Triple Grain Junctions Research Articles

Creep lifetime prediction of virgin and service-exposed Super304H austenitic stainless steel boiler tubes based on hierarchical multiscale analysis and creep cavitation model

ABSTRACTCreep deformation of Super304H austenitic stainless steel boiler tubes in thermal power plants is evaluated using crystal elasto-viscoplastic finite element analysis (CEV-FEA). The material properties of Super304H at elevated temperatures are obtained from experiments and computations at different scales as follows: anisotropic elastic constants of Super304H at elevated temperatures (500–700°C) are obtained from molecular dynamics simulations; microstructures of virgin and service-exposed Super304H boiler tubes are observed using scanning electron microscopy; and macroscopic stress–strain and creep strain curves at 650°C are measured from tensile and miniature creep tests, respectively. Stress concentration evolution at a triple junction of grains featuring various combinations of crystallographic orientations in polycrystalline Super304H is evaluated using CEV-FEA during creep deformation. The remaining creep lifetime of polycrystalline Super304H austenitic stainless steel boiler tubes is estimated from the creep cavitation model, where the stress concentration at the triple junction is used as a representative stress driving creep cavitation.

Optimized composition and improved magnetic properties of Ce-Fe-B alloys

Good magnetic properties of Ce17Fe76.5B6Nb0.5Cu0.5Ga0.5 alloy were obtained in our previous study. In order to increase the Curie temperature of Ce-Fe-B alloy without reducing the magnetic performance, Co element was added to optimize the composition of Ce-Fe-B based alloy. The effect of Nb, Cu, Ga, Co co-addition on phase structures, magnetic characteristics, microstructure and Ce valence state weight of Ce-Fe-B alloy are discussed comprehensively. Magnetic properties of Jr = 0.43 T, Hci = 486 kA/m, (BH)max = 30 kJ/m3 and TC = 437.8 K were obtained in Ce17Fe74.5B6Nb0.5Cu0.5Ga0.5Co2.0 alloy. An obvious remanent magnetization enhancement, decreased recoil permeability and fully closed recoil loops are observed for the optimized alloys. The strengthened short range exchange coupling interaction between adjacent grains contributes to the increased remanence ratio. The synchrotron radiation X-ray absorption near-edge structure fitting result showed that the magnetic Ce3+ valence weight of optimized alloy increased, while the nonmagnetic Ce4+ valence weight decreased compared with the original alloy. Microstructure observations showed that the triple junctions of tetragonal main phase grains are rich in Cu, while Co element prefers to enter into CeFe2 phase. The improved magnetic properties are mainly due to the increased Ce3+ valence weight and strengthened intergranular exchange coupling interactions.

Crystal defects responsible for mechanical behaviors of a WC-Co composite at room and high temperatures - a simulation study.

The microstructure evolution and changes in the structures of crystal defects of the nanocrystalline WC-Co composite in the process of uniaxial compression were studied by simulations at both room and high temperatures. The deformation processes were demonstrated as a function of stress and temperature for the stages prior to and after yielding of the composite. The Peierls stresses were evaluated for Co and WC dislocations with increasing temperature. The deformation mechanisms for each stage of the stress-strain curve were disclosed, in which the effect of temperature was clarified. It was found that with the increase of stress, from elastic deformation to plastic deformation then to yielding of the composite, the dominant mechanisms are grain boundary migration, formation and motion of dislocations in Co, concurrent motion and reaction of dislocations in Co and WC, and then rotation of WC grains in combination with motion of Co and WC dislocations. At the yielding stage, sliding of WC grain boundaries plays an increasingly important role in the contribution to plastic deformation at high temperatures. With strain the proportion of mobile dislocations decreases, and dislocations pile up at triple junctions of WC grains, WC/WC grain boundaries and WC/Co phase boundaries in priority order, leading to the nucleation and propagation of microcracks in these regions.

Unveiling the formation of basal texture variations based on twinning and dynamic recrystallization in AZ31 magnesium alloy during extrusion

Commercial Mg extrusions usually develop various types of basal textures, affecting the formability and mechanical properties. The mechanisms for the formation of basal texture variations were systematically investigated for the first time based on the twinning and dynamic recrystallization (DRX) during extrusion using electron back-scatter diffraction (EBSD) characterization. The results indicate that {101¯2} extension twins with various variants played a key role in the formation of [101¯0] fiber texture, while twin-induced DRX mechanism associated with {101¯1} twins made a limited contribution to overall texture evolution. The microstructure developed consecutively as a result of continuous DRX (CDRX) and discontinuous DRX (DDRX) after twinning with different nucleation of new orientations. In the unDRXed region, new DRXed grains with 30° [0001] GBs preferentially nucleated via CDRX, forming preferred selection of [21¯1¯0] fiber orientation. Consequently, [101¯0] fiber texture became weakened with [21¯1¯0] fiber component strengthening, progressively leading to the development of [101¯0]-[21¯1¯0] double fiber texture. In the DRXed region, DDRX further occurred along the serrated grain boundaries and at the triple junctions of the parent grains by bulging, forming fresh DRXed grains without preferred orientation selection. Thus, the [101¯0]-[21¯1¯0] double fiber texture became randomized and gradually transformed into non-fiber texture at the late stage of extrusion. Besides, typical nucleation sites for DRX and subsequent grain growth were also discussed in this study. These findings unveil that basal texture variations are attributed to the twinning and DRX mechanisms during extrusion, which leads to new insight to design new wrought Mg alloys with high performance by grain refinement and texture modification.

Model for partitioning slip patterns at triple junctions of grains

A rate insensitive model based on interfacial compatibility of plastic strains is formulated to predict partitioning slip patterns in interaction of polyhedral grains at triple junctions. The model imposes macroscopic deformation on a skeleton of grain boundaries that, in turn, prescribes a particular common projection of plastic strains on both sides of any boundary facet. To accommodate three independent components of this projection, each of two adjacent domains deform with three slip systems. Unlike Taylor's model, such slip patterns at different facets of the same grain prove to be dissimilar and hence trigger the crystal orientation splitting near triple lines. For a case study of this phenomenon, the model applies to a grain junction of low deformed IF steel regardless of slip patterns in grain cores. Allowance for the latter is finally discussed that will be indispensable for the model extension to entire grains and, eventually, the entire polycrystal.

Microstructure and corrosion behavior of a Zr-Sn-Nb-Fe-Cu-O alloy fabricated by α+β quenching processing

In this work, the microstructure of a Zr-Sn-Nb-Fe-Cu-O alloy fabricated by α+β quenching processing (ABQ sample) was investigated by the combined utilization of scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results showed that the polygonal grains evenly distributed in ABQ sample and triangular grains distributed at triple junctions of polygonal grains with densely second phase particles (SPPs). The textures of <0002> directions tilted approximately 25° from normal direction and the rolling direction spreading along the were found in the sample, which was also detected in the regularly cold rolled and annealed sample. The occurrence of corrosion kinetics transition of ABQ sample was sight earlier than that of RAX sample. Due to a higher solid solubility of β-Zr, SPPs re-dissolved into β-Zr during α+β annealing and precipitated out afterward in those areas. Finally, discrepant corrosion rate of metal matrix and SPPs led to the formation of protrusions of matrix, which could concentrate stress and generate cracks in the oxide.

Effect of NH4F additive on purification of AlN ceramics

In order to overcome the purification difficulty of aluminum nitride (AlN) ceramics, the sintering of AlN ceramics with ammonium fluoride (NH4F) as an additive had been studied. The results demonstrate that the addition of NH4F evidently affects the phase compositions, the microstructure of grains and the contents of oxygen and nitrogen in the AlN sintered samples. NH4F not only removes oxygen out of AlN grains but also reduces the total oxygen content in AlN ceramics. It is found that relatively high purity of AlN can be acquired when the molar ratio of NH4F/O (oxygen element in raw AlN powder) increases to 0.8. With adequate amount of NH4F, the Al–O–N phases are removed. SEM and TEM results show the hexagonal structures of AlN grains with clean triple-grain junctions. The oxygen content decreases to 0.55 wt% and nitrogen content increases to 33.7 wt%. Thermodynamic analysis illustrates the oxygen removing effects of NH4F by the reaction of NH3 and Al2O3, which inhibits the formation of Al–O–N. NH4F should be at least 2/3 of the oxygen content.

Anisotropic behavior of grain boundary diffusion in hot-deformed Nd-Fe-B magnet

Grain boundary diffusion process using Nd70Cu30 was applied to the c-plane and ab-plane of hot-deformed Nd-Fe-B magnet. The coercivity was increased from 1.38T to 2.44 and 2.20T for the ab-plane and c-plane diffused specimens, respectively. The corresponding diffusion depth characterized by Cu content in the magnet was about 4.5 and 2.3mm for the two specimens. Hierarchical coupling between “river-like” Nd-rich phase on the powder flake surface in micron-scale with the diffusion channels at the grain boundaries and triple junctions of the matrix grains in nano-scale is the fundamental cause of the anisotropic grain boundary diffusion behavior.

Low-temperature sintering of high potential gradient B2O3-doped ZnO varistors

High voltage gradient ZnO–Bi2O3-based varistor ceramics doped with different amounts of B2O3 were prepared by the conventional ceramics processing at low temperature and their phase transformation, microstructure and electrical properties were investigated. B2O3 addition promoted the formation of a liquid phase along the grain boundary and triple junctions of the ZnO grains and improved electrical properties. ZnO–Bi2O3-based varistors with nonlinearity coefficients (α) exceeding 45, the leakage current (IL) not more than 0.1 μA and breakdown fields ranging from 880 to 1450 V/mm were obtained by doping with 0.5–2 wt% B2O3 at the sintering temperature of 850 °C and dwelling time of 5 h.

Creep failure mechanism and life prediction of lead-free solder joint

The reliability of solder joints in electronic products has received a lot of concern. The creep behavior is an important property for lead-free solder. In this research, creep tests were conducted to the Sn3.0Ag0.5Cu solder joints. Creep failure mechanism was analyzed and creep life was evaluated based on the Monkman–Grant and Larson–Miller parameter model. The results show that separation is easy to take place at dendrite grain boundaries. Creep cavity initiates at triple junctions of dendrite grains in solder matrix near the intermetallic compound layer. The creep fracture morphology at low temperature and high stress comprises three regions: the shear plane, shear lip and the instantaneous rupture zone. However, the shear plane region is disappeared with increasing the temperature and decreasing the stress. The rupture surface is characterized by the shear lip and inter-granular failure morphology. The Monkman–Grant equation and Larson–Miller parameter model can be used to predict the solder joint creep life.

Cover Image, Volume 97, Issue 2

False‐color scanning electron microscopy image of a fracture surface in a transparent polycrystalline magnesium aluminate spinel. Areas of sub‐micrometer grains (~500 nm) were observed within a matrix of larger grains (~25 μm); suggesting the presence of an impurity phase. Impurities were detected at grain boundaries and triple junctions of the smaller grains with transmission electron microscopy and these can lead to opacity in the spinel. Micrograph courtesy of Marc Rubat du Merac. image

Microstructure evolution during high strain rate tensile deformation of a fine-grained AZ91 magnesium alloy

A fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing is subjected to high temperature tensile test at 623K and 2×10−2s−1 to intermediate strains of 270%, 510%, 750% and failure strain of 990%, and microstructure evolution of the experimental material during tensile test is investigated. The initial grain size is about 1.2μm. Microstructures within the gauge region are much finer than those of grip region, and the grain aspect ratios remain approximately 1.0 in the whole superplastic deformation. With the tensile strains increasing, the average size of β-Mg17Al12 particles increases, and the density of the β-Mg17Al12 particles decreases. Due to the pinning effect of β-Mg17Al12 particles and the occurrence of DRX, the fine microstructures are maintained in the whole superplastic deformation process. Grain boundary sliding is the main deformation mechanism, and cavities are formed in the triple junctions of grains and around the second phase particles during deformation. The excellent high strain rate superplasticity of the AZ91 magnesium alloy is mainly attributed to its initial fine microstructure and good thermal stability.

Dy拡散処理におけるNdFeB系焼結磁石の組織改善効果

Dy-diffusion process is one of effective methods to improve the coercivity of NdFeB sintered magnets with a small amount of Dy usage. In order to achieve further improvements of coercivity, we investigated microstructural factors such as grain size and impurity of NdFeB sintered magnets after Dy-diffusion treatment. One of our results indicates that oxygen, as existing impurity in NdFeB magnets, reacts with the diffused Dy to generate Nd-Dy oxide which mainly remains at triple junctions of NdFeB grains. As the oxidization of Dy could not work to improve coercivity, reducing the oxygen content is effective to enhance the Dy-diffusion effect. On the other hand, we tried another technique to protect from oxidization of Dy, in which rare-earth fluorides were doped into NdFeB sintered body. We found that the doped fluoride could trap oxygen such as the formation of NdOF. After that, Dy could smoothly come into grain boundaries because of non-reaction with oxygen. Then, coercivity of magnets showed to be remarkable improvement.

Characterization and creep properties of proton-conducting Yb-doped barium cerate

Polycrystals of Yb-doped barium cerate with composition BaCe 0.95Yb 0.05O 3− δ have been synthesized via a solid-state reaction route. The ceramic has a single orthorhombic perovskite phase, and the crystallographic unit cell is almost unperturbed with respect to undoped BaCeO 3. The compound exhibits a homogeneous distribution of equiaxed and submicron grains. The high-temperature mechanical properties have been studied for the first time. Mechanical tests were carried out in compression between 1100 and 1250 °C in air at constant initial strain rate. A gradual brittle–ductile transition was observed with increasing temperature and/or decreasing strain rate. Grain boundary sliding is the main deformation mechanism in the ductile region. In this regime, the true stress–true strain curves display an unusual behavior, with an initial strength drop followed by an extensive steady-state stage. This behavior is accompanied by the emergence of new, fine grains along the boundaries and triple junctions of the original grains during deformation.

Phase Field Study on the Austenite/Ferrite Transition in Low Carbon Steel

A phase field model for the austenite/ferrite transition, including the elastic effect caused by the volume mismatch between the austenite and the ferrite, has been presented and applied to the nucleation and growth of the ferrite in the austenite with various cooling rates. The ferrite particles nucleated at the triple junctions of the austenite grains initially grow fast along the grain boundaries. After the austenite grain boundaries are completely wetted by the ferrite, the ferrite starts to grow to the inside of the austenite grain. While it suppresses the growth of the ferrite at the early stage, the elastic field due to the austenite-ferrite volume difference enhances the growth of the ferrite particles at the late stage.

Thermal stability of submicrocrystalline structure in 4Kh14N14V2M steel

At 700°C, the recrystallization of the high-temperature austenitic 4Khl4N14V2M steel with a submicrocrystalline structure takes place under conditions of a high-rate decomposition of the supersaturated solid solution. In this case, the perfeetnese of its structure inocreases due to the improvement of the boundaries and the shape of micro crystallite s at a low rate of their growth. The high rate of the decomposition of the supersaturated solid solution results in the precipitation of chromium carbides at grain boundaries and at triple junctions of grains, efficiently retarding grain growth. Such a structure remains submicrocrystalline in the course of even long heating of the steel at 700°C, with the maximum size of grains being no more than 1 μm. At lower temperatures (600°C), only few recrystallization centers are capable of growing; the carbides precipitate predominantly in the bulk of re crystallized grains and do not impede their growth. Grains several microns in size are formed before the termination of primary recrystallization, and the submicrograin state is not realized.

Dynamic Recrystallization Behavior of GH4586 Superalloy during Hot Compression

Compression test for GH4586 superalloy was carried out at 1273 K and 1373 K with stain from 20% to 60%. The characteristics of dynamic recrystallization were investigated by confocal laser scanning microscope, TEM and EBSD. It was found that at 1273 K dynamic recrystallization mainly took place along preexisting grain boundaries creating a necklace structure. It was confirmed by TEM observations that the first layer of the necklace structure was formed by the mechanism of bulging of preexisting grain boundaries and the following layers were nucleated at the triple junctions of former dynamic recrystallization grains. When specimens were deformed with different strains at 1373 K, dynamic recrystallization was almost completed. Our observation showed that twinning played an important role during dynamic recrystallization by promoting dynamic recrystallization grains growing. Substructures of grains were constructed by the reconfiguration of dislocations hindered at large ' particles in interior of initial grains during dynamic recovery.

Morphological evidence that Goss abnormally growing grains grow by triple junction wetting during secondary recrystallization of Fe–3% Si steel

Microstructural features near the growth front of Goss abnormally growing grains (AGG) were investigated in the initial stage of secondary recrystallization of Fe–3% Si steel. In order to obtain three-dimensional information about these microstructures, serial section images were obtained by repeated polishing, which revealed that the grains with negative curvatures are three-dimensionally connected to the Goss AGG. These morphologies indicate that the Goss AGG underwent solid-state wetting along the triple junction of small matrix grains at its growth front.

Structure of Triple Junctions of Grains, Nanoparticles in them and Bending - Torsion in Metal Nanopolycrystals

The paper presents the results of the transmission electron microscopic (TEM) investigation of the structure and phase composition of nanocrystalline copper obtained by the severe plastic deformation using the high pressure torsion (HPT) method. Special attention is paid to the triple junctions of grain boundaries. It was established that the triple junctions contained the partial disclinations and particles of the secondary phases. The dependences of such junction fractions on the deformation were measured of the formation at nanocrystalline copper. The phase analyses of the secondary phase structure were carried out, the sizes of the phase particles and their volume fractions were determined. The bending – torsion of the crystal lattice arising near the triple junctions was measured. The problem of the long – range stress field screening was considered.

TEM and STEM investigation of grain boundaries and second phases in barium titanate

An intergranular layer was found in 0.4 mol% Y-doped BaTiO3, which was sintered below the eutectic temperature in air. The layer possessed a crystal structure similar to Ba6Ti17O40 with a thickness of 0.7 nm. Plate-like second phases of Ba6Ti17O40 were also observed at triple-grain junctions and an orientation relationship of and was determined between the tetragonal BaTiO3 and the monoclinic Ba6Ti17O40. The detailed structure of the interface between BaTiO3 and Ba6Ti17O40 was resolved by scanning transmission electron microscopy and high-resolution transmission electron microscopy investigations.