Onset Of Abnormal Grain Growth Research Articles

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.

Cross-rolling mediated microstructure and texture evolution in severely cold-rolled and annealed ultrafine pearlite

Effect of cross-rolling on the evolution of microstructure and texture in pearlite was investigated in the present work. For this purpose, near pearlitic steel was cold-rolled to 95% reduction by unidirectional cold-rolling (UCR) and different cross-rolling routes. The UCR processed material showed a predominantly lamellar structure, while the microstructure development in selected cross-rolled materials was featured by the presence of profuse shear bands and severely bent lamellae. While the UCR processed materials showed the presence of the typical RD (//〈110〉) and ND (//〈111〉) fibers, the texture formation in different cross-rolled materials was featured by the strong presence of a {001}〈110〉 component of the RD-fiber. The presence of the prominent {001}<110 > component could be explained by the stability of this component under cross-rolling. Recrystallization resulted in progressive spheroidization of the cementite. The pinning of the high angle boundaries by the cementite particles hindered the long-range migration of boundaries, thus inhibiting typical discontinuous recrystallization but favoring a recovery dominated softening process. The recrystallization textures in different cross-rolled materials were featured by the retention of deformation texture, namely the prominent {001}<110 > component of the RD-fiber, which amply corroborated a recovery dominated mechanism. Extended annealing resulted in the onset of abnormal grain growth in selected cross-rolled materials, which appeared to affect the microtexture, while the global texture remained mostly unaffected.

Characterization of a nanocrystalline NiCo electroformed sheet metal

A novel electroformed nanocrystalline nickel cobalt alloy (n-NiCo) developed for sheet metal applications was investigated in terms of microstructure, as well as mechanical, thermal stability, and corrosion properties. The as-received material with a grain size of 18 ± 5 nm exhibited enhanced hardness compared with conventional polycrystalline NiCo mainly due to the Hall–Petch strengthening effect. Differential scanning calorimetry and annealing treatment results revealed that the n-NiCo was stable up to 200 °C for at least 1 h, and the onset of abnormal grain growth was observed when the material was annealed to 250 °C. While low-temperature annealing treatment was shown to increase the hardness of the material slightly, annealing at temperatures of 300 °C and above resulted in a reduction in hardness due to rapid normal grain growth. The n-NiCo sheet exhibited mixed Ni/Co anodic polarization behavior in environments of varying pH.

Grain boundary evolution of highly nanotwinned alloys: Effect of initial twinned microstructure

The effects of initial twinned microstructure on the grain growth behavior of four sputtered nanotwinned copper alloys were compared. As-deposited films were characterized by transmission electron microscopy and the microstructural evolution of the annealed samples was investigated through electron backscatter diffraction. Texture showed a pronounced effect, where films with strong {111} textures exhibited abnormal grain growth that was not observed in a randomly textured film. Additionally, the stability induced by twin boundaries appears to be limited, as the excess energy generated by a large twin density could drive the early onset of abnormal grain growth and recrystallization.

Abnormal grain growth in thin wires of commercially pure iron with anisotropic microstructure

This work reports the behavior of annealed commercially pure cold-drawn thin iron wires. Primary recrystallization is complete after holding for 180 s at 1123 K. After recrystallization, grain growth takes place within the central region but not close to the surface. For annealing times up to 14400 s, the grain size distribution broadens and the texture strengthens. The resulting texture is intense along the direction <110> parallel to the wiredrawing direction (WD). For longer times, abnormal grain growth takes place. The center of the wire is the preferred place for abnormal grain growth. By contrast, the grains closer to the surface do not experience any growth, and abnormal grains cannot consume them. Throughout the microstructure, one finds AlN and MnS particles. Close to the wire surface, there is a significantly higher particle density than in the center. We discuss mechanisms for the onset of abnormal grain growth, considering particle pinning and crystallographic texture. We observed two kinds of abnormal grains. Most abnormal grains belonged to the <110 > // WD fiber texture component, whereas a minority did not. We propose two mechanisms of abnormal grain growth initiation. The first mechanism explains the initiation of the abnormal grains with the assistance of the texture. The second mechanism associates the formation of a candidate abnormal grain to the topological path of the largest grains during normal grain growth preceding the onset of abnormal grain growth.

The influence of Mg and Mn content on abnormal grain growth in AA5182 type alloys

The occurrence of abnormal grain growth (AGG) in AA5182 alloy during annealing imposes severe restrictions on processing parameters and deteriorates mechanical properties. In this work, we investigated the effect of chemical composition on the appearance of abnormal grain growth by varying Mg and Mn content in the range of composition limits for standard AA5182 alloy, 4.0-5.0% Mg, and 0.2-0.5% Mn, respectively. Thermo-mechanical processing of alloys included cold rolling with reductions ranging from 40 to 85%, followed by annealing in the temperature range from 350 to 520 °C. The results showed that the rise in alloying elements content drives the onset of abnormal grain growth toward higher temperatures. The increase in the cold rolling reduction degree promotes abnormal grain growth and lowers its onset temperature. Abnormal grain growth and grain boundary mobility showed strong anisotropy related to rod-like shape and alignment of Al6Mn(Fe) dispersoids through Zener pinning.

Phase Field Modelling of Abnormal Grain Growth

Heterogeneous grain structures may develop due to abnormal grain growth during processing of polycrystalline materials ranging from metals and alloys to ceramics. The phenomenon must be controlled in practical applications where typically homogeneous grain structures are desired. Recent advances in experimental and computational techniques have, thus, stimulated the need to revisit the underlying growth mechanisms. Here, phase field modelling is used to systematically evaluate conditions for initiation of abnormal grain growth. Grain boundaries are classified into two classes, i.e., high- and low-mobility boundaries. Three different approaches are considered for having high- and low-mobility boundaries: (i) critical threshold angle of grain boundary disorientation above which boundaries are highly mobile, (ii) two grain types A and B with the A–B boundaries being highly mobile, and (iii) three grain types, A, B and C with the A–B boundaries being fast. For these different scenarios, 2D simulations have been performed to quantify the effect of variations in the mobility ratio, threshold angle and fractions of grain types, respectively, on the potential onset of abnormal grain growth and the degree of heterogeneity in the resulting grain structures. The required mobility ratios to observe abnormal grain growth are quantified as a function of the fraction of high-mobility boundaries. The scenario with three grain types (A, B, C) has been identified as one that promotes strongly irregular abnormal grains including island grains, as observed experimentally.

The influence of grain boundary area on the complexion time-temperature-transformation diagram of Eu-doped magnesium aluminate spinel

We demonstrate the sensitivity of complexion time-temperature-transformation (TTT)-diagrams to composition and grain size in Eu-doped MgAl2O4. 100 ppm and 500 ppm Eu-doped MgAl2O4 undergo the same complexion transition, which causes abnormal grain growth, but exhibit different nucleation and grain growth rates. The Eu content per grain boundary area (Eu/GBA) was calculated for various heat treatments using a simple model based on average grain size. Despite their differences, a common Eu/GBA is calculated at the onset of abnormal grain growth. This common grain boundary excess has important implications for complexion TTT-diagrams and provides a new metric for predicting abnormal grain growth.

Recrystallisation towards a single texture component in heavily cold rolled tungsten (W) sheets and its impact on micromechanics

In this study, we present the recrystallisation behaviour of heavily cold rolled tungsten sheets and show that the material recrystallises towards a single texture component, i.e. the rotated cube component {001}<110>. This result is remarkable as it distinguishes from the classical concept of recrystallisation and is likely to have a strong impact in various fields in science and technology.The thermal stability of as-rolled tungsten sheets featuring a thickness reduction of 98% (i.e. 4.1 in the logarithmic notation) was investigated. Annealings were performed in a vacuum and in the temperature range of 400 °C (673 K) to 2000 °C (2273 K) for a time period of 6 min up to 500 h. The annealing-induced evolution of the microstructure is displayed by hardness (HV0.1) and electron backscatter diffraction (EBSD) measurements. The results show that after an annealing of 6 min at 800 °C (1073 K), the hardness drops from 666 ± 6.3 to 625 ± 7.9 HV0.1.The evolution of the hardness is fitted by classical kinetic models for recovery (logarithmic kinetics) and recrystallisation. The apparent activation energy of half of the hardness loss, EΔHV/2, was found to be 356 kJ/mol (3.69 eV), which can be associated with the activation energy of grain boundary diffusion in tungsten.Furthermore, the hardness data allows for the distinction between two stages. In stage I, the material rapidly develops an equiaxed grain structure by the break-up and spheroidisation of the high aspect ratio grains. We do not observe the formation of nuclei in the classical sense. Moreover, the recrystallisation processes can be described as grain growth. We suggest the dominant driving force to be the reduction of the internal stored energy. Finally, the transition from stage I to stage II is caused by the onset of abnormal grain growth.

The Influence of Grain Boundary Area on the Complexion Time-Temperature-Transformation Diagram of Eu-Doped Magnesium Aluminate Spinel

We demonstrate the sensitivity of complexion time-temperature-transformation (TTT)-diagrams to composition and grain size in Eu-doped MgAl2O4. 100 ppm and 500 ppm Eu-doped MgAl2O4 undergo the same complexion transition, which causes abnormal grain growth, but exhibit different nucleation and grain growth rates. The Eu content per grain boundary area (Eu/GBA) was calculated for various heat treatments using a simple model based on average grain size. Despite their differences, a common Eu/GBA is calculated at the onset of abnormal grain growth. This common grain boundary excess has important implications for complexion TTT-diagrams and provides a new metric for predicting abnormal grain growth.

Limit conditions for the onset of abnormal grain growth in a homogeneous microstructure: Theory and Experiments

In previous papers it has been demonstrated that abnormal grain growth in a polycrystalline microstructure is a natural phenomenon even in the presence of an homogeneous microstructure. Namely, in a statistical sense, each portion of the material has the same grain size distribution and no local heterogeneities are involved. The only necessary condition is the presence of a grain growth inhibitor which can be represented by second phase particles (Zener Drag), homogeneously distributed as well, or by segregating atoms on the grain boundary (Atoms drag).Considering specifically the Zener Drag effect, it can be shown that in the presence of an evolution of the restraining force ( even if extremely slow), due to Ostwald ripening or to dissolution of the particles, the corresponding grain growth process allows the grain size distribution to continuously evolve reaching a critical shape. Such peculiar shape corresponds to a “structural” instability which irreversibly determines the onset of an abnormal growth phase with a drastic change of grain growth kinetics (kinetics exponents for the mean radius largely above the canonical 0.5).In this context the mechanism of Abnormal Grain Growth onset is discussed as a general physical phenomenon and a set of experimental data on several steels will be presented for supporting such conclusion.

Monte Carlo study on abnormal growth of Goss grains in Fe-3%Si steel induced by second-phase particles

The selective abnormal growth of Goss grains in magnetic sheets of Fe-3%Si (grade Hi-B) induced by second-phase particles (AlN and MnS) was studied using a modified Monte Carlo Potts model. The starting microstructures for the simulations were generated from electron backscatter diffraction (EBSD) orientation imaging maps of recrystallized samples. In the simulation, second-phase particles were assumed to be randomly distributed in the initial microstructures and the Zener drag effect of particles on Goss grain boundaries was assumed to be selectively invalid because of the unique properties of Goss grain boundaries. The simulation results suggest that normal growth of the matrix grains stagnates because of the pinning effect of particles on their boundaries. During the onset of abnormal grain growth, some Goss grains with concave boundaries in the initial microstructure grow fast abnormally and other Goss grains with convex boundaries shrink and eventually disappear.

The onset and evolution of fatigue-induced abnormal grain growth in nanocrystalline Ni–Fe

Conventional structural metals suffer from fatigue-crack initiation through dislocation activity which forms persistent slip bands leading to notch-like extrusions and intrusions. Ultrafine-grained and nanocrystalline metals can potentially exhibit superior fatigue-crack initiation resistance by suppressing these cumulative dislocation activities. Prior studies on these metals have confirmed improved high-cycle fatigue performance. In the case of nano-grained metals, analyses of subsurface crack initiation sites have indicated that the crack nucleation is associated with abnormally large grains. However, these post-mortem analyses have led to only speculation about when abnormal grain growth occurs (e.g., during fatigue, after crack initiation, or during crack growth). In this study, a recently developed synchrotron X-ray diffraction technique was used to detect the onset and progression of abnormal grain growth during stress-controlled fatigue loading. This study provides the first direct evidence that the grain coarsening is cyclically induced and occurs well before final fatigue failure—our results indicate that the first half of the fatigue life was spent prior to the detectable onset of abnormal grain growth, while the second half was spent coarsening the nanocrystalline structure and cyclically deforming the abnormally large grains until crack initiation. Post-mortem fractography, coupled with cycle-dependent diffraction data, provides the first details regarding the kinetics of this abnormal grain growth process during high-cycle fatigue testing. Precession electron diffraction images collected in a transmission electron microscope after the in situ fatigue experiment also confirm the X-ray evidence that the abnormally large grains contain substantial misorientation gradients and sub-grain boundaries.

Fatigue stress concentration and notch sensitivity in nanocrystalline metals

Abstract

Improved densification and mechanical properties of spark plasma sintered carbon nanotube reinforced alumina ceramics

Dense magnesium oxide (MgO) doped multiwalled carbon nanotube (MWCNT) reinforced alumina (Al2O3) nanocomposites were fabricated using spark plasma sintering (SPS). Sintered nanocomposites possessed refined microstructure due to the presence of uniformly dispersed CNTs and ability of MgO to increase densification rate before onset of abnormal grain growth. Williamson-Hall analyses of XRD patterns indicated that matrix crystallite size (LC) and lattice micro-strain (εC) of the nanocomposites decreased by ∼40% and >30%, respectively, than those of pure Al2O3 (LC ≈ 75 nm, εC = 1.54 × 10−3). Present investigation also depicted the suitability of CNT addition in Al2O3 towards achieving higher density of nanocomposites using low temperature (1300 °C) SPS. Addition of CNT (especially, at ≥0.6 vol.%) in highly electrically insulating matrix established an electrical percolating network that helped in local heating of matrix particles during SPS and led to higher densification. The highest changes in indentation fracture toughness at 1 kgf and wear rate at 20N normal load were obtained at only 0.6 vol.% MWCNT loading which were >22% higher and 35% lower, respectively, compared to pure Al2O3.

Thermal stability of friction stir processed ultrafine grained AlMgSc alloy

Abstract Two different aspects of the thermal stability of grains were studied — (i) thermal stability of grain structure evolution during friction stir processing (FSP), and (ii) thermal stability of grains after FSP. It was concluded that Al 3 (Sc,Zr) dispersoids are effective in limiting grain growth to ultrafine grained regime (UFG) by stabilizing the microstructure during FSP. The presence of these dispersoids before processing was more effective in refining the grain size than the case in which these dispersoids precipitated during FSP. The mean grain sizes for these cases were 0.40 ± 0.17 μm and 0.50 ± 0.25 μm, respectively. Isothermal grain growth study revealed that UFG grain structure was maintained up to 723 K even after 16 h of annealing whereas most of the UFG alloys reported in the literature show extensive grain growth above 473 K. To understand the role of Al 3 (Sc,Zr) dispersoids in this alloy, 5086Al H32 was friction stir processed and subjected to identical annealing conditions. Annealing of FSP 5086Al H32 at 623 K for 1 h resulted into abnormal grain growth (AGG). The onset of AGG was observed for UFG sample only after annealing at 823 K for 1 h. This was rationalized using Humphreys' model for AGG [F.J. Humphreys, Acta Mater., 45 (1997) 5031].

Homogeneous Abnormal Grain Growth in Polycrystalline Materials

The controversy about the start up of the abnormal growth as heterogeneously engendered in the microstructure or produced by peculiar continuous incubation process has still not been solved. In this work the statistical theory of grain growth has been applied to treat the case of grain growth in the presence of an homogeneously unstable Zener drag. By the simulations presented it will be shown as abnormal grain growth is a result out of a continuous and homogeneous process without requiring any heterogeneity in the microstructure presumed to give local advantages to some grains. The mechanism by which during an incubation period the preconditions for the unstable growth are built up in the microstructure is clarified and discussed. Moreover the peculiar shape of Grain Size Distribution (GSD) approaching the “structural instability” will be also analytically defined and compared with experimental results obtained in a grain oriented Silicon Iron just before the onset of abnormal grain growth.

Relating Grain‐Boundary Complexion to Grain‐Boundary Kinetics I: Calcia‐Doped Alumina

Because of the complex nature of internal interfaces it has been a continual challenge to link the grain growth behavior of alumina (especially the onset of abnormal grain growth) to the internal interface structure and chemistry, and the associated atomic transport rate. The present work considers the problem of normal and abnormal grain growth development in calcia‐doped alumina, a system noted for its complex abnormal grain growth behavior, in terms of the new concept of interface complexions. Calcia‐doped alumina was shown to exhibit four distinct grain‐boundary complexions in the temperature range of 1325°–1870°C. All four complexions may coexist at a single temperature. Each complexion is associated with a characteristic grain‐boundary mobility, all of which enhances the grain growth kinetics relative to undoped alumina. It was found that the activation energy for the different complexions (normal and abnormal grain growth) was approximately the same in each case (∼450 kJ/mol). This is discussed in the context of interface‐ versus diffusion‐controlled grain growth, and it is concluded that normal and abnormal grain growth in this system is diffusion controlled.

Abnormal Grain Growth in Metals

Grain growth may occur in two forms, normal grain growth, characterized by a constant grain size distribution during growth, and abnormal grain growth, where one or more abnormally large grains may form in the microstructure. The presence of abnormally large grains in an otherwise uniform microstructure may be detrimental to the mechanical properties of a polycrystalline structure. Little is understood of the exact cause of abnormal grain growth. The annealing conditions leading to the onset of abnormal grain growth have been investigated via a series of grain growth experiments carried out on an Al-4wt%Cu alloy. The structure of which consisted of equiaxed grains (&lt;8μ) pinned by a fine dispersion of sub-micron second phase particles, which may dissolve upon annealing. Minority texture components may experience accelerated growth due to a higher energy and mobility compared to the surrounding grain structure. The combination of these two events may result in the abnormal growth of some grains. SEM imaging and EBSD data has then made it possible to characterize the influence of particle dissolution and grain boundary misorientation on the onset of abnormal grain growth. The stability of ‘island grains’ found to exist internally in abnormally large grains has also been investigated in relation to the misorientation relationship and localized second phase volume fraction found there. There was only weak evidence of special misorientation relationships between the island grains and the abnormally large grains in which they exist, and although there was evidence of an enhanced fraction of pinning particles at island grain boundaries, this was also true of boundaries in general. The larger size of island grains is their dominant characteristic, and grains which become island grains may have been incipient abnormal grains.

Abnormal Grain Growth Mechanism in Alumina

During the sintering process, the onset of abnormal grain growth is depend on various factors. The abnormal grain growth behavior, however, was focused only the growth stage compared to nucleation stage. Because most of abnormal grain growth occurred to a rapid mode, the observation of abnormal grain growth was impossible. In this study, we intend to in-situ observe the nucleation stage of abnormal grain using porous alumina. As a result, the liquid phase due to unintended impurities plays a main role for the formation of abnormal nuclei by the rearrangement of small grains due to the capillary force.