed to a decrease of Σ3 twin boundary density due to a low deposition rate. The results provide insightful information to better understand the relationship between structural boundaries and the evolution of internal stress upon deposition of thin films.

Effect of microstructure and internal stress on hydrogen absorption into Ni thin film electrodes during alkaline water electrolysis

Efforts to improve the cell efficiency of hydrogen production by water electrolysis continue to address the electrochemical kinetics of the oxygen and hydrogen evolution reactions in detail. The objective of this work is to study a parasitic reaction occurring during the hydrogen evolution reaction (HER), namely the absorption of hydrogen atoms into the bulk electrode. Effects of the electrode microstructure and internal stress on this reaction have been addressed as well in this paper. Ni thin film samples were deposited on a Si substrate by sputter deposition with different deposition pressures, resulting in different microstructures and varying levels of internal stress. These microstructures were first analyzed in detail by Transmission Electron Microscopy (TEM). Cathodic chrono-amperometric measurements and cyclic voltammetries have then been performed in a homemade electrochemical cell. These tests were coupled to a multi-beam optical sensor (MOS) in order to obtain in-situ curvature measurements during hydrogen absorption. Indeed, since hydrogen absorption in the thin film geometry results in a constrained volume expansion, internal stress generation during HER can be monitored by means of curvature measurements. Our results show that different levels of internal stress, grain size and twin boundary density can be obtained by varying the deposition parameters. From an electrochemical point of view, this paper highlights the fact that the electrochemical surface mechanisms during HER are the same for all the electrodes, regardless of their microstructure. However it is shown that the absolute amount of hydrogen being absorbed into the Ni thin films increases when the grain size is reduced, due to a higher grain boundaries density which are favourite absorption sites for hydrogen. At the same time, it was concluded that H2 evolution is favoured at electrodes having a more compressive (i.e. a less tensile) internal stress. Finally, the subtle effect of microstructure on the hydrogen absorption rate will be discussed as well.

Enhanced tensile properties and electrical conductivity of Cu-CNT nanocomposites processed via the combination of flake powder metallurgy and high pressure torsion methods

Using flake powder metallurgy (FPM) technique, combined with high pressure torsion, super high strength-ductile Cu-CNT nanocomposite with high electrical conductivity is developed. The nanocomposite with 4 vol% CNT showed high tensile strength of ~474 MPa, high electrical conductivity of ~82.5% IACS as well as appreciable ductility of ~11%. According to microstructural studies, the excellent properties of the nanocomposite are attributed to the formation of trimodal grains, high density of twin and low angle grain boundaries, improvement in CNT and Cu interfacial bonding, and appropriate distribution and maintaining the microstructure of the nanotubes in the production process. The results of this work provide a new pathway to produce strong, conductive, and ductile metal matrix nanocomposites.

Effect of Annealing Temperature on Annealing Twins and Shape Memory Effect in Hot-Forged Co-23.9Ni-5.6Si Alloy

Effects of annealing temperature on shape memory effect (SME), annealing twins, and deformation behavior in a hot-forged Co-23.9Ni-5.6Si alloy were studied. The results showed that a low density of the annealing twin boundaries tended to cause a good SME in the hot-forged Co-23.9Ni-5.6Si alloy. The best SME was obtained in the hot-forged Co-23.9Ni-5.6Si alloy annealed at 1073 K. A high recovery strain of 4 pct was realized when deformed at 77 K. The low density of the annealing twin boundaries and lots of introduced stacking faults were responsible for this large recovery strain. The pre-existence of too many tangled dislocations was the main reason why the SME was poor in the as-forged alloy and the alloy annealed below 1073 K although their densities of annealing twin boundaries were also low. The significant improvement of SME after deformation at 77 K resulted from the remarkable increase of the critical stress for slip deformation due to the drop in the thermal activation. When the critical temperature of thermal activation (T0) was above the forward martensitic transformation temperature (Ms), a negative temperature dependence of the critical stress for stress-induced martensitic transformation occurred between the Ms and T0 temperatures.

Extraordinary thermoelectric performance in MgAgSb alloy with ultralow thermal conductivity

MgAgSb-based materials are ideal candidates for thermoelectric applications due to several advantages, such as rich elements, low cost and excellent mechanical robustness. Recently, the all-scale hierarchical architecture and strong anharmonicity in bonding are realized as effective strategies to reduce the lattice thermal conductivity greatly. Here, a design of the all-scale hierarchical architectures, in which the phonon is scattered by the high density of grain boundaries, dislocation, stacking faults, twin boundaries and nanopores, and enhancement of Grüneisen parameter have been demonstrated in reducing the lattice thermal conductivity of MgAgSb materials in the whole temperature range, resulting in an ultralow lattice thermal conductivity ∼ 0.45 W m−1 K−1 at 473 K. Furthermore, the carrier concentration and mobility are also optimized by Zn-doping and heat-treating. The simultaneous optimization of electrical and thermal transport properties contributes to a tremendous enhancement of average ZT to about 1.3 in the range from 323 K to 548 K (the maximum ZT is about 1.4 at 423 K) in the sample Mg0.97Zn0.03Ag0.9Sb0.95 with heat-treating for 10 days. The method we designed not only boosts the thermoelectric application of MgAgSb-based materials but also enables a synergetic strategy for designing thermoelectric materials with high thermoelectric performance.

Enhanced irradiation and corrosion resistance of 316LN stainless steel with high densities of dislocations and twins

The irradiation and corrosion damages of nuclear materials have been a major challenge for the nuclear energy industry. Here, rotationally accelerated shot peening was used to improve the radiation and corrosion resistance of 316LN austenitic stainless steel by introducing high densities of dislocations and nanoscale twin boundaries. The dislocations and twins not only impeded the formation of helium bubbles and shear bands during irradiation, but also facilitated the formation of a superior passivation film, which significantly improved its corrosion performance. These observations provide an effective approach for designing irradiation- and corrosion-resistant nuclear materials.

The Annealing Twins of Fe-20Mn-4Al-0.3C Austenitic Steels during Symmetric and Asymmetric Hot Rolling

The present work investigates the annealing twins of Fe-20Mn-4Al-0.3C austenitic steels in symmetric hot rolling (SHR) and asymmetric hot rolling (ASHR). The average grain size is 26 (±9.6) μm and 11 (±7.0) μm for the tested steel in SHR and ASHR processes. The density of high angle grain boundary (HAGB) and annealing twin boundary increase with the decrease of grain size. The annealing twin is obviously higher in ASHR than in SHR. The linear relation model between the logarithm of twin boundary density and the logarithm of the grain size is established. The grain boundary migration is continuously generated during recrystallization in SHR process. The coincident site lattice (CSL) boundary proportion increases with local grain boundary continuing bugling and the migration direction of bugling grain boundary constantly changes. The tensile property of the tested steel is improved due to the effective grain refinement and high density of annealing twins caused by the severe strain in the ASHR process. The purpose of high density HAGB for austenitic steels is helpful to an improvement in mechanical properties.

Understanding the effects of recrystallization and strain induced boundary migration on ∑3 twin boundary formation in Ni-base superalloys during iterative sub-solvus annealing

The formation of ∑3 twin boundaries in an experimental low stacking fault Ni-based superalloy containing 24 wt% Co was investigated. Normalized microstructures were hot deformed to strain limits of 0.10 and 0.50 at a strain rate of 0.1/s and a temperature of 1020 °C. The hot-deformed microstructures were characterized using electron backscatter diffraction, and were subsequently subjected to a series of iterative sub-solvus anneal heat treatments at 1100 °C for one to two minutes. By characterizing the changes in the microstructure and grain boundary character distribution as a function of annealing time, this investigation sought to quantify and detail how twin boundary generation and growth occurs during annealing. This behavior was investigated in the same alloy, but with two distinct as-deformed starting microstructures associated with the different strain limits. The starting microstructures possessed varying levels of intragranular misorientation networks, which were found to have greatly affected the resultant formation of ∑3 annealing twin boundaries. Strain induced boundary migration (SIBM) was observed to have been restricted in the iteratively annealed sample deformed to a strain of 0.10. Due to the sub-solvus heat treat temperature, grain boundary precipitates were present in the microstructure which limited the mobility of the grain boundaries and suppressed grain growth. As a result, the expansion in the length fraction of ∑3 twin boundaries was limited as a function of annealing time. The sample deformed to 0.50 strain, however, was shown to have recrystallized both upon deformation and subsequent annealing. Although recrystallization was found to largely remove much of the pre-existing twin boundaries, the mobility of the newly formed boundaries during growth of the recrystallized grains resulted in a dramatic increase in both the density and length fraction of twin boundaries.

Radiation-resistant nanotwinned austenitic stainless steel

A key strategy to increase the radiation resistance of materials has been to introduce a high density of interfaces that can act as sinks for radiation-induced defects. Twin boundaries are a type of interface that can be introduced through deformation but are usually considered to be ineffective sinks. Using heavy ion irradiation and transmission electron microscopy, this study investigates the influence of a high area per unit volume of twin boundaries on the radiation-induced swelling response of an austenitic stainless steel. The study shows that swelling can be suppressed in regions containing a high density of closely-spaced deformation twin boundaries.

Application of ICME to Engineer Fatigue-Resistant Ni-Base Superalloys Microstructures

Critical rotating components used in the hot section of gas turbine engines are subject to cyclic loading conditions during operation, and the life of these structures is governed by their ability to resist fatigue. Since it is well known that microstructural parameters, such as grain size, can significantly influence the fatigue behavior of the material, the conventional processes involved with the manufacture of these structures are carefully controlled in an effort to engineer the resulting microstructure. For a commercial Ni-base superalloy, RR1000, the development of process models and deformation mechanism maps has enabled not only control of the resultant grain size but also the ability to tailor and manipulate the resulting grain boundary character distribution. The increased level of microstructural control was coupled with a physics-based fatigue model to form an integrated computational materials engineering framework that was used to guide the design of damage-tolerant microstructures. Simulations from a 3D crystal plasticity finite element model were used to identify microstructural features associated with strain localization during cyclic loading and to guide the design of polycrystalline microstructures optimized for fatigue resistance. Conventionally processed and grain boundary engineered forgings of a commercial Ni-based superalloy, RR1000, were produced to validate the design methodology. For nominally equivalent grain sizes, high-resolution strain maps generated via digital image correlation confirmed that the high density of twin boundaries in the grain boundary engineered material were desirable microstructural features as they contribute to limiting the overall length of persistent slip bands that often serve as precursors for the nucleation of fatigue cracks.

Microstructure and mechanical properties of a multilayered CoCrNi/Ti coating with varying crystal structure

Medium entropy alloys (MEAs), such as CoCrNi, have been demonstrated to combine high hardness and excellent ductility, thereby outperforming many high entropy alloys reported to date. In this study, a multilayered CoCrNi/Ti coating was deposited onto a M2 steel substrate using a DC magnetron sputtering system. Columnar grains can be observed in both the CoCrNi and Ti layers. A high density of periodic twin boundaries, aligned in a direction normal to the growth direction, was also observed within the columnar CoCrNi grains. Moreover, different crystal structures were identified for different CoCrNi layers. The outermost CoCrNi layer exhibited a FCC structure, whilst in contrast, both the middle and bottom CoCrNi layers exhibited a BCC structure. It was assumed that Shockley partial dislocations were responsible for the FCC to BCC transition occurring in both the bottom and middle CoCrNi layers. A high hardness of ~7.6 GPa and elastic modulus of ~233 GPa were determined for this multilayered coating by nanoindentation testing. Further, extraordinary damage tolerance was found in the multilayered CoCrNi/Ti coating under indentation loading. The steady shear banding behaviour during deformation may benefit energy dissipation and promote structural plasticity.

Optimal microstructural design for high thermal stability of pure FCC metals based on studying effect of twin boundaries character and network of grain boundaries

Three nickel electrodeposits with comparable grain size were synthesized by tailoring the electrodeposition conditions. Thorough microstructural characterizations including electron backscatter diffraction, ion channeling contrast imaging, electron channeling contrast imaging, transmission Kikuchi diffraction, transmission electron and high annular dark-field imaging were applied. The deposits contain a high density of twin boundaries with similar microstructures in terms of grain boundary character. These materials were annealed at various temperatures to study the microstructural evolution, and hence, their thermal stability. The differences in the character of twin boundaries and morphology of the grain boundaries in as-deposited state and their influence on the microstructural evolution at elevated temperatures are analyzed. The importance of incoherent twin boundaries, and the interaction of mobile general high angle boundaries with stationary boundaries are discussed. Finally, an optimal design for high thermal stability is proposed, based on the mechanisms that were inferred from the results.

The effect of surface step and twin boundary on deformation twinning in nanoscale metallic systems

The effect of surface step and twin boundary on the mechanical twinning process in nanoscale face-centred cubic metallic films was studied using atomistic simulations. Aluminium was considered as a model material but comparisons were made with silver and copper. Surface steps were identified as privileged sites for twin nucleation at lower stresses, leading to the formation of only one large twin in defect-free films. In presence of a coherent twin boundary which acts as a strong barrier to the propagation of dislocations, the extension of nucleated twins is much more limited but the density of secondary twin boundaries is found higher. The key role played by Lomer dislocations, resulting from the interaction between incipient twins and the coherent twin boundary, on the nucleation of new twins was demonstrated. These findings shed light on some elementary mechanisms that can be involved in the elaboration of nanotwinned materials with interesting mechanical properties.

High damping capacity of single crystalline iron-palladium alloy exhibiting a weak first-order martensitic transformation

We have investigated the influence of twin boundary properties on the damping behavior of single crystalline Fe-31.2Pd (at%) alloy, which exhibits a weak first-order martensitic transformation. Different static tensile stress was applied by dynamic mechanical analyzer to adjust the twin boundary density and hydrogen was doped to modify the twin boundary mobility. A high damping capacity (tanδ > 0.1) in a wide temperature range from about 140 K to 230 K was observed under a low static stress of about 0.3 MPa. Hydrogen doping further increased the maximum value of the damping capacity by introducing a relaxation tanδ peak. The damping capacity decreased with increasing static stress probably due to the decrease in twin boundary density.