Showing papers on "Grain growth published in 2009"

Experimental observations of stress-driven grain boundary migration.

Abstract: In crystalline materials, plastic deformation occurs by the motion of dislocations, and the regions between individual crystallites, called grain boundaries, act as obstacles to dislocation motion. Grain boundaries are widely envisaged to be mechanically static structures, but this report outlines an experimental investigation of stress-driven grain boundary migration manifested as grain growth in nanocrystalline aluminum thin films. Specimens fabricated with specially designed stress and strain concentrators are used to uncover the relative importance of these parameters on grain growth. In contrast to traditional descriptions of grain boundaries as stationary obstacles to dislocation-based plasticity, the results of this study indicate that shear stresses drive grain boundaries to move in a manner consistent with recent molecular dynamics simulations and theoretical predictions of coupled grain boundary migration.

High Total Proton Conductivity in Large-Grained Yttrium-Doped Barium Zirconate

Abstract: Barium zirconate has attracted particular attention among candidate proton conducting electrolyte materials for fuel cells and other electrochemical applications because of its chemical stability, mechanical robustness, and high bulk proton conductivity. Development of electrochemical devices based on this material, however, has been hampered by the high resistance of grain boundaries, and, due to limited grain growth during sintering, the high number density of such boundaries. Here, we demonstrate a fabrication protocol based on the sol−gel synthesis of nanocrystalline precursor materials and reactive sintering that results in large-grained, polycrystalline BaZr_(0.8)Y_(0.2O3−δ) of total high conductivity, 1 × 10^(−2) Scm^(−1) at 450 °C. The detrimental role of grain boundaries in these materials is confirmed via a comparison of the conductivities of polycrystalline samples with different grain sizes. Specifically, two samples with grain sizes differing by a factor of 2.3 display essentially identical grain interior conductivities, whereas the total grain boundary conductivities differ by a factor of 2.5−3.2, depending on the temperature (with the larger-grained material displaying higher conductivity).

Microstructural evolution in high purity aluminum processed by ECAP

Abstract: High purity (9999%) aluminum was processed by equal-channel angular pressing (ECAP) through 1–12 passes and examined using orientation imaging microscopy The results reveal two distinct processing regimes: from 1 to 4 passes the microstructure evolves from elongated subgrains to an essentially equiaxed array of ultrafine grains and from 4 to 12 passes there is no measurable change in the average grain size and grain aspect ratio The boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 4 passes and at a slower rate from 4 to 12 passes Anomalously large grains were visible in the central region of the billet pressed through 12 passes due to dynamic recovery and grain growth The results suggest optimum processing is achieved by pressing through 4–8 passes

A Review on the Sintering and Microstructure Development of Transparent Spinel (MgAl2O4)

Abstract: A review of the densification mechanisms and the microstructural development for transparent spinel made by free sintering and by hot pressing is given. The paper is divided into two main parts. The first part considers spinel without any sintering additives because there still is some controversy concerning the role of cation stoichiometry on sintering and grain growth. The second part discusses the role of the classic sintering aid, LiF, in processing transparent spinel. LiF is shown to have multiple behaviors: (1) it initially wets spinel and forms a liquid phase at relatively low temperatures, which affects early-stage densification and also grain growth; (2) upon cooling from intermediate temperatures, or even from higher temperatures if microstructure evolution (e.g., formation of closed porosity) prevents volatization, the LiF-containing liquid dewets and resides in isolated pockets; (3) LiF alters the cation stoichiometry, thereby enhancing diffusion via an increase in the concentration of oxygen vacancies; this affects both the densification rate and grain growth; and (4) it reacts with impurities in the system, thereby acting as a cleanser. For the production of transparent spinel, it is critical that LiF or associated reaction products not be retained as a secondary phase.

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Abstract: Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s−1 was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary { 1 0 1 ¯ 2 } tensile twins and { 1 0 1 ¯ 1 } compression twins, and then { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg17Al12 phases at grain boundaries occurs together with the precipitation of discontinuous Mg17Al12 phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.

Effects of heating rate on microstructure and transparency of spark-plasma-sintered alumina

Abstract: Commercial alumina powder was densified by spark plasma sintering (SPS) at 1150 °C. During SPS processing, the effects of the heating rate were examined on microstructure and transparency. With decreasing heating rate, the grain size and the residual porosity decreased, while the transparency increased. At a heating rate of 2 °C/min, the grain size was 0.29 μm, and the in-line transmission was 46% for a wavelength of 640 nm. The mechanisms for the fine microstructure and low porosity at slow heating, which are conflicting with some existing results, were explained by considering the role of defect concentration and grain-boundary diffusion during densification.

Hot Isostatic Pressing of Transparent Nd:YAG Ceramics

Abstract: This paper demonstrates that fine-grained (2–3 μm), transparent Nd:YAG can be achieved at SiO2 doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO2 levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO2, especially at higher temperatures, because the SiO2 solubility limit is exceeded.

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Abstract: Nanocrystalline Y 2 O 3 powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol −1 and associated diffusion coefficients higher by 10 3 than expected for Y 3+ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.

A Huge Effect of Weak dc Electrical Fields on Grain Growth in Zirconia

Abstract: We show that the application of a modest dc electrical field, about 4 V/cm, can significantly reduce grain growth in yttria-stabilized polycrystalline zirconia. These measurements were made by annealing samples, for 10 h at 1300°C, with and without an electrical field. The finding adds a new dimension to the role of applied electrical fields in sintering and superplasticity, phenomena that are critical to the net-shape processing of ceramics. Grain-growth retardation will considerably enhance the rates of sintering and superplasticity, leading to significant energy efficiencies in the processing of ceramics.

On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content

Abstract: In this work the influence of pressureless sintering on the Vickers hardness and fracture toughness of ZrO 2 reinforced with Al 2 O 3 particles (ATZ) and Al 2 O 3 reinforced with ZrO 2 particles (ZTA) has been investigated. The ceramic composites were produced by means of uniaxial compacting at 50 MPa and the green compacts were heated to 1250 °C using a heating rate of 10 °C min −1 , then to 1500 °C at 6 °C min −1 and maintained at this temperature during 2 h. After sintering, relative density over 94%, hardness values between 9.5 and 21.9 GPa, and fracture toughness as high as 3.6 MPa m 1/2 were obtained. The presence of TZ-3Y particles on the grain boundaries suggests that they inhibit notably the alumina grain growth. The grain sizes of pure Al 2 O 3 and TZ-3Y as well as Al 2 O 3 and TZ-3Y in the 20 wt% Al 2 O 3 +80 wt% TZ-3Y composite were 1.27 ± 0.51 μm, 0.57 ± 0.12 μm, 0.65 ± 0.19 μm and 0.41 ± 0.14 μm, respectively. The 20 wt% Al 2 O 3 + 80 wt% ZrO 2 + 3 mol% Y 2 O 3 (TZ-3Y) composite showed a hardness of 16.05 GPa and the maximum fracture toughness (7.44 MPa m 1/2 ) with an average grain size of 0.53 ± 0.17 μm. On the other side, the submicron grain size and residual porosity seem to be responsible for the high hardness and fracture toughness obtained. The reported values were higher than those obtained by other authors and are in concordance with international standards that could be suitable for dental applications.

Microstructural Evolution During Sintering with Control of the Interface Structure

Abstract: This paper reports recent theoretical perspectives and experimental results on microstructural evolution during sintering in terms of the interface structure, which is either rough (atomically disordered) or faceted (atomically ordered). The paper presents theoretical predictions and calculations of grain growth during liquid-phase sintering based on crystal growth theories. It is shown that various types of grain growth behavior, which may be normal, abnormal, or stagnant, can appear as a result of the coupling effects of the maximum driving force for growth and the critical driving force for appreciable growth. The predictions are also shown to be valid in the case of solid-state sintering. A number of experimental observations showing the effect of some critical processing parameters have been found to be in excellent agreement with the predictions. Principles of microstructure development (grain growth control) during sintering are suggested. In addition, the effect of the interface structure on densification is briefly described and discussed.

Failure by simultaneous grain growth, strain localization, and interface debonding in metal films on polymer substrates

Abstract: In a previous paper, we have demonstrated that a microcrystalline copper film well bonded to a polymer substrate can be stretched beyond 50% without cracking. The film eventually fails through the coevolution of necking and debonding from the substrate. Here we report much lower strains to failure (approximately 10%) for polymer-supported nanocrystalline metal films, the microstructure of which is revealed to be unstable under mechanical loading. We find that strain localization and deformation-associated grain growth facilitate each other, resulting in an unstable deformation process. Film/substrate delamination can be found wherever strain localization occurs. Therefore, we propose that three concomitant mechanisms are responsible for the failure of a plastically deformable but microstructurally unstable thin metal film: strain localization at large grains, deformation-induced grain growth, and film debonding from the substrate.

Doping behaviors of dysprosium, yttrium and holmium in BaTiO3 ceramics

Abstract: The difference in doping behaviors of intermediate rare-earth ions and their effects on the dielectric property and microstructure of BaTiO3(BT)–MgO–Re2O3 (Re = Dy, Ho, Y) system were investigated. Compared to Y and Ho, Dy ions provided BT ceramics with a high rate of densification and much enhanced shell formation due to their high solubility in BT. However, the microstructure of the Dy doped specimen was unstable at high temperatures in terms of grain growth. Until the specimen was densified, the tetragonality of Dy doped specimen was remarkably decreased and the substitution amount of Dy ions for A-site was larger than that of Y and Ho ions. After complete densification, the tetragonality was increased again and the B-site incorporation of Dy ions was increased far more than that of Y and Ho ions resulting in grain growth. This different behavior was considered to result in temperature coefficient of capacitance curves in the Dy doped specimen different from that of typical core–shell grains.

Influence of Sintering Temperature on Grain Growth and Phase Structure of Compositionally Optimized High‐Performance Li/Ta‐Modified (Na,K)NbO3 Ceramics

Abstract: Microstructure characteristics, phase transition, and electrical properties of (Na 0 . 535 K 0 . 485 ) 0 . 926 Li 0 . 074 (Nb 0 . 942 Ta 0 . 058 )O 3 (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20-30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°-1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d 33 is enhanced to 276 pC/N, which is a high value for the Li-and Ta-modified (Na,K)Nb0 3 ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k p = 46.2%, thickness electromechanical coupling factor k t = 36%, mechanical quality factor Q m = 18, remnant polarization P r = 21.1 μC/cm 2 , and coercive field E c = 1.85 kV/mm.

Spark-Plasma-Sintering Condition Optimization for Producing Transparent MgAl2O4 Spinel Polycrystal

Abstract: By controlling the heating rate at 1300°C, the coalescence of the residual pores and the precipitation of second phases, which are caused by rapid grain growth, degrade the transparency. The present study demonstrates that although the high heating rates have been regarded as a primary advantage for the SPS processing, the low heating rate is highly effective in attaining a high transparency in the spinel even at low temperatures and for short sintering times.