Showing papers on "Grain size published in 1975"

Metallurgical factors affecting fracture toughness of aluminum alloys

Abstract: Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1 μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus, b) the size of the ligaments which is related tof c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging conditions are altered for the samef cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established. In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with grain size and is accompanied by a loss of fracture toughness.

Theoretical single-domain grain size range in magnetite and titanomagnetite

Abstract: A theoretical model of single-domain (SD) grain sizes is applied to magnetite and titanomagnetite. In this model, transition to a two-domain configuration takes place at the SD threshold d0. This two-domain configuration is shown to be more applicable to fine-grained magnetites in igneous rocks than previous models involving transition to a circular spin configuration at d0. Calculations of the stable SD grain size range were accomplished by calculating the superparamagnetic threshold size ds by Neel's relaxation equation and calculating the SD threshold d0 at which SD to two-domain transition occurs. For cubic magnetite particles the SD range is extremely narrow and occurs at very small grain size. At room temperature, ds ≃ 0.05 μm, and d0 ≃ 0.076 μm. For cubic magnetite particles just above d0 a two-domain configuration is predicted in which a 180° domain wall occupies ∼60% of the particle volume. No SD range exists for cubic magnetites at T > 450°K. These results are in good agreement with experimental determinations of SD limits in equant magnetites and also agree with experimental observations of thermoremanent magnetization in submicron pseudo-single-domain (PSD) magnetites. The SD range increases rapidly with particle elongation. For a length : width ratio of 5 : 1, SD limits of ds ≃ 0.05 μm and d0 ≃ 1.4 μm are calculated. Both d0 and the SD range for titanomagnetites (Fe3−x Tix04) increase with Ti content. For cubic titanomagnetites of x = 0.6, ds ≃ 0.08 μm, and d0 ≃ 0.3 μm. Comparison of the calculated SD range with the available high-resolution grain size distributions of opaque grains in igneous rocks suggests that elongated SD grains or submicron PSD grains are the major carriers of stable natural remanence in igneous rocks.

Flux pinning centers in superconducting Nb3Sn

Abstract: Very high critical current densities (Jc=8×105 A/cm2 at 4.2 K and 6 T) have been achieved in commercial Nb3Sn tape and more recently in multifilamentary Nb3Sn superconductors. The relative contributions of two types of pinning centers, grain boundaries and second phase particles, to flux pinning have been determined by correlating Jc with microstructure. It was found that grain boundaries were the only defect present in sufficient density to account for flux pinning in the multifilamentary Nb3Sn material. The dependence of Jc on grain size was measured, and the relationship between pinning force and grain size is discussed in terms of current theories for flux pinning in type‐II superconductors.

Stability of Anhysteretic Remanent Magnetization in Fine and Coarse Magnetite and Maghemite Particles

Abstract: Summary Further experiments have been performed to investigate the biasing-field dependency of alternating field demagnetization curves of anhysteretic remanent magnetization as a simple test for the domain state of magnetite and maghemite particles. The biasing-field dependency in fine-grained particles was opposite to that in coarse-grained particles. The experiments were conducted on well sized synthetic specimens in the single domain, pseudo-single domain and multi-domain grain size ranges. A single domain-like biasing-field dependency was observed in equidimensional particles up to 0.2,~ in mean grain size and up to 0.4~ in elongated grains. Either the single domain/pseudo-single domain boundary lies above at least 0.2~ grain size or this field dependency test does not distinguish between single domain and pseudo-single domain states. A multidomainlike trend was observed in very coarse magnetite. The test may possibly distinguish the change from pseudo-single domain to multi-domain states. If both fine and coarse fractions are present a confusing overlap of the demagnetization curves occurs.

Creep behaviour in the superplastic Pb–62% Sn eutectic

Abstract: The creep behaviour of the superplastic Pb-62% Sn eutectic was investigated for grain sizes from 5·8 to 14·5 μm and at temperatures in the range from 336 to 422 K. The results showed a sigmoidal relationship between strain rate and stress. At intermediate strain rates (∼ 10−5–10−2 sec−1), the stress exponent was ∼1·65, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for grain boundary diffusion. At very low strain rates (≲10−5 sec−1), the stress exponent was ∼3·0, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for lattice self-diffusion. The results are not entirely consistent with either of the two major theories of superplasticity, but suggest instead the sequential operation of two different deformation processes.

Tensile strength and work hardening of ultrafine‐grained high‐purity copper

Abstract: Sputter‐deposited high‐purity copper specimens with grain sizes from 8.4 to 0.056 μm were tested in tension to investigate the influence of grain size on yield strength for small grain sizes. The smallest grain size in the sputter‐deposited copper was a factor of 40 smaller than the smallest grain size previously available for yield‐strength–grain‐size studies of high‐purity copper. The 0.2% offset yield strengths varied from 73.4 MPa for the coarsest‐grained copper to 481 MPa for the finest‐grained copper. The hardening was related to grain‐ and twin‐boundary spacings. The yield strength obeyed the Hall‐Petch relation even for the finest grain size of 0.056 μm, giving σy=6.39+3.74(l)−1/2 MPa, where σy is the 0.2% offset yield strength and l is the mean intercept length (mm) between boundaries. The data agreed with the Ashby model, σ∼ (e/l)1/2, only at very low strains (e<0.001) for the specimens with very fine grain sizes (l⩽0.077 μm).

Grain growth in dispersion strengthened superalloys by moving zone heat treatments

Abstract: The high temperature strength of dispersion strengthened materials is limited by grain boundary sliding and transverse boundary rupture. These effects are minimized through development of materials with elongated or fibrous grain structures. Thermomechanical processing and grain growth heat treatments are used to develop coarse elongated grain structures in dispersion strengthened nickel-base superalloys made from mechanically alloyed powder. Grain size and aspect ratio are increased by using a moving hot zone to accomplish grain growth. The application of this technique to extruded bar of two nickel-base superalloys, IN-853* (Alloy A) and a developmental alloy (Alloy B) with composition: 15Cr, 4.5A1, 2.5Ti, 2Ta, 3.5Mo, 4W, 0.l5Zr, 0.01B, 1.1Y2O3 is discussed, and the relation between grain aspect ratio and mechanical properties in these materials is presented.

Single‐domain grain size limits for metallic iron

Abstract: The theory of superparamagnetic (SP) threshold calculations and single-domain (SD) to nonuniform spin threshold calculations is considered. The SD upper grain size limit and the SP-to-SD threshold size are calculated. The size and shape criteria for the stable SD behavior of metallic iron have implications for lunar magnetism and the mechanism of iron-silicate fractionation in the solar nebula.

The high-temperature oxidation of Ni-20%Cr alloys containing various oxide dispersions

Abstract: The oxidation behavior of Ni-20%Cr alloys containing approximately 3 vol.% Y2O3, ThO2, and A12O3 as dispersed particles has been examined in the temperature range 900 to 1200° C in slowly flowing oxygen at 100 Torr. The results show that the oxidation behavior of the Y2O3-, ThO2-, Al2O3-, and Ce02-containing alloys is very similar and that some anomalies in the behavior of the ThO2-containing alloy might be explained by the slower rate of chromium diffusion in this coarse-grained alloy. Two Al2O3-containing alloys were studied. One with a relatively coarse dispersoid size behaved in a manner analogous to a dispersion-free Ni-30% Cr alloy at 1100°C. The other alloy contained a dispersion of fine Al2O3 particles and behaved exactly like the Y2O3-containing alloy at 1000 and 1100°C, but at 1200° C oxidized at a faster rate. It has been shown that the adherent scales on dispersion-containing alloys have a stabilized fine grain size, whereas the nonadherent scales on dispersion-free alloys undergo grain growth.

High‐Temperature Creep of Y2O3‐Stabilized ZrO2

Abstract: The compression creep behavior of Y2O3-stabilized ZrO2 (YSZ) was studied at temperatures to 2000 ° C. The function of Y2O3 content and grain size was tested in specimens with various impurity concentrations and porosity distributions. For relatively fine-grained specimens, creep rates increased with the 1.5 power of the applied stress at low stresses and with the third power at high stresses. The results for coarse-grained specimens can, in general, be fit by the cube dependence. The 1.5 power can be reduced to a linear dependence by correcting for an apparent threshold stress, which decreases with increasing temperature. Creep activation energies for YSZ are 128 ± 10 kcal/mol, independent of Y2O3 content, impurity level, grain size, and porosity distribution. In addition, over a broad range of temperatures and stresses the absolute values of the steady-state creep rates are influenced only by grain size and O2 partial pressure.

The grain size dependence of flow and fracture in a Cr-Mn-N austenitic steel from 300 to 1300K

Abstract: The influence of polycrystal grain size in the range 18 µm to 184 µm on the tensile behavior of an austenitic stainless steel containing by wt pct 21 Cr, 14 Mn, 0.68 N and 0.12 C has been investigated over the temperature range 298 to 1273 K. Decreasing grain size has been shown to increase the flow stress at small strains in accordance with the Hall-Petch relationship at temperatures up to 873 K. The variation of the Hall-Petch constants with temperature is influenced by dynamic strain ageing between 575 and 775 K. Above 875 K, especially at low strain-rates a reversal of the Hall-Petch correlation occurs and the flow stress decreases with decreasing grain size. The relationship between ductility and temperature is marked by a minimum ductility at about half the absolute melting temperature and intergranular cavitation is observed. A decrease in grain size generally enhanced the ductility in this temperature regime whilst at fine grain sizes this trend was reversed. These results are explained in terms of a combination of a Griffith-Orowan type fracture criterion and an intergranular void sheet mechanism of fracture.

Grain refinement through thermal cycling in an Fe-Ni-Ti cryogenic alloy

Abstract: A thermal cycling technique which allows the grain refinement of Fe-12 Ni-0.25 Ti alloy from 40∼60 µm (ASTM #5∼6) to 0.5 ∼ 2 µm (ASTM #1∼18) in four cycles has been developed. The process consists of alternate annealing in γ range and (α + γ) range with intermediate air cooling. The transformation behavior, the change of microstructures and cryogenic mechanical properties on each cycling step are described. Due to the ultrafine grain size, the ductile-brittle transition temperature of this ferritic alloy in Charpy impact testing was suppressed below 6 K. In fracture toughness testing at 77 K, the mode of fracture was altered from brittle quasi-cleavage to complete ductile rupture through the grain refining.

The Effect of Grain Size and Retained Austenite on the Ductile-Brittle Transition of a Titanium-Gettered Iron Alloy.

Abstract: The effect of microstructural changes on the ductile-brittle transition temperature (DBTT) was studied in a titanium-getter ed Fe-8Ni-2 Mn-0.15 Ti alloy. A fairly strong grain size dependence of the transition temperature, 8°C/mm−1/2, was found. Grain size refinement from 38 μm (ASTM #6.5) to 1.5 μm (ASTM #15.5) through a four-step thermal treatment lowered the transition temperature by 162°C. A small amount of retained austenite was introduced into this grain-refined microstructure, and the transition temperature was reduced by an additional 120 ~ 150°C. The reduction of the DBTT due to retained austenite was smaller when the austenite was in a large-grained structure (64°C). The distribution and stability of retained austenite were also studied.

Size‐sorting during suspension transportation—lognormality and other characteristics

Abstract: Grain size distributions of the suspended loads above a bed of bimodal size distribution (size range 2-00-0.04 mm) were studied in a laboratory flume at water velocities varying from 42 to 160 cm/s. With increase of velocity the phi (logarithmic) size distribution of the suspended particles (at 5-20 cm above the bed) changed from a strongly skewed to a nearly symmetrical, unimodal form (nearly lognormal) through an intermediate bimodal stage. At low velocity the skewness of the distribution changed from positive to negative with increase of height. The experiments indicate that lognormality of‘weight frequency’ distribution of grain sizes is a transitional feature, attained through size sorting within a critical range of velocity and height above a sand bed of a given composition. The observed changes in the size distribution patterns were effected by a differential rate of increase in weight in the different size classes in suspension with increase of flow velocity. The phenomenon could be explained by the equation of relative suspension concentration which relates the relative concentration of a suspended particle of a particular diameter to the flow velocity of the turbulent fluid and the height of suspension above the bed.

Ultrafine grain Al-Mg alloy product

Abstract: This invention is directed to an Al-Mg alloy product containing about 0.75-5% aluminide forming transition elements, such as iron, cobalt and nickel. After cold working, the alloy product readily recrystallizes at elevated temperatures to an average grain size less than 15 microns, usually 11 microns or less to provide high mechanical properties and a degree of superplasticity.

The sintering of open and closed porosity in UO2

Abstract: High green density UO2 powder compacts were annealed over a range of temperatures to establish the sintering mechanisms for open and closed porosity. During the intermediate stage of sintering the interconnected porosity was found to sinter at a rate consistent with a volume diffusion process dependent on the instantaneous grain size. The apparent activation energy for open pore sintering was significantly higher than that for volume or grain boundary diffusion, this being attributed to systematic differences resulting from the grain growth process. The fraction of closed porosity increased to a maximum with the collapse of the open porosity then decreased during the final stage of sintering.

Studies of grain production in Sorghum bicolor (L. Moench). III.* The relative importance of assimilate supply, grain growth capacity and transport system

Abstract: In field and glasshouse experiments with grain sorghum (cv. RS610), the assimilate supply was varied by increasing or decreasing radiation and carbon dioxide supply; the potential grain storage capacity was altered by spikelet removal; and the transport system was reduced by incision of the culm. Plants grown at four population densities in the field were manipulated to increase (by removing neighbouring plants) or decrease (by shading) the supply of photosynthates during grain filling. These treatments affected grain size and thus yield. Removal of some of the spikelets at three-quarter anthesis resulted in a significant increase in the size of those grains remaining at maturity. From anthesis onward, a reduction in the capacity of the transport system in the culm had no significant effect on grain yield. These results are interpreted as evidence that grain yield is not limited by the storage capacity of the grain, or by the transport system involved in moving material from the stem to the grain. Treatments which altered the demand for assimilates by the grain, relative to the supply, did not affect net photosynthesis. Dry matter produced in excess of grain requirements accumulated in other plant parts, including the root. Potential grain size was influenced by interspikelet competition operating within 1 week after three-quarter anthesis. *Part II, Aust. J. Agric. Res., 22: 39-47 (1971).