Showing papers on "Grain growth published in 1977"

Theory and Applicability of a Recrystallized Grain Size Paleopiezometer

Abstract: An approximate theoretical relation is derived which relates stress during steady state creep to both subgrain size and dynamically recrystallized grain size. The relation results from equating the dislocation strain energy in the grain boundary to that in the enclosed volume. Available data on metals and silicates are in excellent agreement with the theory. For paleopiezometry, the recrystallized grain size must be preserved by quenching, by cooling under stress, or by inhibition of grain growth by intimate mixture of two or more phases. In general, stress may be underestimated using rocks in which grain size has been reduced by dynamic recrystallization, especially if the grain size is very small. Stress may be overestimated using coarse grained rocks in which the grain size has increased toward the steady state value. Quantitative limits remain to be established. The theoretical relation can in principle be applied to any metal or mineral if only the effective isotropic elastic moduli and the Burgers vector are known. When used as a paleopiezometer, the technique indicates that high stresses on the order of 100 MPa are not infrequently associated with mantle diapirism and with large scale thrust faulting. Consideration of the Mt. Albert ultramafic body suggests that texturally inferred stresses from peridotite massifs and from ultramafic xenoliths in alkali olivine basalts might reflect either horizontal variations in stress across a rising diapir or else a vertical variation in stress as defined by the pyroxene geobarometer (Mercier et al. 1977). In either case the stresses are probably characteristic of local diapirism. Stresses characteristic of global upper mantle flow might be inferred from xenoliths originating from above kimberlite-producing diapirs.

Factors Influencing the Rate and Duration of Grain Filling in Wheat

Abstract: Controlled-environment conditions were used to examine the effects of cultivar and of temperature and illuminance after anthesis on grain setting and on the duration and rate of grain growth. After an initial lag period, which did not differ greatly between cultivars, grain dry weight increased linearly under most conditions until final grain weight was approached. Growth rate per grain depended on floret position within the ear, varied between cultivars (those with larger grains at maturity having a faster rate), and increased with rise in temperature. With cultivars in which grain number per ear was markedly affected by illuminance, light had relatively little effect on growth rate per grain. With those in which grain number was less affected by illuminance, growth rate per grain was highly responsive to it, especially in the more distal florets. In both cases there was a close relation between leaf photosynthetic rate as influenced by illuminance, the rate of grain growth per ear, and final grain yield per ear. The duration of linear grain growth, on the other hand, was scarcely influenced by illuminance, but was greatly reduced as temperature rose, with pronounced effects on grain yield per ear. Cultivars differed to some extent in their duration of linear growth, but these differences accounted for less of the difference in final weight per grain than did those in rate of grain growth. Under most conditions the cessation of grain growth did not appear to be due to lack of assimilates.

The influence of temperature and light intensity on grain growth in relation to the carbohydrate and nitrogen economy of the wheat plant

Abstract: The response of wheat grain growth to temp and light intensity was studied under controlled conditions within the ranges 10-25 deg C and 64-188 W/m2, resp Warmth hastened plant senescence and enhanced the initial growth rate of the grains Additional light promoted the rate of grain growth more at high than at low temp; under the latter conditions there was a considerable accumulation of carbohydrates in the stem (up to 40%) from anthesis onwards The rate of grain growth ranged from 070 to 164 mg/day grain The duration of grain growth was prolonged by decrease from 25 to 10 deg ; the increase in growth duration from about 30 to 80 days corresponded with a relatively stable temp sum Temp and light also affected the redistribution of assimilates and the chemical composition of the grain The rate of protein synthesis was promoted more by warmth than the rate of starch synthesis This resulted in an increased N content of the grain The final content of total non-structural carbohydrates (starch and sugars) was slightly decreased by warmth Additional light raised the carbohydrate content of all parts of the plant and so decreased the N content of these parts However, light intensity had less effect on N distribution and yield than temp (Abstract retrieved from CAB Abstracts by CABI’s permission)

Nitrogen, Phosphorus and Water Contents During Grain Development and Maturation in Wheat

Abstract: Plants of five cultivars of wheat were grown under controlled-environmental conditions in order to analyse the effect of cultivar and of temperature and illuminance after anthesis on the accumulation of nitrogen and phosphorus by grains in relation to dry matter. The water relations of the grain during maturation were also examined, using calcium content as an index of water entry. The nitrogen and phosphorus contents of grains increased linearly throughout the grain growth period. The percentage of nitrogen and phosphorus in grains fell sharply during the first few days after anthesis but rose progressively thereafter. The higher the temperature, and the lower the illuminance, the higher was the percentage of nitrogen in the grain of all cultivars. Such conditions also reduce final grain size, but their effects on nitrogen concentration in the grain were apparent early in grain development. No evidence was found of a flush of nitrogen or phosphorus into the grain late in its development. Water entry into the grain continued at a steady rate until maximum grain dry weight was reached, then ceased suddenly. No evidence was found of an increased rate of water loss by the grain at that stage, and the rapid fall in water content at the cessation of grain growth may have been due to blockage of the chalazal zone of entry into the grain by the deposition of lipids. Accumulation of dry matter, nitrogen and phosphorus and entry of water into the grain all ceased at the time of lipid deposition in the chalazal zone.

Metallurgical factors affecting the crack growth resistance of a superalloy

Abstract: During creep loading of IN-792, grain boundary morphology in conjunction with grain size strongly affected crack propagation. Compositional variations and fabrication techniques showed no significant effect. A primary requirement for materials to be used in gas turbine engine discs is satisfactory resistance to crack growth resistance in the 650 to 760°C range. Both conventional smooth and machine notched stress-rupture samples and dead weight loaded fatigue precracked fracture toughness specimens were evaluated in this study. Creep fractures took place by grain boundary cracking followed by rapid transgranular fractures. Composition variations had only very slight effects on crack propagation. Materials hot worked from castings had the same properties as those made by powder metallurgy techniques. The primary factors influencing the crack growth behavior were the grain size and grain shape. Increasing grain size markedly improved the toughness. By slow cooling through the gamma prime solvus a serrated grain boundary structure was developed that also improved the cracking resistance. Earlier creep fracture toughness studies have shown that the slow crack growth behavior can be described by a critical strain model in which the crack propagation is controlled by the yield strength, grain size, and a critical strain parameter. The present results are consistent with this model, with serrated grain boundaries introducing a four-fold increase in the critical strain parameter over that of smooth grained material.

Superplastic deformation of Ti-6Al-4V alloy

Abstract: The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium. Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight line when plotted as log (ekTd* 2/DgbGb3) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.

Reduction of grain boundary recombination in polycrystalline silicon solar cells

Abstract: The possibility of increasing the carrier collection efficiency in polycrystalline silicon by means of a heavily doped region near the grain boundaries is investigated. Phosphorous dopant is preferentially introduced into the grain boundaries of p‐type material by a low‐temperature diffusion process. A subsequent high‐temperature diffusion forms a highly n‐doped skin covering each grain. The resulting junction around each grain surface collects electrons which might otherwise recombine at the grain boundaries. This grain boundary doping scheme makes possible an increase in the conversion efficiency of polycrystalline silicon solar cells in which the grain structure is columnar.

The growth and structure of oxide films formed on Fe in O2 and CO2 at 550°C

Abstract: A study has been made of the structure of oxide layers formed at different times on abraded Fe oxidized in 1 atm O2 and CO2 at 550°C. A duplex Fe3O4 layer was formed and the inner layer was considered to grow by an oxide dissociation mechanism. The growth of both layers has been explained by a model, which correlates the overall kinetics with oxide grain growth. Derived values of the parabolic rate constant for lattice diffusion have been used to calculate self-diffusion coefficients, which were in good agreement with literature values for Fe diffusion in Fe3O4, but were very much larger than the values for either Fe or O in α-Fe2O3.

Grain boundary diffusion creep in magnesium

Abstract: The creep of pure polycrystalline magnesium has been investigated at stresses beluw 6 MPa and temperatures from 150 to 325°C using both spring and tensile specimens. Examination of the grain size and temperature dependence of the creep rate revealed that, at the lower stresses, deformation occurred by the grain boundary diffusion creep mechanism of Coble, although the rate depended upon stress in a Bingham rather than a Newtonian manner, i.e. there was a threshold stress below which no diffusion creep occurred. At the lowest temperatures, threshold stresses ∼ 1 MPa were found in fine-grained material, decreasing to ∼0.1 MPa as the grain size and temperature were increased. Existing theories were unable to account for the magnitude of the threshold stress. Calculation of magnesium grain boundary self-diffusivity from the creep data gives a value of wDgb = 8 × 10−10 exp -(l05 × 103/RT) m3 S−1 where R is in J mol−1 K−l and w is the grain boundary width.

Effect of slight deformations on grain growth in iron

Abstract: The influence of slight deformations on grain growth after primary recrystallization has been investigated in pure iron specimens. A stage of abnormal growth is always observed in the range of deformations studied (about 2 to 10% in tension), whilst the absence of deformation leads to normal growth. Some kinetic considerations are made, on the basis of current theories.

Prior austenite grain growth in heat-affected zone of a O.5Cr-Mo-V steel

Abstract: A weld thermal-simulation technique has been used to produce prior austenite grain sizes similar to those observed in the heat-affected zones (HAZs) of O.5Cr-Mo-V welds. Grain-size measurements have been used to develop an Arrhenius equation for grain growth in artificial HAZs. This equation is in reasonable agreement with an ‘impurity drag’ theory for grain growth. In applying the equation to weld HAZs it was found that the steep thermal gradients present resulted in a ‘thermal pinning’ effect which inhibited growth. A final expression has been obtained which describes prior austenite grain growth in manual metal arc weld HAZs in 0.5 Cr-Mo- V steel as a function of heat input. The practical implications of this equation are discussed in terms of grain-size control in the HAZ during welding and these indicate that finegrained HAZs can be produced by low heat inputs and a suitable weld-bead overlap technique. In principle, the approach described

On the formation of the diamond grain configuration during high temperature creep and fatigue

Abstract: A study has been made of the influence of test variables on the formation of the diamond grain configuration during high temperature creep and fatigue deformation of a wide variety of metals. The proposed mechanisms for the formation of this interesting grain morphology are reviewed. It is concluded that the diamond grain configuration arises from a balance between grain-boundary sliding, grain-boundary mobility, intragranular deformation and defect imbalance across the grain boundaries and that it tends to be stabilized by intergranular cavitation. While the phenomenon occurs during high temperature fatigue in a variety of metals irrespective of their crystal structure, during creep it has been observed only in to h c p metals. It is surmised that the occurrence of the diamond array of grain boundaries during creep deformation in h c p metals is aided by the limited number of slip systems which leads to high defect imbalances in adjacent grains and consequently high driving forces for grain-boundary migration. On the basis of quantitative metallography involving measurements of the number of edges per grain section, the number of grains meeting at vertices, angular distribution histograms and grain-boundary lengths in different angular orientations with respect to the stress axis in "annealed" and "diamond" microstructures, it is concluded that the shape of the "diamond" grain is essentially the same as that of the "annealed" grain but in a distorted form.

Grain Oriented Silicon-Iron with a Unique Inhibition System for Texture Development

Abstract: The new high induction grain oriented silicon-iron alloys are characterized, as compared to the conventional alloy, by a heavier final cold reduction and increased restraint to normal grain growth. It is shown that the conventional alloy possesses sufficient restraint to normal grain growth to undergo secondary recrystallization when it contains manganese sulfide but not when it contains sulfur only as solute. However, the presence of boron, nitrogen and uncombined sulfur leads to strong grain growth inhibition and, when the strip is heavily cold rolled, to a high induction. A deficiency in any one of the three elements results in grain coarsening rather than a nearly constant grain size over a broad temperature range prior to secondary recrystallization.

Austenite grain growth in medium and high-carbon steels microalloyed with niobium

Abstract: The influence of niobium (0.03

The morphology and crystallography of directionally solidified pb-sn eutectic alloys

Abstract: Directional solidification of Pb-Sn eutectic alloys at high temperature gradients has shown that two distinct eutectic morphologies occur, a regular lamellar structure and a wavy lamellar structure, termed degenerate. An electron channelling technique was used to characterize the crystallography of the two morphologies. The degenerate grains grow in advance of the regular grains and are the preferred growth morphologies in spite of the fact that coarsening experiments revealed that the degenerate grains possessed a higher lamellar interface energy. This result conflicts with the Zener model of eutectic growth. A possible rationalization of this conflict based on the anisotropy of thermal conductivity in Sn is presented.

Development and characterization of an improved Ir--0. 3% W alloy for space radioisotopic heat sources

Abstract: An Ir--0.3% W alloy has been doped with minor alloying additions to develop an improved cladding material for space radioisotopic heat sources operating at temperatures to 1450/sup 0/C. Tensile tests of a series of doped alloys at slow strain rates indicated that the DOP-4 alloy containing 40 Al, 30 Th, 80 Fe, 10 Ni, and 75 Rh ppm was most resistant to the brittle fracture associated with grain-boundary separation. In addition, the DOP-4 dopants raise the recrystallization temperature, and retard grain growth at high temperatures. The impact properties of DOP-4 and undoped Ir--0.3 W alloys were determined as a function of grain size, test temperature, impact velocity and long-term heat treatment. All the results indicate that the impact properties of the DOP-4 alloy are far superior to those of the undoped alloy. The improvement results from the segregation of beneficial dopant(s) (such as thorium) to grain boundaries and precipitation of second-phase particles. The development of the DOP-4 alloy greatly improves the safety margin of the Multi-Hundred-Watt heat sources to be used in a space probe to Jupiter and Saturn in 1977.

Creep mapping in a polycrystalline ceramic: application to magnesium oxide and magnesiowustite

Abstract: The construction of deformation mechanism maps for a polycrystalline ionic solid in which anion and cation transport are coupled has been demonstrated. Because of anioncation ambipolar coupling, two regimes of Coble creep are possible in systems where anion grain boundary transport is rapid: (1) rate-controlled at low temperatures and small grain sizes by cation grain-boundary diffusion, and (2) rate-limited at high temperatures and large grain sizes by anion grain-boundary diffusion. A new type of deformation mechanism map was introduced in which the temperature and grain size were primary variables. This map was shown to be particularly useful for materials which deform primarily by diffusional creep mechanisms. Ambipolar diffusional creep theory was used to construct several deformation mechanism maps for polycrystalline MgO and magnesiowustite over wide ranges of stress, grain size, temperature and composition.

Positron lifetime as a function of grain size

Abstract: Published data on positron annihilation lifetime in copper as a function of grain size have been analyzed to show that there is a linear relationship between the internal grain boundary surface area, per unit volume,S v, and the positron lifetime, τ. The analysis indicates that grain boundaries are important in the trapping of positrons. It is suggested that the slope of the resulting straight line,dS v/dτ, can be used to determine the annihilation rate of the grain boundaries.

Properties of superplastic Cr-Ni steel

Abstract: Superplastic flow has been investigated in the steel X5 CrNiTi 26.6 by means of the hot tensile test. Particular attention has been paid to the magnitude of flow stresses, strain-rate sensitivity, and total elongation with its dependence on microstructure (controlled by technological treatment), strain rate, and temperature. Of all the microstructural parameters examined, the matrix grain size seems to have the predominant influence. The steel exhibits superplastic behaviour if the matrix has a grain size Dα, ≤4.5μm,if the strain rate e is <10−1 min−1, and if the temperature is above 875° C. At temperatures ≥ 1000°C in the α- and γ- regions, grain growth occurs, and the superplastic flow consequently terminates. As temperature and strain rate are connected in an Arrhenius-type relationship, an activation energy for flow processes in the ferrite-austenite phase mixture can be determined for the range of ferrite grain sizes Dα,= 3.2 to 5.4 μm.The activation energy depends linearly on matrix grain si...