Discontinuous Grain Research Articles

Microstructure studies on Ti4+- and Zr4+-substituted Li-Zn ferrites

XRD and microstructure studies were carried out on Ti4+ and Zr4+-substituted Li-Zn ferrites prepared by standard ceramic technique. All the ferrite compositions exhibit single phase formation. The lattice parametera increases linearly with the content of Zn2+Zr4+ and Zn2+Ti4+, which is attributed to the ionic volumes of the cations involved. With substitution by Zr4+ the average size decreases, while with substitution by Ti4+ the grain size increases. In both the series grain size varies with the composition. Excess substitution of Zr4+ (x>0·4) leads to the formation of secondary images and discontinuous grain growth. Both Zr4+ and Ti4+ compositions obey Kurtz theory.

Effect of additives on densification and deformation of tetragonal zirconia

The effect of additives (Bi2O3, Fe2O3) on densification and creep rates of tetragonal ZrO2-Y2O3 has been investigated. In Bi2O3-doped Y-TZP, a reactive liquid forms at temperatures above 800–900‡C, which leads to a strong enhancement of densification for concentrations of 1–2 mol % Bi2O3. However, during cooling from the processing temperature a strong, undesirable transformation of the tetragonal to the monoclinic phase occurs. The addition of 0.6–1.2 mol % FeO3/2 promotes densification without destabilizing the tetragonal phase. A concentration of 1.2 mol %, however, induces discontinuous grain growth, while this is not the case for 0.6 mol %. Creep rates of Y-TZP were enhanced by a factor of 4–6 by adding 0.6 mol % FeO3/2.

Influence of Potassium on Preparation and Performance of PTC Resistors

Different amounts of K2CO3 were added to (Ba,Sr)TiO3‐based PTCR (positive temperature coefficient of resistance) ceramics to investigate their influence on the microstructural and electrical properties. Experimental results showed that the incorporation of K acted as an A‐site acceptor‐type dopant. In addition to enhancing discontinuous grain growth, the increase of K2CO3 was found to raise the room‐temperature resistivity which was dominated by grain‐boundary resistance rather than grain resistance. By adjusting to a suitable amount of donor dopant, the inherent contamination of K in raw material can be compensated to achieve a high‐quality PTC resistor.

Numerical analysis of interface energy-driven coarsening in thin films and its connection to grain growth

Interface energy-driven coarsening in thin films is investigated using numerical analysis of a mean field growth model. The development of preferred orientation in an initially randomly oriented film is demonstrated. The coarsening process may also lead to the development of bimodal particle size distributions, but this is shown to depend sensitively on the shape of the interface energy vs orientation function. The rate of transformation to a state of preferred orientation is dependent on quantities such as the film thickness and the ratio of the interface energy anisotropy to the particle boundary energy. We discuss the applicability of the model, which was developed to describe coarsening of islands in a discontinuous thin film, to surface or interface energy-driven grain growth in a continuous thin film.

Effect of Cr on the ordering behavior and ductility of an Ni-Ni4Mo alloy

Two approaches have been identified to improve the ductility of polycrystalline intermctallic compounds depending upon the embrittlement mechanism. Microalloying involving small additions of B has been proved effective in those cases where the embrittlement is associated with intrinsic weak cohesion along grain boundaries, as in the case of intermetallics based upon the L! ~ superlattice, such as Ni3AI m and Ni3Si. m Macroalloying is used when the embrittlement is caused by a restricted number of slip systems imposed by the crystal structure. For example, the crystal structure of Co3V could be changed from hexagonal (low ductility) into cubic (high ductility) by substituting Ni and Fe for some Co. TM In contrast to the cases of Ni3A1 and Co,V, most evidence points out that the embrittlement of Ni-Ni4Mo alloys is caused by a discontinuous grain boundary reaction. 14'~'61 Such a reaction can be described in terms of strain-induced recrystallization to minimize the strain energy associated with matrix ordering. 17'~} It is the objective of this communication to demonstrate that macroalloying involving an addition of Cr to a Ni-27Mo alloy which otherwise orders to Ni4Mo stabilizes a PhMo-type superlattice, producing about the same level of strengthening as with NiaMo ordering but with considerably improved ductility. Alloys examined included Ni-27Mo and Ni-27Mo-5Cr in all weight percent. Table 1 summarizes their chemical compositions. At the hypostoichiometric Ni-27Mo composition, Ni4Mo (tetragonal, D I , superlattice) precipitates from the parent face-centered cubic (fcc) lattice. Both alloys were prepared by vacuum melting and then processed into 1.5-mm-thick sheets by hotand coldrolling. Experimental specimens were solution annealed for 15 minutes at 1065 ~ in argon atmosphere and then water quenched. To examine the ordering behavior of the two alloys, annealed specimens were exposed at 700 ~ for up to 1000 hours. Mechanical properties were determined from roomand elevated-temperature tensile tests (50.8-ram gage length). Transmission electron microscopy (TEM) was used to characterize the ordered microstructure. Thin-foil specimens were prepared by the jet polishing technique in a solution consisting of 30 pet nitric acid in methanol at about 2 0 ~ Fractography was conducted in a scanning electron microscope. Both alloys were found to contain short-range order in the as-quenched condition, as was evidenced from the observation of {I I / 2 0} diffuse intensity maxima in

The influence of iron impurities on grain boundary self-diffusion in zirconium below 1138 K

The creep behavior of α-zirconium at high temperatures is not understood. Recently, steady-state stress exponents between 5 and 7 have been suggested over a range of elevated temperatures, indicating the predominance of dislocation climb (dynamic recovery) as the restoration mechanism. However, the activation energies are significantly higher than those of self-diffusion of pure Zr, as expected from climb-controlled mechanisms. This discrepancy and the observations of increased high-angle grain boundary area with straining have been attributed to the possible occurrence of discontinuous recrystallization and/or grain growth as additional restoration mechanisms. Tension, torsion and creep tests to small and large strains were performed at temperatures from 400 to 800 °C. The microstructure of the deformed samples was characterized by optical microscopy, transmission electron microscopy, as well as texture analysis using X-ray and electron backscatter diffraction. Dynamic recovery through dislocation climb appears to be the prevailing restoration mechanism. The increase in high angle boundary area with larger strains is a consequence of geometric dynamic recrystallization.

Determination of microtexture of initial stages of secondary recrystallisation in Fe–3 wt-%Si by etch pitting

AbstractRegular grain oriented silicon iron, produced by a two stage cold rolling technique, has been the subject of many investigations owing to its economic significance as sheet steel for transformer cores. In particular, .many investigations of the texture development through the various processing stages have been carried out by calculation of the orientation distribution functions. The final processing stage is a high temperature box annealing, by which discontinuous grain growth (secondary recrystallisation), controlled by a fine dispersion of second phase particles, leads to a very sharp Goss texture. To investigate the conditions for secondary recrystallisation, material from two stages of processing has been examined: the primary recrystallised material and the material just before the start of secondary grain growth. Utilising the etch pit method – a well established and powerful technique for single orientation determination – in addition to grain size measurements, both applied to scanning el...

The effects of sintering atmosphere on impurity phase formation and grain boundary resistivity in Y2O3-fully stabilized ZrO2

Abstract X-ray photoelectron spectroscopy and impedance studies have been used to monitor the migration of the impurity silicate phase in the grain boundary network of fully stabilized Y 2 O 3 -ZrO 2 sintered at different temperatures in air. The nature of the grain boundary phase formed and its influence on the grain boundary resistivity were considerably different under air compared with the previously reported argon-sintered specimens. As a function of the sintering temperature, two peaks were observed in the XPS atomic ratios (Y/Zr, Si/Zr, O/Zr) at the fractured surfaces of air-sintered specimens, while in argon-sintered specimens only one peak was present. There is a high degree of correlation between the grain boundary resistivity and XPS atomic ratios. The activation energy for the grain boundary resistivity is independent of the sintering temperature and atmosphere and is similar to that for the lattice resistivity. No evidence was found for perfect wetting of grains by the impurity glass phase at any sintering temperature. Instead, the results strongly support a discontinuous grain boundary phase model.

低温焼結ＮｉＣｕＺｎフェライトの微細構造に及ぼすＡｇの影響

Effect of Ag on the electromagnetic properties of low temperature sintered NiCuZn ferrite has been studied with special reference to the micro- and nano-structure. It is shown that As accelerates the densification of NiCuZn ferrite and promotes the dissociation of Cu from the ferrite, which causes discontinuous grain growth. Silver addition to NiCuZn ferrite brings about not only distortion of lattice near the grain boudaries but also the compression of sintered body.

Stability of Nanoscale Cobalt Silicide Film Formation on Polysilicon

ABSTRACTWe have extended the recent work1,2 on grain boundary grooving, agglomeration and islanding during the reaction of thin Co films on single crystal Si<100> substrates (source/drain contacts) to the case of poly silicon substrates3 (gate contact and local interconnects). The stability of 27 to 50 nm CoSi2 films formed via rapid thermal annealing on polysilicon substrates with and without P+ or Ge+ implantation was examined using XTEM, RBS, XRD, and Four-probe measurements. For an undoped polysilicon, partial agglomeration of a 30 nm CoSi2 film has been observed, similar to that seen on a single crystal Si substrate.2For heavily Phosphorus doped polysilicon, a transition from low resistance, stoichiometric cobalt disilicide films (700 °C, 10 sec RTA) to high resistance films at 700 °C, 30 sec RTA (600–1000 Ω per square and discontinuous suicide grain growth) has been observed. Concurrent suicide formation on preamorphized polysilicon (Ge+ I/I and no anneal) has been found to suppress agglomeration of nanoscale cobalt suicide layers, since small grain size and low grain boundary energy are favorable for improving thermal stability4. These continuous layers also exhibited the lowest sheet resistance corresponding to a resistivity of ≤ 21 μΩ-cm.

Effect of Pore–Grain‐Boundary Interactions on Discontinuous Grain Growth

Pore behavior as influenced by migration of an abnormal grain boundary was studied. The pore behavior during the discontinuous growth of hot‐pressed spinel (MgAl2O4) — whether the pore can move attached to the grain boundary — is particularly governed by the pore size and the matrix grain size. These phenomena can be understood from the interaction between the pore and the abnormal grain boundary.

Discontinuous grain growth of quartz in metacherts: the influence of mica on a microstructural transition

Abstract The microstructure of quartz in metacherts of the Ryoke metamorphic belt in central Japan develops from polygonal, through duplex to irregular with increasing metamorphic grade. The polygonal microstructure is composed of small (mostly 90–160 μm), equant, equigranular, polygonal quartz grains, whereas the irregular microstructure is characterized by large (>300 μm) grains with irregular grain boundaries. The duplex microstructure is a mixture of small polygonal and large irregular grains. The development of these microstructures is interpreted as being due to secondary recrystallization. The size of polygonal grains is greatly influenced by the presence of second‐phase minerals, such as mica, whereas that of large irregular grains is unaffected by second‐phase minerals. There seems to be a critical grain size for quartz to occur as polygonal aggregates: no polygonal aggregates occur in rocks with larger than the critical grain size. The size (about 140 μm) decreases slightly with increasing volume fraction of mica. The mean grain sizes of polygonal quartz (D) and coexisting mica (d) in the duplex microstructure are systematically related to the volume fraction of mica (f) by D= 0.728d(1/f)0.629.

Microstructural refinement of ?-sintered and Ti-6Al-4V porous-coated by temporary alloying with hydrogen

A series of thermochemical treatments, in which hydrogen was used as a temporary alloying element to refine the lamellar microstructure of β-sintered and porous-coated Ti-6Al-4V was formulated. Each step of the treatment sequence (hydrogenation, eutectoid decomposition and dehydrogenation) was studied separately, on uncoated specimens and then on porous-coated specimens. The resultant microstructures can have α-grain sizes less than 1 μm, aspect ratios near unity and discontinuous grain boundary α (GBα), microstructural attributes which increase the fatigue strength. Microstructural refinement occurs because hydrogen-alloying reduces the (α+β)↔β transition temperature and enables a eutectoid decomposition reaction to occur. The optimal hydrogenation temperature is 850 °C, because hydrogen concentrations of 0.71 to 0.85 wt% are in-diffused and β-transformation is achieved. These weight percentages are in the optimal range for efficient eutectoid decomposition kinetics, β-transformation obviates the need for a separate β-transformation treatment step. A separate eutectoid decomposition treatment step may be used, or eutectoid decomposition may be combined with dehydrogenation. The finest eutectoid microstructures are obtained if hydrogen concentrations are in the range 0.5 to 0.8 wt%. The criteria for dehydrogenation are efficient removal of hydrogen, with minimal grain growth and absence of GBα. These criteria are best met by using dehydrogenation temperatures <700 °C. Altering the sintering temperature or adding a porous coating does not affect the parameters of the hydrogen-alloying treatment steps.

Control of microstructure with seed grains

The addition of seed grains to a ceramic powder is an effective means to control the final grain size by supporting discontinuous grain growth. Seed grains are widely single crystalline particles having the same composition as the sintered powder, but showing larger grain size. The effect of seed grains can be very useful in the production of ZnO and BaTiO 3 based on nonlinear resistors (PTC, VDR).

Mode I fracture of concrete: Discontinuous crack growth and crack interface grain bridging

In the present paper results are presented of vacuum impregnation tests using single-edge-notched concrete plates subjected to uniform boundary displacement. A realistic fracture model for cement-based composites should be based on correct micro-structural observations of cracking and localization. Macroscopic structural effects such as non-uniform opening and non-uniform drying out have a pronounced influence on the specimen behaviour. The results of the vacuum impregnation tests reveal that the fracturing of the specimens is a three-dimensional growth process. Cracking starts from the outer surfaces of a specimen, most likely due to non-uniform drying out and the related tensile eigenstresses near the specimens surface. The macrocracks are highly discontinuous cracks with debonding near larger aggregates and intact material bridges between them. The load carrying capacity of a tensile specimen for average crack openings larger than 50 μm can be explained from distributed crack interface grain bridging. The failure of the grain bridges is a process involving bending and frictional pull-out. Next to this an ‘intact core’ was observed in some specimens loaded up to average axial deformations between 50 and 100 μm. The effects of maximum aggregate size on softening was significant for the range of specimen sizes and aggregate sizes tested. The increased post-peak load carrying capacity for coarse grained mixes can be explained from the larger size (and thus the increased flexural capacity) of the crack interface grain bridges.

Sintering and microstructure of ultrafine yttriazirconia compacts

Sintering behaviour of ultrafine yttria-zirconia solid solution compacts, with yttria contents ranging between 4 and 10 mol % have been investigated and the microstructural developments during sintering were followed. The 4 mol % yttria-zirconia compacts sinter at 1200 ° C to a high density with a relatively fine uniform microstructure. Firing at 1400 ° C produces no improvement in densification and discontinuous grain growth develops. The lowest density values are obtained with the 10 mol % yttria-zirconia compacts, with clustering in the grains and greater porosity in the microstructure. The activation energies were determined for both 4 and 10 mol % Y2O3 -containing bodies in the temperature range 800 to 1000 ° C.

Effect of Green States on Sintering Behavior and Microstructural Evolution of High‐Purity Barium Titanate

Compacts prepared from three differently agglomerated powders were studied. Hg‐penetration results and SEM observations were employed to compare the uniformity of powder compacts and to investigate the pore‐size evolution and the microstructural development during sintering. It was found that the more nonuniform the powder compact, the higher the degree of pore growth in the initial and at the beginning of intermediate stages of sintering. Moreover, a higher sintering temperature and a nonuniform microstructure with larger grains could not be avoided. Microstresses might develop because of the differential shrinkage, but they would be released thereafter via the change of grain morphology. It was observed that the aggregate and pore‐boundary separation might not be the primary reason for the initiation of discontinuous grain growth.

Effect of partial substitution of Ge4+ for Zn2+ on the microstructure and temperature stability of Mn-Zn ferrites

The effects of substitution of GeO2 for ZnO on the microstructure and magnetic properties of Mn-Zn ferrite have been studied. Results suggest that, to a very limited extent, by the substitution of Ge and a proper choice of sintering condition, good temperature coefficients can be obtained in a temperature range from 20 to 80 °C. Experimental results show that the substitution of GeO2 promotes discontinuous grain growth, and the lattice parameter is slowly reduced due to the substitution of from 0.5 to 2.0 mol % GeO2 for ZnO. The temperature coefficient is extremely process sensitive. For appropriate Ge substitution (0.5%), optimum initial permeability and the temperature coefficient are dependent on the sintering conditions. Samples with 0.5 mol % GeO2 sintered at 1290 °C using a rapid heating rate and short-time sintering yielded the best temperature coefficient of 0.1×10−6 °C−1 over the range 20–80 °C.

The effect of silica characterization on the microstructure of BaFe12O19 ferrites

The anisotropically formed BaFe12O19 ferrites were prepared from the hot-rolled mill scale and silica was added to the ferrite during fine milling in the range 0.15 to 0.50 wt%. These ferrites were sintered at 1220° C for 2 h. The grain growth of the ferrites is dominantly influenced by the sizes of the silica added. Coarse-grain (≈ 1μm) silica tends to promote discontinuous grain growth, which increases drastically with slightly increasing amounts of silica added, while fine-grain (≈ 0.013μm) silica tends to retain fine grain microstructures with the same increasing amount of silica. The average grain size of the ferrite without silica addition was 8 to 10μm. The size was increased to as large as 30μm on addition of 0.15% coarse-grain silica and the microstructure was full of extremely large grains on the addition of 0.50% coarse-grain silica.

Cu-Be合金の粒界反応析出と粒界性格

Cu-Be合金の粒界反応析出と粒界性格