Search

A new sintering technique as a combination of ultra-fast spark plasma sintering and high-pressure sintering was employed to densify UO 2 fuel pellets. UO 2 pellets with a relative density of 97.5 % and inhibited grain growth were fabricated at 1000 °C under 150 MPa within only 4.5 min. Both short sintering time and high pressure might explain the inhibited grain growth and hence finer microstructures. UO 2 fuel pellets prepared in this work exhibit excellent mechanical properties. [ABSTRACT FROM AUTHOR]

The challenge of producing dense fine-grained high-entropy ceramics is hereby addressed by a combined cold and conventional pressureless sintering method for the first time. It provides nearly fully dense high-entropy pyrochlores with better microstructural homogeneity, and only half the grain size of conventional pressurelessly sintered samples with identical density. Both the finer and more homogeneous microstructures benefit better mechanical properties in hardness and reduced Young's modulus. The kinetics and activation energies of both densification and grain growth were quantified and discussed to understand how cold sintering affects the microstructural evolution during the following pressureless sintering. More importantly, cathodoluminescence analysis suggested that cold sintering could cause more defects on the surface of the original particles, which could promote the grain boundary diffusion and have drag effects on grain boundary motion. The lessons learned here offer scientific understanding and technological guidance towards pressureless sintering of dense bulk high-entropy ceramics. [ABSTRACT FROM AUTHOR]

The challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering is hereby addressed by a simple two-step sintering method. Ultrafine-grained (Ce 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2) 2 Zr 2 O 7 high-entropy zirconate with 99.2 % theoretical density and 135 nm grain size has been fabricated by pressureless two-step sintering for the first time. Compared to the conventional sintering, two-step sintering provides finer grain sizes and better microstructural uniformity, which yield excellent comprehensive mechanical properties with high hardness of 16.0 GPa, large elastic modulus of 262 GPa and high fracture toughness of 2.8 MPa·m1/2. It is believe that pressureless two-step sintering can be utilized to prepare other high-entropy ceramics with fine grain size and high quality. [ABSTRACT FROM AUTHOR]

Notwithstanding the impressive phase stability and low thermal conductivity exhibited by lanthanum zirconate (LZ), its low coefficient of thermal expansion is deemed a paramount limitation. In the present study, (La 1-x Y x) 2 Zr 2 O 7 with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, as new TBCs, were synthesized by reverse co-precipitation and calcination method. The XRD and Raman analyses elucidated that the substitution of Y3+ ions occurred exclusively at the La3+ positions. In addition, in accordance with FESEM and DTA findings, the incorporation of Y 2 O 3 has induced a reduction in particle size and an enhancement in crystallization resistance. The evaluation of the coefficient of thermal expansion (CTE) demonstrated a notable improvement following the introduction of Y 2 O 3. Specifically, the CTE of LZ increased from 9.34 × 10−6 °C-1 to 10.5 × 10−6 °C-1 in the case of (La 0.5 Y 0.5) 2 Zr 2 O 7 at 1100oC. Investigations of phase stability following a 50 h heat treatment at 1300oC indicated that Y 2 O 3 had no impact on phase stability. All compounds, similar to LZ, exhibited excellent phase stability. Furthermore, the addition of Y 2 O 3 significantly amplified the grain boundary diffusion mechanism during heat treatment, leading to an accelerated grain growth rate. The grain growth rate increased from 1.7 nm/h at LZ to 2.2 nm/h in the case of (La 0.6 Y 0.4) 2 Zr 2 O 7. [ABSTRACT FROM AUTHOR]

Core-rim structure has distinct advantage to improve the performances of KNN-based piezoceramics. Whereas the state of the core-rim structure is difficult to control during sintering. Here, the core-rim structured 0.96(K 0.51 Na 0.47 Li 0.02)(Nb 0.8 Ta 0.2)O 3 -0.04CaZrO 3 (KNLNT) ceramics were both obtained by conventional sintering (CS) and rapid sintering (RS). KNLNT ceramics prepared by rapid sintering exhibit more outstanding controllability on grain growth and core-rim structure and can hold the core/rim size ratio in a stable and favorable level due to extended effective range of grain boundary diffusion in densification. Benefiting from the controllable microstructure, the RS method prepared samples show excellent performance. The unipolar strain value of RS-1240 (S max =0.252%) is 2.07 times as much as CS-1110 (S max =0.122%). Large strain, low hysteresis and low dielectric permittivity features make the core-rim structured KNLNT ceramics a potential material for pulse drive applications and demonstrate that the manual precise control of core-rim structure could create many possibilities on materials design. [Display omitted] [ABSTRACT FROM AUTHOR]

The effect of the flash event on grain growth behavior was examined in 8 mol% yttria-stabilized cubic zirconia (8Y-CSZ) polycrystals under direct and alternating electric fields/currents. Even at the same specimen temperature, the rate of the grain growth is accelerated in the flash event than in the heat treatment without electric field/current (0 V), suggesting that the non-thermal effect caused additionally under the flash event contributes to the acceleration of the grain growth. Although the grain growth behavior can be characterized by the empirical equation (d t n – d 0 n = kt) with the same grain growth exponent of n = 3 irrespective of the electric field/current, the activation energy for the grain growth is lowered under the flash event. This suggests that the non-thermal effect caused by the flash event does not affect the mechanism of the grain growth, but would accelerate the rate-controlling process of the grain growth. The non-thermal effects can be further accelerated in fine-grained specimens under the alternating current flash and/or by increasing the input power density. Since the grain growth is rate-controlled by the grain boundary cation diffusivity, the non-thermal effect under the flash event would accelerate the grain boundary cation diffusivities through the formation of excess oxygen vacancies depending on the polarity and the power density. [ABSTRACT FROM AUTHOR]

Skorohod and Olevsky's constitutive relation for viscous sintering (SOVS) is employed to simulate the shrinkage and relative density evolution of ceramics during the sintering process. While this model effectively simulates shrinkage and density evolution, the original formulation of the SOVS model does not explicitly incorporate grain growth or microstructural evolution. This paper introduces an enhanced version of the SOVS model, incorporating a novel calibration method to obtain the temperature dependence of the viscosity parameter. It considers grain growth as well as the evolution of the microstructure by adding new parameters. Simulations were conducted using Code_Aster, an open-source finite element software, in conjunction with MFront, a tool for formulating material behaviour equations. Dilatometer tests were performed with probes of porcelain stoneware composition to calibrate the model. The predictions of dimensional shrinkage and relative density evolution showed a high accuracy of the model (> 95%), enabling its use in the manufacturing process. [ABSTRACT FROM AUTHOR]

The ability to control the particle size of titanium dioxide nanoparticles (TNPs) is crucial for their practical use. This study presents a method for preparing monodisperse amorphous and anatase TNPs with high surface areas and tunable grain sizes. The method combines a sol-gel process with a hydrothermal process, using benzoic acid (BA) as a structure-directing agent. By adjusting the hydrolysis and condensation rates of the titanium alkoxide, we can synthesize monodisperse amorphous TNPs with a controllable size range of 200–800 nm. Additionally, the amorphous TNPs synthesized through this method serve as raw material, enabling the morphology of anatase TNPs to be inherited from amorphous TNPs with a hydrothermal reaction time of 1 h. When the hydrothermal reaction time is extended to 10 h, the closely linked primary titanium dioxide is fully separated and replaced by ultra-fine anatase TNPs. This optimal scheme show promise for self-assembly and large-scale synthesis of monodisperse amorphous and anatase TNPs. [ABSTRACT FROM AUTHOR]

Two strategies have been extensively employed to develop advanced accident tolerant fuels (ATF): improving thermal conductivity and producing large grain sized pellets. However, there are few reports on the simultaneous utilization of both strategies. In this work, we fabricated Mo–Cr alloy reinforced UO 2 (UMC) composite pellet with both high thermal conductivity and large grain size by a simple in-situ alloying method for the first time. The average grain size of UO 2 increased from 6 μm to 113 μm. Thermal conductivity of the UMC pellet (with 2 vol% Mo) at 1200 °C increased by 36.46% compared with pure UO 2. The increase rate of thermal conductivity per 1 vol% dopant reached about 18%. The Mo–Cr alloy formed continuous micro-cell layer around UO 2 particles. Such continuous micro-cell Mo–Cr alloy layer and increase of UO 2 grain size both helped improve the thermal conductivity of UMC composite pellets. Thermal expansion coefficient of the UMC under the operating temperature decreased by 11.90% compared with pure UO 2. The oxidation resistance of the UMC pellet under high temperature steam was also improved. This work provides a new strategy towards fabricating ATF with both high thermal conductivity and large grain size. [ABSTRACT FROM AUTHOR]

Bi 1.6 Pb 0.4 Sr 2 Co 2 O 8 samples have been textured by the Laser Floating Zone (LFZ) process using Nd:YAG, and CO 2 laser radiation. Using different wavelengths resulted in significant structural and microstructural modifications. Powder XRD patterns showed that the thermoelectric phase is the major one in both cases. Microstructural studies revealed that all samples presented the same phases but with much lower content of secondary ones in those processed with the CO 2 laser. Electrical resistivity showed different behavior for the two types of samples, being in general, lower for the CO 2 grown rods. Seebeck coefficient is lower for the CO 2 grown samples up to 300 °C, and higher in the high-temperature range, reaching 240 μV/K at 650 °C, which is one of the highest values obtained so far in these compounds. Moreover, thermal conductivity at 600 °C for these samples (0.93 W/K m) is among the lowest reported in the literature. As a consequence, ZT values at 600 °C reached 0.42 in CO 2 textured materials, about two times higher than the obtained in Nd:YAG ones. This value is among the highest reported so far in the literature, and is comparable to the performance attained for the same composition containing nanoparticles addition. All these properties, combined with the fact that the processed materials can be directly integrated into thermoelectric modules, render them highly attractive for industrial production. [ABSTRACT FROM AUTHOR]

LiNi x Co y Mn 1-x-y O 2 (NCM) has been regarded as promising cathode materials for high energy density lithium ion battery. Apart from the capacity deterioration during cycling process, NCM particle microcracks' generation resulting from anisotropic strain in crystal lattice is also regarded as the crucial issue from mechanical scale. As the original materials of synthesizing NCM, NCMOH precursor decides much of the prepared NCM materials in both electrochemical and mechanical scale while mechanical degrading mechanism inside crystal lattice is missing in related literature. In this study, the LiNi 0.7 Co 0.1 Mn 0.2 O 2 single crystalline particles are calcined from NCMOH precursors with difference in mechanical stacking morphology and physical property (specific surface area and tapping density) through the same appropriate sintering condition. The evolution of lattice microstrain calculation and elastic modulus for NCM materials through structure and mechanical evaluation has been studied. It is indicated that larger size and higher rate capability (fast charging) of single crystalline LNCMO materials can be obtained from precursor with smaller specific surface area and higher tapping density while the larger microstrain and less elastic modulus degradation are determined during electrochemical charging process. Stronger structure maintaining ability of NCM lattice under large electric field force (during fast charging process) is beneficial to the capacity performance related with higher-rate capability. This might provide a novel insight of understanding the mechanical effect of NCMOH precursors on the synthesis and electrochemical performance of Ni-rich LiNi x Co y Mn 1-x-y materials. [ABSTRACT FROM AUTHOR]

It has been a common method to improve the mechanical properties of metals by manipulating their microstructures via static recrystallization, i.e., through heat treatment. Therefore, the knowledge of recrystallization and grain growth is critical to the success of the technique. In the present work, by using in-situ high temperature EBSD, the mechanisms that control recrystallization and grain growth of an extruded pure Mg were studied. The experimental results revealed that the grains of priority for dynamic recrystallization exhibit fading competitiveness under static recrystallization. It is also found that grain boundary movement or grain growth is likely to show an inverse energy gradient effect, i.e., low energy grains tend to swallow or grow into high energy grains, and grain boundaries of close to 30° exhibit superior growth advantage to others. Another finding is that {10–12} tensile twin boundaries are sites of hardly observed for recrystallization, and are finally swallowed by adjacent recrystallized grains. The above findings may give comprehensive insights of static recrystallization and grain growth of Mg, and may guide the design of advanced materials processing in microstructural engineering. [ABSTRACT FROM AUTHOR]

Ferrite materials are urgently demanded in high-frequency miniaturized power electronics for their high electromagnetic properties and high electrical resistivity. However, traditional NiZn ferrites are faced with low resistivity and high loss at low-temperature sintering, which severely restrict their applications. In this study, CdO was substituted into Ni 0.22 Cu 0.2 Zn 0.58 Fe 2 O 4 ferrites at a presintered stage to refine the grain size and to achieve superior performance at low-temperature sintering. Interestingly, the electrical, magnetic and gyromagnetic properties of the NiCuZn ferrites exhibited a strong dependence on the concentration of Cd2+ ions. The Cd2+ ions entered into the lattice and substituted for Fe3+ at the octahedral site. SEM image results showed that CdO-substituted NiCuZn ferrites for the first sintering stage possess inhibition of abnormal grain growth, which can effectively solve the problem of shrinkage and grain compaction of ferrite materials in LTCC technology. Furthermore, to promote the grain growth at low sintering temperature, the optimized Bi 2 O 3 additives were added at final sintering stage. The resistivity of the optimized Cd2+ ions is significantly enhanced from 2.41 × 108 Ω cm to 8.51 × 108 Ω cm. Finally, the NiCuZnCd (x = 0.15)-1.2 wt % Bi 2 O 3 ferrites with the highest densification possess ρ = 8.51 × 108 Ω cm, ε′ = 18, tan δ ε = 6.0 × 10−3, μ′ = 81, f r = 117 MHz, tan δ μ = 4.8 × 10−2 and ΔH = 281 Oe. This work provides a new doping method for the design of the high frequency LTCC device. [ABSTRACT FROM AUTHOR]

Preparation of high-entropy ceramics (HECs) requires long duration at high temperatures, which limits their wide application. In this study, high-entropy (La 0.2 Ce 0.2 Nd 0.2 Sm 0.2 Eu 0.2) 2 Zr 2 O 7 ceramics was prepared at low temperatures of 762–885 °C in very short time of 60 s using the flash sintering. The effects of electric field strength and current limit on the densification and grain growth, as well as the phase transformation were systematically investigated. Both density and grain size were increased with the increase of the electric field strength and current limit. The phase transformation from defective fluorite to pyrochlore occurred at large current limit. The flash-sintered bulks exhibited good mechanical properties. This study demonstrates that flash sintering is an effective method for rapid preparation of HECs at low temperatures. Moreover, the microstructure of HECs can be controlled by adjusting the electric field strength and current limit. [ABSTRACT FROM AUTHOR]

Solid-state sintering of Y 2 O 3 is difficult and requires high-temperature heat treatment or the use of a sintering additive. In this study, liquid-phase sintering was utilized to produce dense Y 2 O 3 bodies with fine-grained microstructure. Al 2 O 3 and SiO 2 were added to a commercial nano-Y 2 O 3 powder using aluminum nitrate and tetraethylorthosilicate (TEOS) as the precursors; the samples were uniaxially pressed at 100 MPa and sintered at 1500 °C and 1650 °C. The addition of the sintering aids increased the relative density of the samples sintered at lower temperatures; also, the sintering aids suppressed grain growth. The dissolution of Y 2 O 3 into the liquid phase changes the solid-liquid interfacial energy resulting in the formation of isolated yttrium-silicate/yttrium-aluminate grains and accounts for the suppression of the growth of yttria grains. [Display omitted] [ABSTRACT FROM AUTHOR]

• Superplasticity of fine-grained Mg-10Li alloy was studied. • Severe plastic deformation was used to achieve the fine grains. • Its deformation mechanisms for superplastic flow were analyzed. • Deformation mechanism maps were constructed in 2D and 3D. The superplastic behavior and associated deformation mechanisms of a fine-grained Mg-10.1 Li-0.8Al-0.6Zn alloy (LAZ1011) with a grain size of 3.2 µm, primarily composed of the BCC β phase and a small amount of the HCP α phase, were examined in a temperature range of 473 K to 623 K. The microstructural refinement of this alloy was achieved by employing high-ratio differential speed rolling. The best superplasticity was achieved at 523 K and at strain rates of 10−4 -5 × 10−4 s−1, where tensile elongations of 550–600% were obtained. During the heating and holding stage of the tensile samples prior to tensile loading, a significant increase in grain size was observed at temperatures above 573 K. Therefore, it was important to consider this effect when analyzing and understanding the superplastic deformation behavior and mechanisms. In the investigated strain rate range, the superplastic flow at low strain rates was governed by lattice diffusion-controlled grain boundary sliding, while at high strain rates, lattice diffusion-controlled dislocation climb creep was the rate-controlling deformation mechanism. It was concluded that solute drag creep is unlikely to occur. During the late stages of deformation at 523 K, it was observed that grain boundary sliding led to the agglomeration of the α phase, resulting in significant strain hardening. Deformation mechanism maps were constructed for β-Mg-Li alloys in the form of 2D and 3D formats as a function of strain rate, stress, temperature, and grain size, using the constitutive equations for various deformation mechanisms derived based on the data of the current tests. [ABSTRACT FROM AUTHOR]

Li 2 TiO 3 ceramic pebbles with uniform structure, high density and good mechanical property were successfully fabricated by wet process. The Li 2 TiO 3 pebbles sintered at 1000 °C for 4 h had satisfactory density (91% T.D.) and crush load (a.v. 27.1 N). The apparent activation energy of Li 2 TiO 3 grain growth was 201 ± 7 kJ/mol. Accelerated densification was observed in a reducing atmosphere, this might be attributed to the oxygen vacancies generated by the reduction of Ti4+ to Ti3+ in 0.1%H 2 +He atmosphere, which facilitate the diffusion of ions through the lattice. The grain growth was controlled by lattice diffusion under reducing atmosphere, and the apparent activation energy was 168 ± 6 kJ/mol. The mechanical results show that the crush load of Li 2 TiO 3 pebbles decreased with the increase of temperature, and the average crush load at 500 °C was 13.9 N. In addition, the thermal cycling and long-time annealing experiments suggested that Li 2 TiO 3 pebbles have good structural and performance stability. The Li 2 TiO 3 ceramics prepared by wet process were satisfactory tritium breeder materials with ideal application potential. [ABSTRACT FROM AUTHOR]

• HPT-processed Mg-Gd-Y-Zn-Zr nanostructure exhibits an excellent thermal stability. • Mg-Gd-Y-Zn-Zr nanostructure exhibits an unique phase transformation upon annealing. • The formation of solute segregation and β phase retards grain growth during annealing. • A hardness peak is achieved by annealing due to the effects of solute segregation. A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy is processed by solution treatment and high pressure torsion (HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K, grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573 K, appears to be nearly one order of magnitude better than for the other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than those of SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg 5 RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from the other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries. [ABSTRACT FROM AUTHOR]

The Th 1- x Sc x O 2- x /2 (0 ≤ x ≤ 0.1) ceramic system with high density was originally prepared via a co-precipitation route and simple sintering method. XRD analysis indicates that the Th 1- x Sc x O 2- x /2 ceramics exhibited a single cubic fluorite structure and a slight increase of cell parameters. SEM images show that Sc doping contributed a decreasing pores and more uniform grain distribution. The calculated relative densities could reach up to a value higher than 99.0% for x = 0.01 during the isothermal process while the maximum actual relative density was only about 77.4% during the non-isothermal sintering process. And the analysis of grain growth kinetics demonstrates that Th 1- x Sc x O 2- x /2 was a solid solution with a high solubility of Sc for x = 0.01 and a low solubility of Sc for both x = 0.05 and x = 0.1, which resulted in a strongly slow grain growth for x = 0.1 due to the solute drag effect. The sintering mechanism investigation also illustrates that low amount (x = 0.01) of Sc doping could significantly enhance the densification sintering of ThO 2 -based ceramics and reduce the sintering temperature by nearly 150 °C. Such strategy has issued the bottleneck problem existed in the densification sintering of ThO 2 -based ceramics at a lower temperature without any other special sintering atmosphere, which even could be applicable for other ceramics difficult to sinter. [ABSTRACT FROM AUTHOR]

The effects of both nuclear energy loss and electronic energy loss need to be taken into consideration in the ceramic-based waste forms under repository environment. However, the irradiation responses of ceramic-based waste forms to each type of energy loss are somewhat different. In this study, the microstructure evolutions of ultrafine nano and micro Gd 2 Zr 2 O 7 -based waste forms were systematically studied under predominant electronic energy loss simulated by multi-energy He+ irradiation, and compared to those under predominant nuclear energy loss. The results reveal that the fewer He bubble chains, ribbon-like He bubbles and smaller microcracks were observed in the irradiated nano-grained sample. Additionally, nano-grained sample displayed a lower degree of amorphization and higher atomic order compared to micro-grained samples when subjected to predominant electronic energy loss. Moreover, the irradiation dominated by nuclear energy loss can easily induce the grain growth of nano-grained Gd 2 Zr 2 O 7 -based waste form, but in the present study this phenomenon was not observed under multi-energy He+ irradiation. Consequently, under predominant electronic energy loss, the thermodynamic instability and driving force for grain growth due to excess surface energy in the ultrafine nano sample can be suppressed. As a result, the sample demonstrated enhanced irradiation resistance due to the more efficient absorption and elimination of defects at grain boundaries induced by electronic excitation. We elucidated that enhanced irradiation resistance of the waste forms by tailoring the grain size requires the consideration of the effects of electronic energy loss and nuclear energy loss, which can provide guidance for the design and optimization of highly irradiation-resistant nuclear waste forms. [ABSTRACT FROM AUTHOR]

Textured SiC with anisotropic crystallographic texture is proposed as a material with improved mechanical properties. Textured SiC was created via alignment of platelet seed particles during aqueous suspension material extrusion, also known as direct ink writing, and subsequent pressureless liquid phase sintering and annealing. The microstructure and texture of the SiC fabricated with and without 5 vol% platelet seeds, and with and without annealing at 2050 °C and 2150 °C was explored via SEM, XRD, and EBSD. All samples fabricated had over 95% theoretical density. Annealing leads to the development of large, high aspect ratio plate-shaped grains among a matrix of many finer, low aspect ratio grains. Higher annealing temperatures and addition of platelet seeds both increased the size of the large grains. Samples were found to be textured regardless of having platelet seeds. Via XRD and EBSD, unseeded SiC was found to have texture where the crystallographic direction [0001] had a preferred orientation perpendicular to the normal direction. This occurred for both direct ink written and cast SiC, so the texture development must have occurred during sintering, though the mechanism is unknown. For seeded SiC, platelet seeds aligned in direct ink writing seeded the grain growth to develop crystallographic texture. The texture was mainly influenced by the alignment of platelet seed particles via shear stresses in the print nozzle, causing a one-dimensional texture where [0001] is perpendicular to the printing direction. However, it was found that the texture was not the expected concentric alignment of platelet particles in direct ink writing, so the shear stresses in the nozzle are not solely responsible for the texture developed. [ABSTRACT FROM AUTHOR]

This study investigates the variations in the microstructural characteristics of a pre-twinned Mg alloy with the temperature of the subsequent annealing treatment. To this end, a rolled AZ31 alloy is compressed to 3% plastic strain along the rolling direction (RD) to activate {10-12} twinning and is subsequently annealed at 200, 250, 300, 350, and 400 °C. Numerous {10-12} twins are formed throughout the compressed material, leading to the formation of a RD-oriented texture. At an annealing temperature of 200 °C, no microstructural variations occur during annealing. As the annealing temperature increases from 250 to 400 °C, the residual strain energy and remaining twin boundaries of the annealed material decrease owing to the promoted static recovery and the increased area fraction of twin-free grown grains. Consequently, an increase in the annealing temperature results in a gradual microstructural transition from a fully twinned grain structure to a completely twin-free grain structure. The microstructural evolution during annealing is predominantly governed by the movement of high-angle grain boundaries via a strain-induced boundary migration mechanism, and a few twin boundaries migrate above 350 °C because of their lower boundary energy. The boundary migration behavior and resultant microstructural evolution are discussed in detail based on the variations in boundary mobility and driving force for boundary migration with annealing temperature. [ABSTRACT FROM AUTHOR]

The uniaxial viscosity is an important material property that has to be determined in order to understand the densification of a material during the sintering process. It depends simultaneously on temperature, relative density and grain size which complicates its determination. To determine its dependence on these three factors, bending creep tests are performed on pre-sintered 3Y-TZP specimens with different relative densities and grain sizes up to the corresponding pre-sintering temperatures. The uniaxial viscosity of 3Y-TZP specimens is calculated after obtaining the deflection rates. It is found that the uniaxial viscosity decreases with temperature according to the Arrhenius equation. In addition, the tetragonal to cubic phase transformation at temperature above 1300 °C leads to a decrease of the activation energy. Apart from that, the contribution of densification and grain growth to the increase of the uniaxial viscosity is quantitatively determined. When grain boundary diffusion dominates, the contribution of grain growth is up to 1.8 times that of densification to the uniaxial viscosity. Furthermore, it is found that Rahaman's model best fits the normalized uniaxial viscosity. At the end, an evolution profile of the uniaxial viscosity for polycrystalline materials during the sintering process is proposed to facilitate the analysis of the sintering shrinkage rate. [ABSTRACT FROM AUTHOR]

We investigated the effect of very high pressure on the sintering temperature, phase transition and the grain growth during Spark Plasma Sintering (SPS) of a 15 nm TiO 2 nanopowder. Using in situ synchrotron X-ray diffraction during sintering at 1.5 and 3.5 GPa, we followed the evolution of the crystalline phases and the crystallite size as a function of temperature. In comparison, in the laboratory, SPS experiments were performed on two original facilities: A Paris-Edinburgh press and a high-pressure module adapted to standard SPS equipment. We studied the effect of the pressure on the sintering in the range 76 MPa to 3.5 GPa. We have shown that highly dense nanostructured ceramics can be prepared under very high pressure at low sintering temperatures. At 1 GPa, we limited the grain growth to an average size of 233 nm by heating at only 560 °C, and achieved a relative density of 98 %. [ABSTRACT FROM AUTHOR]

Critical properties of metallic materials, such as the yield stress, corrosion resistance and ductility depend on the microstructure and its grain size and size distribution. Solute atoms that favorably segregate to grain boundaries produce a pinning atmosphere that exerts a drag pressure on the boundary motion, which strongly affects the grain growth behavior during annealing. In the current work, the characteristics of grain growth in an annealed Mg-1 wt.%Mn-1 wt.%Nd magnesium alloy were investigated by advanced experimental and modeling techniques. Systematic quasi in-situ orientation mappings with a scanning electron microscope were performed to track the evolution of local and global microstructural characteristics as a function of annealing time. Solute segregation at targeted grain boundaries was measured using three-dimensional atom probe tomography. Level-set computer simulations were carried with different setups of driving forces to explore their contribution to the microstructure development with and without solute drag. The results showed that the favorable growth advantage for some grains leading to a transient stage of abnormal grain growth is controlled by several drivers with varying importance at different stages of annealing. For longer annealing times, residual dislocation density gradients between large and smaller grains are no longer important, which leads to microstructure stability due to predominant solute drag. Local fluctuations in residual dislocation energy and solute concentration near grain boundaries cause different boundary segments to migrate at different rates, which affects the average growth rate of large grains and their evolved shape. [ABSTRACT FROM AUTHOR]

The present study aims to investigate the influence of grain growth during heat treatment on the phase stability of AZO: A 2 Zr 2 O 7 compounds at 1300°C. To this end, three cations of La3+, Gd3+, and Yb3+ (corresponding to large, medium and small cations, respectively) were placed in the A-site of the AZO compounds through co-precipitation–calcination technique at 1000°C for 3 h to achieve LaZO: La 2 Zr 2 O 7 , GdZO: Gd 2 Zr 2 O 7 , and YbZO: Yb 2 Zr 2 O 7 , respectively. The phase stability of LaZO, GdZO, and YbZO was assessed under 50 h of heat treatment at 1300°C. Characterization techniques such as conventional and high-temperature XRD, Raman and Fourier transform infrared (FTIR) spectroscopies, simultaneous thermal analysis (DTA/TG), and field-emission scanning electron microscopy (FESEM) were employed to assess the synthesized compounds. FESEM results of LaZO, GdZO and YbZO calcinated powders indicated orderly particles, uniform in shape and size, with a relative tendency of agglomeration and with an average particle size of less than ∼100 nm. XRD results of LaZO and YbZO powders after calcination indicated the formation of pyrochlore and defect fluorite structures, respectively. The placement of Gd3+ (GdZO) led to the formation of a dual pyrochlore-defect fluorite structure. The current research also indicated an enhancement in the tendency to form pyrochlore structure in GdZO and YbZO compounds along with the growth of the grains after 20 (mean grain size of ∼250 nm) and 50 h (mean grain size of ∼800 nm) of exposure to the temperature of 1300°C, respectively. [ABSTRACT FROM AUTHOR]

Preparation of Gd 2 Zr 2 O 7 nanoceramics using a simple and low energy-consuming approach is significant for their wide range of applications. In this work, flash sintering (FS) and two-step flash sintering (TSFS) were first utilized to prepare Gd 2 Zr 2 O 7 nanoceramics. The effects of electric field intensity and current limit on the densification and grain growth were investigated. The grain size was decreased with the increase of electric field intensity. Meanwhile, both grain size and density were increased with the increase of current limit. The Gd 2 Zr 2 O 7 nanoceramics with an average grain size of 98 nm and a relative density of 92.8% were prepared by FS. By contrast, the Gd 2 Zr 2 O 7 nanoceramics with the higher relative densities of 96.3–97.5% and the smaller average grain sizes of 85–90 nm were prepared by TSFS. Compared to FS, TSFS can achieve suppressed grain growth and fast densification. The as-prepared Gd 2 Zr 2 O 7 nanoceramics showed excellent hardness and fracture toughness. The sample breakage caused by current localization can be avoided by controlling the current ramp after the occurrence of the flash event. [ABSTRACT FROM AUTHOR]

In-depth investigation of the sintering phenomena in ceramic powders remains challenging, with the typical size of the individual particles being around 1 µm or less, i.e., at the resolution limit of X-ray micro-tomography (μCT). This has been dealt with, thanks to the state-of-the-art hard X-ray nano-analysis beamline at the upgraded European Synchrotron Radiation Facility (ESRF). Complete 3D images were obtained for representative ceramic powder systems with a voxel size as low as 25 nm, so as to depict particles and pores with adequate details and follow the entire sintering process. Subsequent quantitative image analyses were used to explore microstructural changes, including the evolution of relevant sintering parameters with respect to the grains and the pores. Notably, a study adopted in this research on the advancement of pore curvatures can be linked to tracking the stages of sintering. [ABSTRACT FROM AUTHOR]

The effects of dopants with different valences on the flash sintering behavior of α-Al 2 O 3 are investigated. The results indicate that regardless of their valence and ionic radius, all the tested dopants reduce the onset temperature more effectively compared to undoped Al 2 O 3. Interestingly, the relative density and grain size of the flash-sintered samples exhibit an inverse linear relationship. The data for the samples doped with non-trivalent cations fall on a different line than those for the samples doped with trivalent cations and the undoped samples. Dopants have an effect on the flash sintering behavior of Al 2 O 3 because flash sintering increases the solubility of the dopants in alumina, creating more point defects, and thereby increasing the electrical conductivity of the material. The mechanisms for point defect generation in trivalent and non-trivalent dopants are different. [ABSTRACT FROM AUTHOR]

In functional ceramics, the impact of dopants on bulk crystals is generally well understood. Their impact on grain boundaries is less well known. The present study investigates the impact of acceptor dopants on grain growth in strontium titanate. Scanning electron microscopy and analytical (scanning) transmission electron microscopy have been used to gain knowledge on Fe segregation behavior, grain sizes, and grain size distributions of SrTiO 3. While undoped microstructures show normal grain growth at low temperatures (<1350 °C), doped microstructures evolve bimodally. With increasing acceptor dopant concentration, an increasing population of small grains develops. It is shown that Fe segregates to the grain boundaries due to its negative charge and a positive boundary potential. Thus, the experimental findings seem to be well explained by the theory of solute drag: The diffusion of segregated defects ('solutes') at grain boundaries can retard grain boundary migration. [ABSTRACT FROM AUTHOR]

This paper aims at elucidate the role of calcium as sintering aid for YAG ceramics elaborated by a reaction-sintering process. From structural and microstructural analyses, the solubility limit of calcium was found to be close to 0.065 ± 0.015 at% at 1700 °C. For calcium content higher than this value up to 1 at%, CaAl 12 O 19 crystallized precipitates with a melting temperature close to 1850 °C were observed. Whatever its content, calcium segregates at grain boundaries as evidenced by STEM and NanoSIMS chemical analyses at nanometric scale, and strongly decreases densification and grain growth kinetics of YAG ceramics. This result was explained by a solute-drag mechanism due to the segregation of complex C a 2 + - V O.. clusters with low mobility at grain boundaries. Finally, calcium appears as a very efficient sintering additive to limit grain growth in YAG, thus avoiding the formation of intragranular porosity that is a critical point for the manufacturing of highly transparent ceramics. [ABSTRACT FROM AUTHOR]

Avoiding grain growth during sintering of ceramic nano-powders is of great technological interest. Although two-step sintering is an effective technique to achieve this goal, the mechanisms at play are not well understood. This study adapts our previous discrete model to investigate the conventional and two-step sintering of nano-powders. The densification and grain growth results agree qualitatively well with experimental data on α -alumina. Simulations confirm that faster heating rates retard grain growth in conventional sintering of nano-alumina. Our results support the hypothesis that the success of nano-alumina two-step sintering relies on the sharp increase in the activation energy of the grain boundary mobility at low temperatures. Simulations indicate a transition temperature of 1100 °C and that at least a 2.5-fold increase in activation energy is required to explain the suppression of grain growth. The relative weights of surface diffusion and of grain boundary motion for grain growth are clarified. [ABSTRACT FROM AUTHOR]

In metal additive manufacturing (MAM), laser direct energy deposition (L-DED) and wire arc direct energy deposition (W-DED) are commonly used methods that also possess challenges in the part quality due to the effect of different process parameters. Very little knowledge is currently available on the single-layer thermal behavior, cooling rate, melt pool dimension and their correlation with the microstructure for the DED processes (L-DED and W-DED). In this study, a comparative analysis of the microstructural characteristics along with hardness of austenitic stainless steel (SS 316L) samples is conducted to understand the variations between the L-DED and W-DED processes. It is observed that the microstructure of the W-DED samples possesses more columnar dendritic structure than the L-DED due to slower cooling rate in W-DED. It is also found that the ferrite (δ) phase fraction is 23% higher in W-DED than L-DED. There is also the formation of carbide precipitation at the fusion boundary in the L-DED samples. In L -DED, the grains are mostly oriented along the<001>/<111>direction, whereas in W-DED, the dominant orientation is<111>/<101>. The W-DED samples have significantly higher HAGB fraction (49%) than the L-DED samples. The detailed texture analysis of the samples showed that the L-DED has lower average grain misorientation than the W-DED along with ND rotated cube, twin copper, and cube structures. The hardness data reveals that the W-DED samples possess much higher hardness compared to the L-DED samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • This communication is focused on development of CMT-based PV cells with CdCl 2 treatment. • Devices were fabricated having structure of ITO/CdS/CMT/Au and then chlorine treatment was performed. • Devices without and with CdCl 2 treatment demonstrated the efficiency of 7.83% and 8.76%. • Deep trap states in the devices were analyzed with the help of Mott-Schottky plots. • Grain growth was also explored with chlorine treatment by analyzing SEM images. The suitability of the metal rear contact, the density of surface states, the rate of recombination of the absorber layer, and open circuit voltage loss are all innate issues with typical CdTe solar cells. These problems can be addressed with small Mn doping in CdTe, making it a Cd 1-x Mn x Te (CMT) trinary semiconductor, which can tune the band gaps. With the purpose of give an alternative to traditional CdTe material, CMT PV cells having device structures of ITO/CdS/CMT/Au have been investigated in this study with CdCl 2 treatment. The devices were fabricated by vacuum evaporation, and CMT PV cells without and with CdCl 2 treatment demonstrated the efficiency of 7.83% and 8.76%. The capacitance–voltage data were also used to calculate the doping concentration and depletion layer width, while deep trap states in the devices were analyzed with the help of Mott-Schottky plots. The Quantum efficiency measurements were done at zero bias to identify the optical or electronic losses in devices with CdCl 2 treatment. The grain growth was also explored with chlorine treatment by analyzing SEM images. For further improve the device performance, the states responsible for recombination must be located and eliminated to enhance the fill factor and efficiencies of the devices. The devices' repeatability was also examined and was found to be satisfactory with a 6–10% deviance. This work demonstrates the need for additional research to enhance the performance of CMT PV cells with CdCl 2 treatment for usage as an affordable alternative photovoltaic source. [ABSTRACT FROM AUTHOR]

Pure and low α-Al 2 O 3 /SiO 2 co-doped ceria specimens were prepared by conventional ceramic processing using powder metallurgy. This study investigated the effect of co-doping on the microstructural and structural properties of cerium dioxide was investigated by means of optical microscopy, scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The co-addition of alumina and silica promoted liquid phase sintering and significantly contributed to grain growth, even in the small concentrations of the present study (1 and 2 wt%). A glassy look phase precipitated at the grain boundaries, a characteristic feature of liquid phase sintering. The addition of dopants to the formulation greatly enhanced the grain growth process without disturbing the CeO 2 structure. [ABSTRACT FROM AUTHOR]

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al 2 O 3 –ZrO 2) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al 2 O 3 –ZrO 2 composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al 2 O 3 –ZrO 2 composite ceramic was fabricated via AC flash sintering under 60 mA mm−2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering. [ABSTRACT FROM AUTHOR]

YBa 2 Cu 3 O 7−δ (YBCO) bulk superconductors are regarded as the most promising alternatives for conventional permanent magnets as they provide high field trapping capacity at low temperatures. However, for further improving their superconducting properties, it remains a significant challenge to enlarge the c growth sector by large-sized seeds or multi-seeds while maintaining the microstructure with a low porosity or clean grain boundaries that enables their performance. Here, we report a novel seeding assembly approach by vertically-connecting strip-seeds for inducing potential [110]-oriented overgrowth. Initially, from naturally right-angled corners, YBCO rapidly crystallized to form kite-liked convex areas, which, together with the original seed, effectively functioned as a large-sized seed to generate a sizable c growth sector. The novel-seeded sample exhibited a promising structure not only free of grain boundaries, but also with a favorable porosity of 20.29%. Such a low value is similar to 19.77% of the small-seeded one and much lower than 34.20% of the large-seeded one. Finally, due to structure tailoring towards an enlarged c growth sector with maintained pore characteristics, high and mono-peak trapped fields of 16 mm diameter YBCO bulks were reliably attained, up to a record value of 0.7025 T. This work offers insights into design principles for boosting magnetic properties of other members in the YBCO family. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a hybrid cold sintering/spark plasma sintering (CSP-SPS) process to densify ZnO ceramic with controlled grain growth. The densification of ZnO is initially activated at 85 °C, and high densities (>98%) are achieved at 200–300 °C in only 5 min with a low assisted pressure of 3.8–50 MPa. The microstructure of ZnO grains experiences a mild coarsening from ~205–680 nm during the CSP-SPS. In comparison, a much higher temperature (>770 °C) is required to sinter ZnO ceramic via SPS, and the grain size exhibits an obvious overgrowth to ~10 µm. The calculated apparent activation energy of grain growth using CSP-SPS is 69.3 ± 6 kJ/mol, which is much lower than that of SPS samples with 296.8 ± 59 kJ/mol. In addition, the conduction mechanism of the CSP-SPS and SPS samples is investigated using impedance spectroscopy. Overall, CSP-SPS is promising for the fabrication of fine ceramics with mild sintering conditions. • A hybrid cold sintering/spark plasma sintering (CSP-SPS) is proposed to densify ZnO ceramics. • CSP-SPS enables highly dense ZnO ceramics at 200–300 °C for only 5 min under 3.8–50 MPa. • ZnO grains experience a mild coarsening from ~205–680 nm during the CSP-SPS. • The apparent activation energy of grain growth using CSP-SPS is only 69.3 ± 6 kJ/mol. [ABSTRACT FROM AUTHOR]

Current grain growth models have evolved to account for the relationship between grain boundary energy/mobility anisotropy and the five degrees of grain boundary character. However, the role of grain boundary networks on overall growth kinetics remains poorly understood. To experimentally investigate this problem, a highly textured Al 2 O 3 was fabricated by colloidal casting in a strong magnetic field to engineer a unique spatial distribution of grain boundary character. Microstructural evolution was quantified and compared to an untextured sample. From this comparison, a prevalence of (0001)/(0001) terminated grain boundaries with anisotropic networks were identified in the textured sample. These boundaries and their networks were found to be driving grain growth at a faster rate than predicted by models. These findings will allow better modelling of grain growth in real systems by experimentally exploring the impact thereon of grain boundary plane anisotropy and relative energy/mobility differences between neighboring boundaries. [ABSTRACT FROM AUTHOR]

• More impurities reduce conductivity in annealed small crystallite (<33 nm) films. • More impurities raise conductivity in annealed large crystallite (>68 nm) films. • Laser annealing stimulates impurity diffusion more than thermal annealing. • Laser-annealed films lose more effects from impurities than thermal-annealed films. In this paper, the changes in microstructure and conductivity of the films before and after thermal and laser annealing are observed, which corresponds to various combinations of crystallite sizes and impurity densities. The results show that the impurities promote grain boundary migration at larger initial crystallite sizes, which contradicts previous reports. This is because the rate of grain growth is determined by the driving force and resistance to grain growth, the former provided by the energy of grain boundaries and the impurities outside the nanograin boundaries, and the latter by the impurities enriched within the boundaries. Additionally, laser annealing is more effective than thermal annealing in stimulating impurity diffusion, resulting in a reduced influence of impurities on grain growth. [ABSTRACT FROM AUTHOR]

• A microstructural analysis demonstrates that the grain growth of the samples is influenced by the amount of Bi 2 O 3 addition resulting an increase of the average grain size from 1.12 (for x = 0.0 wt%) to 2.74 μ m (for x = 0.6 wt%). • The magnetic hysteresis measurement shows that the saturation magnetization increases from 72.1 (for x = 0.0 wt%) to 77.6 emu/g (for x = 0.6 wt% Bi 2 O 3). However, it declined at x = 0.8 wt%, which may be ascribed to the dilution effect. • The complex magnetic permeability measurement shows that an enhancement of the initial permeability is clearly observed with the addition of Bi 2 O 3 content. • The RQF is also shown a larger value for x = 0.6 wt% Bi 2 O 3 content, which is good agreement along with the microstructural feature. Influence of Bi 2 O 3 addition on the microstructure and magnetic properties of Li-Cu-Mg-Zn ferrites having the compositions Li 0.25 Cu 0.10 Mg 0.10 Zn 0.30 Fe 2.25 O 4 + xBi 2 O 3 (where x = 0.0, 0.3, 0.4, 0.6 and 0.8 wt%) have been reported in this study. The structural property has been studied with the X-ray diffraction technique, which reveals that all the samples have shown the cubic spinel phase without any impurity peak. A microstructural analysis demonstrates that the grain growth of the samples is influenced by the amount of Bi 2 O 3 addition resulting an increase of the average grain size from 0.84 (for x = 0.0 wt%) to 2.47 µ m (for x = 0.6 wt%). The development of the grain size could be attributed to the densification of sample due to the formation of the liquid phase of Bi 2 O 3 in the sintering process. The magnetic hysteresis measurement shows that the saturation magnetization increases from 72.1 (for x = 0.0 wt%) to 77.6 emu/g (for x = 0.6 wt% Bi 2 O 3). However, it declined at x = 0.8 wt%, which may be ascribed to the dilution effect. The complex magnetic permeability measurement shows that an enhancement of the initial permeability is clearly observed with the addition of Bi 2 O 3 content. It is inferred that a correlation has been found between the grain growth and the magnetic properties of the investigated samples. Fourier transforms infrared absorption reveals that the frequency band appears in the high frequency (539–555 cm−1) range which can be attributed to the stretching vibrations at the tetrahedral site. The band appearing in the low frequency (359–373 cm−1) range is for the octahedral site, which is due to bending vibration. The elastic constants such as bulk modulus, Young's modulus, rigidity modulus, Poisson's ratio and Pugh's ratio have been calculated for all the studied samples. It shows that all synthesized samples are in ductile nature. [ABSTRACT FROM AUTHOR]

Grain boundaries play a significant role during deformation and can exhibit both beneficial and adverse effects on deformation behaviour during irradiation. For instance, macroscopic irradiation growth is increased by the presence of a high density of grain boundaries. Grain boundaries can act as sinks for point defects and barriers to dislocation motion. Thus, a mechanistic assessment of the impact of grain boundaries is essential for better understanding irradiation-induced deformation. The interplay between irradiation-induced microstructure and mechanical properties has been an area of research for many decades, but many uncertainties remain. In the present work, the deformation fields adjacent to grain boundaries in pure zirconium are investigated by using high angular resolution electron backscatter diffraction (HR-EBSD) after a period of irradiation growth. It is observed that the extent of grain boundary deformation is associated with the crystallographic orientation difference between adjacent grains. High-angle grain boundaries function as a deformation constraint, whereas a significant grain boundary migration occurs at some low-angle grain boundaries, which likely act as an active diffusion path for the irradiation-induced point defects. In addition, the change in residual strain fields associated with the irradiation damage is quantified with respect to a non-irradiated reference. A net change in the state of residual elastic strain in the order of ∼ 10 − 3 has been estimated for ∼8 dpa proton irradiation. Furthermore, the origin of residual elastic strain and lattice misorientation concentration along (0002) traces are discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

Within this study, the effects of non-isothermal annealing procedures, especially different heating rates, were studied for cold rolled pure and potassium doped tungsten sheets. The sheets had a thickness of 0.2 mm. The microstructure was studied via etching of polished cross sections as well as electron backscatter diffraction. The potassium-bubble distribution in the doped tungsten sheets was investigated using scanning electron microscopy. Regarding the microstructure, the doped tungsten sheets presented an elongated, layered microstructure, that was considerably affected by the heating rate. A decrease of the heating rate from 14,400 K/min to 20 K/min resulted in a grain size shift in rolling direction from 70 to 513 μm. On the other hand, pure tungsten sheets showed no significant dependence on the heating rate. Moreover, additional isothermal annealing periods at 2400 °C did generally not affect the microstructure of doped tungsten sheets, which in turn led to a significant increase of the grain size in pure tungsten. This difference was attributed to the grain growth mechanisms, normal grain growth for pure and secondary recrystallization for K-doped tungsten. However, the potassium bubble structure, being responsible for the microstructural differences, was in turn only affected by the maximum annealing temperature. The heating rate differences led to no significant changes in the bubble size distribution. Furthermore, additional isothermal annealing at 2400 °C led to comparably minor changes in the average bubble diameter. • Microstructure of K-doped W-sheets can be controlled by the temperature increase rate. • K-bubbles in tungsten are not affected significantly by the temperature increase rate. • K-doped tungsten sheets preserve γ-grains during annealing. [ABSTRACT FROM AUTHOR]

• E a of recrystallization of hybrid AZ31/Mg-0.6Gd was estimated as 112 ± 10 kJ/mol. • Mg 17 Al 12 and Mg 2 Zn phases in the AZ31 region obstructs the grain growth process. • The effect of Mg 5 Gd and Mg 12 Gd phases in the mg-0.6gd region is less evident. The recrystallization and grain growth activation energies of the hybrid AZ31/Mg-0.6Gd (wt.%) alloy were calculated using differential scanning calorimetry analyses and scanning electron microscopy, respectively, after fabricating by high-pressure torsion up to 20 turns and then subjecting to an isochronal annealing treatment from 423 to 723 K for 1 h. The DSC results show one exothermic peak belonging to the static recrystallization of the AZ31 region with an activation energy of 112 ± 10 kJ/mol. The grain growth kinetics for the AZ31 and Mg-0.6Gd regions were described by the Arrhenius equation. The calculation with a grain growth exponent equal to 4 gave values for the activation energies in both the AZ31 (146.2 ± 8.4 kJ/mol) and Mg-0.6Gd (90.9 ± 13.5 kJ/mol) regions. The present results reveal the heterogeneity of the thermal stability of the AZ31/Mg-0.6Gd hybrid material. [Display omitted] [ABSTRACT FROM AUTHOR]

• A critical size for base metal grain size could be defined, above which grain growth does not occur outside the FL. • In spot laser welding, critical grain size is independent from laser power density. • In continuous laser welding processes, the heat input per unit length of the weld controls critical grain size. • As heat input per unit length of the weld decreases, critical grain size decreases. • The heating step is more crucial in the HAZ grain growth in both spot and continuous welding processes. One of the biggest challenges in the welding of strengthened alloys by grain refinement is the grain growth phenomenon in the heat-affected zone (HAZ), which would result in the degradation of mechanical properties. The goal of this study is to comprehend the HAZ evolution in the fine-grained aluminum alloy during laser welding as a low heat input process. Spot laser welding of fined grain 1050 aluminum alloy revealed that grain growth and HAZ formation occurred in samples with 4 µm grain size, but no grain growth occurred outside of the fusion line (FL) in larger grain samples. The microstructure of HAZ was simulated using the numerical phase-field technique. The numerical model's findings demonstrated that a critical grain size (D cr) for the base metal can be established, above which grain growth would not occur outside FL. To predict grain size distribution outside FL, an analytical model was also used. According to both models, as grain size increases, the HAZ width decreases, and above a critical grain size, no HAZ will form outside of FL. In spot welding, D cr was independent of power density but for continuous welding processes, D cr has a direct relationship with heat input per unit length of weld. Moreover, HAZ grain growth mainly occurred in heating step for both types of processes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Equiaxed grain copper foil undergoes self-annealing-induced recrystallization at room temperature, leading to a decrease in mechanical properties. • Columnar grain copper foil exhibits good mechanical stability at room temperature. • The internal stress caused by nanocrystals and high-density dislocations is the key to driving the recrystallization of equiaxed copper foil. • Internal stress drives the movement of dislocations, and the interaction between dislocations and interstitial atoms creates dislocation rings. • The reaction between dislocation loops and twin boundaries leads to the transformation of twins from nanosheets to twin steps. Copper foils demonstrate a tendency for self-annealing, which adversely impacts the stability of the products. The self-annealing induced recrystallization of copper foil is closely related to the grain growth mode. When copper foil grows in equiaxed grain mode, resulting in a decrease in tensile strength from 780 MPa to 330 MPa. Conversely, the columnar grain copper foil exhibits excellent room-temperature stability in mechanical properties. The mechanism of copper foil room temperature self-annealing behavior was put forward. It was found that the internal stress generated in the process of preparing copper foils is the key factor driving the recrystallization of equiaxed grains. This internal stress originates from nanocrystals and high-density dislocations, with the latter arising from the doping of organic elements. The initial high stress values and the plasticity of dislocations contributed to a noteworthy decrease in residual stress. As dislocation plasticity ensues, the anchoring of interstitial atoms gives rise to dislocation loops. The interaction between dislocation loops and coincident twin boundaries induces the formation of twin steps. The quantity of twins of equiaxed copper foil increases greatly after self-annealing. Conversely, the initial dislocation density of columnar grain copper foil is an order of magnitude lower than that of equiaxed grains. [ABSTRACT FROM AUTHOR]