Showing papers on "Grain growth published in 2004"

Templated Grain Growth of Textured Piezoelectric Ceramics

Abstract: Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6, and (N...

Semiconductor to metal phase transition in the nucleation and growth of VO2 nanoparticles and thin films

Abstract: The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.

Processing and Mechanical Properties of Ti3SiC2: I, Reaction Path and Microstructure Evolution

Abstract: In this article, the first part of a two-part study, we report the reaction path and microstructure evolution during the reactive hot isostatic pressing of Ti3SiC2, starting with titanium, SiC, and graphite powders. A series of interrupted hot isostatic press runs have been conducted as a function of temperature (1200°–1600°C) and time (0–24 h). Based on X-ray diffractometry and scanning electron microscopy, at 1200°C, the intermediate phases are TiCx and Ti5Si3Cx. Fully dense, essentially single-phase samples are fabricated in the 1450°–1700°C temperature range. The time-temperature processing envelope for fabricating microstructures with small (3–5 μm), large (∼200 μm), and duplex grains, in which large (100–200 μm) Ti3SiC2 grains are embedded in a much finer matrix, is delineated. The microstructure evolution is, to a large extent, determined by (i) the presence of unreacted phases, mainly TiCx, which inhibits grain growth; (ii) a large anisotropy in growth rates along the c and a directions (at 1450°C, growth normal to the basal planes is about an order of magnitude smaller than that parallel to these planes; at 1600°C, the ratio is 4); and (iii) the impingement of grains. Ti3SiC2 is thermally stable under vacuum and argon atmosphere at temperatures as high as 1600°C for as long as 24 h. The influence of grain size on the mechanical properties is discussed in the second part of this study.

Phase transformation and growth of mullite in kaolin ceramics

Abstract: The phase transformation and growth of mullite (3Al 2 O 3 ·2SiO 2 ) in kaolin ceramics have been investigated using X-ray diffractometer, transmission electron microscope, select area electron diffractometer, energy dispersion spectrometer and differential thermal analysis. The mullite which was transformed from kaolin appears at 1050 °C by XRD and tallies with DTA. The initial mullite crystal showed a plate-like morphology. The Al 2 O 3 content in mullite crystal increased from 49.57 to 71.37 wt.% but the lattice parameters of a , b and c axes decreased from 8.085, 8.106 and 3.215 A to 7.882, 7.974 and 2.946 A, respectively, with the grain width increasing from 20 to 70 nm when the kaolin was sintered at 1300 °C for 30 min. The nonisothermal activation energy of mullite crystallization in kaolin ceramics was 1182.3 kJ mol −1 . The growth morphology parameters n and m were both about 2.0, indicating that the bulk nucleation was dominant in mullitization and the crystal growth was controlled by diffusion. Seemingly, this study has been attempted to provide an integrative presentation of the thermal–structural characterization together with detailed kinetic and mechanistic interpretations.

The Distribution of Internal Interfaces in Polycrystals

Abstract: Recent advances both in experimental instrumentation and computing power have made it possible to interrogate the distribution of internal interfaces in polycrystals and the three dimensional structure of the grain boundary network with an unprecedented level of detail. The purpose of this paper is to review techniques that can be used to study the mesoscopic crystallographic structure of grain boundary networks and to summarize current findings. Recent studies have shown that grain surfaces within dense polycrystals favor the same low energy planes that are found on equilibrium crystal shapes and growth forms of crystals in contact with another phase. In the materials for which comprehensive data exists, the distribution of grain boundaries is inversely correlated to the sum of the energies of the surfaces of the grains on either side of the boundary.

The influence of time, temperature, and grain size on indentation creep in high-purity nanocrystalline and ultrafine grain copper

Abstract: Microhardness measurements have been carried out on high purity Cu samples with average grain sizes ranging from ∼10 to ∼200nm, over temperatures from liquid nitrogen to ambient, and dwell-times of the indenter in the sample from 5 s to 39 h. The Vickers hardness diminishes approximately linearly with the logarithm of the dwell-time. At short dwell-times the hardness increases significantly with decreasing grain size and with decreasing temperature, but the influence of these variables substantially diminishes at longer times. Investigation by transmission electron microscopy shows that rapid grain growth under the indenter most likely is responsible for the strong and long-lasting indentation creep.

Microstructural instability and strength of an AZ31 Mg alloy after severe plastic deformation

Abstract: Equal channel angular pressing (ECAP) technique, which involves a simple large shear deformation during passage through two intersecting channels, was applied to the AZ31 Mg alloy to make an ultrafine-grained microstructure. ECAP temperature was controlled to decrease with pass number to maximize the grain refinement efficiency with preventing cracking. The first and second pressings were conducted at 593 K, while the third and fourth pressings were conducted at 523 and 473 K, respectively. The degree of grain refinement and homogeneity of grain-size distribution increased with pass number. After four passes, the reasonably homogeneous microstructure composed of fine and equiaxed grains was obtained. The stability of the ECAPed structure at elevated temperatures was examined by annealing the four-passed materials over a wide range of temperature between 473 and 773 K. Measurement of activation energies for static grain growth shows the presence of three different values depending on the temperature range investigated: Q = 0.78 Q gb (activation for grain boundary diffusion) in the low temperature range 473–523 K, Q = 0.27 Q gb in the intermediate temperature range 523–673 K and Q = 0.84 Q L (activation for lattice diffusion) or Q = 1.23 Q gb in the high temperature range 673–773 K. The abnormally low Q value in the intermediate temperature range may not represent the true activation energy. Progressive decrease in dislocation density by enhanced recovery with increasing temperature may be the cause of the result. After ECAP, the yield stresses (YS) of the ECAPed AZ31 alloys decreased while their elongations increased. Enlarged strain hardening exponent after ECAP is the key factor considered to bring in the tensile-elongation increase, while modification of texture for easier slip on basal planes during ECAP is believed to be responsible for the yield stress decrease.

Microstructural evolution of a Ti–6Al–4V alloy during β-phase processing: experimental and simulative investigations

Abstract: The microstructural evolution of a Ti–6Al–4V alloy during thermomechanical processing in the β-phase field was investigated using both experimental and modelling methods. The experimental results show that dynamic and/or metadynamic recrystallization occurred when the alloy was processed in the β-phase field. A model embedding fundamental metallurgical principles of dynamic recrystallization (DRX) within the cellular automaton (CA) method was able to simulate quantitatively and topographically the microstructural evolution and the flow stress–strain relationship during the thermomechanical processing. In the simulation, the dislocation density variation and the grain growth kinetics of each dynamically recrystallizing grain (R-grain) was calculated on the physical model of DRX, and the plastic flow curve was calculated directly from the dislocation density variation of the matrix grains and the R-grains. The equiaxed growth of R-grains was simulated using the CA method. The influence of strain rate and temperature on the microstructural evolution and the flow stress during dynamic recrystallization was studied, and the results compared with experiments.

Fabrication and Electrical Properties of Textured Sr0.53Ba0.47Nb2O6 Ceramics by Templated Grain Growth

Abstract: Textured Sr0.53Ba0.47Nb2O6 ceramics with a relative density of >95% were fabricated using templated grain growth (TGG). Acicular KSr2Nb5O15 template particles synthesized via a molten salt process were aligned by tape casting in a mixture of solid-state-synthesized SrNb2O6 and BaNb2O6 powders. The resulting ceramics possessed strong fiber texture along the polar axis ([001]) of the strontium barium niobate. Samples with 15.4 wt% templates attained a textured fraction of 0.82 after sintering at a temperature of 1450°C for 4 h. These materials showed peak dielectric constants of 7550 at 1 kHz, remanent polarizations of 13.2 μC/cm2, saturation polarizations of 21 μC/cm2 (60%–85% of the single-crystal value), piezoelectric strain coefficients of 78 pC/N (70%–85% of the single-crystal value), and room-temperature pyroelectric coefficients of 2.9 × 10−2μC·(cm2·°C)−1 (52% of the single-crystal value). These results show that TGG is a viable option for accessing single-crystal properties in polycrystalline ceramics.

Consolidation and properties of binderless sub-micron tungsten carbide by field-activated sintering

Abstract: The rapid sintering of nano-structured WC hard materials in a short time is introduced with a focus on the manufacturing potential of this spark plasma sintering process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. A dense pure WC hard material with a relative density of up to 97.6% was produced with simultaneous application of 60 MPa pressure and electric current of 2800 A within 2 min. A larger current caused a higher rate of temperature increase and therefore a higher densification rate of the WC powder. The finer the initial WC powder size the higher is the density and the better are the mechanical properties. The fracture toughness and hardness values obtained were 6.6 MPa m1/2 and 2480 kg/mm2, respectively under 60 MPa pressure and 2800 A using 0.4 μm WC powder.

Effect of Silicon Substitution on the Sintering and Microstructure of Hydroxyapatite

Abstract: The substitution of between 0 and 1.6 wt% silicon (Si-HA) in hydroxyapatite (HA) inhibited densification at low temperatures (1000°–1150°C), with these effects being more significant as the level of silicon substitution was increased. For higher sintering temperatures (1200°–1300°C), the sintered densities of HA and Si-HA compositions were comparable. Examination of the ceramic microstructures by scanning electron microscopy (SEM) showed that silicon substitution also inhibited grain growth at higher sintering temperatures (1200°–1300°C). The negative effect of silicon substitution on the sintering of HA at low temperatures (1000°–1150°C) was reflected in the hardness values of the ceramics. However, for higher sintering temperatures, e.g., 1300°C, where sintered densities were comparable, the hardness values of Si-HA compositions were equal to or greater than that of HA, reflecting the smaller grain sizes observed for the former.

Effects of SiO2, CaO2, and MgO Additions on the Grain Growth of Alumina

Abstract: When pure alumina is sintered at 1620°C, normal grain growth occurs with equiaxial grains and curved grain boundaries. When 100 ppm of SiO 2 together with 50 ppm of CaO is added, abnormal grain growth (AGG) occurs with large grains elongated with straight grain-boundary segments in the direction of the basal planes. Some of the fine matrix grains also have straight grain boundaries, and ∼10% of the grain boundaries of the matrix grains are faceted when observed by transmission electron microscopy (TEM). Some of these grain boundaries are expected to be singular with low-energy structures corresponding to the cusps in the polar plot of the grain-boundary energy against the inclination angle. No frozen liquid is found at the grain triple junctions and grain boundaries by TEM. When 600 ppm of MgO is added together with 100 ppm of SiO 2 and 50 ppm of CaO normal growth occurs. The grain boundaries are curved when observed via optical microscopy and TEM and show that all the grain boundaries are defaceted, indicating that they become atomically rough. When sintered at 1900°C after adding 150, 300, or 500 ppm of SiO 2 , AGG occurs with straight and faceted grain boundaries, similar to the specimens sintered at 1620°C after CaO and SiO 2 are added. When MgO is added together with SiO 2 and sintered at 1900°C, normal grain growth occurs with rough grain boundaries. High-resolution TEM observation shows no frozen liquid layer at a grain boundary. The results indicate that the occurrence of AGG in alumina with SiO 2 or together with CaO is correlated with the formation of faceted and straight (on large and atomic scales) grain boundaries. It is proposed that these grain boundaries have singular ordered structures with low boundary energies and their growth by lateral step movement can cause the AGG. The addition of MgO causes grain-boundary roughening and, thus, normal grain growth. The grain boundaries in pure alumina also appear to be rough, and, hence, normal grain growth occurs.

A generalized self-consistent polycrystal model for the yield strength of nanocrystalline materials

Abstract: Inspired by recent molecular dynamic simulations of nanocrystalline solids, a generalized self-consistent polycrystal model is proposed to study the transition of yield strength of polycrystalline metals as the grain size decreases from the traditional coarse grain to the nanometer scale. These atomic simulations revealed that a significant portion of atoms resides in the grain boundaries and the plastic flow of the grain-boundary region is responsible for the unique characteristics displayed by such materials. The proposed model takes each oriented grain and its immediate grain boundary to form a pair, which in turn is embedded in the infinite effective medium with a property representing the orientational average of all these pairs. We make use of the linear comparison composite to determine the nonlinear behavior of the nanocrystalline polycrystal through the concept of secant moduli. To this end an auxiliary problem of Christensen and Lo (J. Mech. Phys. Solids 27 (1979) 315) superimposed on the eigenstrain field of Luo and Weng (Mech. Mater. 6 (1987) 347) is first considered, and then the nonlinear elastoplastic polycrystal problem is addressed. The plastic flow of each grain is calculated from its crystallographic slips, but the plastic behavior of the grain-boundary phase is modeled as that of an amorphous material. The calculated yield stress for Cu is found to follow the classic Hall–Petch relation initially, but as the gain size decreases it begins to depart from it. The yield strength eventually attains a maximum at a critical grain size and then the Hall–Petch slope turns negative in the nano-range. It is also found that, when the Hall–Petch relation is observed, the plastic behavior of the polycrystal is governed by crystallographic slips in the grains, but when the slope is negative it is governed by the grain boundaries. During the transition both grains and grain boundaries contribute competitively.

Grain-boundary character and grain growth in bulk tin and bulk lead-free solder alloys

Abstract: Grain-boundary deformation is the primary failure mode observed in solder joints. Understanding the effects of alloy composition variations and cooling rates on microstructural stability and deformation processes will allow development of improved joints. The effects of these variables on grain-boundary character were investigated in a pure-tin ingot and a reflowed sample; ingots of Sn-3.5wt.%Ag and Sn-3.8wt.%Ag-0.7wt.%Cu; and solder balls with 1.63-wt.% or 3-wt.%Ag. The microstructure was characterized using orientation imaging microscopy (OIM). After aging (150°C for 200 h), the fine-grained polycrystalline microstructure in both pure-tin specimens grew considerably, revealing preferred misorientations and ledge formation at grain boundaries. Aging of the alloy ingots showed only slight grain growth caused by precipitate pinning. The solder balls showed similar phenomena. The role of alloying elements, cooling rate, and the anisotropy of the coefficient of thermal expansion (CTE) in tin on microstructural evolution, grain-boundary character, and properties of solder joints are discussed.

Thermal Conductivity of ß-Si3N4: I, Effects of Various Microstructural Factors

Abstract: Calculations based on a simple modified Wiener's model for thermal conductivity of a composite material predict that the thermal conductivity of β-Si 3 N 4 decreases quickly as the grain-boundary film thickness increases within a range of a few tenths of a nanometer and also that it initially increases steeply with increased grain size, then reaches almost constant values. Because of the faceted nature of the β-Si 3 N 4 crystal, the average grain-boundary film thickness is much greater than that in equilibrium. The present study demonstrates both theoretically and experimentally that grain growth alone cannot improve the thermal conductivity of β-Si 3 N 4 .

Densification and grain growth in pulse electric current sintering of alumina

Abstract: Pulse electric current sintering (PECS) method was used to sinter an ultrafine and high purity α-Al2O3 powder. The powder compact was either slowly heated (50 °C/min) or rapidly heated (300 °C/min) to temperatures ranging between 1000 and 1400 °C and then rapidly cooled down without holding. The densification and grain growth behaviors at different stages of the PECS process were investigated. At the earlier stages, fast heating greatly promoted the formation of necking between particles and enhanced both densification and grain growth; at the late stage, faster heating resulted in smaller grain sizes.

Sintering of nanophase WC–15vol.%Co hard metals by rapid sintering process

Abstract: A sintering method for the rapid sintering of a nanostructured hard metals in short time is introduced with a focus on the manufacturing potential of this novel high-frequency induction heated sintering (HFIHS) process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. In this work, we developed a new process of sintering for nanostructured WC–15vol.%Co hard metals. A highly dense WC–15vol.%Co with a relative density of up to 99.4% was produced with simultaneous application of 60 MPa pressure and induced current within 1 min. The average grain size was about 258 nm and mean free path was about 11.6 nm. The fracture toughness and hardness values obtained were 11.9 MPa m1/2 and 1992 HV30, respectively. Also, we produced dense WC–Co hard metals with a relative density of 97% without applying pressure.

Microstructural Evolution and Grain Boundary Structure During Static Recrystallization in Synthetic Polycrystals of Sodium Chloride Containing Saturated Brine

Abstract: The effects of brine on recrystallization in halite are well known. However, properties of brine such as morphology, connectivity, diffusivity and the resulting influences on deformation mechanisms are still a matter of debate. This paper presents a microstructural study of dense, statically recrystallizing synthetic polycrystalline halite containing small amounts of brine. We used powders of two different grain size classes: <10 µm and 200–355 µm. The aggregates were compacted to brine-filled porosities less than about 2% and annealed at room temperature, without an external stress field. Coarse-grained samples undergo recrystallization manifested by the growth of large (up to 300 µm) strain-free grains into the deformed old grains. The new grains are frequently euhedral, with mobile grain boundaries moving at rates up to 6 nm/s. Their mobility is interpreted to be high due to the presence of water. Grain surfaces are smooth and the width of the water-rich zones is usually below the resolution of the SEM (less than 50 nm). The evolution of fine-grained samples starts with primary recrystallization and a reorganization of grain boundaries. After this stage, which lasts a few hours, normal grain growth effectively stops, and no significant increase of grain size is observed even after several months. Microstructural observations indicate contact healing at the grain boundaries, with dihedral angles ranging between 20 and 110°. We interpret these boundaries to be fluid-free, with the brine residing in a network of triple junction tubes. This system of triple junctions is interconnected and associated with significant permeability. While grain growth is inhibited in the fine-grained samples, after a few hours of annealing exaggerated grain growth is commonly initiated. This is manifested by the growth of large, euhedral grains replacing the fine-grained matrix. These grains also grow with low-index facets and their boundaries are also interpreted to be mobile due to the existence of a water-rich phase.

Influence of Yttria–Alumina Content on Sintering Behavior and Microstructure of Silicon Nitride Ceramics

Abstract: The present study investigates the influence of the content of Y2O3–Al2O3 sintering additive on the sintering behavior and microstructure of Si3N4 ceramics. The Y2O3:Al2O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering-additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β-Si3N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction.