Showing papers on "Grain growth published in 2008"

Dynamic recrystallization during high temperature deformation of magnesium

Abstract: As a consequence of the high critical stresses required for the activation of non-basal slip systems, dynamic recrystallization plays a vital role in the deformation of magnesium, particularly at a deformation temperature of 200 °C, where a transition from brittle to ductile behavior is observed. Uniaxial compression tests were performed on an extruded commercial magnesium alloy AZ31 at different temperatures and strain rates to examine the influence of deformation conditions on the dynamic recrystallization (DRX) behavior and texture evolution. Furthermore, the role of the starting texture in the development of the final DRX grain size was investigated. The recrystallized grain size, measured at large strains (ɛ ∼ −1.4) seemed to be more dependent on the deformation conditions than on the starting texture. In contrast to pure magnesium, AZ31 does not undergo grain growth at elevated deformation temperatures, i.e. 400 °C, even at a low strain rate of 10−4 s−1. Certain deformation conditions gave rise to a desired fully recrystallized microstructure with an average grain size of ∼18 μm and an almost random crystallographic texture. For samples deformed at 200 °C/10−2 s−1, optical microscopy revealed DRX inside of deformation twins, which was further investigated by EBSD.

Quantitative analysis of grain boundary properties in a generalized phase field model for grain growth in anisotropic systems

Abstract: A good choice of model formulation and model parameters is one of the most important and difficult aspects in mesoscale modeling and requires a systematic and quantitative analysis. In this paper, it is studied how the model parameters of a generalized phase field model affect the landscape of the free-energy density functional, the phase field profiles at the grain boundaries, and the corresponding trajectory along the free-energy landscape. The analysis results in quantitative relations between the model parameters, on one hand, and grain boundary energy and mobility, on the other hand. Based on these findings, a procedure is derived that generates a suitable set of model parameters that reproduces accurately a material's grain boundary energy and mobility for arbitrary misorientation and inclination dependence. The misorientation and inclination dependence are formulated so that the diffuse interface width is constant, resulting in uniform stability and accuracy conditions for the numerical solution. The proposed model formulation and parameter choice allow us to perform quantitative simulations with excellent controllability of the numerical accuracy and therefore of the material behavior.

Stabilization of nanocrystalline grain sizes by solute additions

Abstract: This paper will review the grain growth in nanocrystalline materials with emphasis on the grain size stabilization that can result from solute additions. The grain growth in nominally pure nanocrystalline metals will be presented followed by descriptions of the stabilization of nanocrystalline grain sizes by kinetic approaches and thermodynamic strategies. The descriptions of nanocrystalline grain size by solute additions will be taken from the literature as well as from recent research in the authors’ laboratory. Examples of kinetic stabilization, which involves reduction of the grain boundary mobility, include second phase drag, solute drag, chemical ordering, and grain size stabilization. The thermodynamic stabilization, which is due to the lowering of the specific grain boundary energy by solute segregation to the grain boundaries, will be described for systems including Pd–Zr, Fe–Zr, Ni–W, Ni–P, and Co–P. Recrystallization during grain growth will be presented for the Ti–N system. Finally, a summary of alloys where nanocrystalline grain sizes can be maintained at annealing temperatures close to the melting point will be presented.

Densification and grain growth during sintering of nanosized particles

Abstract: The sintering of nanosized particles is a scientific and technological topic that affects the manufacture of bulk nanocrystalline materials and the understanding of the stability of nanoparticles. Owing to their extremely small size and the high surface to volume ratio, nanoparticles during sintering exhibit a number of distinctively unique phenomena compared to the sintering of coarse powders. Particularly, it is generally found that the sintering temperatures of nanosized particles are dramatically lower than that of their micrometre or submicrometre sized counterparts. Research has also shown that the grain growth during nanosintering consists of an initial dynamic grain growth stage that occurs during heating up and the normal grain growth stage that occurs mostly during isothermal holding. For nanoparticles, the effect of the initial grain growth cannot be ignored because that it is sufficient to cause the material to lose nanocrystalline characteristics. This review aims to bring to focus th...

A thermal spike model of grain growth under irradiation

Abstract: The experimental study of grain growth in nanocrystalline metallic foils under ion irradiation showed the existence of a low-temperature regime (below about 0.15–0.22Tm), where grain growth is independent of the irradiation temperature, and a thermally assisted regime where grain growth is enhanced with increasing irradiation temperature. A model is proposed to describe grain growth under irradiation in the temperature-independent regime, based on the direct impact of the thermal spikes on grain boundaries. In the model, grain-boundary migration occurs by atomic jumps, within the thermal spikes, biased by the local grain-boundary curvature driving. The jumps in the spike are calculated based on Vineyard’s analysis of thermal spikes and activated processes using a spherical geometry for the spike. The model incorporates cascade structure features such as subcascade formation, and the probability of subcascades occurring at grain boundaries. This results in a power law expression relating the average grain ...

Grain growth during the early stage of sintering of nanosized WC–Co powder

Abstract: Rapid grain growth during the early stage of sintering has been found in many nano material systems including cemented tungsten carbide WC–Co. To date, however, there have been few reported studies in the literature that deal directly with the kinetics or the mechanisms of this part of grain growth. In this work, the grain growth of nanosized WC during the early stages of sintering was studied as a function of temperature and time. The effects of other influencing factors, such as the initial grain size, cobalt content, and the grain growth inhibitor VC, were investigated. The kinetics of the grain growth process was analyzed and the evolution of the morphology of WC grains during heating-up was studied using high resolution scanning electron microscopy. The results showed that the grain growth process consists of an initial stage rapid growth process which typically takes place during heat-up and the normal grain growth during isothermal holding. The initial rapid grain growth is at least partially attributed to the process of coalescence of grains via elimination common grain boundary. The preferred orientation between WC grains within the aggregates is considered a favorable condition for coalescence of grains, hence rapid grain growth. The solution–reprecipitation process is considered a mechanism of coalescence.

Deformation-induced grain rotation and growth in nanocrystalline Ni

Abstract: Nanobeam electron diffraction and a series of dark field images techniques were used to investigate the deformation mechanisms of nanocrystalline (nc) Ni in response to in situ tensile deformation under transmission electron microscopy (TEM). The experiments exhibit the complete processes of individual grain rotation and neighboring grain rotation/growth. Deformation-induced grain rotation and growth as one of plastic deformation mechanisms in nc materials was revealed. At the same time, these results were confirmed further by ex situ TEM observation on deformed sample and were also better understood by physical deformation model.

Densification and grain growth of nanocrystalline 3Y-TZP during two-step sintering

Abstract: Two-step sintering (TSS) was applied on nanocrystalline yttria tetragonal stabilized zirconia (3Y-TZP) to control the grain growth during the final stage of sintering. The process involves firing at a high temperature (T1) followed by rapid cooling to a lower temperature (T2) and soaking for a prolonged time (t). It is shown that for nanocrystalline 3Y-TZP (27 nm) the optimum processing condition is T1 = 1300 °C, T2 = 1150 °C and t = 30 h. Firing at T1 for 1 min yields 0.83 fractional density and renders pores unstable, leading to further densification at the lower temperature (T2) without remarkable grain growth. Consequently, full density zirconia ceramic with an average grain size of 110 nm is obtained. XRD analysis indicated that the ceramic is fully stabilized. Single-step sintering of the ceramic compact yields grain size of 275 nm with approximately 3 wt.% monoclinic phase. This observation indicates that at a critical grain size lower than 275 nm, phase stabilization is induced by the ultrafine grain structure.

Sintering and grain growth in SiO2 doped Nd:YAG

Abstract: Densification and grain growth in pure YAG, SiO 2 doped YAG and SiO 2 doped Nd:YAG were explored. The activation energy for densification (235 kJ/mol) in pure YAG is lower than that of grain growth (946 kJ/mol) which is unusual in ceramic systems. Consequently, pure YAG sinters to near full density (>99.9%) at 1700 °C with little grain growth (1.2 μm average grain size). The remaining large pores (radius > 2 μm) were determined to be thermodynamically stable because their coordination number with grains was >6. The stability of these pores underscores the importance of powder processing and forming in fabricating transparent YAG. SiO 2 doped YAG sinters to near full density 100 °C lower than pure YAG because SiO 2 enables liquid phase sintering and the removal of large pores. The addition of Nd 2 O 3 further enhances both densification and grain growth at temperatures below 1700 °C. Above 1700 °C higher concentrations of Nd 3+ suppressed grain growth, possibly due to solute drag.

The influences of alloying additions and processing parameters on the rolling microstructures and textures of magnesium alloys

Abstract: The combined effects of lithium additions (1–3 wt%) and processing parameters (rolling temperature, annealing) on the microstructural and texture evolution of pure Mg and Mg–3 wt% Al–1 wt% Zn alloy have been studied. Following rolling the basal planes were aligned with the sheet surface, although the basal poles were split and rotated towards the rolling direction. Lithium additions increased the rotation of basal poles in the rolling and transverse directions; an increase in the rolling temperature was associated with decreased rotation in the rolling direction and some broadening of texture in the transverse direction. Recrystallization during rolling varied between alloys, but had little influence on the texture. Recrystallization, and particularly grain growth, during annealing resulted in a single peak in the basal poles replacing the split observed following rolling. Texture is interpreted in terms of deformation, recrystallization and grain growth. Microstructural and texture evolution during industrial forming processes are also discussed.

NbC as grain growth inhibitor and carbide in WC–Co hardmetals

Abstract: WC–NbC–12 wt%Co hardmetals with 0.45–60 wt%NbC were prepared by solid state pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS), for 2 min at 1240 °C and conventional sintering (CS) for 1 h at 1420 °C. The role of NbC as grain growth inhibitor and major carbide addition was investigated in terms of the densification behaviour, the microstructure and mechanical properties. Experimental work revealed that the addition of more than 5 wt%NbC inhibits pressure assisted solid-state densification compared to WC–Co based hardmetals. The addition of NbC limits WC grain growth during PECS and conventional sintering, whereas substantial (Nb, W)C grain growth was observed in the hardmetals with ⩾0.9 wt%NbC addition. The influence of the NbC content on the hardness, strength and toughness of the WC–NbC–12 wt%Co hardmetals was explained in terms of WC grain growth inhibition and the formation of coarse (Nb, W)C grains.

Thickness dependence of structural, electrical and optical behaviour of undoped ZnO thin films

Abstract: Undoped ZnO thin films of different thicknesses were prepared by r.f. sputtering in order to study the thickness effect upon their structural, morphological, electrical and optical properties. The results suggest that the film thickness seems to have no clear effect upon the orientation of the grains growth. Indeed, the analysis with X-ray diffraction show that the grains were always oriented according to the c (0 0 2)-axis perpendicular to substrate surface whatever the thickness is. However, the grain size was influenced enough by this parameter. An increase in the grain size versus the thickness was noted. For the electrical properties, measurements revealed behaviour very dependent upon thickness. The resistivity decreased from 25 to 1.5×10 −3 Ω cm and the mobility increased from 2 to 37 cm 2 V −1 s −1 when the thickness increased from 70 to 1800 nm while the carrier concentration seems to be less affected by the film thickness and varied slightly remaining around 10 20 cm −3 . Nevertheless, a tendency to a decrease was noticed. This behaviour in electrical properties was explained by the crystallinity and the grain size evolution. The optical measurements showed that all the samples have a strong transmission higher than 80% in the visible range. A slight shift of the absorption edge towards the large wavelengths was observed as the thickness increased. This result shows that the band gap is slightly decreases from 3.37 to 3.32 eV with the film thickness vary from 0.32 to 0.88 μm.

Quantitative Phase-Field Approach for Simulating Grain Growth in Anisotropic Systems with Arbitrary Inclination and Misorientation Dependence

Abstract: A phase-field approach for quantitative simulations of grain growth in anisotropic systems is introduced, together with a new methodology to derive appropriate model parameters that reproduce given misorientation and inclination dependent grain boundary energy and mobility in the simulations. The proposed model formulation and parameter choice guarantee a constant diffuse interface width and consequently give high controllability of the accuracy in grain growth simulations.

Influence of zirconia addition on the microstructure of K0.5Na0.5NbO3 ceramics

Abstract: We prepared (K0.5Na0.5)NbO3 (KNN) ceramics with the addition of 1 mass% ZrO2 with the aim to hinder the exaggerated grain growth encountered in KNN. Both KNN and KNN-ZrO2 ceramics sintered at 1115 °C and 1125 °C, respectively, had relative density exceeding 95%. KNN had a bimodal microstructure, with a population of fine grains of a few 100 nm and the large grains of about 20 μm, while the microstructure of KNN-ZrO2 was fine and uniform, with the median grain size of 0.3 μm and the largest grains of about 1.3 μm. We attribute the refinement of the microstructure to the matrix grain-growth inhibition by ZrO2 addition. The influence of ZrO2 is twofold: sub-100 nm sized ZrO2 particles, located at the KNN grain junctions hinder the matrix grain growth. Additionally, the enrichment of the boundary regions of the matrix grains with Zr relative to the grain interiors, confirmed by TEM/EDXS analysis, is also a probable reason for the decreased mobility of the grain boundaries. The dielectric permittivity and losses, measured at 10 kHZ, and piezo d33 constant of KNN-ZrO2 are 905, 0.04 and 100 pC/N, respectively.

Precipitation and its effect on the mechanical properties of a cast Mg-Gd-Nd-Zr alloy

Abstract: Microstructural characterization of a Mg–11Gd–2Nd–0.5Zr (wt.%) alloy processed by isothermal aging at 250 °C has been performed using transmission electron microscopy, and the mechanical properties of the alloy after aging for different time have been evaluated using Vickers hardness, room- and high-temperature tensile tests. A four-stage precipitation sequence of the alloy can be described as follows: supersaturated α-Mg solid solution → β″(D019) → β′(bco) → β1(fcc) → β(fcc), where the later precipitate can coexist with the former. The precipitation inside the grains is accompanied by the formation and growth of grain boundary precipitates (GBPs) and precipitation free zones (PFZs). Discussion on the structure–properties relationships reveals that microstructure with dense β′ precipitates in triangular arrangement is the optimum strengthening structure for the alloy. The existence of GBPs and PFZs in the alloy deteriorates the mechanical properties.

Mechanics of grain-size reduction in fault zones

Abstract: [1] Recent observations of nanometer-scale particles in the cores of exhumed fault zones raise questions about how such small particles are formed and how they survive, especially if significant shear heating is produced during an earthquake. Commercial crushing and grinding operations encounter a grind limit near 1 mm below which particles deform plastically rather than fracturing. A fragmentation model and low-temperature plasticity mechanics indicate that it is not possible to produce under compressive loading and short timescale significantly smaller particles at any strain rate. However, shock loading and subcritical crack growth can produce nanometer-scale fragments in compression. Under tensile loading the fragment size is determined by a competition between the nucleation of cracks and stress relaxation in their neighborhoods. Therefore higher tensile strain rates produce smaller fragments. The ultimate limit is determined by the availability of elastic strain energy, which does not place a significant constraint on the minimum grain size. Grain growth kinetics suggests that survivability of grains is very temperature sensitive. A 10 nm quartz fragment will double its size in 0.1 s at 1000C, in 20 s at 800C, in 14 h at 600C, and in 10 years at 400C. The observation of grains smaller than 10 nm places meaningful constraints on the dynamic fields and permeability of the fault zone during a large earthquake. Microstructural analysis of the grains and rock damage may be used to infer whether fragmentation occurred under macroscopic tension or compression.

Spark Plasma Sintering of Zirconium Diborides

Abstract: The densification behavior and grain growth of single-phase ZrB2 ceramics by a spark plasma sintering (SPS) process were systematically investigated. The sintering was conducted at temperatures ranging from 1800° and 1950°C in vacuum under a pressure of 50 MPa. The heating rate and the soaking time varied from 50° to 430°C/min and from 0 to 20 min, respectively. The microstructure was observed by field emission scanning electron microscopy. A near-fully dense ZrB2 ceramic can be obtained at a lower temperature, within a much shorter time, than those used in conventional hot-pressing process. The density of the sintered ceramics increases with increase in sintering temperature, heating rate, and holding time. On the other hand, microstructural examination shows a strong dependence of grain growth on sintering temperature, heating rate, and holding time. The fully dense ZrB2 ceramic with fine grain and homogeneous structure could be achieved by the processing conditions as follows: sintering temperature of 1900°C, holding time of 3 min, and heating rate of 200°–300°C/min.