Boundary Triple Junctions Research Articles

Interface and grain-boundary amorphization in the Al/Fe bimetallic system during pulsed-magnetic-driven impact

Using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS), we have investigated the microstructures of the bonding interface in Al/Fe clads fabricated by pulsed-magnetic-driven impact. A ∼10nm thick interfacial layer of amorphous Al–Fe alloys has been observed to form at the interface of crystalline Al/Fe bimetallic system. Amorphization of polycrystalline Al takes place concurrently within the grain boundary triple junctions. It is suggested that amorphization at the bonding interface is initially driven by the collapse of the lattice during the high-strain-rate impact process and then followed by the interdiffusion reaction at temperature below the melting point.

Thermodynamics and Kinetics of 1D Structural Elements and Stability of Nanocrystalline Materials

Grain boundary triple junctions are the structural elements of a polycrystal. Recently it was recognized that they can strongly impact the microstructural evolution, and therefore there engender new opportunities to control and to design the grain microstructure of fine-grained and nanocrystalline materials due to their effect on recovery, recrystallization and grain growth. The measurement of triple junction energy and mobility is thus of great importance. The line energy of a triple junction constructs an additional driving force of grain growth. Taking the triple line energy into account, a modified form of the Zener force and the Gibbs-Thomson relation can be derived to reveal the influence of the triple line energy on second phase particles and the change of the equilibrium concentration of vacancies in the vicinity of voids at a grain boundary. The impact of triple junctions on the sintering of nanopowders is discussed. The role of “grain boundary - free surface” triple lines in the adhesive contact formation between spherical nanoparticles is considered. It is shown that there is a critical value of the triple line energy above which the nanoparticles do not stick together. Based on this result, a new nanoparticle agglomeration mechanism is proposed, which accounts for the formation of large agglomerates of crystallographically aligned nanoparticles during the nanopowder processing.

Structural stability and energetics of grain boundary triple junctions in face centered cubic materials

We present a systematic study to elucidate the role of triple junctions (TJs) and their constituent grain boundaries on the structural stability of nanocrystalline materials. Using atomistic simulations along with the nudge elastic band calculations, we explored the atomic structural and thermodynamic properties of TJs in three different fcc materials. We found that the magnitude of excess energy at a TJ was directly related to the atomic density of the metal. Further, the vacancy binding and migration energetics in the vicinity of the TJ were examined as they play a crucial role in the structural stability of NC materials. The resolved line tension which takes into account the stress buildup at the TJ was found to be a good measure in predicting the vacancy binding tendency near the TJ. The activation energy for vacancy migration along the TJ was directly correlated with the measured excess energy. Finally, we show that the resistance for vacancy diffusion increased for TJs with larger excess stored energy and the defect mobility at some TJs is slower than their constituent GBs. Hence, our results have general implications on the diffusional process in NC materials and provide new insight into stabilizing NC materials with tailored TJs.

G0301402 繰り返し変形における振幅が主すべり系と二次すべり系の転位相互作用に及ぼす影響

Slip deformation in tri-crystal models subjected to cyclic deformation is investigated by a finite element crystal plasticity analysis code. Accumulation of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) are studied in detail. Results of the analysis shown that non-uniform deformation and the high density of GNDs and SSDs accumulated in the form of band around the grain boundary triple junction, these dislocation pattern formation shown that develop during increase of number of cyclic deformation and extend of development of dislocation pattern depend on the strain amplitude. Mechanism of dislocation pattern formation and extend of development of dislocation pattern are discussed from the viewpoint of influence of strain amplitude on dislocation interaction of primary and secondary slip systems.

On the orientation dependence of grain boundary triple line energy in Cu

Abstract Triple lines are the lines of intersection of three interfaces, either external interfaces or internal interfaces of a bulk material. They have been recognized as important microstructural features with specific kinetic and thermodynamic properties. Utilizing atomic force microscopy, the line tensions, i.e. the energy of grain boundary-free surface triple lines and grain boundary triple junctions for different crystallographic systems in copper were determined. The line tension of grain boundary triple junctions in copper was found to be positive and of the order of 10−9 J m−1. Junctions including low energy boundaries, twin boundaries and low angle boundaries revealed a substantially lower line tension than triple junctions comprised only of random high angle boundaries. A simple model based on a constant grain boundary energy density is proposed to account for the orientation dependence of triple line energy.

Thermal dissolution mechanisms of AlN/CrN hard coating superlattices studied by atom probe tomography and transmission electron microscopy

AlN/CrN superlattices with a B1 cubic crystal structure and a bilayer period of 4nm were deposited by reactive radiofrequency magnetron sputtering. The coatings were investigated with respect to their thermal stability and changes in microstructure and chemical composition at 900°C. The AlN layers show high chemical stability but undergo dissolution by pinching off at grain boundaries. A transformation from cubic to hexagonal AlN with subsequent coarsening at grain boundary triple junctions is observed. In contrast to AlN, the CrN layers show poor chemical stability and their compositions are shifted towards Cr2N upon annealing in a protective argon atmosphere due to nitrogen loss. However, even after establishing Cr2N stoichiometry the crystal structure of the layers remains cubic.

The line tension of grain boundary triple junctions in a Cu-Ni alloy

Grain boundary triple junctions are defects in their own right with specific thermodynamic and kinetic properties. The line tension of a grain boundary triple junction will cause impurity segregation, which, in turn, affects the triple line energy and gives rise to a drag effect of solute atoms on grain boundary motion. This can strongly impact microstructural development, in particular of fine grained and nanocrystalline materials. We measured the line tension of grain boundary triple junctions which comprised the intersection of three random high angle boundaries in a Cu-Ni alloy (Ni content about 2.2%) to be 7.8×10−9J/m. This energy is slightly higher than the line tension of a comparable triple line in pure Cu.

The roles of grain orientation and grain boundary characteristics in the enhanced superelasticity of Cu71.8Al17.8Mn10.4 shape memory alloys

Through texture and grain boundary control by continuous unidirectional solidification, the continuous columnar-grained polycrystalline Cu71.8Al17.8Mn10.4 shape memory alloys were prepared and possess a strong 〈001〉 texture along the solidification direction and straight low-energy grain boundary. The alloys show excellent superelasticity of 10.1% improved from 3% for ordinary polycrystalline counterpart and with a tiny residual strain of less than 0.3% after unloading. There are some reasons for the enhanced superelasticity: (1) The martensitic transformation of all grains with strong 〈001〉-oriented texture occur at the same time under the tensile loading, which can avoid the significant stress concentration problem and transformation strain incompatibility at the grain boundaries due to the high elastic anisotropy in ordinary polycrystalline alloy. (2) High phase transformation strain can be obtained along 〈001〉 grain orientation. (3) Straight low-energy grain boundary and the absence of grain boundary triple junctions of continuous columnar-grained polycrystals can significantly reduce the blockage of martensitic transformation at the grain boundaries. These results provide a reference to structure design of high-performance polycrystalline Cu-based shape memory alloys.

Void formation of nanocrystalline materials at the triple junction of grain boundaries

A theoretical model of void formation at the triple junction of grain boundaries is described. Based on the combined effects of grain boundary diffusional creep and triple junction diffusional creep as well as dislocation climb, void formation time and void growth rate are derived. The results indicate that vacancy concentration increases with increasing creep strain rate and the angle of the Burgers vector to its dislocation line, and with decreasing grain size. It sharply declines at low creep strain rates, then the asymptotic behavior approaches a constant at high rates. It is also found that the dislocation density is noticeable for small grain sizes in nanocrystalline Cu, and the void growth rate decreases with creep strain rate and time, which are qualitatively consistent with the conclusions in previous work (Dongare et al 2010 J. Appl. Phys. 108 113518; Du et al 2010 Mater. Sci. Eng. A 527 4837).

Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe–Mn–Al–C light weight austenitic steel

Hydrogen embrittlement of a precipitation-hardened Fe–26Mn–11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.

Stress induced creep cavity

Studies monitoring cavity nucleation with strain after creep deformation has limited information with varying applied stresses. In the present communication, a large number of experiments were performed to investigate cavity nucleation at different stresses for a primary reformer tube. It has been found that with the increase in stress, cavity formation increases drastically, which is more prone to nucleate at grain boundary triple junctions rather than grain boundaries where the extent of initial inclusions/other second phase particles was kept unaltered.

G0310101 繰り返し変形における振幅が転位パターン形成に及ぼす影響(材料力学部門一般セッション:変形・ひずみ(1))

Slip deformation in tri-crystal models subjected to cyclic deformation is investigated by a finite element crystal plasticity analysis code. Accumulation of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) are studied in detail. Results of the analysis shown that non-uniform deformation and the high density of GNDs accumulated in the form of band around the grain boundary triple junction, SSDs accumulated in the form of band during cyclic deformation. Mechanism of dislocation pattern formation is discussed from the viewpoint of influence of strain and stress amplitude.

Inhomogeneous deformation behavior in intercrystalline regions in polycrystalline Ni

We report on three-dimensional spatially resolved X-ray microdiffraction measurements of inhomogeneous deformation within individual polycrystalline grains. These measurements provide new insights into the role of grain boundaries. Particularly striking is the qualitatively different mechanical behavior of subgrain volumes near grain boundaries and triple junctions compared to bulk-grain mechanical behavior. These differences are studied by characterizing the evolution of the local orientation distribution in a polycrystalline Ni sample using polychromatic synchrotron X-ray microdiffraction. Dependence of deformation on grain boundary types and triple junctions is determined. Quantitative distinctions in deformation behavior between low-angle boundaries, special low-energy high-angle boundaries, general high-angle boundaries and triple junctions are characterized in terms of spatially resolved misorientation development and extensions of grain boundary effects. In general, larger lattice misorientations are observed near boundaries with higher grain boundary energy. Special high-coincidence grain boundaries exhibit smaller deformation misorientations than general high-angle grain boundaries. These observations are consistent with the observed increases in ductility in grain boundary engineered materials.

Micromechanical investigations and modelling of a Copper–Antimony-Alloy under creep conditions

In many practical applications, creep damage is the limiting factor of a component’s lifetime. A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal within the framework of finite element analysis. The model considers grain boundary cavitation and sliding according to a micromechanically motivated cohesive zone model while creep deformation of the grains is described following the slip system theory. The model can be applied to idealised polycrystalline structures, such as a Voronoi tessellation or, like demonstrated here, to real grain structures of miniature creep specimens. Creep tests with pure Cu single crystals and with a coarse-grained polycrystalline Cu-1wt.% Sb alloy at 823K have been performed and used to calibrate the polycrystal model. The grain structure of the polycrystalline Cu–Sb specimens has been revealed by the EBSD method. Extensive grain boundary sliding and cavitation has been observed in the crept specimens. Grain boundary sliding has been found to promote wedge-type damage at grain boundary triple junctions and to contribute significantly to the total creep strain. Furthermore, the assumed stress sensitivity of the models grain boundary cavity nucleation rate strongly influences the development of wedge-type damage.

Formation of Surface Nanoaustenite and Properties of 3Cr13 Steel Induced by Pulsed Electron Beam Irradiation under Melting Mode

The surface of 3Cr13 martensitic stainless steel was irradiated by high current pulsed electron beam (HCPEB). The microstructures of the irradiation surface were characterized by X-ray diffraction and electron microscopy. After HCPEB irradiation, formation of a melting layer with depth of about 4 μm on the irradiated surface was determined. Further microstructural investigations indicate that the surface melted layer consists of nanoaustenite and ultrafine carbide particles, which primarily appear at grain boundary triple junction. Additionally, the microhardness and corrosion resistance of the irradiated surfaces was improved significantly. The formation of the nanoaustenite layer induced by HCPEB irradiation was believed to be the dominating reasons for the improvement of comprehensive performance of the material surface.

Creep deformation and mechanisms in Haynes 230 at 800 °C and 900 °C

Creep was studied in Haynes 230, a material candidate for the very high temperature reactor’s intermediate heat exchanger, at 800°C and 900°C. This study focused on the differences between the behavior at the two elevated temperature, and using the microstructure, grain boundary serrations and triple junction strain concentrations were quantitatively identified. There was significant damage in the 900°C samples and the creep was almost entirely tertiary. In contrast, the 800°C sample exhibited secondary creep. Using an Arrhenius equation, the minimum creep rate exponents were found to be n≈3 and n≈5 for 900°C and 800°C, respectively. The creep mechanisms were identified as solute drag for n≈3 and dislocation climb for n≈5. Strain concentrations were identified at triple junctions and grain boundary serrations using high resolution digital image correlation overlaid on the microstructure. The grain boundary serrations restrict grain boundary sliding which may reduce the creep damage at triple junctions and extend the creep life of Haynes 230 at elevated temperatures.

Grain boundary junctions, ridges and facets: their kinetics and effects on grain microstructure evolution

The kinetics of grain boundary triple junctions, grain boundary ridges and grain boundary facets are investigated. A theoretical concept and results of experimental observations and computer simulations of the behavior of these interfacial elements are presented, and their effect on the evolution of grain microstructure in 2D and 3D polycrystals is considered.

Nanostructure and surface effects on yield in Cu nanowires

The yield strengths of nanomaterials are highly sensitive to their internal and surface structures. However, it is difficult to identify a priori which structural feature will govern plastic yield. We employ very large scale molecular dynamics simulations to explicitly identify the relevant yield mechanisms for Cu nanowires with four distinct, experimentally realizable nanostructures: single crystal (SC), nanotwinned single crystal (NTSC), nanocrystal (NC) and nanotwinned nanocrystal (NTNC). By characterizing the deformation at the yield point on the atomic scale, our simulations elucidate the effects of surface defects, nanotwins and grain boundaries on the commencement of yield and reveal several critically important features of the yielding process. First, the initial yields in all nanowires occur via dislocation nucleation at different characteristic nanostructural features. SC and NTSC nanowires yield via dislocation nucleation from surfaces or surface defects, while NC and NTNC nanowires yield via dislocation nucleation from grain boundary triple junctions. Second, our simulations highlight the relative potency of stress concentrators arising from different imperfections in modulating the yield strength of nanowires. Grain boundary triple junctions are as effective as surface defects at acting as stress concentrators. However, the higher density of triple junctions in NC and NTNC nanowires renders these structures considerably weaker than their SC and NTSC counterparts. Third, the presence of nanotwins only marginally enhances the yield strength of nanocrystalline Cu nanowires, which is in line with experimental observation in NTNC Cu nanowires but contrary to that in bulk ultrafine-grain nanotwinned Cu. The reason for this divergent behavior is that in nanowires yield strength is governed by dislocation nucleation from triple junctions in contrast to dislocation propagation in the bulk. Finally, excellent agreement is obtained between the relative yield strengths, stress–strain behavior and dislocation nucleation conditions of nanowires in our simulations and existing experimental data. This suggests that our predicted atomistic processes controlling yield in our simulations may also control yield in experiments.

G031051 三重結晶の繰り返し変形に伴うSS転位パターン形成([G031-05]材料力学部門,一般セッション : 破壊力学,転位)

Slip deformation in tri-crystal models subjected to cyclic deformation is investigated by a finite element crystal plasticity analysis code. Accumulation of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) are studied in detail. Results of the analysis showed that the high density of GNDs and SSDs accumulated in the form of band around the grain boundary triple junction, and dislocation pattern are developed during cyclic deformation. Mechanism of dislocation pattern formation is discussed from the viewpoint of dislocation interaction between primary and secondary slip systems.

Identical Area Observations of Deformation-Induced Martensitic Transformation in SUS304 Austenitic Stainless Steel

In this study, an identical area of a SUS304 austenitic stainless steel specimen was observed by electron backscattering diffraction measurements at different strains in tensile test at ambient temperature, in order to investigate the details of deformation-induced martensitic transformation. Firstly, a number of thin deformation twins were formed in austenite grains. Most of the martensite crystals were observed either near grain boundary triple junctions or inside the deformation twins. Secondly, it was found that martensite crystals preferentially appeared in the austenite grains whose < 001> crystal directions were close to the tensile direction. Furthermore, when austenite grains had several martensite crystals inside, only one or two variants were observed among 24 variants possible under Kurdjumov-Sachs orientation relationship, which indicated the existence of variant selection rules. Patel and Cohen model and Bogers–Burgers model were examined to understand the variant selection, but both models could not explain the variant selections. The result suggests that complicated stress states govern the deformation-induced martensitic transformation in polycrystalline austenite.