The development of macrostructure in ingots of increasing size

A series of Al-Cu ingots ranging in volume from approximately 50 to 500 and to 5000 cc has been conventionally prepared in two ways, by normal chill casting or by using a magnetic field to control the convective currents. Comparisons allow us to differentiate among the various mechanisms proposed for the origin of equiaxed grains and for the columnar-to-equiaxed transition in castings: i) In normal casting, with the natural convection associated with standard superheats, the majority of grains is provided by the mechanism suggested by Chalmers (and also known as the Big Bang), or by crystal settling from a free surface as suggested by Southin. ii) If new grains, no matter how they may form, are inhibited from mixing with the rest of the liquid, no significant equiaxed structure appears. iii) Unless some special or preselected conditions prevail, other mechanisms such as those involving constitutional supercooling or dendrite remelting, do not seem operative in the formation of a central equiaxed zone.     

The effect of deformation on abnormal grain growth in tungsten ingots

This paper reports a study of abnormal grain growth in tungsten ingots that had been deformed in compression at elevated temperatures. The results show that very large grains form in these samples directly from the polygonized structure when the deformed samples are annealed. A critical amount of deformation is required to begin this process. Beyond that point, the resulting grain size decreases with increasing amounts of deformation and decreasing test temperature. Abnormal grain growth occurs first in the regions of the sample that have undergone the most strain. Two factors appear to provide the driving force for the formation of these large grains. One is the elimination of grain boundary area. This effect would be present in any system undergoing regular or abnormal grain growth. The other is the elimination of grains that are more highly strained by grains that are less strained. In this way, this process is similar to the large grain growth resulting from strain annealing.     

Control over grain size in memory reporting--With and without satisficing knowledge.

When answering questions from memory, respondents strategically control the precision or coarseness of their answers. This grain control process is guided by 2 countervailing aims: to be informative and to be correct. Previously, M. Goldsmith, A. Koriat, and A. Weinberg Eliezer (2002) proposed a satisficing model in which respondents provide the most precise answer that passes a minimum-confidence report criterion. Pointing to social-pragmatic considerations, the present research shows the need to incorporate a minimum-informativeness criterion as well. Unlike its predecessor, the revised, "dual-criterion" model implies a distinction between 2 theoretical knowledge states: Under moderate-to-high levels of satisficing knowledge, a grain size can be found that jointly satisfies both criteria--confidence and informativeness. In contrast, under lower levels of unsatisficing knowledge, the 2 criteria conflict--one cannot be satisfied without violating the other. In support of the model, respondents often violated the confidence criterion in deference to the informativeness criterion, particularly when answering under low knowledge, despite having full control over grain size. Results also suggest a key role for the "don't know" response which, when available, can be used preferentially to circumvent the criterion conflict. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

NS3105合金晶粒度极差控制趋向探讨

自日本福岛核电站事故后,我国已将以AP1000为代表的第三代核电堆型作为今后大力发展的堆型,并以此为基础着重研发具有自主知识产权的更大型的CAP1400/CAP1700核电堆型.第三代核电机组设计使用寿命60年,大大高于目前普遍使用和建设的二代改进型CPR1000核电机组40年的使用寿命,其在NS3105合金U形管材的各项技术指标均有大幅的提高,其中对晶粒度组织首次提出了极差控制且在窄范围的要求,而NS3105高镍耐蚀合金的晶粒度组织极差控制是一个难点.从材料主要成分、源料的晶粒度、冷加工热处理制度控制方向等方面探讨了成品晶粒度如何控制才能达到窄范围极差的方向.     

On the relation between grain size and grain topology

Experimental studies of the topology of grains in polycrystals have indicated that the topological complexity of a grain is related to its diameter, as opposed to its surface area or volume. This paper presents additional experimental documentation of this correlation and a theoretical derivation of the empirically observed relationship.     

A tight-binding study of grain boundaries in silicon

A simple empirical tight-binding model for silicon is propounded and used to compute the atomic and electronic structures of three symmetrical tilt grain boundaries and the intrinsic stacking fault. The ability of the model to describe silicon in a variety of crystal structures is tested and it is shown to be satisfactory for simulating defects in the diamond structure. The effect of charge transfer on the energy and stability of the grain boundaries is assessed. Interatomic forces and energies are computed in real space using a rotationally invariant formulation of the recursion method. Five proposed reconstructions of the (112̄) symmetrical tilt boundary are studied in detail and good agreement is achieved with results from electron microscopy and diffraction. The (13̄0) and (111) symmetrical tilt boundaries have also been modelled successfully. Comparison is made between the computed electronic structures of the boundaries, reported in this work and by other authors, and experimental measurements of the densities of states at grain boundaries. The existence of band tails and midgap continua in the experimental measurements and the absence of both of these features in the models are two notable points of disagreement. Some fundamental questions about localisation of electronic states at grain boundaries are raised.     

The study of grain size dependence of yield stress of copper for a wide grain size range

Room temperature yield strength of copper has been measured by means of miniaturized disk-bend test as a function of grain size ranging from about 30 nm to 180 μm. It has been established that grain size dependence of strength does not obey the Hall-Petch relation in the entire range of grain sizes studied. The results obtained suggest that a gradual change of deformation mechanism takes place with decreasing grain size. Nanostructured samples appear to be rather ductile.     

Predicting the ferrite grain size in low-alloy steel

Mathematical models are proposed for calculating the onset temperature of ferrite transformation and the ferrite grain size as a function of the cooling conditions and the initial austenite grain size. The models are calibrated on the basis of experimental data for steels with ferrite-pearlite microstructure. The models are integrated in HSMM software, with considerable gain in computational power when considering the mechanical properties of steel rolled on the 2000 mill at OAO Severstal??.     

Evolution of grain size and grain shape during thermomechanical processing in a powder metallurgical nickel-based superalloy

With a strain rate range of 0.01-10 s-1and a deformation temperature range of 1110-1200℃,the isothermal compression test was performed on one powder metallurgy ...     

The effect of extrinsic grain boundary dislocations on the grain size strengthening in polycrystals

The grain size dependence of flow stress at room temperature in the regime of small strains (0–2%) has been examined in as-annealed and 2% pre-strained and subsequently annealed specimens of 316L stainless steel. Grain sizes in the range of 3.4–22.4 μm were considered. The stress-strain curves exhibit linear hardening characteristic beyond 0.2% plastic strain. The analysis of the Hall-Petch parameters show a linear increase in σ₀(ϵ) and a linear decrease in K(ϵ) with strain in the as-annealed specimens. The increase in σ₀(ϵ) has been associated with both, the work-hardening processes in the grain interior and the long range stress field of extrinsic grain boundary dislocations (EGBDs). The EGBDs also act as sites of stress concentration thereby making it easier to generate dislocations in the vicinity of grain boundaries. Therefore, K(ϵ) which is function of the stress required to generate dislocations decreases with increasing strain. The observed drop in flow stress as a result of annealing of pre-stained specimens at 800°C has been related with the annihilation of dislocations at and in the vicinity of grain boundaries. Annealing at 550°C (this temperature is sufficient for the delocalization of the cores of EGBDs) does not have any significant effect on the density of dislocation at and in the vicinity of grain boundaries. Therefore, no significant change in the flow stress was obsereved.     

Stochastic modeling of the independent roles of particle size and grain size in transgranular cleavage fracture

The independent roles of grain size and particle size on sharp crack and rounded notch toughness are investigated over a range of temperatures from the lower shelf into the early ductile/brittle transition region. The results are interpreted in terms of a weakest link statistical model wherein the onset of failure coincides with the critical propagation of a particle microcrack into the matrix. It is shown that, for a fixed particle size distribution, both sharp-crack and rounded-notch toughness decrease with increasing grain size. However, at fixed grain size, the sharp-crack toughness increases, while the rounded-notch toughness decreases with increasing particle size. Such effects result primarily from the difference in the number of activated particles in the plastic zone. Formerly Graduate Student in the Department of Materials Science and Mineral Engineering, University of California, Berkeley. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2–6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

Abnormal grain growth in bulk Cu—The dependence on initial grain size and annealing temperature

The dependence of abnormal grain growth (AGG), also termed secondary recrystallization, on annealing temperature in the range between 600 °C and 1050 °C has been observed in pure bulk Cu specimens compressed to various levels between 5 and 75 pct. There is no grain texture after annealing. The average grain size after primary recrystallization, which represents the initial grain size for secondary recrystallization during further annealing, decreases with increasing deformation and is nearly independent of the annealing temperature, in agreement with previous observations. The incubation time for AGG decreases and the number density of abnormally large grains increases with increasing deformation (hence, a decreasing initial grain size) and increasing annealing temperature. At low temperatures, most of the grain boundaries are faceted, with some facet planes probably of singular structures corresponding to cusps in the polar plots of the grain-boundary energy vs the grain-boundary normal. With increasing temperature, the grain boundaries become defaceted and, hence, atomically rough. The observed grain-growth behavior appears to be qualitatively consistent with the movement of faceted grain boundaries by two-dimensional nucleation of boundary steps. The temperature dependence appears to be consistent with roughening of grain boundaries. Before the onset of AGG, stagnant growth of the grains occurs at low rates, probably limited by slow two-dimensional nucleation of boundary steps, and, at low deformations and low annealing temperatures, the stagnant growth persists for 100 hours. The specimens with relatively small initial grain sizes (because of high deformation) show double AGG when annealed at high temperatures.     

Weibull尺寸分布晶粒组织演变的仿真研究

采用一种改进的Potts模型Monte Carlo算法,对具有Weibull尺寸分布(参数β=3.47)的晶粒组织进行了3D正常晶粒长大过程的仿真研究.仿真结果表明:整个晶粒长大过程遵循抛物线长大规律,晶粒生长指数为0.501,非常接近理论值0.5.晶粒长大过程可分为过渡阶段与准稳态长大两个阶段.Weibull尺寸分布参数β由过渡阶段的3.47逐渐演变为准稳态阶段的2.76,准稳态阶段晶粒尺寸分布参数保持β=2.76不变.晶粒的平均面数〈f〉随仿真时间的增加而增大,在准稳态阶段后期趋近于稳定数值.晶粒面数分布为Lognormal分布,最高频率面数f为10,个体晶粒面数范围为3~43.     

Strategic regulation of grain size memory reporting.

To increase their report accuracy, rememberers may either withhold information that they feel unsure about or provide relatively coarse information that is unlikely to be wrong. In previous work (A. Koriat & A Goldsmith, 1996c), the authors delineated the metacognitive monitoring and control processes underlying the decision to volunteer or withhold particular items of information (report option) and examined how these processes are used in the strategic regulation of memory accuracy. This article adapts that framework to address control over the grain size (precision- coarseness) of the information that people report. Results show that rememberers strategically regulate the grain of their answers to accommodate the competing goals of accuracy and informativeness. The metacognitive processes underlying this regulation are elucidated. (PsycINFO Database Record (c) 2010 APA, all rights reserved)