Showing papers on "Grain growth published in 2001"

Accretion disks around young objects. iii. grain growth

Abstract: We present detailed models of irradiated T Tauri disks including dust grain growth with power-law size distributions. The models assume complete mixing between dust and gas and solve for the vertical disk structure self-consistently including the heating effects of stellar irradiation as well as local viscous heating. For a given total dust mass, grain growth is found to decrease the vertical height of the surface where the optical depth to the stellar radiation becomes unity and thus the local irradiation heating, while increasing the disk emission at mm and submillimeter wavelengths. The resulting disk models are less geometrically thick than our previous models assuming interstellar medium dust, and agree better with observed spectral energy distributions and images of edge-on disks, like HK Tau/c and HH 30. The implications of models with grain growth for determining disk masses from long-wavelength emission are considered.

Grain size reduction by dynamic recrystallization: can it result in major rheological weakening?

Abstract: It is widely believed that grain size reduction by dynamic recrystallization can lead to major rheological weakening and associated strain localization by bringing about a switch from grain size insensitive dislocation creep to grain size sensitive diffusion creep. Recently, however, we advanced the hypothesis that, rather than a switch, dynamic recrystallization leads to a balance between grain size reduction and grain growth processes set up in the neighborhood of the boundary between the dislocation creep field and the diffusion creep field. In this paper, we compare the predictions implied by our hypothesis with those of other models for dynamic recrystallization. We also evaluate the full range of models against experimental data on a variety of materials. We conclude that a temperature dependence of the relationship between recrystallized grain size and flow stress cannot be neglected a priori. This should be taken into account when estimating natural flow stresses using experimentally calibrated recrystallized grain size piezometers. We also demonstrate experimental support for the field boundary hypothesis. This support implies that significant weakening by grain size reduction in localized shear zones is possible only if caused by a process other than dynamic recrystallization (such as syntectonic reaction or cataclasis) or if grain growth is inhibited.

A few remarks on the kinetics of static grain growth in rocks

Abstract: Static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature. Given the relative simplicity of the protocol for grain growth experiments, measurements of grain boundary mobility show surprising variations. Boundary mobilities during grain growth are affected by solute and impurity chemistry, chemical fugacity of trace and major elements, pore size and number, pore fluid chemistry, the presence of melts, and the presence of solid second phases, as well as temperature and pressure. All of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments. Provided that the necessary kinetics parameters are known, bounds may be placed on the interface mobility when pores, partial melts, or solutes are present. To predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature.

Mechanisms of grain growth in nanocrystalline fcc metals by molecular-dynamics simulation.

Abstract: To elucidate the mechanisms of grain growth in nanocrystalline fcc metals, we have performed fully three-dimensional molecular-dynamics simulations with a columnar grain structure and an average grain diameter of 15 nm. Based on the study of coarse-grained materials, the conventional picture is that grain growth is governed by curvature-driven grain-boundary migration. However, our simulations reveal that in a nanocrystalline material grain rotations play an equally important role, at least during the early stages of grain growth. By eliminating the grain boundary between neighboring grains, such rotations lead to grain coalescence and the consequent formation of highly elongated grains. A detailed analysis exposes an intricate coupling between this mechanism and the conventional grain-boundary-migration dominated mechanism. Incorporation of these insights into mesoscopic models should enable more realistic mesoscopic simulations of grain growth in nanocrystalline materials. (A short movie showing the overall evolution of the grain microstructure can be viewed at http://www.msd.anl.gov/im/movies/graingrowth.html.)

Piezoelectric properties of 〈001〉 textured Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics

Abstract: The piezoelectric properties of (1−x) Pb(Mg1/3Nb2/3)O3–xPbTiO3 (x=0.3–0.35), ceramics with a high degree of 〈001〉 fiber texture were investigated for possible actuator applications. Piezoelectric coefficients (d33) in excess of 1200 pC/N associated with strain levels up to >0.3% were observed in samples prepared by a reactive templated grain growth process. No excess PbO was used in the starting composition. A high degree of fiber texture was achieved using 〈001〉 oriented BaTiO3 template particles in a fine-grained precursor for the PMN–32PT matrix. High densities together with texture resulted in a significant increase in strain levels and d33 values compared to their polycrystalline counterparts. Peak dielectric constants on the order of 22 000 with losses of ∼2% and well-saturated hysteresis loops with a Pr∼27 μC/cm2 were recorded on the textured samples. These domain engineered, textured ceramics have tremendous potential for high-performance actuators.

Size-Dependent Grain-Growth Kinetics Observed in Nanocrystalline Fe

Abstract: Measurements of grain growth in nanocrystalline Fe reveal a linear dependence of the grain size on annealing time, contradicting studies in coarser-grained materials, which find a parabolic (or power-law) dependence. When the grain size exceeds approximately 150 nm, a smooth transition from linear to nonlinear growth kinetics occurs, suggesting that the rate-controlling mechanism for grain growth depends on the grain size. The linear-stage growth rate agrees quantitatively with a model in which boundary migration is controlled by the redistribution of excess volume localized in the boundary cores.

Grain refinement of AZ31 and ZK60 Mg alloys — towards superplasticity studies

Abstract: Low temperature superplastic (SP) behavior (mechanical and deformation mechanisms) of two commercial Mg-based alloys (AZ31 and ZK60) was characterized. The two alloys were tested in the as extruded condition with initial grain size of 15 μm (AZ31) and fine (2 μm) and coarse (25 μm) grains mixed randomly for the ZK60. Strain rate was activated in the range 10−5–1 s−1 at 450 K (0.49Tm) in order to determine the deformation capacity curves (elongation to failure vs. strain rate), and to evaluate the strain rate sensitivity coefficient, m, from the stress versus strain rate curves. Optical, scanning and transmission electron microscopy observations (SEM and TEM) were performed to elaborate on the dynamic recrystallization (DRX) grain growth, fracture modes and deformation mechanisms at the SP mode. In addition, X-ray diffraction was utilized to track for microstructural classification. Although low temperature was applied, the ZK60 exhibited superplastic-like behavior and the maximum peak of elongation (220%) was detected at 1×10−5 s−1 with m equal to 0.2. In AZ31 SP behavior was suppressed due to grain growth, while for ZK60, DRX was detected. However, for the latter alloy, it was observed that the coarse/fine grain interface was the trigger for microcracking initiation. Actually, this phenomenon reduces the SP capacity of the ZK60 alloy. Surface observations and TEM findings indicate that grain boundary sliding with homogeneous character is the controlling SP deformation mode. Typical dislocation features have supported this deformation mode, mainly by grain boundary dislocation pile-ups. More sophisticated extrusion processes as equal angular channel extrusion (EACE) is likely to be considered in the future as a mean to improve grain homogeneity and produce ultra fine grain microstructure.

Mechanical Behavior and Superplasticity of a Severe Plastic Deformation Processed Nanocrystalline Ti-6Al-4V Alloy

Abstract: Mechanical behavior of a severe plastic deformation (SePD) processed nanocrystalline Ti–6Al–4V alloy has been studied in the temperature range 25–675°C Compared with the microcrystalline state, the nanocrystalline state material had higher strength up to 400°C and comparable strength above that The ductility was significantly higher for the nanocrystalline state above 500°C, including superplasticity above 600°C Transmission electron microscopy showed considerable grain growth and dislocation activity during superplastic deformation A comparison of the superplastic data across the nanocrystalline and microcrystalline range showed an interesting discrepancy in the kinetics of superplastic deformation Contrary to the general expectation, the kinetics of superplastic deformation was slower in ultrafine grained materials after normalizing the data for grain size and temperature dependence The slower superplastic deformation kinetics in the nanocrystalline materials is discussed in terms of the difficulty associated with slip accommodation of grain boundary sliding

Tensile behavior and fracture in nickel and carbon doped nanocrystalline nickel

Abstract: The potential engineering applications of nanocrystalline materials need more detailed study on deformation and fracture mechanisms at room and elevated temperatures under tensile loading. This paper reports results of a series of experiments carried out on nickel and carbon doped nanocrystalline nickel with different carbon concentrations from 500 to 1000 ppm at room temperature to 300°C. Grain growth was observed in nanocrystalline nickels as the testing temperature increases. A fast grain growth was noticed at 300°C. Pure nanocrystalline nickel experienced an abnormal grain growth at 500°C and its tensile properties reduced to a very low level. The addition of carbon exerted a potential effect to enhance the stability of the microstructure in nanocrystalline nickel at intermediate temperatures. However, carbon doped nickels exhibited lower tensile properties. Nanocrystalline nickels displayed a conventional Hall–Petch relationship. The results are discussed in relation to microstructural characteristics by using TEM and SEM.

Grain growth and strain release in nanocrystalline copper

Abstract: Grain growth and strain release processes in the electrodeposited nanocrystalline (nc) Cu specimen with a high purity were investigated by means of differential scanning calorimetry, x-ray diffraction, electrical resistance measurement, and high-resolution transmission electron microscopy. It was found that for the as-deposited nc Cu, the grain growth started at about 75 degreesC, at which the microstrain in (111) plane (e(111)) began to release, while the mean microstrain and that in (100) plane (e(100)) began to release at a higher temperature (150 degreesC). With an increment in microstrain in the nc Cu introduced by cold rolling, the grain growth onset temperature increased while the strain release onset temperature dropped obviously. These results showed an evident correlation between the grain size stability and the microstrain in the nc materials. The activation energy for the grain growth was determined by using Kissinger analysis and isothermal kinetics analysis, being about 86 kJ/mol, implying that the grain growth process is dominated by grain boundary diffusion. (C) 2001 American Institute of Physics.

Electromigration in Cu interconnects with very different grain structures

Abstract: To determine the effects of grain structures on the rate of electromigration-induced failure of Cu interconnects, scanned laser annealing (SLA) has been used to produce Cu interconnects with very different grain structures. SLA, in which a moving hot-zone induces local grain growth, can be used to produce interconnects with fully bamboo grain structures that have bamboo grain lengths up to ten times the interconnect width. Electromigration experiments have been carried out on interconnects with very-long-grained bamboo structures, as well as on interconnects with polygranular structures in which the average grain size is less than the linewidth. Such differences are known to lead to orders of magnitude changes in lifetimes for Al-based interconnects. However, no significant differences in the failure rates were found for these Cu interconnects. This result supports earlier work that suggested that electromigration in Cu interconnects with now-standard liners and interlevel diffusion-barrier layers occurs ...

Sol-Gel Processed TiO2-Based Nano-Sized Powders for Use in Thick-Film Gas Sensors for Atmospheric Pollutant Monitoring

Abstract: Sol-gel routes were used to prepare pure and 5 at% and 10 at% Ta- or Nb-dope TiO2 nano-sized powders. The thermal decomposition behaviour of the precursors was studied using simultaneous thermogravimetric and differential thermal analysis (TG/DTA). X-ray diffraction (XRD) analysis showed that the powders heated to 400°C were crystalline in the anatase TiO2 structure. The pure TiO2 powder heated to 850°C showed the rutile structure. The addition of Ta and Nb inhibited the anatase-to-rutile phase transformation up to 950–1050°C. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired for 1 h at different temperatures in the 650–1050°C range. Scanning electron microscopy (SEM) observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at a nanometric level even at 950°C, though grain growth was observed with increasing temperature. The gas-sensitive electrical response of the thick films were tested in laboratory, in environments with CO in dry and wet air. Conductance measurements showed a good gas response only for the nanostructured titania-based films. For field tests, the prototype sensors were placed beside a conventional station for atmospheric pollutant monitoring. The electrical response of the thick films was compared with the results of the analytical instruments. The same trend was observed for both systems, demonstrating the use of gas sensors for this aim.

Microstructural characteristics and thermal stability of ultrafine grained 6061 Al alloy fabricated by accumulative roll bonding process

Abstract: An accumulative roll bonding process was employed to introduce a ultrafine grained structure into a commercial 6061 Al alloy. In performing the accumulative roll bonding process, the alloy was rolled with a 50% reduction ratio. Then, the rolled sheet was cut, stacked to be the initial thickness and the stacked piece was rolled again with the same reduction ratio. This procedure was repeated five times so that an effective strain of 4 was accumulated into the alloy. By 5-passes rolling, the grain size of ∼0.4 μm was obtained when the grain size was measured on the rolling plane and a remarkable enhancement in hardness was achieved, over twice than that before rolling. The microstructural examination revealed that, at relatively low strain, fine dislocation cells were formed by an operation of multi-directional slip. With increasing strain, dislocation cells were developed into ultrafine subgrains. At large strains, the microstructural change was dominated by a conversion of low-angled subboundaries to high-angled boundaries, rather than grain refinement. The 1 h static annealing treatment was carried out at temperatures of 373–773 K in order to examine the thermal stability of ultrafine grained 6061 Al alloy. The present ultrafine grained 6061 Al was found to be thermally stable up to 473 K. The microstructural change of 6061 Al alloy during accumulative roll bonding was compared with that observed in ultrafine grained Al alloys fabricated by the equal channel angular pressing technique which is another representative technique for fabricating ultrafine grained bulk materials. In addition, thermal stability of ultrafine grained 6061 Al alloy was discussed in terms of the grain growth kinetics.

High grain size stability of nanocrystalline Al prepared by mechanical attrition

Abstract: Grain growth in nanocrystalline (nc) Al with a grain size of 26 nm produced by cryogenic mechanical milling was studied through x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. Grain growth kinetics resembled those of ball-milled nc Fe. For homologous temperatures (T/TM) of 0.51–0.83, the time exponent n from D1/n − D01/n = kt was 0.04–0.28, tending toward 0.5 as T/TM increased. Two grain-growth regimes were distinguished: below T/TM = 0.78 growth ceased at an approximate grain size of 50 nm while at higher temperatures, grain growth proceeded steadily to the submicrometer range. Grain growth over the range of temperatures studied cannot be explained in terms of a single thermally activated rate process. The observed high grain size stability was attributed primarily to impurity pinning drag associated with the grain growth process.

Sintering features of cemented carbides WC–Co processed from fine powders

Abstract: Since the last 30 years, the cemented carbides WC–Co are processed from finer and finer powders. Both densification behaviour and microstructure evolution along the liquid-phase sintering (LPS) are different for materials prepared from powders of micronic or submicronic size. The typical features induced by the use of submicronic particles – large contribution of the solid-state densification, abnormal growth – are presented. The major questions, related to the understanding of densification hindrance and grain growth inhibition, are listed.

Microstructure development in calcium hexaluminate

Abstract: Calcium hexaluminate (CA6) was prepared from alumina and calcium carbonate powders. The influence of processing method and firing temperature on calcium hexaluminate grain morphology was studied. A significant correlation was found between grain morphology and green density, porosity distribution and presence of agglomerates. Platelet grains were observed for low green densities and large pores, while more equiaxed grains were found when green density was increased. A model is proposed for the formation of equiaxed or platelet grains. The model is based on the number of contact areas between alumina and calcium carbonate grains in green specimens as well as on the free space available for calcium hexaluminate to grow.

High-temperature effects in the fracture mechanical behaviour of silicon carbide liquid-phase sintered with AlN–Y2O3 additives

Abstract: It has been shown that pressureless sintering of SiC to theoretical density is possible with sintering additives from the system AlN–Y2O3. While commonly a combination of oxides is used such as Al2O3–Y2O3 (–SiO2), the oxynitride additives offer the advantage that only a nitrogen atmosphere is required instead of a powder bed for thermochemical stabilisation at the sintering temperature. The thermal decomposition of AlN is suppressed quite effectively when a moderate nitrogen overpressure is applied, resulting in very small mass loss during densification which only depends on the oxygen content of the SiC starting powder. By varying the mass ratio of β-SiC to α-SiC and applying dedicated post-densification heat treatments, a platelet-strengthened microstructure is obtained which shows enhanced fracture toughness. The platelet formation is attributed to a solution / precipitation process with simultaneous phase transformation from β-SiC to α-SiC, followed by anisotropic grain growth of α-SiC. In the present work, recent progress in the mechanical properties of these materials is reported. By means of a simple surface treatment-annealing in air — it is possible to obtain four-point bending strengths in excess of 1 GPa in liquid phase sintered SiC. The strength retention at temperatures around 1200°C is significantly improved.

Effects of heating rate on densification and grain growth during field-assisted sintering of α-Al2O3 and MoSi2 powders

Abstract: Two types of powders, electrically conductive MoSi2 and insulating α-Al2O3, were sintered by a field-assisted sintering technique (FAST) using heating rates from 50 °C to 700 °C/min. The Al2O3 powders were sintered to 99 pct density at 1100 °C for 2 minutes under 45 Mpa pressure. For Al2O3, no exaggerated grain growth was observed and the final grain size inversely scaled with the heating rate. Such a grain growth behavior fits the literature models based on multiple transport mechanisms for constant-heating-rate sintering. The density reached by MoSi2 under similar sintering conditions was 91 pct. The grain size was independent of the heating-rate value. Specific electrical field and pressure effects are shown to contribute to enhanced densification and minimal coarsening in each material.