Grain Growth Mechanism Research Articles

The curious mechanism of irradiation-induced cryogenic grain growth in tungsten thin films: A pathway to single crystals

With high resistance against interdiffusion, electromigration, creep and fatigue, single-crystal films of refractory metals are highly desired for applications in opto- and micro-electronic devices. Their fabrication is nevertheless very challenging, primarily due to their high melting points. Requiring no external thermal input, ion-irradiation presents an alternative route for single-crystal films by converting from their polycrystalline counterparts through irradiation-induced selective grain growth. The incomplete poly- to single-crystal conversion is however a common problem which limits the application of this method. Here we use refractory W thin films, the element with the highest melting point, as a model system. We systematically investigate the influences of film microstructure (texture and grain size) and irradiation temperature (77 K and room temperature) on the evolution of selective grain growth under 4.5 MeV Au+ ion-irradiation. We find that the driving force for selective grain growth can be increased by narrowing the texture spread of the films, refining the grain size and decreasing the irradiation temperature. Following this guidance, a complete conversion of a polycrystalline W film into a single crystal is achieved. This study therefore provides insights into the mechanism of selective grain growth and proves that it is an effective technique for microstructure engineering in thin film materials. The concepts can be applied to all crystalline metal films.

Multi-scale simulation of grain/sub-grain structure evolution during solidification in laser welding of aluminum alloys

This work developed a multi-scale model, combining the macro-scale model for heat transfer and fluid flow with the micro-scale phase-field model for polycrystalline alloy solidification, to investigate the grain/sub-grain structure evolution during solidification in laser welding of 5083 Al sheets. Special attentions were paid to the planar to cellular and dendritic growth. Results demonstrated that, during planar-to-cellular transition, the initial planar instability primarily resulted from the rapid change in growth rate. The surface tension anisotropy can significantly affect the initial planar instability. With the increase of misorientation angle, the interfacial stiffness tended to increase, which enhanced the stabilizing effects of surface tension and thus made the S/L interface more and more stable. Furthermore, Results elucidated how solidification morphology develops during cellular-to-dendritic transition. Two important phenomena, namely, tertiary branching and tip-splitting, which were responsible for the decrease in primary dendritic arm spacing, were discovered. Finally, results illustrated how multiple grains with various orientations grow in competition in laser weld pool. Two competitive mechanisms of grain growth were revealed, namely, lateral expansion of favorably-oriented (FO) grain and blocking its unfavorably-oriented (UO) grain neighbors, and the elimination of UO grain by FO grain at the converging grain boundary.

Microstructural and mechanical properties of magnesium alloy processed by severe plastic deformation (SPD) – A review

Abstract A review on Mechanical properties of ultra-fined grained (UFG) Magnesium based alloy using SPD process is presented in this paper. The properties discussed comprise of microstructural, optical, physical and mechanical behaviour of Ultra- fined grained magnesium alloy and effects of grain size, heat treatment, Extrusion and Annealing, quenching, Normalizing process on these properties. The outcome obtained by many Researchers showed that Magnesium alloy processed by Severe Plastic Deformation (SPD) showed a way to improve the mechanical properties, by refining the grains. The Dislocation density, Hardness, Compressive, Tensile and texture were observed to be increased up to certain limit after that loss of microstructure stability, decrease of micro hardness and decrease in dislocation density were observed. The Mechanical properties are well improved by Extrusion, Annealing, ECAP, ARB and HPT processes. Mechanism of Grain growth, Activation energies for grain growth, stress corrosion rate and Thermal stability were observed by previous Researchers during Fractography analysis on several specimens of Ultra-fined grained Magnesium alloy.

Analysis of oxide layer structure in nitrided grain-oriented silicon steel

The production of low-temperature reheated grain-oriented silicon steel is mainly based on the acquired inhibitor method. Due to the additional nitriding process, a high nitrogen content exists in the oxide layer, which changes the structure of the oxide layer. In this study, the structure of the surface oxide layer after nitriding was analyzed by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), glow discharge spectrometry (GDS), and X-ray diffraction (XRD). The size and orientation of ferritic grains in the oxide layer were characterized, and the distribution characteristics of the key elements along the thickness direction were determined. The results show that the oxide layer of the steel sample mainly comprised particles of Fe2SiO4 and spherical and lamellar SiO2, and Fe4N and fcc-Fe phases were also detected. Moreover, the size and orientation of ferritic grains in the oxide layer were different from those of coarse matrix ferritic grains beneath the oxide layer; however, some ferritic grains exhibited same orientations as those in the neighboring matrix. Higher nitrogen content was detected in the oxide layer than that in the matrix beneath the oxide layer. The form of nitrogen enrichment in the oxide layer was analyzed, and the growth mechanism of ferritic grains during the oxide layer formation is proposed.

Influencing factors of abnormal grain growth in Mg alloy by phase field method

In this study, abnormal grain growth of single phase AZ31 Mg alloy was simulated by phase field modeling during the real spatiotemporal process. The influencing factors of abnormal grain growth were discussed to figure out ways for controlling secondary recrystallization in the microstructure evolution, as well as clarifying the mechanisms of abnormal grain growth in real alloys. The simulation revealed that abnormal grain growth was controlled by three key factors: strain restored energy, grain boundary energy, and grain boundary mobility. The grains with certain orientations in the microstructure with locally high restored energy, local low boundary energy or local high boundary mobility would induce secondary recrystallization after annealing treatment. However, no secondary recrystallization in the microstructure was generated at local restored energy E0’ with certain grain orientation ≤ 1.1E0, local boundary energy σ0’ ≥ 0.98σ0 or local boundary mobility L’ ≤ 2L.

Mechanical strength stability of tritium breeding materials under high temperature deuterium conditions

The mechanical strength of solid tritium breeding materials is one of the key factors for D-T fusion reactors. The crushing of ceramic breeder pebbles will block both tritium recovery circuit and thermal transmission. The mechanical strength changes of Li4SiO4 and Li2TiO3 pebbles exposed to high temperature and hydrogen isotope atmosphere were investigated. The crush load of Li4SiO4 decreased to 66 % of the original values when the breeders were exposed in deuterium gas at the temperature higher than 750 oC. Nevertheless, the crush load of Li2TiO3 remained stable at all the designated temperatures. Investigations involving phase analysis, defect analysis, grain size analysis, and lithium content analysis were performed to determine the causes of mechanical strength change. No new phase formation and no obvious lithium decrease were demonstrated by X-ray diffraction (XRD) and Inductively Coupled Plasma - Atomic Emission Spectrometer (ICP-AES). Defects arising from reduction reaction didn't play an important role in mechanical strength reduction. The main reason responsible for the decrease of mechanical strength of Li4SiO4 was attributed to grain growth. The grain growth behavior was understood by grain growth kinetics. The kinetic parameters of grain boundary migration for Li4SiO4 pebbles were obtained as n of 4 and activation energy Q of 140 kJ/mol. The mechanism of grain growth was given to demonstrate the mechanical strength change of ceramic breeder pebbles.

Carbide grain growth in cemented carbides sintered with alternative binders

Cemented carbides are composites made of a hard refractory ceramic phase and a ductile binder, most commonly WC and Co, respectively. Since the use of cobalt in the hard metal industry is questioned by the new European regulation on chemicals, extensive research has been done to develop new grades based on a Co-Ni-Fe binder. With similar mechanical, physical properties and affinity to C and W, nickel and iron are the best candidates for an efficient binder in cemented carbides. As mechanical properties are strongly dependent on the materials microstructure, and especially on the WC grain size, understanding the effect of the binder on the final microstructure is crucial.In this work, the carbide grain growth behaviour of WC-M alloys (M = Co, Ni, Fe) with different carbon contents is discussed from qualitative and quantitative microstructural analyses. Whereas grain growth is more or less inhibited in WC-Fe alloys, increasing carbon content promotes grain growth in WC-Co and WC-Ni alloys, with a slight abnormal grain growth in case of Ni binder. Different mechanisms for grain growth are discussed, in relation with the observed morphology of WC grains after sintering.

Gradient Microstructures and Mechanical Properties of Ti-6Al-4V/Zn Composite Prepared by Friction Stir Processing.

In this work, a biomedical Ti-6Al-4V (TC4)/Zn composite with gradient microstructures was successfully prepared by friction stir processing (FSP). The microstructures and mechanical properties of the composite were systematically studied using scanning electron microscope (SEM), X-ray diffractometer (XRD), transmission electron microscope (TEM), atom probe tomography (APT), and microhardness test. The results show that TC4/Zn composite can be successfully prepared, and gradient microstructures varying from coarse grain to nanocrystalline is formed from the bottom to the upper surface. During FSP, adding Zn can accelerate the growth of β phase region, and the grain size significantly increases with the increasing rotation rate. The grain combination is the main mechanism for grain growth of β phase region. The deformation mechanisms gradually change from dislocation accumulations and rearrangement to dynamic recrystallization from the bottom to the upper surface (1.5 mm–150 μm from the upper surface). The composite exhibits slightly higher microhardness compared with the matrix. This paper provides a new method to obtain a TC4/Zn composite with gradient surface microstructures for potential applications in the biomedical field.

Combustion synthesis of MgSiN2 powders and Si3N4‐MgSiN2 composite powders: Effects of processing parameters

AbstractIn the present work, high‐quality MgSiN2 powders were prepared by a combustion synthesis method, based on Mg‐Si3N4‐N2 system. The effects of additive content (NH4Cl or NH4F) and N2 pressure on phase composition and crystal morphology of products were investigated. The results suggested that both NH4Cl and NH4F additives alleviated agglomeration and decreased the grain size of MgSiN2. In addition, NH4Cl did not result in the formation of an extra impurity, while MgF2 residue was found when NH4F was introduced. Therefore, NH4Cl additive was considered as a better choice for the preparation of MgSiN2 powders in comparison with NH4F additive. According to the observation of some MgSiN2 whiskers and notably increased Si content under low N2 pressure, the pressure of N2 over 0.5 MPa was essential to have a complete reaction in the present work. Finally, MgSiN2‐Si3N4 composite powders with quasi‐spherical morphology were prepared by adding excessive Si3N4 and NH4Cl additive in reactant. The formation and growth mechanism of MgSiN2 grains was reasonably speculated based on the experimental results. Because of the remarkable merits, the as‐prepared Si3N4‐MgSiN2 composite powders showed great potential for alternative sintering aid in the sintering of Si3N4‐based ceramics.

Preface

The Seventh International Conference on Recrystallization and Grain Growth is the continuation of a conference series that was initiated in 1990 with Recrystallization ’90, organized in Wollongong, followed in 1991 by ’Grain Growth in Polycrystalline Materials”, organized in Rome. Since 2001, both conference series were merged with the organization of the ‘1st International Conference on Recrystallization and Grain Growth’ in Aachen, which has ever since been a regularly recurring triannual event.During the three decades that have passed since its launch, this conference series has shaped an active and versatile research community with active participation from fundamental and applied research groups and industrial R&D organizations. Recrystallization and grain growth are defining factors in shaping the microstructure of both structural and functional polycrystalline materials, metals and nonmetals alike, and therefore of paramount importance for a wide variety of microstructurally sensitive material properties. Thus, an intensive and mutually fruitful collaboration has emerged between applied R&D groups, often associated with industrial organizations, and more fundamentally oriented scientists. Other scientific fields, such as geology, mineralogy and archeology have increasingly studied microstructures that ensue from thermally activated processes such as recrystallization and grain growth, which may provide clues as to how materials were created and what circumstances prevailed during their history.A more detailed understanding of the basic mechanisms of recrystallization and grain growth also contributes to the development of new materials with improved and more versatile functional properties. For newly emerging processing technologies, such as additive manufacturing, a better knowledge on recrystallization and grain growth, particularly in far out-of-equilibrium conditions, is of crucial importance for microstructural control of 3D printed metal objects and hence for the successful introduction of additive manufacturing to all its potential markets.

Modeling and experimental study of friction surfacing of AA2024 alloy over AA1050 plates

Friction Surfacing (FS) of an AA2024 consumable rod was successfully conducted on an AA1050 substrate. Moreover, ABAQUS was utilized, as a 3D finite element software, in order to predict the thermo-mechanical behavior of the materials during the FS process. Then, several experimental observations and model predictions techniques were employed to further study the mechanical properties and microstructures of the coated samples. The results revealed that as the rotating velocity of the consumable rod is increased, the axial feeding rate or the traverse velocity is declined. Moreover, if the other two parameters are kept constant, the maximum temperature and heat generation increase. It should also be noted that a major part of heat generation in the process is due to plastic strain. On average, plastic deformation accounts for about 80% of the total heat generation. In addition, the high slip rate at the quasi-liquid layer due to the rotational velocity and the axial feeding rate can account for this particular high contribution to the total heat generation. In strains more than 20, the temperature leads to an increase in the grain size by basically enhancing the grain growth phenomenon, however, the mechanism of grain growth is not dominant with respect to the other values of strain. The maximum ultimate shear strength occurs in the plastic strain more than 20 and the temperature around 395 °C in the coating.

Formation of nanoscale structures in thin films deposited by reactive magnetron sputtering of binary metal targets in Ar+O2+N2 gas mixture

Bi-metal targets of ZnSn and SiAl were sputtered in A+O2+N2 gas mixtures at various N2/O2 ratios, and the deposited films were investigated by AFM and XPS. No nitrogen was detected in the films even at the highest nitrogen partial pressure in the gas, and the films were oxides. The concentration of the doping metal component in the deposited film increases with the increase of nitrogen partial pressure in the working gas. These experimental observations were supported by numerical calculations of the sputter-deposition process using a SRIM/SIMTRA+RSD software. It was shown that added nitrogen promotes sputtering of doping metal components of the targets, and the concentration of oxygen remains high enough to substitute the less active nitrogen in the growing film. The films consisted of nanograins with the average diameter, which decreased from 18 nm to 3 nm with increase of nitrogen content in the working gas. This effect was explained by the mechanism of abnormal grain growth: the film consisted of grains of oxides of two metals with different concentrations. The doping metal forms very fine nanograins of oxides, which suppress the growth of oxides of the main metal. With increase of nitrogen in the working gas mixture, the sputtering rate of doping metals from the cathodes and their respective concentration in the growing films increases, so the average diameter of oxide grains forming the coating decreases.

Promoting abnormal grain growth in Fe-based shape memory alloys through compositional adjustments

Iron-based shape memory alloys are promising candidates for large-scale structural applications due to their cost efficiency and the possibility of using conventional processing routes from the steel industry. However, recently developed alloy systems like Fe–Mn–Al–Ni suffer from low recoverability if the grains do not completely cover the sample cross-section. To overcome this issue, here we show that small amounts of titanium added to Fe–Mn–Al–Ni significantly enhance abnormal grain growth due to a considerable refinement of the subgrain sizes, whereas small amounts of chromium lead to a strong inhibition of abnormal grain growth. By tailoring and promoting abnormal grain growth it is possible to obtain very large single crystalline bars. We expect that the findings of the present study regarding the elementary mechanisms of abnormal grain growth and the role of chemical composition can be applied to tailor other alloy systems with similar microstructural features.

Competitive grain growth and dendrite morphology evolution in selective laser melting of Inconel 718 superalloy

Microstructure control is a great challenge for selective laser melting (SLM) mainly because of nonuniform and ultra-rapid heating and cooling in the process. It is related to the control of dendrite morphology evolution and competitive grain growth by adjusting the process parameters of SLM. In this study, a multiscale simulation approach, which combines the finite element method (FEM) and the phase field (PF) method, is performed to investigate the dendrite morphology and stray grain growth under various SLM parameters for Inconel 718 superalloy. The cooling characteristics of melt pool, including the cooling rate, thermal gradient/solidification ratio (G/R), and the slope of the solidification front are investigated. Meanwhile, experiment verification adopting the manufacturing parameters identical to the simulation is performed. It is shown that simultaneously increasing or decreasing the laser power and scan speed leads to a drastic change of melt pool aspect ratio (MPAR), and this in turn affects the competitive growth of stray grains. Overall, a larger MPAR is found to suppress the growth of stray grains more effectively. Also, the experimental results are found to agree well with the simulation results. Three mechanisms of stray grain growth are revealed from the simulation results, and their roles are analzyed. This study provides important insights into the microstructure control in the laser additive manufacturing processes.

Grain growth and Hall-Petch relationship in a refractory HfNbTaZrTi high-entropy alloy

Understanding the effect of temperature variation on the microstructural evolution is particularly important to refractory high-entropy alloys (RHEAs), given their potential high-temperature applications. Here, we experimentally investigated the grain-growth behavior of the HfNbTaZrTi RHEAs during recrystallization at temperatures from 1,000 to 1,200 °C for varied durations, following cold rolling with a 70% thickness reduction. Following the classical grain-growth kinetics analysis, two activation energies are obtained: 205 kJ/mol between 1,000 and 1,100 °C, and 401 kJ/mol between 1,100 and 1,200 °C, which suggests two mechanisms of grain growth. Moreover, the yield strength – grain size relation was found to be well described by the Hall-Petch relation in the form of σy=942+270D−0.5. It was revealed that the friction stress, 942 MPa, in the HfNbTaZrTi HEA is higher than that of tungsten alloys, which indicates the high intrinsic stress in the BCC-RHEA. The coefficient, 270 MPa/μm −1/2, is much lower than that in the 316 stainless steel and Al0.3CoCrFeNi HEAs, which indicates low grain-boundary strengthening.

Synthesis of rod-like ZrB2 crystals by boro/carbothermal reduction

Abstract Rod-like ZrB2 crystals were synthesized at 1600 °C in Ar atmosphere by boro/carbothermal reduction using ZrOCl2⋅8H2O, B4C and carbon powders as raw materials. The optimum molar ratio of raw materials required to form pure ZrB2 grains was found to be 2: 1.2: 3. With increase in temperature and subsequent heat preservation stage, ZrB2 powders grew into a rod-like morphology along the c axis. The rod-like ZrB2 grains obtained at 1600 °C have diameters of 0.5–3 μm and high aspect ratios of >8. Effects of molar ratio of raw materials, heating temperature and holding time on the phase composition and final morphology were investigated. Growth mechanism of rod-like ZrB2 grains was also analyzed.

Mechanisms of grain boundary migration and growth in nanocrystalline metals under irradiation

Atomistic simulations of radiation damage uncover how grain boundaries (GBs) migrate and coalesce under irradiation in bicrystalline Cu. Planar GB migration biased by defect cluster-mediated attraction first leads to slow and steady motion. Subsequently, adjoining GBs coalesce into curved surfaces, where curvature-driven migration with a velocity three orders of magnitude higher than that of a planar boundary dominates motion, triggering rapid grain growth. This study reveals the atomistic mechanisms of radiation-induced grain growth, and has practical implications towards engineering radiation-tolerant nanostructures.

Effect of Extensive and Limited Plastic Deformation on Recrystallized Microstructure of Oxide Dispersion Strengthened Fe-Cr-Al Alloy

Ferritic oxide dispersion-strengthened alloys (ODS) are manufactured using a mechanical alloying process. The development of a coarse-grained microstructure during recrystallization has been detected and discussed by several authors, but the mechanism of grain growth remains uncertain. Recent work has emphasized the large influence of non-uniformities on the development of the recrystallized microstructure. The purpose of the present work is to study the effect of both slight heterogeneous plastic strains and strong directional deformation on recrystallization of a ferritic Fe-base ODS alloy.

Effect of CaCO3 and V2O5 Composite Additives on the Microstructure and Magnetic Property of MnZn Ferrites

Manganese-zinc (MnZn) ferrites have important application in convertors and switching mode power supplies. The core losses ( $\text{P}_{L}$ ) of MnZn ferrites have serious influence on the performance of power conversion devices. It is worth emphasizing that reducing the core losses at high frequency is of great significance for the miniaturization and integration of power conversion devices. Here in this paper, MnZn ferrites have been fabricated by the method of solid-state reaction, and the effect of CaCO3 and V2O5 co-additives on the microstructure and magnetic property has been investigated. This paper illustrated the competition mechanism of grain growth, which originated from the hindrance effect of CaCO3 and acceleration effect of V2O5, as well as the effect of CaCO3 and V2O5 on core losses by means of loss separation method. In addition, as for the MnZn ferrite sample with 0.1 wt% CaCO3 and 0.01 wt% V2O5 co-additives, core losses were suppressed to 46 and 664 kW/m3 corresponding to 3 MHz 10 mT and 3 MHz 30 mT at 25 °C, respectively. The state-of-the-art MnZn ferrites with ultralow core losses own potential application for high-frequency power conversion devices.

Effect of Added Mullite Whisker on Properties of Lithium Aluminosilicate (LAS) Glass-Ceramics Prepared for Dental Restoration.

In this work, lithium aluminosilicate (LAS) glass-ceramics were prepared from lithium disilicate glass powder and mullite whiskers. Lithium disilicate glass powder (28.6Li-68.6Si-2K-0.8La, mol.%) with contents of 5, 10 and 15 wt.% mullite whiskers were hot-pressed at 860 °C for 1 h. The effects of added mullite on phase composition, microstructure, mechanical properties, coefficient of thermal expansion (CTE) and translucency were systematically studied. Also, the mechanism for phase transformation and grain growth was discussed. The LAS glass-ceramic samples exhibited relative densities above 98% with main crystalline phases of lithium disilicate (Li2Si2O5, LD), lithium silicate (Li2SiO3, LM), and β-spodumene (LiAlSi2O6). The β-spodumene whiskers were in-situ synthesized through the reaction between mullite whiskers and glass matrix. As β-spodumene fraction rose, lithium disilicate crystals content decreased and that of lithium silicate increased, resulting in declined bending strength and improved fracture toughness. The composite containing 10 wt.% mullite whisker showed optimal performance with bending strength above 300 MPa and fracture toughness of 2.7 MPa·m1/2. The formation of β-spodumene in the obtained composite declined CTE, with values ranging from 10.5×106 °C-1 for LD glass-ceramic to 6.03×10-6 °C-1 for LAS glass-ceramic doped with 15 wt.% mullite whiskers. Finally, though the real in-line transmission of obtained glass-ceramic composites declined as β-spodumene content rose, the specimens containing less than 10 wt.% mullite whiskers showed favorable translucency, which would suit applications in dental restoration.