Abnormal grain growth in pure materials

Thompson et al. [Acta metall.35, 887 (1987)] have proved that if a large grain of size R_A is introduced in a matrix which has Hillert's [Acta metall.13, 227 (1965)] quasi-stationary grain size distribution this large grain will decrease its size relative to the critical radius of the distribution, R_Cr, so that d(R_A/R_Cr)/dt < 0. In the present work the effect of having a matrix distribution different from Hillert's is examined. It is shown that in such case the behaviour of an abnormally large grain is more complex and either d(R_A/R_Cr)/dt < 0 or d(R_A/R_Cr)/dt > 0 are initially possible. The present analysis is applied to lognormal distributions with different values of side deviation.     

On the analysis of grain size in bulk nanocrystalline materials via x-ray diffraction

The Warren-Averbach (WA) analysis and other simplified methods that are commonly used to determine the grain size of nanocrystalline materials are discussed in terms of accuracy and applicabilities. The nanocrystalline materials used in the present study are prepared by cryomilling of A1 powders and subsequent consolidation (hot isostatic pressing and extrusion). Transmission electron microscopy observations of the as-extruded nanocrystalline A1 reveal a bimodal distribution of grain sizes centered around 50 to 100 nm and 250 to 300 nm. It is shown that the grain size determined by the WA analysis agrees with the lower bound grain size (e.g., 50 to 100 nm) observed experimentally. In the case of the integral method, it is useful to use a parabolic (Cauchy-Gaussian (CG)) relationship to approximate instrumental broadening and separate the intrinsic broadening. Compared to the Cauchy-Cauchy (CC) and Gaussian-Gaussian (GG) approximations, this is shown to give the best results. In addition, the reliability of the Scherrer equation is also discussed.     

Abnormal grain growth in materials containing particles

The behaviour of an abnormally large grain in a matrix containing stable particles in which normal grain growth is initially possible is examined by means of a theoretical model. It is found that an abnormally large grain will initially decrease its size relative to the critical radius of the distribution. As normal grain growth continues and the grain growth rate decreases due to the increasing action of the particles this situation is reversed and the abnormally large grain starts to increase its size relative to the critical radius. The model satisfactorily explains why the occurrence of severe abnormal grain growth in the presence of particles is often apparently “erratic”.     

Evaluation of grain growth criteria in particle-containing materials

The present work consists of a critical evaluation of the theoretical approaches describing the grain growth inhibiting effect of second phase particles in a material. The combination of the most realistic relations giving the driving force for grain growth and the pinning force of the particles, results in a satisfactory estimation of the limiting grain size, as a function of the structure parameters (homogeneity, particle size and volume fraction). Some special particle configurations require a refinement of the general model, e.g. in the case of a specific particle shape or when the precipitates are not homogeneously sized or dispersed. The validity of the general and adjusted grain growth criterion is shown in β-Cu-based shape memory alloys with a particle forming addition.     

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.     

The effect of small deformation on abnormal grain growth in bulk Cu

The effect of small deformation below the level (about 8 pct) required for primary recrystallization on abnormal grain growth (secondary recrystallization) has been investigated in bulk polycrystalline Cu. The starting microstructure, without any texture and with a nearly uniform grain size of 168 μm, has been obtained by compressing a cylindrical Cu specimen and recrystallizing at 800 °C. The fully recrystallized specimen shows distinct abnormal grain growth (AGG) after heat treatment at 800 °C for 12 hours. Most of the grain boundaries are faceted when observed under transmission electron microscopy (TEM), and most of the faceted segments are expected to be singular. A singular grain boundary free of defects will migrate by two-dimensional nucleation of new layers, with its velocity varying nonlinearly with the driving force arising from the grain-size difference. Such a growth mechanism is analogous to the well known process for the growth of crystals with singular surfaces from liquid or vapor. The grains slightly larger than the average size will hardly grow, because the driving force for their growth is not sufficient for nucleation of new crystal layers at the boundaries. Those grains larger than a certain critical size will, however, grow at ever-increasing rates with their increasing size, because of the sufficient driving force for two-dimensional nucleation. Such a selective accelerated growth of large grains results in overall AGG behavior. The specimen deformed to 2 pct shows AGG after heat treatment for only 5 minutes at 800 °C, and after 1 hour, large impinged grains are obtained. The grain boundaries show many extrinsic dislocations even after the heat treatments. As proposed earlier by Gleiter, Balluffi, Smith, and their colleagues, the extrinsic grain-boundary dislocations increase the grain-boundary mobilities even at low driving forces, and, hence, even those grains slightly larger than the average size can rapidly grow at the early stages of the heat treatment, in agreement with the observation. In the specimens deformed to 4 to 8 pct, below the level for primary recrystallization, all grains grow steadily without producing distinct AGG. With high densities of extrinsic dislocations at the grain boundaries even after long heat treatments, all grains can readily grow, resulting in overall growth patterns resembling the normal growth. When deformed to 20 and 50 pct, primary recrystallization occurs, and the subsequent AGG behavior depends on the grain size obtained at the completion of the primary recrystallization. Similar small-deformation effects are observed with heat treatment at 600 °C.     

Modeling grain growth dependence on the liquid content in liquid-phase-sintered materials

A model for grain growth during liquid-phase sintering (LPS) is presented. A Rayleigh grain size distribution is assumed based on both experimental and theoretical results. This asymmetric distribution provides a continuous driving force for coarsening. The model uses the solid grain contiguity to calculate the relative solid-state and liquid-phase contributions to coarsening. The level of grain agglomeration affects both the mean diffusion distance and interface area over which diffusion occurs. A cumulative grain growth rate is calculated assuming independent solid and liquid contributions to coarsening. Consequently, only the liquid volume fraction and solid-liquid dihedral angle are required to predict the change in grain coarsening rate with solid-liquid ratio. A prior empirical correlation between the grain growth rate constant and the liquid volume fraction is compared to the resulting analytic form, showing excellent agreement. The new model is projected to be generically applicable to microstructure coarsening in multiple phase materials, including porous structures.     

电子散料秤在码头散装物资商贸计量中的应用

首先简要介绍一种用于码头散装物资运输计量的高精度的计量系统——电子散料秤的工作原理、结构、功能特点,并将其性能与其它常用于散装物资计量的设备进行比较,然后给出了散料秤在湖北新冶钢有限公司200万t码头卸船和装船物资散装物料计量中的应用效果。     

超声空化对陶瓷刀具材料晶粒生长的影响

为了研究超声无压烧结陶瓷刀具材料时超声空化对晶粒生长的影响,分析了气孔在熔融金属中发生超声空化的条件,建立了含有空化泡的晶粒模型,讨论了超声空化对晶粒的作用,并采用蒙特卡罗法模拟了未施加和施加超声下的晶粒生长过程,研究了超声空化对晶粒生长过程的影响。结果表明:当空化泡的半径介于1 μm～2 μm时,超声波的声压阈值为8.02×106 Pa,频率阈值为2.00×106 Hz;超声空化可增大晶格振动频率和振动能量,阻碍晶粒生长,起到细化晶粒和减小孔洞的作用。     

粗细晶WC基体对WC-Co非均匀结构显微组织形成的影响

本文研究了粗晶WC基体与细晶WC基体对WC-8Co合金的非均匀结构组织形成的影响。结果表明,粗晶WC为基体,细晶WC添加量35%以上时,形成粗、细WC晶粒尺寸分别为7.0～8.0μm、2.0～2.5μm的非均匀结构显微组织。细晶WC为基体,粗晶WC添加量为45%时,形成粗、细WC晶粒尺寸分别为6.0～7.0μm、0.4～0.5μm的非均匀结构显微组织。在合适的工艺参数条件下,可获得粗、细晶分别为8.0～9.0μm、2.0～3.0μm的非均匀结构的合金组织。     

A kinetic and mechanistic study of the hydrolysis of camptothecin and some analogues

Quantitative rate and equilibrium constants for the hydrolysis of the lactone (ring E) in camptothecin (1) and analogues (2-5, all substituted in the A ring of 1) at 25 degrees C in aqueous solution were determined by high-performance liquid chromatography with UV-visible detection and by UV-visible spectrophotometry. It was shown that the lactone was converted to the carboxylate in a pH-dependent equilibrium. No major differences were observed in rate and equilibrium constants for 1-5, a result suggesting that the mechanism of lactone hydrolysis is independent of a variety of substituents on the A ring. The conversion of the lactone to its carboxylate form occurred under neutral and basic conditions and appeared to be largely dependent on hydroxide ion. The conversion of the carboxylate to the lactone was observed under neutral and acidic conditions and was pH independent at pH values greater than 5 and dependent on hydronium ion at pH values less than 5. Significant incorporation of oxygen-18 into the lactone ring of 3, a water-soluble analogue of 1, was observed for the ring opening and subsequent ring closing of 3 in H2(18)O. This finding strongly suggests that the mechanism of lactone ring hydrolysis involved acyl cleavage rather than alkyl cleavage. Kinetic solvent isotope effects (kH2O/kD2O, where kH2O and kD2O are the rate constants in H2O and D2O, respectively), obtained over the pH-rate profile for 1, were used to speculate about the nature of the transition states involved in the opening and closing reactions of the lactone ring.     

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.     

Taking into account grain growth during the finite-element modeling of the superplastic formation of promising construction materials

A way to take into account the influence of grain growth in the statement of the edge problem for the mechanics of a deformed solid as applied to the superplastic formation of microcrystalline and ultra-fine-grain materials is suggested. The concrete example of a numerical solution for the edge problem of creep theory in a medium of the ANSYS software complex according to the published experimental data is suggested. A satisfactory agreement between the results of the solution to the edge problem and the experimental data is obtained. This solution is analyzed, and certain features of the character of the stress-strained state in the deformation site are revealed. In particular, it is established that complex disproportionate loading occurs in the vicinity of the fastening contour. Under this loading, the stress trajectories in the Il’yushin space have a larger curvature, despite the fact that the curvature of the corresponding deformation trajectories in the deformation space is relatively small.     

The differences in growth of cerebellar vermis between appropriate-for-gestational-age and small-for-gestational-age newborns

Positive and negative symptoms are measurable characteristics that may represent core features of schizophrenia and offer a quantitative approach for studying the genetics of schizophrenia and related disorders. The Positive and Negative Syndrome Scale (PANSS) was used to assess 72 members of five families segregating schizophrenia. The study confirmed high internal reliability of PANSS scales in this sample with diverse lifetime diagnoses. Gender but not alcoholism affected scores. Schizophrenia/schizoaffective and schizophrenia spectrum disorder groups had higher mean scores for the positive and negative scales than other lifetime diagnostic groups, consistent with genetic transmission of these symptoms. Positive and negative symptom patterns did not subtype families. The results support the validity of positive and negative symptom measures as independent dimensions in familial schizophrenia.