Showing papers on "Grain growth published in 2000"

Sintering dense nanocrystalline ceramics without final-stage grain growth

Abstract: Sintering is the process whereby interparticle pores in a granular material are eliminated by atomic diffusion driven by capillary forces. It is the preferred manufacturing method for industrial ceramics. The observation of Burke and Coble that certain crystalline granular solids could gain full density and translucency by solid-state sintering was an important milestone for modern technical ceramics. But these final-stage sintering processes are always accompanied by rapid grain growth, because the capillary driving forces for sintering (involving surfaces) and grain growth (involving grain boundaries) are comparable in magnitude, both being proportional to the reciprocal grain size. This has greatly hampered efforts to produce dense materials with nanometre-scale structure (grain size less than 100 nm), leading many researchers to resort to the 'brute force' approach of high-pressure consolidation at elevated temperatures. Here we show that fully dense cubic Y2O3 (melting point, 2,439 degrees C) with a grain size of 60 nm can be prepared by a simple two-step sintering method, at temperatures of about 1,000 degrees C without applied pressure. The suppression of the final-stage grain growth is achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. Such a process should facilitate the cost-effective preparation of other nanocrystalline materials for practical applications.

An experimental study of the recrystallization mechanism during hot deformation of aluminium

Abstract: Discontinuous dynamic recrystallization (involving nucleation and grain growth) is rarely observed in metals with high stacking fault energies, such as aluminium. In this metal, two other types of recrystallization have been observed: continuous dynamic recrystallization (CDRX, i.e. the transformation of subgrains into grains); and geometric dynamic recrystallization (due to the evolution of the initial grains). The main purpose of this work was to bring clearly into evidence and to better characterize CDRX. Uniaxial compression tests were carried out at 0.7 T m and 10 −2 s −1 on three types of polycrystalline aluminium: a pure aluminium (1199), a commercial purity aluminium (1200) and an Al-2.5wt.%Mg alloy (5052), and also on single crystals of pure aluminium. In addition, 1200 aluminium specimens were strained in torsion. The deformed microstructures were investigated at various strains using X-ray diffraction, optical microscopy, scanning electron microscopy and electron back-scattered diffraction. Observations of the single crystalline samples confirm that subgrain boundaries can effectively transform into grain boundaries, especially when the initial orientation is unstable. In the case of polycrystalline specimens, after separating the effects of the initial and new grain boundaries, it turns out that CDRX operates faster in the 1200 aluminium compared to the two other grades. Moreover, it appears that the strain path does not alter noticeably the CDRX kinetics.

Hot working of commercial Ti–6Al–4V with an equiaxed α–β microstructure: materials modeling considerations

Abstract: The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s−1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol−1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol−1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s−1 and these are manifested as adiabatic shear bands in the α–β regime.

Effects of electric current on solid state phase transformations in metals

Abstract: The influence of an electric current on the following solid state transformations in metals are considered: (1) intermetallic compound formation and growth in diffusion couples, (2) precipitation, (3) crystallization of amorphous alloys and (4) recrystallization and grain growth of cold worked metals. The formation and growth of intermetallic compounds were in qualitative accord with electromigration theory. Regarding precipitation, an electric current can either enhance or retard the precipitation rate, depending on the alloy, the current density and its frequency. Important factors appear to be the effect of current on the quenched-in vacancies and the presence of an internal stress. Both a continuous d.c. current and high current density electropulsing enhanced the crystallization rate of amorphous alloys. The effects are greater than can be explained by simple electromigration theory and suggest the cooperative motion of a larger number of atoms. Electropulsing enhanced the recrystallization rate of cold worked metals, but retarded subsequent grain growth. Enhancement of the recrystallization rate resulted mainly from an increase in the pre-exponential factor of the Arrhenius rate equation, which is considered to refer to the nucleation rate. Retardation of subsequent grain growth resulted from a lower residual dislocation density within the newly-formed grains.

Electroplasticity in metals and ceramics

Abstract: The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75TM and on the superplasticity of fine-grained oxides (MgO, Al2O3 and ZrO2) at T>0.5TM are considered. Regarding NaCl, at T≤0.5TM an electric field E≥10 kV cm−1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm−1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5TM, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5TM, an internal electric field E≤0.3 kV cm−1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.

Effect of particle size and dopant on properties of SnO2-based gas sensors

Abstract: The effect of composition, microstructure, and defect chemistry on sensing performance of gas sensors based on CuO-doped SnO2 is investigated using sol–gel derived nano-sized powders (about 20 nm). The particle size of copper oxide doped tin oxide is varied by annealing at different temperatures and a significant grain growth is observed at temperatures above 1000°C due to the liquid phase sintering effect of copper oxide. The reduction of particle size to nanometers, or to the dimension comparable to the thickness of charge depletion layer, leads to a dramatic improvement in sensitivity and speed of response. It appears that the substitution of Sn by Cu in the cassiterite structure increases the concentration of oxygen vacancies and decreases the concentration of free electrons. In particular, the existence of cuprous ions (Cu+), due to partial reduction of Cu2+ during sintering, plays an important role in enhancing the sensor response to nitric oxide (NO) and CO2.

The effect of substrate temperature and biasing on the mechanical properties and structure of sputtered titanium nitride thin films

Abstract: The mechanical properties of titanium nitride (TiNx) thin films have been investigated using depth sensing nanoindentation tests The effects of substrate temperature (Ts) and of substrate biasing (Vb) on the mechanical properties and the microstructure of the TiNx films were studied Ts and Vb have strong effect on the film's microstructural characteristics such as density, grain size and orientation It was found that deposition at high Ts and Vb promotes the growth of (002) oriented films with density close to the bulk density of stoichiometric TiN, indicating the absence of voids and the growth of stoichiometric TiN The film hardness and elastic modulus were measured using the continuous stiffness measurements technique It was found that there exists a direct correlation between the film's mechanical properties and microstructure The films that exhibit the best mechanical performance are those grown along the (002) orientation and being denser and stoichiometric

Evolution of near-equiaxed microstructure in the semisolid state

Abstract: The microstructure of alloys with a near-equiaxed microstructure, produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process, as it evolves within short times in the semisolid state, is examined by rapid quenching and isothermal soaking experiments. Quenching experiments reveal the morphology and distribution of solid phase at high and medium volume fractions of liquid. At medium liquid content, the microstructure of spray-cast and SIMA alloys consists of discrete equiaxed grains uniformly dispersed in the liquid phase, while the corresponding microstructure of MHD-cast alloys exhibits extensive agglomerates consisting of incompletely spheroidized grains. The connectivity of solid phase and the formation of a solid skeleton in the semisolid state are discussed in terms of grain misorientation. Isothermal soaking experiments investigated grain growth and degree of spheroidization as a function of soaking time and liquid content in the semisolid state. Results demonstrated that MHD-cast microstructures are less equiaxed compared with SIMA and spray-cast alloys even after 5 min of soaking in the semisolid state. It is also shown that the grain growth rate is smaller in spray-cast alloys than in SIMA alloys. The role of coalescence and the effects of alloying elements are also discussed.

Analogous experiments on the stickiness of micron-sized preplanetary dust

Abstract: In the early solar nebula, the formation of planetesimals and cometesimals is believed to be due to inelastic collisions of initially micron-sized grains. The collisions are caused by relative velocities due to size-dependent interactions with the surrounding dilute gas. The grain growth process is determined by the velocity-dependent sticking efficiency upon collisions. Therefore, we performed experiments with eight samples of micron-sized particles consisting of monodisperse silica spheres, of irregularly shaped diamond, enstatite, and silicon carbide grains, and of silicon carbide whiskers. We determined the sticking probability and the energy loss upon bouncing collisions by studying individual grain-target collisions in vacuum. We found a sticking probability higher than predicted by previous theoretical work. Grain size, roughness, and primarily grain shape, i.e., the difference of spherical versus irregular grain shape, is important for the collisional behavior, whereas the material properties are rather unimportant. Our results indicate that the preplanetary dust aggregation is more effective than previously thought.

Influence of pressing temperature on microstructural development in equal-channel angular pressing

Abstract: Experiments were conducted to evaluate the influence of the pressing temperature when materials are processed by equal-channel angular pressing. The tests were conducted using samples of pure Al, an Al–3% Mg alloy and an Al–3% Mg–0.2% Sc alloy at temperatures up to 573 K. All samples were pressed to sufficiently high strains to give homogeneous microstructures. The results show two important effects. First, the measured grain size tends to increase with increasing pressing temperature. Second, grain refinement is achieved at all temperatures but there is a transition from arrays of high angle boundaries at the lower temperatures to low angle boundaries at temperatures at and above 473 K in pure Al and at 573 K in the Al–3% Mg alloy, whereas the Al–3% Mg–0.2% Sc alloy exhibits high angle boundaries at all pressing temperatures. The inability to achieve high angle boundaries at the higher pressing temperatures in these two materials is attributed to the higher rates of recovery which lead to dislocation annihilation rather than the absorption of dislocations into the subgrain walls.

Grain-growth kinetics of nanocrystalline iron studied in situ by synchrotron real-time X-ray diffraction

Abstract: Pulsed electrodeposition (PED) is used to prepare nanocrystalline iron with an average grain size of 19 nm and thermal stability up to 550 K. At 663 K ≤ T ≤ 783 K the kinetics of grain growth, with respect to size and size distribution, is studied in situ by means of real-time synchrotron X-ray diffraction. The Bragg peak line shapes of the large number of diffractograms are analyzed using a Warren/Averbach procedure improved with respect to reliability and efficiency. We observe two regimes of grain growth: at less elevated temperatures grain growth is smooth and moderate up to limiting size values between 50 and 100 nm, depending on temperature. The initially rather narrow width of the size distribution increases slightly, and the activation energy of grain growth, about 100 kJ/mol, corresponds to the literature value for grain boundary self-diffusion in nanocrystalline Fe. At higher temperatures the grains grow first rapidly and then slowly up to limiting values between 200 and 400 nm, depending on te...

Thermal stability and mechanical properties of ultrafine grained low carbon steel

Abstract: Ultrafine grained low carbon steel manufactured by equal channel angular pressing was annealed at 753 K, where negligible grain growth occurred, up to 72 h and the microstructural change and the mechanical properties were examined. This investigation was aimed at providing the guiding information for the effective use of ultrafine grained low carbon steel manufactured by severe plastic deformation processes. Under the present annealing conditions, the microstructural change was dominated by recovery. The tensile behavior of annealed ultrafine grained steel was characterized by much higher strength and the absence of strain hardening compared with that of large grained steel. In addition, the present ultrafine grained steel became mechanically stable by 24 h annealing treatment although recovery was in progress. The microstructure of the deformed sample of annealed ultrafine grained steel exhibited the elongated grains and dislocations distributed densely in the vicinity of grain boundaries. This finding indicated that dynamic recovery during deformation was associated with the absorption of dislocation by grain boundaries. The mechanical behavior of the present ultrafine grained low carbon steel was discussed in light of the recent development explaining that of nanocrystalline materials, i.e. the dislocation bow-out mechanism for high strength and the spreading kinetics of trapped lattice dislocation into grain boundary for the absence of strain hardening.

Behaviour of normal grain growth kinetics in single phase titanium and titanium alloys

Abstract: The grain growth kinetics of commercially pure Titanium, Ti–0.2Pd in the α and β phases, Ti–6Al–4V and Ni–Ti in the β phase have been studied. The perimeter, diameter and area as grain size parameters have been measured by means of the image analysis technique for different heat treatment temperatures and times. The growth exponents and activation energies have also been determined. The influence of the grain size on the mechanical properties have been evaluated and for the Ni–Ti shape memory alloy, the influence of the grain growth on the transformation temperatures, thermodynamic magnitudes and on the transformation stresses has been determined.

Unidirectionally Textured CaBi4Ti4O15 Ceramics by the Reactive Templated Grain Growth with an Extrusion.

Abstract: Dense CaBi4Ti4O15 (CBT) ceramics with a unique texture were prepared by the reactive templated grain growth (RTGG) method using an extrusion technique. Plate-like Bi4Ti3O12 (BIT) particles, a reactive template for CBT, were mixed with other oxide and carbonate powders and unidirectionally aligned by extrusion. During sintering, oriented CBT grains were formed in situ topotaxially on the oriented BIT particles, and the templated grain growth and their densification eventually fabricated textured CBT ceramics. X-ray diffraction measurements and scanning electron microscopic observations revealed that the plate-like CBT grains were unidirectionally oriented parallel to the extruding direction without uniaxial alignment. Textured CBT ceramics poled in the extruding direction exhibited electromechanical coupling coefficients (k33) and piezoelectric coefficients (d33 and g33) more than two times as large as one of nontextured ceramics with the same composition.

Microstructural modelling of aluminium alloys during thermomechanical processing

Abstract: It is well-accepted that modelling of microstructural evolution of aluminium alloys during thermomechanical processing is highly desirable to predict product properties and/or to design process variables based on requirements for the properties. To do so, having soundly based physical models is of interest for both academic research and industrial practice. In the present paper, models for predicting the evolution of internal state variables such as internal dislocation density, subgrain size and misorientation between subgrains, and subsequent recrystallisation behaviour are developed for both constant and transient deformation conditions. In predicting the evolution of the internal state variables under transient deformation conditions, the internal ‘geometrically necessary’ dislocation density is related to the subgrain boundary dislocation density. In the model to predict static recrystallisation behaviour, nucleation of recrystallisation is initially discussed based on experimental results and quantitative metallographic observations. In the calculation of recrystallisation nucleation density, distribution of subgrain size and misorientation between subgrains are key parameters. The predicted evolution of internal state variables and subsequent recrystallisation kinetics and recrystallised grain size using the developed model are in reasonable agreement with experimental data.

Process modelling of grain selection during the solidification of single crystal superalloy castings

Abstract: A process model is described for the grain selection occurring during the solidification of single crystal investment castings, which are now used widely for a number of critical applications in gas turbine engines. The basis of the model is a thermal analysis of the heat transfer in the vicinity of the chill region onto which the molten metal is poured. Subsequently the competitive growth of grains during directional solidification is simulated via a cellular-automaton technique. For the purpose of model validation, processing trials have been carried out on a commercial single crystal casting furnace. The thermal cycles set up in and around the vicinity of the grain selector have been measured, and these are used to choose a number of critical parameters in the thermal model. The evolution of grain structure during competitive growth has been characterised using a number of analytical techniques, including orientation imaging microscopy. The results are compared critically with the predictions from the model. It is shown that the model is able to reproduce the statistical distribution describing the final casting orientation, measured with respect to the 〈001〉 crystallographic pole. The model is used to study the geometrical factors influencing competitive growth and the efficacy of two designs of grain selector, and in particular the conferral of any control of the secondary 〈001〉 orientation.

Triticale grain growth and morphometry as affected by drought stress, late sowing and simulated drought stress

Abstract: The effects on grain filling and morphometry of natural drought, late sowing and simulated drought by means of a chemical treatment with potassium iodide (KI) were compared over 3 years of field trials in triticale ( ¥ Triticosecale Wittmack) cv. Trujillo and three near-isogenic lines derived from it. Grain weight data fitted accurate to a logistic curve. The maximum rate of grain filling was the curve coefficient most sensitive to drought stress, and accounted for 7–50% of grain yield variation. Chemical treatment with KI caused greater variation in grain filling curve coefficients and grain morphometry than did a delay in the sowing date, which in turn caused greater variation than natural drought. The type and magnitude of the effects of the different kinds of stresses on grain growth and morphometry could be related to the time that elapsed from anthesis, at which time the effects were perceptible. KI reduced the maximum rate of grain filling and final grain weight by 38 and 32%, respectively, its effect being significant from 8 d after the treatment. The volume of grain was reduced 11% by KI. The impact of delayed sowing date and drought were significant 29 and 33 d after anthesis, respectively, corresponding to the end of the linear phase of the grain filling curves. Both treatments diminished grain filling duration (13% by delayed sowing, and 6% by drought, respectively), final grain weight (16 and 12%, respectively), grain volume (15% and 8%, respectively), and embryo area (8% in both cases), but neither altered the maximum grain filling rate.

Effect of grain size on tensile behaviour of a submicron grained Al–3 wt-%Mg alloy produced by severe deformation

Abstract: Equal channel angular extrusion has been used to deform an Al–3 wt-%Mg alloy to an effective strain of 10, resulting in a 0.2 µm grain size. In the as deformed condition the yield strength was incr...

High-temperature properties of nanocomposite TiBxNy and TiBxCy coatings

Abstract: High-temperature investigations on nanocomposite TiB x N y and TiB x C y coatings were performed to determine their thermal stability. All coatings investigated were prepared by means of unbalanced DC magnetron co-sputtering using either a segmented TiN/TiB 2 or TiC/TiB 2 target. Recovery and recrystallization behavior were characterized by means of stress measurements during thermal annealing and differential scanning calorimetry (DSC). The nanocomposite coatings used for DSC measurements were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD of these coatings showed a pronounced growth of the individual phases after the DSC measurements. Therefore, the total enthalpy change during DSC can be attributed to grain growth of the nanocrystalline phases. The grain sizes after the DSC treatment as analyzed by XRD and SEM are in excellent agreement with the grain sizes calculated from the exothermic peaks of the DSC measurements. Grain growth occurred for the individual phases in TiB 1.2 N 0.5 and TiB 1.2 C 0.6 coatings during heating up to 1400°C from approximately 4 to 15 nm and 4 to 5 nm, respectively. Recovery effects of the coatings investigated are strongly influenced by the ion bombardment during film growth as well as by the chemical composition of the coatings.

Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering

Abstract: SPS (spark plasma sintering) process was used to sinter nanocrystalline ZrO 2 (3Y). It was found to be different with the usual rapid sintering method, the density of the samples kept increasing with the rising of the sintering temperature. A higher density could be reached at a lower temperature and shorter dwelling time than that by hot-pressing under the similar pressures. In contrast to the samples with a differential densification from edge to center prepared by a rapid hot-pressing, no obvious densification gradient could be found in the samples sintered by SPS. The grain sizes of the Y-TZP obtained by SPS were smaller than those by the pressureless sintering method, while the grain growth speed was much higher under SPS conditions. All these unique sintering behaviors were explained by the special sintering process of SPS.