Showing papers on "Grain size published in 1976"

Grain‐size effects on dielectric properties in barium titanate ceramics

Abstract: Measurements of the dielectric constant κ in high‐purity BaTiO3 ceramics as a function of temperature show that κ is quite dependent on grain size in the ferroelectric state while it is almost independent of grain size in the paraelectric state. In the ferroelectric orthorhombic and rhombohedral phases, κ decreases rapidly with lowering temperature when grain size is reduced smaller than 3 μm. It is also observed that the Curie temperature Tc is decreased very slightly, but the lower two transition points are shifted to higher temperatures as grain size decreases.

The role of microstructure on the strength and toughness of fully pearlitic steels

Abstract: An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.

Grain Size Effects in Hydrogen-Assisted Cracking

Abstract: There is conflicting evidence in the literature with respect to the effect of grain size on hydrogen embrittlement. Differences may arise because of the degree of segregation in different grain size materials, because of different structures obtained in the effort to produce varying grain sizes, or because of the grain-size dependency of diffusion and growth processes. An extremely dirty heat of 4340 steel with 0.07 S and 0.015 P was investigated so that any tramp element segregation or hydrogen recombination poison effects would be present. Measurements were obtained on cathodically-charged samples with average grain sizes of 20, 50, 90 and 140 μm. In general, tramp element effects were not controlling. For those cases where the grain diameter was significantly larger than the plastic zone, increased grain size improved resistance. This was reflected by a slight increase in threshold stress intensity and an inverse grain-size squared dependence of crack velocity. Although the data are consistent with a pressure tensor hydrogen-assisted migration model, they could also be interpreted in terms of high austenitizing temperatures promoting retained austenite.

The effect of uniaxial compression on the initial susceptibility of rocks as a function of grain size and composition of their constituent titanomagnetites

Abstract: An ac bridge method has been used to study the effect of uniaxial compression on the initial magnetic susceptibility of rocks and separated titanomagnetites. The samples used were basalts containing titanomagnetite with varying grain size and morphology and dispersions of magnetite and titanomagnetite powders in epoxy. The results indicate that, (l) the effect of pressure on susceptibility decreases with decreasing grain size, not in a continuous manner but rather depending upon whether the dominant magnetic grain size is multidomain, pseudo single-domain or single-domain, (2) the pressure response increases with the composition parameter x of the titanomagnetite in the solid solution series xFe2TiO4 (1−x)Fe2O4, and (3) the morphology of the grains influences the pressure response. The results are interpreted in terms of the behavior of the multidomain, pseudo single-domain and single-domain material. Changes in thermoremanent magnetization and the acquisition of pressure remanent magnetization under uniaxial compression, which were observed in conjunction with the susceptibility studies, suggest that rocks containing coarse grain titanomagnetites as the dominant magnetic mineral phase are the most efficient stress transducers. Hence a definitive seismomagnetic experiment would be easiest in a seismically active region in which such rocks are prominent.

Cryogenic fracture toughness of 9Ni steel enhanced through grain refinement

Abstract: A thermal cycling technique was used to refine the grain size of commercial “9Ni” cryogenic steel. The grain refined alloy was then tempered, a treatment which introduced a small admixture of retained austenite. The reprocessed alloy shows an excellent combination of strength and toughness to temperatures as low as 6 K, and shows little evidence of embrittlement in liquid helium. The microstructure and mechanical properties of the reprocessed alloy are described and compared to those obtained through conventional treatment.

Measurement of average grain volume and certain topological parameters by serial section analysis

Abstract: Serial section analysis is suitable for the measurement of such topological parameters of a polycrystalline aggregate as number of grains, of grain faces, of grain edges and of grain corners in unit volume of the material. The number of grains in unit volume is of particular interest, because its reciprocal is the average grain volume, which is the only available shape-insensitive measure of grain size. An experimental approach to the serial section analysis of polycrystalline structures is developed.

Grain size and film thickness of Nb3Sn formed by solid‐state diffusion in the range 650–800 °C

Abstract: A simple etching and replication technique for the determination of grain size in thin films of Nb3Sn has been established. The resulting data compares well with previous reports of grain size as a function of reaction temperature and of the pinning force Fpmax as a function of grain size. The possibility of an optimum grain size for Fpmax is indicated.

Wear resistant alloy

Abstract: The alloy of the invention is of the type wherein 30-70% by volume of hard components are homogeneously dispersed in a matrix of binder metal (Fe, Co or Ni). The hard components are carbides or carbonitrides and/or borides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W. The mean grain size of the hard component particles is between 0.01 and 1.0 micron and their grain size distribution, represented by the standard deviation S, in which S 2 ≦(M/1+1.5 M z ) 2 μm 2 , not more than 15% of the grains are larger than 1.2 microns.

Carbide morphology in p/m IN-792

Abstract: Adequate tensile and smooth stress rupture properties were achieved by heat treating p/m IN-792 to produce serrated grain boundaries pinned with a globular M23C6 precipitate. A notch-strengthened condition with Kt = 3.5 at 1400‡F (1033 K)/90 ksi (620 MN/m2) was achieved when the material was heat treated to produce a grain size larger than about 76 Μm (ASTM G.S. #6). To obtain this grain size using realistic solution time-temperature heat treatments, the carbon content of the IN-792 should be maintained below 0.1 pct and preferably about 0.04 pct.

The effect of grain size on the shock-loading response of 304-type stainless steel

Abstract: The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes—23, 55, 85 and 187 µm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both ɛ and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pet cold rolling, the strengthening efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation to α martensite during subsequent tensile deformation.

Fabrication of translucent magnesium aluminate spinel and its compatibility in sodium vapour

Abstract: The sintering behaviour of magnesium aluminate spinel on both sides of the stoichiometry has been investigated. It is found that the composition close to the stoichiometric composition, but slightly enriched in magnesia can be sintered to translucency in a free flowing hydrogen environment. The grain size decreases with magnesia content; the addition of yttria, however, results in a highly bimodal grain structure. Prolonged exposure of the spinel in a high temperature sodium vapour environment forms Na2O · 5Al2O3 and some traces of Na2O · 11Al2O3 predominantely at the grain boundaries and near the surface of the specimen. Some possible sintering mechanism and factors affecting the in-line transmission are discussed.

Thermal fluctuation analysis: A new technique in rock magnetism

Abstract: Thermal fluctuation analysis is a new method of determining ‘magnetic grain size’ v and microscopic coercive force HK in particles too small for direct domain observations. When it is used with single-domain particles, thermal fluctuation analysis is a method of magnetic granulometry, since v is the physical grain volume and HK is usually closely related to grain shape. For multidomain particles, v corresponds to one Barkhausen jump of a domain wall, and HK describes the opposition to wall motion. The technique requires three steps: (1) measurement of coercive force Hc and saturation magnetization over a broad temperature range, (2) separation of the contributions of HK and the ‘fluctuation field’ Hq to HC, in a manner suggested by the Neel (1949) theory, and (3) calculation of average values of v and room temperature HK from Hq(T) and HK(T). Thermal fluctuation analysis is the only simple means of calculating average v; and HK separately. If thermal fluctuations are important, as is always the case for the fine particles or small wall displacements involved in low-field thermoremanence or viscous remanence, thermal and alternating field demagnetization do not resolve v and HK unambiguously, because observed coercivities and blocking temperatures TB depend on both v and HK. The method is tested by applying it to single-domain materials of known grain size. Fluctuation analysis is then used to infer the possible domain structure of 500- to 2200-A magnetite particles and to point up a deficiency in the Stacey and Banerjee (1974) procedure for detecting ordering of lattice defects impeding domain wall motion. Potential broader applications include determining mean grain sizes of single, and under favorable conditions, double populations of magnetic carriers in rocks, defining average elongation for single-domain particles in rocks and synthetic dispersions (e.g., magnetic recording tapes), and deducing the mechanism of anisotropy (shape, crystalline or magnetoelastic) or of opposition to wall motion (strain, inclusion or magnetostatic). The main limitations to the method are that the averages of v and HK become nonstationary if Hc measurements are made within the TB range and that step 2 (separation of HK(T) and Hq(T)) is difficult unless HK is predominantly due to a single anisotropy or wall opposition mechanism.

Effect of Microstructure on Rate of Machining of Ceramics

Abstract: The effect of microstructure on the machinability of a variety of ceramics was studied. The machining difficulty, i.e. the inverse of sawing or grinding rate at constant force, increased with decreasing porosity and grain size. For materials with nonuniform distributions of grain size or porosity, machining rates were controlled by the smaller grains and the least porous regions. The grain-size dependence of the machining difficulty was generally greater for harder materials. The effect of porosity on machining difficulty generally followed the same trend as its effect on other mechanical properties. The results obtained were interpreted in terms of the Petch relation, which provided a rationale for the occurrence of flaw sizes independent of grain size. The results also indicate that theories of the machinability of ceramics must include the effects of compressive strength and, especially, hardness.

Size distribution of quartz in mudrocks

Abstract: Sixteen penecontemporaneously deposited mudrock-sandstone pairs ranging in age from Devonian to Cretaceous were treated chemically to isolate the quartz and chert fraction from the other constituents of the rocks. The amount of crystalline silica was determined from the chemical treatments; its size distribution was determined using a combination of normal and Micro Mesh sieving and settling tube analysis; the crystalline silica coarser than 10 μu was examined petrographically to determine the amount of chert. Percentages of crystalline silica in the mudrocks range from 6·7 to 46·7 % and average 27·6 (σ= 10·7). Mean grain size ranges from 4·4 φ to 7·3 φ and averages 6·1 φ. The crystalline silica fraction is a poorly sorted medium to fine silt consisting of one-eighth sand, six-eighths silt, and one-eighth clay size sediment. Percentage of crystalline silica and mean size of crystalline silica in the thirty-two mudrocks and sandstones are positively correlated; r= 0·685, which is significant at the 99 % level. The best fit linear regression line is: Y= 102·7–11·3X. Extrapolation of the regression line indicates that, on the average, crystalline silica is lacking in mudrocks in grain sizes finer than 9·1 φ (1·μm), a result consistent with observations by clay mineralogists. The crystalline silica fraction of both the mudrocks and associated sandstones averages 4% chert in the >10 μm portion. There is no correlation between the mean grain size of the crystalline silica and the percentage of chert in it.

Effect of internal boundaries on the yield strengths of spheroidized steels

Abstract: The combined effects of cementite particles, grain boundaries, and subgrain boundaries on the room temperature yielding behavior of spheroidized, plain carbon steels were investigated. Spheroidization by austenitizing and quenching, followed by annealing at temperatures just under theA 1 temperature, produced a subgrain-connected cementite particle distribution. The subgrain size, λl,p, stabilized by the particles governs the yield stress via the relationσ y = 9.5 + 1.33 λl,p -1/2, kgf/mm2 In contrast, austenitizing and quenching, followed by thermal cycling about theA 1 temperature, produced microstructures with a large fraction of intraboundary, subgrain-free cementite particles. The lower yield stress of these steels could not be accounted for by either the Orowan or the Ansell-Lenel theory. The yield stress is predominantly controlled by the ferrite grain size, λg, via the relationσ y = 12.4 + 1.87 λg -1/2, kgf/mm2 The intraboundary particles contribute only a small strain-hardening term which increases the value of the friction stress (12.4 kgf/mm2) over that associated with grain boundary strengthening alone (8.8 ± 0.8 kgf/mm2).

Bed topography and grain size in open channel bends

Abstract: Engelund's (1974a, b) theoretical bed topography model for curved channels is modified and shown to fit natural channels when using values for the dynamic bed load friction coefficient of about 0·4–0·5. The dependence of this coefficient on grain size for flow in the lower flow regime is discussed. Using the bed topography model simultaneously, Allen's (1970a, b) theory for grain size variation over point bar profiles is shown to be broadly applicable. The interaction between sediment, flow and bed topography in bends is therefore adequately described.

Settling tubes for size analysis of fine and coarse fractions of oceanic sediments

Abstract: : Instrumentation for rapid, high precision size analysis of silt and sand size particles has been acquired and/or developed by the marine geology group of the School of Oceanography of Oregon State University. A Cahn automatic electrobalance with particle sedimentation accessories is used for size analysis of the silt fraction of pelagic sediment; a large diameter settling tube developed at Oregon State University is used for analysis of the sand size fraction. The data generated with these systems are recorded by a strip chart recorder as well as by a high speed paper tape punch. Computer programs to deal with large volumes of data and to calculate various size parameters for each sample have been developed for this instrumentation and are documented in this report. Instrumentation has also been developed to permit specific size fractions to be separated from the bulk sediment to allow compositional studies of the different size modes. All documentation necessary to understand this instrumentation system is presented in this report, and thus is available to all students interested in textural classifications of pelagic sediments. (Author)

Precipitation of equilibrium phases in vapour-quenched Al-Ni. Al-Cu and Al-Fe alloys

Abstract: In situ annealing experiments in the transmission electron microscope have been used to investigate the thermal stability and precipitation behaviour of non-equilibrium phases in vapour-quenched Al-Ni, Al-Cu and Al-Fe alloys prepared by co-sputtering. The non-equilibrium phases exhibit surprisingly high thermal stability and the general nature of precipitation is different from that in liquid-quenched or conventional alloys. These phenomena are caused by the very small as-quenched grain size.