Abnormal Grain Research Articles

Suppressing abnormal grain growth and switching precipitation behaviors in ZK60 Mg profile by inducing pre-tension

The pre-tension was induced to suppress the abnormal grain growth (AGG) of ZK60 Mg profile during solution treatment. The effects of pre-tension on the microstructure evolution and mechanical properties were studied, and the suppressing mechanism was discussed. If the pre-tension strain was larger than 10%, AGG during solution could be effectively inhibited, resulting in a sharp decrease in the grain size. The suppression effects were realized by restricting the orientation dependent of AGG and promoting static recrystallization. The pre-tension reset the distributions of stored energy and sizes of the grains with 〈11–20〉 and 〈10–10〉 orientations, and thus retarded the orientation dependent of AGG. Moreover, the pre-tension introduced a mass of dislocations, twins, and stacking faults, all of which promoted the occurrence of static recrystallization, and the grain structure was further refined. The pre-tension accelerated the precipitation kinetics during aging, resulting in fine and dense precipitates. With the increase of pre-tension strain, the strength of ZK60 Mg profile monotonically increased.

Temperature-dependent mechanical behavior of aluminum AM structures generated via multi-layer friction surfacing

Multi-layer friction surfacing (MLFS) is a solid state layer deposition technology for metals. In order to make use of the potential of MLFS as technology for additive manufacturing, the material properties of MLFS built structures have to be investigated and understood in detail. This study presents a comprehensive analysis of the mechanical properties of MLFS deposited material from micro-flat tensile testing (MFTT) at elevated temperatures. The specimens obtained from the fine-grained MLFS structures show a slightly higher tensile strength at room temperature but lower tensile strength at testing temperatures of 300 °C and above compared to the stud base material. No significant gradient along the MLFS structure could be observed in terms of mechanical properties. The analyses of fracture surfaces and microstructure of tested MFTT specimens provide insights to deformation mechanism of MLFS deposited and consumable stud material. Especially at high testing temperatures of 500 °C, MLFS deposited structure shows abnormal grain growth which results in the observed tensile behavior.

Effect of cyclic heat treatment on abnormal grain growth in Fe-Mn-Al-based shape memory alloys with different Ni contents

Cyclic heat treatment that can continuously promote abnormal grain growth is widely used for the preparation of single‐crystal Fe‐Mn‐Al‐based shape memory alloys. However, it takes a long time to prepare large‐size Fe‐Mn‐Al‐based alloy single crystals via the reported cyclic heat treatments. Meanwhile, the long‐time cyclic heat treatment at high temperatures leads to the development of defects including oxidation and a decrease in Mn, which would deteriorate superelasticity in the Fe‐Mn‐Al‐based shape memory alloys. To shorten the fabrication time of single crystals, the effect of the cyclic heat treatment process on the abnormal grain growth in the Fe‐Mn‐Al‐based alloys with different Ni contents was systematically investigated. It is found that the abnormal grain growth of Fe‐Mn‐Al‐based alloys was not significantly affected by the Ni contents (within 2.1 at.%–6.2 at.%). In addition, the abnormal grain growth could be promoted by 1–2 °C min–1 cooling rate, high solution temperature, and multiple cycles, while it was insensitive to other processes including heating rate, dual‐phase time as well as long‐time solution treatment. These findings can guide optimizing the fabrication process of single crystals by cyclic heat treatment. Finally, the Fe41.9Mn37.8Al14.1Ni6.2 single crystal prepared by the optimized cyclic heat treatment showed a recoverable strain of about 4%.

Preparation, Characterization, and Application of AlN/ScAlN Composite Thin Films

Piezoelectric aluminum nitride (AlN) thin film, as a commonly used material for high-frequency acoustic resonators, has been a research hotspot in the RF field. Doping Sc elements in AlN is one of most effective methods to improve the piezoelectricity of the material. In this work, the first principal calculation and Mori-Tanaka model are used to obtain the piezoelectric constants of AlN, ScAlN, and AlN/ScAlN composites. Then, five types of AlN/ScAlN thin films are prepared on 8 inch silicon substrates. The crystal quality, roughness, and stress distribution are measured to characterize the film quality. The results show that composite film can effectively solve the problem of abnormal grains and reduce the roughness. Finally, a lamb wave resonator with an AlN/Sc0.2Al0.8N composite working at 2.33 GHz is fabricated. The effective electromechanical coupling coefficient Keff2 is calculated to be 6.19%, which has the potential to design high-frequency broadband filters.

β Grain Size Inhomogeneity of Large Scale Ti-5Al-5V-5Mo-3Cr Alloy Bulk after Multi-Cycle and Multi-Axial Forging in α + β Field

In order to fabricate homogeneous large-scale Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy bulk with fine and equiaxial β grain, we performed a series of multi-axial α + β field forging with 62 forging cycles on the large-scale Ti-5553 billet by using 12.5 MN high-speed hydraulic press. The β-annealed microstructure was the starting microstructure of the billet. After the 6th forging cycle, β grain deformed dramatically, and the grain-boundary network developed within the irregular β grain. As the forging cycle increased to 44, the volume fraction of the fine and equiaxial β grain that is less than 20 μm, which is caused by dynamic recrystallization, increased gradually. However, the incomplete dynamic recrystallization region within the original β grain could not be eliminated. As the forging cycle further increased, the volume fraction of the fine and equiaxial β grain did not increase. In contrast, the abnormal grain growth of the β phase occurred during 50th~62nd forging cycle. Here, we attribute the formation of the incomplete dynamic recrystallization region and the abnormal grain growth of the β phase to the high deformation rate of the α + β forging. The refining behavior of β grain and the abnormal coursing β grain, which is found during the multi-cycle multi-axial forging of large-scale Ti-5553 alloy billet, are seldom reported in the isothermal compression of small-scale Ti-5553 alloy specimen. The findings of the paper are instructive for improving the sub-transus forging strategy that is used to fabricate the large-scale homogeneity Ti-5553 alloy billet with fine and equiaxial β grain.

Effects of rapid heating on the phase transformation and grain refinement of a low-carbon mciroalloyed steel

The present study aims to clarify the possibility of rapid heating in the preparation of low-carbon ultrafine (UFG) steel. The effects of heating rate on the phase transformation, microstructure and precipitation of a low-carbon microalloyed steel were elaborately investigated. The results manifest that rapid heating potentially provides a simple and innovative process for the preparation of UFG steels just by increasing the heating rate. The parent austenite grains were refined due to the more reserved (Nb, Ti, V)C precipitates and dislocations within a short heating process by rapid heating. However, an effective holding time was proposed in the rapid heating to ensure the refined austenite grains. Moreover, the synthetic influences of initial increased grain boundaries and redissolved (Nb, Ti, V)C carbides provoked the abnormal grains with weakened thermostability. Redissolution of (Nb, Ti, V)C carbides were apparent during austenite nucleation at a slow heating rate, and it was verified during the isothermal holding stage by rapid heating. In addition, the austenitic phase transition occurred at higher temperature with the increase of heating rate, by which the transformation rate was accelerated.

Abnormal grain growth and electrical properties of Ba-excessive La-doped BaTiO3 ceramics prepared from nanopowder synthesized by sol-gel method

Abnormal grain growth and electrical properties of Ba-excessive La-doped BaTiO3 ceramics prepared from nanopowder synthesized by sol-gel method

Abnormal grain growth induced by orientation of AZ31 magnesium alloy

ABSTRACT The abnormal grain growth behaviour and preferential orientation of AZ31 Mg alloy were systematically examined. Abnormal grain growth was observed after annealing at 500°C for 9 h, and the grain growth rate was 1.3 mm h−1. Due to the abnormal grain growth, the activation energy of grain growth was reduced from 172 to 104 KJ mol−1. The abnormal grains with <110> orientation grew preferentially, which resulted in the increase of the <110> type texture intensity and the decrease of the <0001>. The energy difference of the misorientation is the main driving force for the formation of abnormal grains with <110> orientation.

In situ annealing studies on the microstructural evolution of a macro defect free electroformed nanocrystalline Ni-Co sheet metal and its relation to tensile properties

The effect of annealing at temperatures between 200 – 400 °C on the microstructural evolution and tensile properties of a recently developed macro defect free electroformed nanocrystalline Ni-32at%Co free-standing sheet metal was investigated. The tensile strength of the materials exhibited an extrinsic Hall-Petch (HP) to inverse Hall-Petch (IHP) transition due to grain growth. In situ transmission electron microscopy (TEM) annealing revealed for the first time that at lower annealing temperatures, in addition to reordering of atoms at grain boundaries, grain rotation and grain recovery were also found to be mechanisms for grain boundary relaxation (GBR). Combined, these mechanisms contribute to the increase in ultimate tensile strength (UTS) observed in the IHP regime of the Ni-32at%Co alloy. TEM energy dispersive X-ray spectroscopy (EDX) and in situ synchrotron X-ray diffraction (XRD) annealing analyses showed that the drop in UTS and ductility in the HP regime after annealing at higher temperatures was associated with the onset of abnormal grain growth and sulfur impurity segregation to random high angle boundaries. Atom probe tomography (APT) further showed that there is no sulfur impurity segregation in the electroformed material before annealing. Increasing in annealing temperature led therefore to a transition from ductile to brittle fracture manifested as a change in dimple to mixed and eventually to faceted brittle fracture surfaces at 400 °C.

Abnormal grain growth of FeMnAlNiCo shape memory alloys during directional recrystallisation

Polycrystalline FeMnAlNi-based alloys generally exhibit poor pseudoelasticity owing to grain constraint. A promising method to improve the pseudoelasticity involves increasing the grain size by directional recrystallisation (DR). To improve the efficiency of obtaining large grain size, the effect of DR parameters—hot zone temperature (HZT) and drawing velocity—on the columnar grain size and the mechanisms of grain growth were investigated. The results showed that samples of FeMnAlNiCo shape memory alloys (SMAs) with an average grain size of over 20 mm and the largest grain size exceeding 50 mm were obtained after DR. The apparent activation energy of the columnar grain growth for DR was ≈135 kJ/mol, which was much lower than that of the equiaxed grain growth for isothermal annealing (≈384 kJ/mol). The lateral growth of the columnar grains was probably because the boundaries between columnar grains tend to form random high-angle grain (HAG) boundaries. By incremental compression test, at an applied strain of 7.0%, nearly <104> oriented single crystal exhibited a recoverable strain of ≈6.0%, while the polycrystal had a recoverable strain of only ≈3.4%. This work provides important guidance and inspires new strategies for microstructural control in FeMnAlNi-based SMAs.

Investigation of initial metallurgical factors on the dynamic impact response and adiabatic shear bands formation of the 6061 Al alloy

The effect of multi-pass friction stir processing and initial base metal condition on the dynamic impact response of AA6061-T6 alloy at high strain rates was investigated in this study. The Split-Hopkinson Pressure Bar (SHPB) was used in dynamic experiments with strain rates ranging from 1600 to 4200/s. The results revealed that the base metal alloy's (BM) dynamic flow stress was comparable to all the friction stir processed AA6061-T6 (FSP'd) specimens as the strain rate and true strain increased. The general increase in stress response levels as strain rates and true strain increases can be attributed to strain hardening. The higher values of strain rate sensitivity in multi-pass FSP'd specimens are an indication that they are more ductile in comparison to the BM condition. The optical and electron backscattered diffraction (EBSD) analysis showed that adiabatic shear bands were formed by heterogeneous deformation. The EBSD analysis revealed that the FSP'd specimens showed more abnormal grain growth and dynamic recrystallized sites than BM. The substantial decrease in dynamic flow stress, and high strain rate sensitive values of multi-pass FSP'd specimens compared to BM for all the investigated strain rates attributed to softening effect by dynamic recrystallization (DRX), and the emergence of abnormal grain growth during friction stir process.

Effect of R2O3 (R=Y, La) on the microstructure, optical, and photoluminescent properties of MgAl2O4:Cr3+ transparent ceramics

MgAl2O4:Cr3+ transparent ceramics were prepared by a hot-pressed sintering process with Y2O3 and La2O3 as sintering aids, and the effects on the microstructure, densification, optical transmission, and photoluminescent properties were investigated. The sintering aids of Y2O3 and La2O3 enhanced the relative density, reaching 99.33% with 2.4 wt% Y2O3 and 99.24% with 0.8 wt% La2O3. La2O3 also inhibited abnormal grain growth and obtained ceramics with uniform grain size. In addition, the optical transmission was also improved with Y2O3 and La2O3 as sintering aids. The transmission of the ceramic with 2.4 wt% Y2O3 exceeded 78% in the region of 700–6000 nm and reached 83% at 1450 nm. Moreover, Y2O3 and La2O3 both contributed to improving the quantum yield of the ceramics, especially for Y2O3, and the quantum yield was about 0.354, which was approximately two times higher than that without R2O3. In summary, La2O3 was helpful for improving sintering process, inhibiting grain growth, and homogenizing the grain size, while Y2O3 was significant for enhancing the optical and photoluminescent properties.

Особенности формирования микроструктуры алюминиевых сплавов АК4-1 и АК12Д после их совместной обработки трением с перемешиванием

Friction stir processing is one of the modern methods of local modification of the surface of aluminum alloys in the solid-phase state, which provides the dispersion of structural components. In heat-hardened aluminum alloys with a matrix type structure, heat treatment following after friction stir processing can lead to abnormal grain growth in the stir zone. However, in alloys with the structure close to microduplex type, a fine-grained structure can be formed after friction stir processing and heat treatment. This work is aimed at evaluating the possibility of increasing the microstructure thermal stability of the AK4-1 (Al–Cu–Mg–Fe–Si–Ni) matrix-type aluminum alloy. For this purpose, AK12D (Al–Si–Cu–Ni–Mg) aluminum alloy with the structure close to microduplex type was locally mixed into the studied alloy by friction stir processing. Subsequent Т6 heat treatment was carried out according to the standard mode for the AK4-1 alloy. Studies showed that the stir zone had an elliptical shape with an onion-ring structure. This structure comprised alternating rings with different amounts and sizes of excess phases. At the same time, in the stir zone center, the width of rings and the average area of excess phases were larger compared to the stir zone periphery, where the width of rings and the average area of particles were smaller. The average area of excess phases in the rings with their higher content was smaller than in the rings with their lower content. This distribution of excess phases leads to the formation of a fine-grained microstructure, where the average size of grains depends on the interparticle distance in the α-Al solid solution.

Effect of forging sequence and heat treatment on microstructure of high-duty power-plant shaft made of Cr-Mo ultra-high strength steel

The paper presents results of modeling and testing of a heavy weight part made of Cr-Mo, which was V-modified ultra-high strength steel grade AISI 4140, processed through a novel open-die forging schedule and two alternative routes of two-stage heat treatment cycles designed to meet requirements of high-duty components for energy sector. Taking advantage of unconventional forging conditions based on assumption of large feed and reduction ratios and the modification of chemical composition better control of austenite grain was achieved to minimize abnormal grain growth and/or strain uniformity problems. With aid of Finite Element Modeling of multi-stage sequence of upsetting and cogging strain distribution was optimized so as to minimize strain fluctuations on the length to range 2.2?2.7,and correlated with microstructure produced at every major stage on the large cross-sections of the shaft. Designed with aid of finite element method processing cycles was verified in full-scale physical modeling with use of 16 ton forging ingot, including two alternative quenching strategies: oil vs. water spray and air. Examination of material in as-forged, normalized and heat-treated condition was carried out to observe response of hot-worked material and the effect of cooling conditions on microstructure during final heat treatment. As observed, employing large feed ratios on cogging and varied cooling allowed suppress detrimental effect of inevitable abnormal grain growth which resulted in 1-2 ASTM grain in as-forged conditions to reach 6 ASTM in normalized and 8/9 ASTM after quenching in oil and water spray, respectively, which allows producing after tempering, correspondingly, 44?48 and 85?122 J/cm2 V-notch impact strength in the critical area of the forged shaft.

Overexpression of a pear B-class MADS-box gene in tomato causes male sterility

B-class MADS-box genes are sufficient for the specification of petals and stamens; however, the role of <italic>Tomato MADS-box protein 6</italic> (<italic>TM6</italic>) in seed formation in pear remains largely unknown. In this study, <italic>PbTM6a</italic> and <italic>PbTM6b</italic>, characterized as negative regulators of the response to GA₄₊₇, were identified as classic B-class MADS-box genes. Additionally, both of the genes encoding proteins carried the highly conserved MADS-box domain, and showed high expression levels in anther, petal and filament of 'Dangshansu'. Overexpression of <italic>PbTM6a</italic> in tomato reduced the number of seeds per fruit. Analysis of the anatomical structure of floral organs revealed that the reduction in seed number in transgenic fruits might be attributed to an obstacle of pollen release due to strongly formed stomium and limited ovary space of ovule development. Moreover, triphenyl tetrazolium chloride (TTC) staining and <italic>in vitro</italic> germination tests of pollen grains indicated that <italic>PbTM6a</italic> overexpression reduced pollen viability and germination rates. Reciprocal crosses showed that the reduction in seed number in transgenic fruits was dominantly caused by the decreased fertility of pollen grains. Subsequent scanning electron microscopy showed that sterile pollen grains were caused by abnormal pollen grains. Additionally, the reduced levels of jasmonic acid (JA), abscisic acid (ABA), indole-3-acetic acid (IAA) and gibberellin A3 (GA3) in transgenic stamens contributed to the development of sterile pollen. Collectively, our results reveal the role of <italic>PbTM6a</italic> genes in controlling male infertility and broaden our understanding of the mechanism underlying the function of B-class MADS-box genes in pear.

The effect of plasma-assisted ball milling on preparation and sintering behavior of (Zr0.1429Hf0.1429Ce0.1429Y0.2857La0.2857)O2-δ high entropy fluorite oxide

The sintering process of high entropy fluorite oxide ceramics prepared by the conventional solid-phase reaction method suffers from high single-phase synthesis temperature, low densities, and abnormal grain growth. To improve the sintering characteristics of high entropy fluorite oxide ceramics, the oxide mixture was dealt with dielectric barrier discharge plasma-assisted ball milling. The samples were analyzed using XRD, SEM, TEM, TG/DTG, visual high-temperature deformation analyzer, and nanoindentation meter. The results show that the plasma field activates the raw material powder and promotes powder finer. The rapid heating effect of the plasma during the ball milling process causes micro-area sintering and micro-area melting on the powder surface, which increases the active sites of the solid phase reaction of the material, accelerates the mass transfer process, and improves the sintering activity. Compared with conventional ball milling, plasma-assisted ball milling reduces the single-phase synthesis temperature of high entropy ceramics from 1500 °C to 1300 °C. The sintered ceramic pellets have intact grain growth, no noticeable abnormal grain growth, and improved density and mechanical properties.

Effects of ion diffusion and dielectric composite on properties of highly tunable Ba2SiO4– Ba0.4Sr0.6TiO3 composite ceramics

xBa2SiO4-(1−x)Ba0.4Sr0.6TiO3 (x = 10–50 wt%) composite ceramics were prepared via solid-state and liquid sintering. Ion diffusion between Ba2SiO4 (BS) and Ba0.4Sr0.6TiO3 (BST40) phases occurred in all the composites, and was enhanced during the liquid sintering process. A additionally, molten BS inhibited abnormal grain growth in BST and accelerated the densification of the composites. The increase in the Ba/Sr ratio in BST with x, owing to the A site ion diffusion, promoted an increase in the Curie temperature of the composite ceramics, with a concurrent increase in their tunability. The recombination effect maintained the Q value of the composite ceramics above 200, offsetting the negative effect of ion diffusion on the Q value, for x > 30 wt %. The 50 wt% BS-50 wt% BST samples exhibit a high comprehensive performance with dielectric permittivity, tunability, and Q value of 376, 32.6%, and 428, respectively.

Abnormal grain growth through cyclic heat treatment in a Mg–Sc alloy

Enlarging the grain size in Mg–Sc alloys can greatly improve superelasticity. The impact of heat treatment conditions on abnormal grain growth (AGG) caused by phase transformation through cyclic heat treatment (CHT) was investigated in this study. In the CHT process between temperatures in a β single-phase region (Tβ) and α + β two-phase region (Tα+β), lowering the Tα+β resulted in a uniform huge grain size after AGG. In the heating process of the CHT from the Tα+β to the Tβ, the decrease in the heating rate was found to be ineffective in realizing huge grain size because the number of grains that can start AGG increases, which causes the collision of the abnormally growing grains with each other. We also revealed one of the key factors for the AGG to occur. Subgrains, which were formed through CHT, not only act as an additional driving force for grain growth but also shorten the required time for AGG nucleation.

Properties of sintered zinc hydroxyapatite bioceramic prepared using waste chicken eggshells as calcium precursor

In the recent years, research on the development of hydroxyapatite (HA) using calcium from natural resources such as limestone, mammalian bones, marine shells and avian eggshells have been extensively studied. However, many studies focused on the properties of prestine HA without incorporation of dopants for strengthening effect. In this work, HA bioceramic was prepared using waste chicken eggshells calcium with addition of various concentrations of zinc dopant (1, 3 and 5 mol% Zn). In this work, the zinc-doped HA (ZnHA) was synthesized using a wet-chemical precipitation technique followed by oven drying and unixial pressing to formed green compacts. Pressureless sintering was carried out at 1200, 1250 and 1300 °C. The results showed that 5 mol% ZnHA (5ZnHA) exhibited the overall best properties after sintering at 1250 °C. The improvement in the fracture toughness was attributed to the formation of β-TCP phase when zinc ion was incorporated into HA, combined with enhanced densification. It was observed that the HA grains were coarser and more densely when sintered at 1250 °C, indicating that there was strong interaction between pores and grain boundaries. However, fracture toughness slightly declined after sintering at 1300 °C due to rapid and abnormal HA grain growth.

Simulation of strain induced abnormal grain growth in aluminum alloy by coupling crystal plasticity and phase field methods

A mesoscale modeling methodology is proposed to predict the strain induced abnormal grain growth in the annealing process of deformed aluminum alloys. Firstly, crystal plasticity finite element (CPFE) analysis is performed to calculate dislocation density and stored deformation energy distribution during the plastic deformation. A modified phase field (PF) model is then established by extending the continuum field method to consider both stored energy and local interface curvature as driving forces of grain boundary migration. An interpolation mapping approach is adopted to transfer the stored energy distribution from CPFE to PF efficiently. This modified PF model is implemented to a hypothetical bicrystal firstly for verification and then the coupled CPFE-PF framework is further applied to simulating the 2D synthetic polycrystalline microstructure evolution in annealing process of deformed AA3102 aluminum alloy. Results show that the nuclei with low stored energy embedded within deformed matrix tend to grow up, and abnormal large grains occur when the deformation is close to the critical plastic strain, attributing to the limited number of recrystallized nuclei and inhomogeneity of the stored energy.