The relative rates of secondary and normal grain growth

Phase-Field Simulation on the Effect of Second-Phase Particles on Abnormal Growth of Goss Grains in Fe-3%Si Steels

A phase-field model was revised to study the abnormal growth of Goss grains during the annealing process in Fe-3%Si steels, in which the interaction between the second-phase particles and Goss grain boundaries (GBs) was considered. The results indicate that the abnormal growth of Goss grains occurs due to the different dissolvability of the particles at Goss GBs compared with the other GBs. Moreover, the degree of abnormal growth increases first and then decreases with an increasing particle content. Meanwhile, the size advantage of Goss grain can further promote the degree of abnormal growth. Two types of island grains were found according to the simulated results, which is consistent with the experimental observations. A proper GB dissolvability of particles is the key factor for the formation of isolated island grains, and a higher local particle density at GBs is the main reason for the appearance of serial island grains. These findings can provide guidance for the desired texture control in silicon steels.

Controlling Crystal Texture in Programmable Atom Equivalent Thin Films

DNA is a powerful tool in the directed assembly of nanoparticle based superlattice materials, as the predictable nature of Watson-Crick base pairing allows DNA-grafted particles to be programmably assembled into unit cells that arise from the complete control of nanoparticle coordination environment within the lattice. However, while the local environment around each nanoparticle within a superlattice can be precisely dictated, the same level of control over aspects of crystallite structure at the meso- or macroscale (e.g., lattice orientation) remains challenging. This study investigates the pathway through which DNA-functionalized nanoparticles bound to a DNA-functionalized substrate reorganize upon annealing to synthesize superlattice thin films with restricted orientation. Preferential alignment with the substrate occurs because of the energetic stabilization of specific lattice planes at the substrate interface, which drives the aligned grains to nucleate more readily and grow through absorption of surrounding grains. Crystal orientation during lattice reorganization is shown to be affected by film thickness, lattice symmetry, DNA sequence, and particle design. Importantly, judicious control over these factors allows for rational manipulation over crystalline texture in bulk films. Additionally, it is shown that this level of control enables a reduction in nanoscale symmetry of preferentially aligned crystallites bound to an interface through anisotropic thermal compression upon cooling. Ultimately, this investigation highlights the remarkable interplays between nanoscale building blocks and mesoscale orientation, and expands the structure-defining capabilities of DNA-grafted nanoparticles.

Programmable Atom Equivalents: Atomic Crystallization as a Framework for Synthesizing Nanoparticle Superlattices

Decades of research efforts into atomic crystallization phenomenon have led to a comprehensive understanding of the pathways through which atoms form different crystal structures. With the onset of nanotechnology, methods that use colloidal nanoparticles (NPs) as nanoscale "artificial atoms" to generate hierarchically ordered materials are being developed as an alternative strategy for materials synthesis. However, the assembly mechanisms of NP-based crystals are not always as well-understood as their atomic counterparts. The creation of a tunable nanoscale synthon whose assembly can be explained using the context of extensively examined atomic crystallization will therefore provide significant advancement in nanomaterials synthesis. DNA-grafted NPs have emerged as a strong candidate for such a "programmable atom equivalent" (PAE), because the predictable nature of DNA base-pairing allows for complex yet easily controlled assembly. This Review highlights the characteristics of these PAEs that enable controlled assembly behaviors analogous to atomic phenomena, which allows for rational material design well beyond what can be achieved with other crystallization techniques.

Recrystallisation behaviour of a fully austenitic Nb-stabilised stainless steel

We have performed an in-depth characterisation of the microstructure evolution of 20Cr-25Ni Nb-stabilised austenitic stainless steel during 1 h isochronal annealing up to 1100°C using scanning electron microscopy. This steel grade is used as cladding material in Advanced Gas-cooled fission reactors, due to its resistance to thermal creep and oxidation. The initial deformed microstructure undergoes recrystallisation via a strain-induced boundary migration mechanism, attaining a fully recrystallised microstructure at 850°C. A number of twins are observed in the vicinity of deformation bands prior to the start of recrystallisation. New Nb(C, N) particles form gradually in the microstructure, and the particle dispersion presents a maximum volume fraction of 2.7% at 930°C. At higher temperatures, the smaller particles become unstable and gradually dissolve in the matrix. Consequently, the Zener pinning pressure exerted on the grain boundaries is progressively released, triggering the growth of the austenite grains up to an average size of ∼47 μm at 1100°C. The observed temperature window for recrystallisation and grain growth can be predicted by a unified model based primarily on the migration of high- and low-angle grain boundaries. LAY DESCRIPTION: Austenitic stainless steel containing high percentage of chromium and nickel is currently used as fuel cladding material in the British Advanced Gas-cooled Reactors (AGR). This material has been chosen because of its high resistance to thermal creep and corrosion, both enhanced by the presence of a fine dispersion of carbonitrides precipitated during the cladding thermomechanical processing. During the time spent in the reactor core, few fuel cladding elements can become susceptible to local chromium depletion at grain boundaries, which is ascribed to the time evolution of the microstructural damage caused by the neutron bombardment in the reactor core. This depletion might increase the susceptibility of this steel to intergranular corrosion attacks during medium-to-long term storage of spent fuel elements in water ponds. The severity of the local chromium depletion depends not only on the irradiation conditions, but also on the grain boundary geometry. We have investigated the recovery, recrystallisation and grain growth of AGR stainless steel during 1 h annealing at selected temperatures relevant for the thermomechanical processing of the steel claddings, focusing on the formation and evolution of grain boundaries and second phases. These two features play a key role in the progression of the neutron damage and the subsequent development of local chromium depletion during reactor service operations. A deep understanding of the mechanisms and conditions behind their formation during the thermomechanical processing of the cladding material and their interaction with each other constitutes the foundation to evaluate, and potentially mitigate, the effect of irradiation on the cladding material.

The evolution of a GaN/sapphire interface with different nucleation layer thickness during two-step growth and its influence on the bulk GaN crystal quality

The role of the nucleation layer thickness on the GaN crystal quality grown by metal organic chemical vapor deposition is explored.

Strain-induced selective growth in 1.5% temper-rolled Fe;1%Si

Strain-induced selective growth was investigated in a 1.5% temper-rolled Fe∼1%Si alloy using the electron backscatter diffraction (EBSD) technique. The EBSD technique was used to quantify the presence of orientation spreads within grains and to show that this particular case of selective growth can be directly related to differences in stored energy as reflected in the geometrically necessary dislocation content. The differences in stored energy were sufficient to give rise to selective growth as evidenced by bi-modal grain sizes.

Simulation of Microstructural Evolution in Polycrystalline Films

This paper will review the topic of computer simulation of the evolution of grain structure in polycrystalline thin films, with particular attention to the modelling of the grain growth process. If the grain size is small compared to the film thickness, then the grain structure is three-dimensional. As the grains grow to become larger than the film thickness, so that most grains traverse the entire thickness of the film, the microstructure may approach the conditions for a two-dimensional grain structure. Both two- and three-dimensional grain growth have been simulated by various authors.

When the grains become large enough for the microstructure to be two-dimensional, the surface energy associated with the two free surfaces of the film becomes comparable to the surface energy of the grain boundaries. In this condition, the free surface may profoundly effect the grain growth. One effect is that grooves may develop along the lines where the grain boundaries meet the free surfaces. This grooving may pin the boundaries against further migration and lead to grain-growth stagnation. Another possible effect is that differences in the free surface energy for grains with different crystallographic orientation may provide a driving force for the migration of the boundaries which is additional to that provided by grain boundary capillarity. Grains with favorable orientations will grow at the expense of grains with unfavorable orientations. The coupling of grain-growth stagnation with an additional driving force can produce abnormal or secondary grain growth in which a few grains grow very large by consuming the normal grains.

A New Look At Amorphous Vs Microcrystalline Structure

Historically, non-periodic models have usually found preference over microcrystalline ones for describing the structure of amorphous materials. Nevertheless, truly microcrystalline materials do exist, and the characteristic, monotonically decreasing isothermal calorimetric signal associated with the growth of the grains, can sometimes be used to identify them unambiguously. An example of the identification of the micro-quasicrystalline structure of some sputtered aluminum-transition metal alloys is discussed.

Amorphous/Crystalline Structure And Phase Transformations In Metastable Semiconducting Ge1−xSnx

The semiconducting crystalline alloys, Ge1−xSnx, are of interest due to theoretical predictions about their electronic band structures which make them useful in infrared photodetectors. However the composition region where these alloys have the desired properties is greater than the equilibrium solid solubility limit of Sn in Ge (x<0.01). We have circumvented the solubility limits and produced thin (2000Å) and thick (4–8Μm) films of Gei.xSnx (x<0.31) by rf sputtering. Differential scanning calorimetry (DSC) Measurements were performed to study grain growth and crystallization processes in these highly metastable semiconductors. X-ray and electron diffraction measurements indicated the materials were amorphous, but the fact that some of the films were fine grained polycrystalline samples only became apparent in their DSC spectra. We present models that describe quantitatively the transformation behavior in both sets of films.

Secondary Recrystallization in Grain-Oriented Silicon Steel

Mechanism of Goss secondary recrystallization in grain-oriented silicon steel has been investigated by temperature gradient annealing and by in situ observation utilizing synchrotron x-ray topography. The results support the selective growth theory. Migration of Goss grains is controlled by second phase particles (inhibitor) and sharper Goss grains, which have higher frequency of CSL boundaries to the matrix, start to grow preferentially while the other matrix grains are stagnated by inhibitor. CSL boundaries are supposed to have lower grain boundary energy, thus suffer lower pinning force from the inhibitor and start to migrate at higher inhibition level. Based on this model, we have made a computer simulation and have found that this model successfully depicts the important features of secondary recrystallization; grain growth behavior of secondary grains, secondary grain size and sharpness of Goss texture.

Local texture changes associated with grain growth

Recently it has been shown that grain growth is accompanied by macroscopic texture changes. Such changes will necessarily produce modifications in the proportions of ‘special’ grain boundaries and consequently may affect significantly the kinetics of grain growth and particularly anomalous growth. After new contributions from computer modelling processes are taken into account, it becomes clear that the onset of grain growth for a fixed driving force is a complex function of particle pinning, effects from solutes, surfaces and residual strains, grain-size distribution and texture-boundary mobility. The aim of this paper is to provide insight into the relation between grain texture-grain misorientation texture and anomalous grain growth in the alloy Nimonic PE16 where, for the particular heat-treatment conditions employed, particle pinning may be ignored. Macroscopic orientation measurements reveal only the overall texture; the present work considers microstructural regions of anomalous growth and texture measurements arecollated on a grain-specific basis (microtexture). From this grain-specific data the grain-misorientation textures (GMT) (i. e. the grain-boundary counterparts of an inverse pole figure) are also computed. The experimental results demonstrate that two types of microtexture may exist for different regions of anomalous growth within the same specimen. These differences are rationalized by noting that further grain growth-inducing heat treatments cause a change from the first texture to the second. Clear evidence is thus provided that there are micro textural changes associated with anomalous grain growth. The grain-boundary textural data show some deviation from a random distribution, particularly for boundaries between small grains (SS boundaries) as compared to boundaries between large and small grains (LS boundaries). These differences between the SS and ls groups extend to the proportions of geometrically special boundaries in each group, with the SS group containing a higher proportion of special (therefore higher than average mobility and lower energy) boundaries than the ls group. This trend was particularly apparent for boundaries that fell into the second texture group. The implication from these results is that in this case anomalous growth will tend to stagnate because the boundaries of small grains surrounding the large grain (SS boundaries) are on average of lower energy than the boundaries that border the large grain (LS boundaries). Finally, it is suggested, by analogy with primary recrystallization, that the definition of grain growth in terms of an energy-reduction criterion should be extended to encompass the sum of all available sources of energy minimization. Hence both grain-boundary textural rearrangements and grain-boundary migration are included.