Large-grained Samples Research Articles

Dielectric Spectroscopy Study Of Znse Grown By Physical Vapor Transport

AbstractZnSe, although generally thought of as a wide band gap semiconductor, is insulating in the as-grown state. It is only after heat treatment in a zinc rich atmosphere that semiconducting properties are observed Therefore, dielectric spectroscopy is an appropriate tool to study the electrical properties of as grown ZnSe. The dielectric properties of large-grained samples of ZnSe grown by physical vapor transport were measured as a function of frequency. Differences can be seen in the dielectric properties of samples grown under different conditions (such as the effect of a seed and the orientation of the gravity field during growth). The spectra of heat treated samples were also acquired and were found to exhibit significant deviations from those of the as grown crystals.

Effects of grain boundary diffusion of copper on nickel creep

We have investigated the effects of surface copper diffusion on the creep of large-grain nickel (average grain size about 20 m) and submicron crystalline nickel (grain size about 0.3 m). For both structural states of nickel we find an acceleration of creep over its value in vacuum, and an increase in plasticity during creep when copper has diffused deeply into the nickel from the surface. The temperature range over which these effects are observed in the submicron nickel is nearly 300° lower than that for the large-grain samples. This result is probably due to a significant increase in the grain-boundary and bulk diffusion coefficients of copper in submicron nickel when compared with large-grain nickel.

Characterization of the 105 K superconductor Tl 2Ba 2CaCu 2O 8 (“2212”)

Homogeneous Tl 1.8Ba 2CaCu 2O 8 ceramics were synthesized by a novel route, starting from Tl 2Ba 2O 5 precursors and using high gas pressures. This method allows tight control of thallium losses, resulting in dense, large-grained samples with sharp superconducting transitions above 105 K. Results of a characterization by X-ray diffraction, optical micrographs and micro-probe analysis are presented, together with selected physical properties.

Microstructural Effects on the Cyclic Deformation of Polycrystalline FCC Materials

AbstractThe effect of crystallographic texture and grain size on the cyclic hardening of FCC polycrystals was studied using copper as a model material. Samples with textures approximately random and two grain sizes were tested under cyclic loading. The cyclic response for high values of the plastic strain amplitude of the random-textured material was softer than that of polycrystals with strong <111>-<100> texture reported in [1], and almost the same at low plastic strains, when similar testing conditions were used. The cyclic behavior of specimens with different grain sizes and similar texture was almost the same as well. This result shows that texture had a much more important effect on the cyclic behavior of polycrystalline copper than the grain size. The effect of ramp loading as a pretreatment was also studied. The results show that large grained samples had a softer cyclic response after the ramp than small grained ones, but the effect was still smaller than that of texture.

In-situ synthesis and rapid consolidation of fully dense MoSi2

Fully dense, large-grained MoSi2 samples can be fabricated by self-propagating high-temperature synthesis (SHS) and rapid consolidation using the Ceracon process. Using this process, several major limitations associated with SHS/hot isostatic pressing to fabricate MoSi2 are overcome.

Production and characterization of large-grain bulk samples of Y1Ba2Cu3O7−x

A heat-treatment method is described which yields superconducting Y1Ba2Cu3O7−x pellets having grain inclusions of a few millimeters in size. Resistivity, magnetic susceptibility, and magnetization display sharp transitions near 90 K for at least 85% of the volume of the samples. X-ray powder patterns reveal the presence of both the superconducting and ‘‘green’’ phases. Laue pictures show sharp spots. The normal state resistance of these samples is less than pellets prepared in the usual way by a factor of ∼10.

Effect of preoxidation and grain size on ductility of a boron-doped Ni 3Al at elevated temperatures

The ductility of a preoxidized Ni 3Al (Ni-23Al-0.5Hf-0.2B. at.%) alloy with various grain sizes (17–193 μm) was evaluated by means of tensile tests at 600 and 760°C in vacuum. The preoxidation does not affect the ductility of the finest-grained material at either temperature, whereas it causes severe embrittiement in the largest-grained material, especially at 760°C. Auger studies revealed very little oxygen penetration along grain boundaries in the finest-grained material but substantial oxygen penetration in the largest-grained one. A continuous, thin Al-rich oxide layer which forms on the fine-grained samples protects the underlying alloy from oxygen penetration, preventing any loss of ductility, whereas the nickel-rich oxide which forms on the large-grained samples allows oxygen to penetrate along grain boundaries, causing severe embrittiement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the change in oxidation product from Ni-rich to Al-rich oxide with decreasing grain size.

Oxidation of Ba2YCu3Ox at High Po2

Dense large‐grain ceramic samples of Ba2YCu2Ox have been oxidized at 400° to 500°C in 10 to 30 MPa of oxygen for 1 to 10 d. The high‐pressure treatment has increased the equilibrium content x slightly above 7 but there is no concomitant increase in Tc. At x = 7.03 the superconducting transition, measured by ac susceptibility, has an onset temperature of ∼91 K and ΔT∼4 K and no change in lattice parameter is observed. High oxygen pressure increases the rate of oxidation.

Demonstration of grain growth induced Microcracking and its role in the electrical response of YBa2Cu3O7 − δ

The effect of grain size on the electrical properties of polycrystalline YBa 2 Cu 3 O 7 − δ has been investigated. A strong decrease in the critical current density has been observed when an average grain size of 15 μm is exceeded. Large grains promote microcracks due to anisotropic volume changes during cooling from the sintering temperature. This causes a reduction in the effective current-carrying cross section of the material. Ultrasonic measurements were used to confirm a significant increase in the microcrack density for large grained samples. Der Effekt der Korngröße auf die elektrischen Eigenschaften von polykristallinem YBa 2 Cu 3 O 7 − δ wurde untersucht. Es wurde ein starker Abfall der kritischen Stromdichte beobachtet, wenn die mittlere Korngröße von 15 μm überschritten wurde. Große Körner verursachen Mikrorisse aufgrund ihrer anisotropischen Volumenänderung während des Abkühlens von der Sintertemperatur. Hierdurch wird die stromtragende Querschnittsfläche des Materials reduziert. Mittels Ultraschall konnte die Mikrorißbildung in Proben mit grobkristallinem Gefüge deutlich erhöht werden. On a étudié l'effet de la taille des grains sur les propriétés électriques du YBa 2 Cu 3 O 7 − δ polycristallin. On a observé une forte décroissance de la densité de courant critique lorsque la taille moyenne des grains dépasse 15 μm. Les gros grains provoquent des microfissures en raison de changements volumiques anisotropiques durant le refroidissement à partir de la température de frittage. Celà entraîne une réduction de la section conductrice efficace du matériau. Des mesures par ultrasons ont confirmé une augmentation significative de la densité des microfissures dans les échantillons à gros grains.

Auger study of grain boundaries in large-grained YBa2Cu3Ox

A scanning Auger microscopy study has been carried out on fractured surfaces of YBa2Cu3Ox samples prepared by standard sintering procedures from powders. It is concluded that in the large-grained samples examined, a majority of the grain boundaries are sufficiently ‘‘open’’ that standard metallographic preparation causes them to become contaminated with C. This suggests that an actual loss of contact at the grain boundaries during sample preparation is responsible for the low Jc values observed in bulk-sized large-grained YBa2Cu3Ox.

Effect of Preoxidation and Grain Size on Ductility of a Boron-Doped Ni3Al at Elevated Temperatures

ABSTRACTThe ductility of preoxidized Ni3Al (Ni-23Al-0.5Hf-0.2B, at.%) specimens with various grain sizes (17∼193 μm) was evaluated by means of tensile tests at 600 and 760°C in vacuum. It was found that the preoxidation does not affect the ductility of the finest-grained material at either temperature, whereas it causes severe embrittlement in the largest-grained material, especially at 760°C. A continuous, thin Al-rich oxide layer, which forms on the fine-grained samples, protects the underlying alloy from oxygen penetration, preventing any loss of ductility, whereas the nickel-rich oxide which forms on the large-grained samples allows oxygen to penetrate along grain boundaries, causing severe embrittlement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the difference in oxidation behavior between fine- and large-grained materials. The embrittlement of large-grained samples can be eliminated through control of oxide formation on Ni3Al surfaces.

Cyclic stress-strain response of polycrystalline copper in constant and variable amplitude fatigue

Cyclic stress-strain response of polycrystalline copper of two different grain sizes was studied in constant, incremental step and variable plastic strain amplitude fatigue tests. The multi-regime ess curves of large-grained samples exhibited short but distinct plateaus both in constant and variable amplitude tests, but not in the incremental step tests. With small-grained samples, the css curve of random amplitude tests did not show the plateau but only a region with a small positive slope. The effective amplitude of the random square distributed loading block was estimated to be about 25% below the maximum amplitude of the block.

FATIGUE OF POLYCRYSTALLINE COPPER AT CONSTANT AND VARIABLE PLASTIC STRAIN AMPLITUDES

Fatigue behavior of polycrystalline copper of two different grain sizes was studied using constant and randomly varying plastic strain amplitude tests. The different regimes in the ess-curves of large-grained samples (d˜ 2.0 mm) were found to be reflected also in the measured fatigue lives in both test modes. The variable amplitude fatigue lives of the small-grained samples (d˜ 0.03 mm) seemed to follow the Coffin–Manson linear law. The damage accumulation rate in variable amplitude tests was found to be lower than in the corresponding constant amplitude tests, e.g. when expressed as cumulative plastic strains to failure. The fatigue life predictions based on the Palmgren–Miner rule, rain-flow analysis and the constant amplitude life data measured in this work were also conservative.

The growth of unusually large grains in thin Al-Cu films

The recrystallization of Al-Cu films with copper contents greater than 77.8% prepared by evaporation onto glass substrates in an ultrahigh vacuum at temperatures between -160 and 200°C was investigated. The film properties were determined by measuring the resistance change during annealing by transmission electron microscopy and by electron diffraction examination. Films with a copper content of 90%–94% which were condensed at -160°C and subsequently annealed at temperatures up to 200°C were transformed into large-grained samples. Grain diameters (flat single crystals) in excess of 100 μm were observed in films about 50 nm thick. The flat crystals were oriented in the [111] direction.

Differences in oxides on large-and small-grained 304 stainless steel

The oxides formed on large-grained (∼40 °m) and small-grained (∼4 μm) 304 stainless steel oxidized in air at 800 °C have been examined and compared by Auger electron spectroscopy to learn more about the role of grain boundaries in the oxidation of the materials. For vacuum preannealed specimens, relatively thick iron oxides formed over the grains and thin, chromium-rich oxides formed over grain boudaries of large-grained material. The oxide formed over the entire surface of the small-grained material was a thin chromium-rich layer similar to that formed over grain boundaries of the large-grained samples. The oxidation of both small- and large-grained samples was consistent with selective formation of Cr2O3 at grain boundaries followed by a lateral diffusion of Cr and spreading of Cr2O3. A protective Cr2O3 layer formed readily on small-grained material but not on large-grained material. In contrast to the differences in oxide morphology for small- and large-grained preannealed specimens, oxide morphologies were similar for small- and large-grained material when the outer surface layer was removed by polishing after annealing and before oxidation tests. Surface differences, not adequately defined by Auger and SEM studies, caused marked changes in oxide morphologies for large-grained material. The difference in oxidation behavior before and after polishing was attributed to enhanced oxidation at grain boundaries during the vacuum preannealing treatment and to differences in defect concentrations in the surface region.

Morphology and Crystallography of Widmanstätten Proeutectoid Ferrite

AbstractThe morphologies and precipitate/matrix orientation relationships of Widmanstatten proeutectoid ferrite precipitates within large-grained samples of an Fe–0·47%C alloy, isothermally transformed above the eutectoid temperature, have been investigated using the Kossel X-ray microdiffraction technique in conjunction w th two-surface metallographic analysis. The Widmanstatten precipitates in the samples studied often have somewhat irregular overall shapes but nevertheless they usually can be described as having habit planes within 0–5° of {111} austenite planes. Individual precipitates have been observed to contain a substructure of lowangle boundaries, the misorientations between the sub-units within a precipitate being generally < 1°. In addition, it has been shown that the precipitate/matrix orientation relationship is not unique, but rather exhibits a range of relationships between the standard Kurdjumov–Sachs and Nishiyama–Wasserman fcc–bcc orientation relationships.

Effect of grain size on the thermal diffusion of copper in aluminum

The effect of grain size upon the thermal diffusion of copper in aluminum has been investigated. It is found that the heat of transport of copper in fine-grained aluminum is 10.8 kcal/mole, and -7.1 kcal/mole is found in large-grained samples. This grainsize dependence is opposite to that found in self-thermal diffusion of aluminum by Matlock and Stark1 where the large-grained samples had the larger heat of transport. This copper result might be expected if vacancies are tightly bound to copper atoms in aluminum since the copper flux is antiparallel to that of aluminum and parallel to the vacancy flux. The difference in the heat of transport of copper in aluminum for small and large-grained samples is apparently equal to the heat of vacancy formation in aluminum. The observed difference of 17.9 kcal/mole is in numerical agreement with the value of 17.5 kcal/mole obtained by Simmons and Balluffi.2

The effect of environment and grain size on the creep behavior of a ni-6 pct w solid solution

The effects of environment and grain size on the steady-state creep and creep rupture properties of a Ni-6 pct W solid solution are examined by testing in vacuum and commercial purity argon at 5000 psi and 900°C. The steady-state creep rate is found to decrease with increasing grain size at small grain sizes, both in vacuum and argon, owing to the effects of grain boundary sliding. At large grain sizes the creep rate is independent of grain size in vacuum and increases with grain size in argon. It is suggested that the increase in creep rate with increasing grain size is associated with fact that large-grained samples tested in argon do not reach steady-state before rupture occurs.

On the relaxation strength of the snoek peak

A calculation was made on the relaxation strength Δ of the Snoek peak when a single crystal specimen is strained by a torsional stress. For a specimen with 〈100〉 orientation Δ vanishes and for a specimen with 〈111〉 orientation Δ takes a non-zero constant value independent of the shape of cross-section of the specimen. However, for a specimen with 〈110〉 orientation Δ varies from zero to a finite value depending on the shape of cross-section of the specimen. These calculated values are compared with those obtained for a tensile stress. Two simplified cases (the uniform stress and the uniform strain approximations) were considered for a random polycrystalline sample of cylindrical shape. In these approximations the relaxation strength under a torsional stress is larger than that under a tensile stress, when the same specimen is measured at the same frequency and temperature. Confirmation of these predictions was obtained from experiments on the single and poly-crystals of α-iron containing nitrogen in solid solution. A general survey of experimental results on polycrystalline specimens shows that the closest agreement with the calculations is obtained when data on large grained samples are compared with the calculations based on the uniform stress approximation.

Fine-Grain Nickel Ferrite for Microwave Applications at High Peak-Power Levels

Fine-grain ferrites were fabricated by hot-pressing ball-milled and flame-sprayed powders of Ni1−xCoxFe2O4 (where 0≤x≤0.04) and Ni0.973Mn0.027Fe2O4. Threshold fields for subsidiary absorption at Ku-band were significantly increased by decreasing the grain size to 5 μ or less. Between 5 and 10 μ, the critical field hcrit decreased gradually and above 10 μ was independent of grain size. The highest value observed was 192 Oe on a fine-grained nickel-cobalt ferrite vs ∼24 Oe for conventionally fired nickel ferrites. Dielectric losses were reduced to values below 5×10−4 at X-band, and low-field magnetic losses were approximately two to four times those of conventionally fired ferrites of the same composition. No systematic variation in threshold field or magnetic loss was observed with cobalt substitution in fine-grain samples, although threshold fields in general were found to be higher when cobalt was present. The threshold field increased linearly with cobalt substitution on large-grain samples. A Ku-band 90° ferrite phase shifter using a fine-grained, hot-pressed ferrite was capable of operating at power levels in excess of 750 kW/750 W, whereas the same device with a conventional ferrite exhibited nonlinearity below 20-kW peak power.