Showing papers on "Grain growth published in 2002"

Spark Plasma Sintering of Alumina

Abstract: A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al 2 O 3 powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al 2 O 3 could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21-22 GPa and fracture toughness values of 3.5 ± 0.5 MPa.m 1/2 were found for the compacts containing submicrometer-sized Al 2 O 3 grains.

Grain nucleation and growth during phase transformations.

Abstract: The mechanical properties of polycrystalline materials are largely determined by the kinetics of the phase transformations during the production process. Progress in x-ray diffraction instrumentation at synchrotron sources has created an opportunity to study the transformation kinetics at the level of individual grains. Our measurements show that the activation energy for grain nucleation is at least two orders of magnitude smaller than that predicted by thermodynamic models. The observed growth curves of the newly formed grains confirm the parabolic growth model but also show three fundamentally different types of growth. Insight into the grain nucleation and growth mechanisms during phase transformations contributes to the development of materials with optimal mechanical properties.

Spark plasma sintering as advanced PM sintering method

Abstract: A short overview of works on spark plasma sintering (SPS) is given in the present paper. SPS is a newly developed rapid sintering technique with a great potential for achieving fast densification results with minimal grain growth in a short sintering time. It is proven by obtained experimental data that enhanced sinterability of powders subjected to SPS mainly associated with particle surface activation and increased diffusion rates on the contact zones caused by applied pulse current. Application of rapid heating results in bypassing of low temperature regions where surface transport controlled sintering is dominant. This preserves the powder surface area to temperature levels where bulk transport is significant. However, the nature of activation effects, especially in its regards to acceleration of diffusion processes, is not clearly established. A lot of research work reports about the occurrence of plasma during the application of pulse current. However, the appearance of thermal plasma during...

Physical Origins of Intrinsic Stresses in Volmer–Weber Thin Films

Abstract: As-deposited thin films grown by vapor deposition often exhibit large intrinsic stresses that can lead to film failure. While this is an “old” materials problem, our understanding has only recently begun to evolve in a more sophisticated fashion. Sensitive real-time measurements of stress evolution during thin-film deposition reveal a generic compressive-tensile-compressive behavior that correlates with island nucleation and growth, island coalescence, and postcoalescence film growth. In this article, we review the fundamental mechanisms that can generate stresses during the growth of Volmer-Weber thin films. Compressive stresses in both discontinuous and continuous films are generated by surface-stress effects. Tensile stresses are created during island coalescence and grain growth. Compressive stresses can also result from the flux-driven incorporation of excess atoms within grain boundaries. While significant progress has been made in this field recently, further modeling and experimentation are needed to quantitatively sort out the importance of the different mechanisms to the overall behavior.

Young’s modulus and mechanical properties of Ti-29Nb-13Ta-4.6Zr in relation to α″ martensite

Abstract: An investigation on the formation of α″ martensite and its influence on Young’s modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported in this article. For ice-water-quenched specimens after solution treatment at 1023, 1123, and 1223 K in the single β-phase field for 1.8, 3.6, 14.4, and 28.8 ks, X-ray diffraction and internal friction measurements showed that the volume fraction of the α″ martensite changes with both solution temperature and time. This effect has been attributed mainly to the influence of grain size of the β-parent phase on the stability of the β phase and, consequently, on the martensitic start (Ms) temperature. A critical grain size of 40 µm was identified for the β phase, below which the martensitic transformation is largely suppressed because of low MS temperature. With the β grain size increasing above this critical value, the volume fraction of the α″ martensite increases significantly at first and then decreases gradually with further grain growth. The α″ martensite was shown to possess good ductility and, compared to the β phase, lower strength and hardness but nearly identical Young’s modulus in the studied alloy.

WC grain growth and grain growth inhibition in nickel and iron binder hardmetals

Abstract: WC grain growth and growth inhibition of an 0.6 μm FSSS WC powder (average SEM size: 0.35 μm) were studied in WC–10 wt% Ni alloys by adding 0–2 wt% of inhibitor carbides (VC, Cr 3 C 2 , TaC, TiC and ZrC). Alloy gross carbon content turned out to be a crucial factor for WC growth in Ni alloys, even with high inhibitor additions. Coarsening was more pronounced in high carbon alloys, compared with low carbon grades, resulting in a significantly lower hardness. VC proved to be by far the most effective grain growth inhibitor in WC–Ni hardmetals, followed by TaC, Cr 3 C 2 , TiC and ZrC. Hardness increased with increasing amount of additive but reached a maximum above which it remained about the same. Experiments on WC–Fe–(VC) alloys revealed that WC grain growth is strongly restricted in Fe-binder alloys, even without additions of growth inhibitors. Binder chemistry thus strongly influences both continuous and discontinuous WC grain growth. This chemistry is determined by the nature of the binder matrix (Fe, Co, Ni), the alloy gross carbon content (which determines the composition of the binder matrix) as well as the inhibitor additive.

Scaling behavior of grain-rotation-induced grain growth.

Abstract: Recent investigations of grain growth in nanocrystalline materials have revealed a new growth mechanism: grain-rotation-induced grain coalescence. Based on a simple model employing a stochastic theory and using computer simulations, here we investigate the coarsening of a polycrystalline microstructure due solely to the grain-rotation coalescence mechanism. Our study demonstrates that this mechanism exhibits power-law growth with a universal scaling exponent. The value of this universal growth exponent is shown to depend on the assumed mechanism by which the grain rotations are accommodated.

Synthesis, characterization and photocatalytic property of nano-sized Zn2SnO4

Abstract: Nano-sized Zn2SnO4 materials have been synthesized using the coprecipitation method. The synthetic conditions and the calcination behaviors of nano-sized Zn2SnO4 materials have been studied. The nano-sized Zn2SnO4 materials have been characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry and differential thermal analysis (TG-DTA) and specific surface area. As a result, the kinetic grain growth equation for nano-sized Zn2SnO4 can be expressed as D4.78 = 9.12 × 1023t exp(−40.6 × 103/T), with an activation energy for grain growth of Q = 337.9 KJ/mol. The nano-sized Zn2SnO4 materials have been used as photocatalysts to decompose benzene in water solution. The results show that Zn2SnO4 can photocatalytically decompose benzene, and the photocatalytic capacity for Zn2SnO4 relates to the grain size, which is discussed in terms of the surface effect and the quantum size effect.

Boundary Mobility and Energy Anisotropy Effects on Microstructural Evolution During Grain Growth

Abstract: We have performed mesoscopic simulations of microstructural evolution during curvature driven grain growth in two-dimensions using anisotropic grain boundary properties obtained from atomistic simulations. Molecular dynamics simulations were employed to determine the energies and mobilities of grain boundaries as a function of boundary misorientation. The mesoscopic simulations were performed both with the Monte Carlo Potts model and the phase field model. The Monte Carlo Potts model and phase field model simulation predictions are in excellent agreement. While the atomistic simulations demonstrate strong anisotropies in both the boundary energy and mobility, both types of microstructural evolution simulations demonstrate that anisotropy in boundary mobility plays little role in the stochastic evolution of the microstructure (other than perhaps setting the overall rate of the evolution. On the other hand, anisotropy in the grain boundary energy strongly modifies both the topology of the polycrystalline microstructure the kinetic law that describes the temporal evolution of the mean grain size. The underlying reasons behind the strongly differing effects of the two types of anisotropy considered here can be understood based largely on geometric and topological arguments.

Effect of particle size distribution on sintering of agglomerate-free submicron alumina powder compacts

Abstract: Experiments were performed with colloidally processed submicron alumina powders to investigate the effect of particle size distribution on their sintering characteristics. The results showed that in the absence of agglomerates and macroscopic size segregation, a broader particle size distribution leads to two opposing phenomena during sintering—enhanced overall sintering characteristics and a higher degree of local differential densification. The former is a result of both the higher initial green density and smaller isolated pores in the final stage of sintering brought about by enhanced grain growth during the intermediate stage. The latter is promoted by a higher degree of variation in local particle packing and may negate the enhanced sintering effect at sufficiently broad particle size distribution. There therefore exists an optimum range of particle size distribution for best sinterability. Since the optimum particle size distribution may vary considerably even for a given powder system, depending on the compaction technique and conditions used, narrow size distribution powder is preferred to monosized or broad size distribution powders for high sinterability and microstructure control of powder compacts, provided that agglomerates in the starting powder are removed by appropriate means. For the agglomerate-free, submicron alumina powder system studied, the optimum particle size distribution was found to have a geometric standard deviation value lying in between 1.6 and 1.9.