Grain Boundary Effects Research Articles

Real-time atomic deformation behavior of nano CoCrCuFeNi high-entropy alloy

Tensile tests were performed by molecular dynamics at various strain rates ranging from 2×108 to 1×1010 s−1 and various temperatures ranging from 10 to 1200 K, and the real-time deformation behavior of a nanocrystalline CoCrCuFeNi high-entropy alloy was investigated. The results indicate that the main deformation mechanism is grain boundary slip at high temperatures and low strain rates. Dislocation slip replaces grain boundary slip to control plastic deformation with decreasing temperature and increasing strain rate, correspondingly enhancing the strength of the alloy. Furthermore, the alloys with different grain sizes were simulated to evaluate the effects of grain boundaries on the mechanical behavior. It is found that the strength of the nanocrystalline high-entropy alloys increases with increasing grain sizes when the grain size is too small, exhibiting an inverse Hall—Petch relationship.

The importance of grain boundaries on the reactive elements effect in β-NiAlHf alloy

Cyclic oxidation of single crystalline and polycrystalline β-NiAlHf alloy was investigated at 1200 °C. The effect of grain boundary on the role of reactive elements was discussed. The results indicate that Hf effectively reduced the oxide scale growth rate and enhanced the scale adhesion in polycrystalline NiAl. But the single crystalline NiAlHf exhibits a higher oxide scale growth rate, severer internal oxidation and catastrophic scale spallation. The absence of the grain boundary leads to the negative effects of Hf.

Lamellar aspect-ratio and thickness-dependent strength-ductility synergy in pure nickel during in-situ micro-tensile loading

In order to overcome the trade-off between strength and ductility in traditional metallic materials, the gradient lamellar structure was fabricated through an ultrasound-aided deep rolling technique in pure Ni with high stacking fault energy after heat treatment. The gradient lamellar Ni was successively divided into three regions. In-situ micro-tensile tests were performed in different regions to reveal the corresponding microscopic mechanical behaviors. Microscopic characterization techniques were adopted to explore the effects of microstructural parameters and defects on mechanical properties. This work demonstrates that the micro-tensile sample with small lamellar thickness and large aspect ratio possesses excellent strength and ductility when the loading direction is parallel to the long side of lamellar grain boundaries. The finding is helpful to the design of metallic material microstructure with superior comprehensive properties. On one hand, the reason for high strength is that the strength increases with the decrease of lamellar thickness according to the Hall-Petch effect. Besides, initial dislocation density also participates in the strengthening mechanism. On the other hand, the deformation mechanisms include dislocation slip, grain rotation, and the effects of grain boundaries on dislocations, jointly contributing to good ductility.

Effect of Segregation on Deformation Behaviour of Nanoscale CoCrCuFeNi High-Entropy Alloy

A molecular dynamics (MD) simulation method is used to investigate the effect of grain boundary (GB) segregation on the deformation behavior of bicrystals of equiatomic nanoscale CoCrCuFeNi high-entropy alloy (HEA). The deformation mechanisms during shear and tensile deformation at 300 K and 100 K are analyzed. It is revealed that upon tensile deformation, the stacking fault formation, and twinning are the main deformation mechanisms, while for the shear deformation, the main contribution to the plastic flow is realized through the GB migration. The presence of the segregation at GBs leads to the stabilization of GBs, while during the shear deformation of the nanoscale CoCrCuFeNi HEA without the segregation at GBs, GBs are subject to migration. It is found that the GB segregation can differently influence the plasticity of the nanoscale CoCrCuFeNi HEA, depending on the elemental composition of the segregation layer. In the case of copper and nickel segregations, an increase in the segregation layer size enhances the plasticity of the nanoscale CoCrCuFeNi HEA. However, an increase in the thickness of chromium segregations deteriorates the plasticity while enhancing maximum shear stress. The results obtained in this study shed light on the development of HEAs with enhanced mechanical properties via GB engineering.

Improved microstructure, mechanical properties and electrical conductivity of the Cu–Ni–Sn–Ti–Cr alloy due to Ce micro-addition

In this paper, new Cu–Ni–Sn–Ti–Cr and Cu–Ni–Sn–Ti–Cr–Ce alloys were prepared and studied. After solution treatment and 60% cold rolling, the Cu–Ni–Sn–Ti–Cr alloy with micro-hardness of 221 HV, a tensile strength of 598.1 MPa, elongation of 9.9% and electrical conductivity of 45.2% IACS was obtained after aging at 500 °C for 60 min. Furthermore, the new Cu–Ni–Sn–Ti–Cr–Ce alloy was also obtained using aging at 500 °C for 60 min, having 245 HV micro-hardness, 645.2 MPa tensile strength, elongation 9.4% and 44.6% IACS conductivity. The comprehensive properties of high strength, hardness, and electrical conductivity of the alloy are attributed to the composite effects of solution, deformation, grain boundary, and precipitation strengthening. Among them, precipitation strengthening plays a major role, and the precipitates are mainly nanometer CuNi2Ti, Cr, and NiTi phases. The interface between the nanometer CuNi2Ti phase and Cu matrix is a coherent, which can improve the strength of the alloy without sacrificing plasticity. In addition, texture type transformation and texture strength change of the alloy are closely related to the increase of micro-hardness. The micro-addition of trace rare earth Ce can refine the alloy grains, and promote dynamic recrystallization to produce finer grains, significantly improving the alloy’s micro-hardness.

Molecular dynamics study of grain boundary and radiation effects on tritium population and diffusion in zirconium

Tritium population thermodynamics and transport kinetics critically define the tritium storage performance of zirconium tritides that can be used for a variety of nuclear applications including tritium-producing burnable absorber rods. Both thermodynamic and kinetic properties can be sensitive to grain sizes of materials and can be significantly altered by irradiated defects during operation under the reactor environments. A thorough experimental characterization of how these properties evolve under different reactor conditions and different initial grain structures is extremely challenging. Here molecular dynamics simulations are used to investigate tritium population and diffusion in zirconium with and without different planar symmetric and asymmetric tilt grain boundaries and irradiated defects. We found that in addition to trapping tritium, the most significant effect of planar grain boundaries is to increase tritium diffusivity on the boundary plane. Furthermore, fine grain structures are found to mitigate the change of tritium diffusivity due to irradiated point defects as these point defects are likely to migrate to and sink at grain boundaries.

Effect of grain boundaries and crystal orientation on etching behavior of 12 µm thick rolled copper foil

Effect of grain boundaries and crystal orientation on etching behavior of 12 µm thick rolled copper foil

Effect of grain boundary and reinforced particles on grain boundary precipitates in TiB2/Al-Zn-Mg-Cu composite

Effect of grain boundary and reinforced particles on grain boundary precipitates in TiB2/Al-Zn-Mg-Cu composite

A partitioned computational framework for damage evolution in stress corrosion cracking utilizing phase‐field

AbstractDissolution‐driven stress corrosion cracking is a complicated multi‐physics phenomenon consisting of coupling between mechanical stress and corrosion. In this coupled electro‐chemo‐mechanical problem, the combined effects of mechanical field, localised corrosion and the microstructure with the effects of grain boundaries are investigated. Therein, corrosion kinetics are accelerated by mechanical straining and also affected by the different material properties along the grain boundaries. A phase‐field modelling approach, a method for regularizing sharp interfaces with smooth gradients, is utilized to describe the metal‐electrolyte interface during localized metal dissolution. The current contribution presents a partitioned computational framework, as an extension of recently published works for solving pitting corrosion and stress corrosion cracking at the micro‐scale. In this extension, mechanically assisted corrosion is coupled bi‐directionally and solved on two software instances facilitating simulation at different time scales, using an open‐source coupling software. The performance of the proposed computational framework is demonstrated through various deterioration model system situations in 2D.

Mesoscale slip behavior in single crystal and bicrystal tantalum

Single-crystal and bicrystal tensile specimens of various orientations were prepared from coarse-grained, pure Ta to investigate fundamental BCC slip behavior at the mesoscale, that is, without considering discrete dislocation mechanisms. Single crystal yield behavior was well-represented by concurrent {110} and {112} slip modes with the initial CRSS for {110} 6% lower than for {112}. A new metric quantitatively representing slip multiplicity in a crystal visco-plastic context was introduced. Initial strain hardening correlates positively with this “activity index” tending to confirm the dominant role of latent hardening in single crystals. Anelastic hysteresis at least as significant as in polycrystal/polyphase alloys was observed, with loop width increasing with pre-strain rather than solely with flow stress as previously supposed. Tensile tests of “triplets” consisting of a bicrystal specimen and its two constituent single crystal specimens revealed the differential effects of grain boundaries (GB) on strength and anelastic hysteresis. Two triplets followed the rule-of-mixtures (ROM); two triplets showed synergistic strength and anelasticity. Crystal plasticity (CP) simulations with two slip modes having different hardening behaviors predicted the single crystal stress-strain curves reasonably for various orientations. Plastic deformation incompatibility was identified as the likely source of deviation from rule-of-mixture behavior. CP simulations predicted the appearance of additional slip systems near the GBs in the synergistic triplets, but did not predict the observed strength and anelastic hysteresis.

An ultra-strong and ductile crystalline-amorphous nanostructured surface layer on TiZrHfTaNb0.2 high-entropy alloy by laser surface processing

Heterogeneous crystalline-amorphous nanostructures have been documented to show superior strength-ductility synergy via the co-deformation cooperative effects of nanograins and amorphous grain boundaries. In this work, a facile laser surface remelting technique with rapid cooling rate was successfully developed to fabricate a ∼ 100 μm-thick gradient nanostructured layer accompanied by phase decomposition on a TiZrHfTaNb0.2 high-entropy alloy, where a ∼ 5 μm-thick crystalline-amorphous nanostructured top surface layer with an average grain size of ∼ 7 nm was obtained. Such crystalline-amorphous nanostructured layer shows an ultrahigh yield strength of ∼ 6.0 GPa and a compression strain of ∼ 25 % during the localized micro-pillar compression tests. The atomic observations reveal that co-deformation cooperative mechanisms include the well-retained dislocation activities in nanograins but crystallization in amorphous grain boundaries, which subsequently lead to the grain coarsening via grain boundary-mediated plasticity. This study sheds light on the development of high-performance high-entropy alloys with novel crystalline-amorphous nanostructures and provides significant insight into their plastic deformation mechanisms.

Effects of rhenium and high-angle grain boundaries upon the elemental distribution and microstructure of Ni-based single-crystal superalloys

Effects of rhenium and high-angle grain boundaries upon the elemental distribution and microstructure of Ni-based single-crystal superalloys

The effect of annealing on the properties of copper-coated carbon fiber

In this paper, we prepared copper-coated carbon fibers (CFs) by electroplating and studied the effect of annealing temperature on the electrical conductivity and mechanical properties of these fibers. The results showed that the defects of the copper layer were reduced after annealing. The XRD peak intensity and grain size of the copper layer increased significantly with the increase in annealing temperature, indicating that annealing can effectively increase crystallinity and reduce the presence of grain boundaries. After annealing for 20 min, the conductivity of the copper-coated CFs increased by 78 times, and the mechanical properties also improved. The effect of grain boundaries on the electronic transport properties of copper was studied using the density functional tight-binding method in combination with the non-equilibrium Green's function method. According to the results, the existence of grain boundaries reduces the transmission probability of electrons and current passing through the system, as well as the uniformity of the atomic potential distribution in the central region of the structure, which hinders the transmission of electrons.

Cyclic plasticity and deformation mechanism of AlCrCuFeNi high entropy alloy

High entropy alloys (HEAs) are newer siblings of multi-component metallic compound alloys with high tensile strength and ductility. The impact of temperature and strain rates under cyclic plasticity of AlCrCuFeNi HEA are investigated using molecular dynamics (MD) simulations. The modeling results show that interactions between partial dislocations in AlCrCuFeNi HEA during cycle deformation cause various lattice disorders. The impact of Bauschinger in the HEA is minimized by lattice disorder, which prevents dislocations from moving in reverse. Temperature, strain rates, and the twin boundary significantly impact Bauschinger's effect, which influences the plasticity cycle of AlCrCuFeNi. Furthermore, the effect of temperature, strain rates, and grain boundaries on lattice disorder is disclosed. The current study provides new insights into the mechanical property of AlCrCuFeNi HEA subjected to cyclic plasticity and deformation mechanism with atomic size.

Effect of Grain Sizes on Electrically Assisted Micro—Filling of SUS304 Stainless Steel: Experiment and Simulation

The filling quality of micro-feature structures has a significant impact on the forming quality of micro-channels. The electrical-assisted forming technology can effectively improve the formability of difficult-to-deform materials. In this research, the electrically driven micro-compression constitutive model of SUS304 stainless steels was established to assign grain boundary and grain interior with different material properties. An electrical–thermal–mechanical coupling model was constructed to simulate the filling process considering the effect of grain boundary and grain size. Compared to the experimental results, the simulation indicated a good agreement in microstructure characteristics and higher filling height for the fine-grained material. The increase in grain boundary density makes the resistivity of the fine grain material larger, causing the current destiny and temperature of the specimen to increase with the decrease in grain size. An ellipsoidal gradient temperature distribution is observed due to the uneven current density. Because of the high geometric dislocation density near the grain boundary, a significant dislocation pile-up causes stress to concentrate. It is observed that the deformation coordination is enhanced between the grain boundary and grain core with the decrease in grain size, thus improving the material formability and forming quality.

Effect of Sn oxides on the thermal conductivity of polycrystalline SnSe

SnSe is a promising thermoelectric material, with intrinsically low lattice thermal conductivity, κL. Surprisingly, in several reports, polycrystalline samples are found to have a higher thermal conductivity than single crystals. This disparity has been attributed to trace amounts of thermally conductive Sn oxides at the grain boundaries of polycrystalline samples. The same culprit was recently proposed to explain the reduction of κL in purified, oxide-free, SnSe polycrystals. Here, we test this hypothesis by: (i) tuning the type of oxide in SnSe by exploiting thermodynamic stability regions, since Sn-rich or Sn-poor compositions favour the formation of SnO or SnO2, respectively; and (ii) varying the quantity of SnO2 by intentionally oxidizing SnSe powder before consolidation, to obtain samples with quantifiable amounts - up to 15% - of SnO2. We find that the κL of SnSe is impervious to changes in the type or the amount of Sn oxide present in the samples. Our results show that a simple “rule of mixtures” cannot be used to estimate the effect of grain boundary oxides on the thermal conductivity of SnSe. These results call for an improved understanding of the intriguing thermal transport mechanisms in SnSe and numerous other systems where a two-phase transport is presumed.

Roles of LPSO phases on dynamic recrystallization of high strain rate multi-directional free forged Mg-Gd-Er-Zn-Zr alloy and its strengthening mechanisms

High strain rate multi-directional free forging (HSMDFF) is an effective method to expand the industrial application of Mg alloys. However, dynamic recrystallization (DRX) and strengthening mechanisms of the HSMDFFed Mg alloys have not been understood. In this work, the evolution of the LPSO phase and its roles on DRX of Mg-8Gd-1Er–1Zn-0.6Zr (wt.%) (GEZ811) alloy at various cumulative strains during the HSMDFF process are studied, and the corresponding strengthening mechanisms are analyzed. As the cumulative strain increases, the lamellar LPSO phase undergoes four stages: kinking, tearing, local breaking, and breaking into submicron particles, while the block LPSO phase is kinked, torn and broken with a long-plate morphology. The LPSO-induced DRX (LDRX) is dominant while the continuous dynamic recrystallization (CDRX) is complementary for the grain refinement of the HSMDFFed alloy. At ∑Δ ε = 2.64, the comprehensive mechanical performance of the HSMDFFed GEZ811 alloy is optimal, and its tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation (EL) are 302.4 MPa, 370.3 MPa and 11.8%, respectively. The combined strengthening effects of grain boundary, LPSO phase and dislocation are responsible for the enhancement of mechanical properties, in which the grain boundary strengthening is dominant. The present work is beneficial to provide a low-cost approach for designing and fabricating forged Mg-RE alloys with advanced mechanical performance.

Excellent corrosion resistance of electron beam welded joint and remelted layer of eutectic high-entropy alloy AlCoCrFeNi2.1

Electron beam welding (EBW) and electron beam remelting (EBR) experiments were performed on the eutectic high-entropy alloy (EHEA) AlCoCrFeNi2.1. Electrochemical corrosion experiments of different regions of the joint and the remelted layer were carried out, the subsequent corrosion behavior, the corresponding microscopic morphology, structural characteristics were compared and analyzed in detail. The results show that the corrosion resistance of both the welded joint and the remelted layer is better than that of the cast EHEA. Additionally, the corrosion resistance of the weld center area and the core area of the remelting layer is significantly enhanced. This is mainly because the grain refinement of the weld and the remelted layer, and the redistribution of elements play a key role. Moreover, the effect of large and small angle grain boundaries and the change of dislocation density on the corrosion resistance of different regions cannot be ignored.

Influence of Cerium substitution on structural, optical, and electrical transport properties of Ni-Nano ferrites prepared by Citrate gel auto combustion method

The citrate-gel auto combustion is chosen to the preparation nanocrystalline materials of NiCeXFe2-X O4(X = 0.0 to 0.040 with variation of 0.010). XRD confirmed the crystalline phase of the prepared samples, and by using Debye Scherrer equation the crystalline size was calculated to be 26–14 nm. Lattice parameter variation was observed for the series of samples, which indicates that it obeys Vegard's law. Agglomerated structures and particles observed in the SEM images are in the nanometre region. Two FTIR bands were found in the FTIR spectrum, which attribute the stretching frequencies of tetrahedral (1) and octahedral (υ2) bands, respectively. The Urbach energy of the present Ce doped Ni NFs was found to decrease with increase in the Ce dopant content. It is an indication that enrichment of rare-earth oxide into ferrite base results in more order and decrease of disorderness. Dielectric studies of the samples carried out by LCR meter, analysed results of AC conductivity (σAC), impedance(Z), dielectric constant (ε/), dielectric loss (Tan δ). The σ AC of the samples increased with increasing temperature and dopant concentration. Impedance spectroscopy revealed semi-circular behaviour, and the effect of grain and grain boundaries on electrical conductivity was investigated. The dielectric permittivity and loss decreased as increasing frequency, which phenomena was explained by Koop’s theory of mechanism.

On the relationship between shock and particle velocities in single and bicrystal systems of Aluminum: A molecular dynamics study

Molecular dynamics (MD) simulations are used to study shock wave propagation in various symmetric tilt and symmetric twist grain boundaries (STGB & STwGB) in Al for a range of impact velocities (1 to 3.5 km/s) using the open source LAMMPS package. The calculated shock velocity (US) to particle velocity (UP) relationship for Al single crystal (SC) matches well with the results of ab-initio and experiments. However for the shock simulations in bi-crystals (biC), the calculated shock velocity range fall in the same order of magnitude but does not show a linear, monotonic, US vs UP relationship. The interaction of the shock with the bi-crystal boundary, and shock propagation along different crystal directions on either side of the bi-crystal boundary are possible reasons for such behaviour. Common Neighbour Analysis (CNA) shows that there are more structural changes occurring in the bicrystal than in the single crystal. This is because the impact is along a different plane than the 〈0 1 0〉 direction as is the case of a single direction. We report effect of grain boundary on the calculated shock velocity.