Computer simulation of grain growth

We present a model calculation which allows us to simulate both steady state and dynamic grain growth. The model yields a steady state size distribution which resembles Hillerts distribution, but is somewhat more sharply peaked. The dynamic relaxation of a non-stationary distribution towards a stationary distribution is also included in the calculations presented.     

The effect of curvature on the grain boundary migration induced by diffusional coherency strain in mo-ni alloy

When a liquid phase sintered Mo-Ni alloy is heat-treated at 1400°C after replacing the liquid matrix with a Cu melt, the grain boundaries between some grains migrate, producing a Ni depleted Mo-Ni solid solution behind them. The phenomenon is same as those commonly referred to as DIGM with the Cu melt acting as the sink for Ni atoms. When Fe of 1% by weight is added to the Cu melt, the grain boundaries do not migrate, because the compressive coherency strain produced by Ni diffusion from the lattice is exactly compensated by the tensile strain due to the Fe diffusion into it. The diffusional coherency strain energy is thus shown to be the driving force for the grain boundary migration. Because Mo is insoluble in liquid Cu, the grain boundaries are pinned at the grooved ends. The grain boundary curvature thus increases during the migration, causing a migration reversal and consequently an oscillatory motion. The observed critical curvature for the migration reversal falls closely into the range predicted on the basis of the generation of misfit dislocations when the migration velocity decreases to a critical value because of the curvature. The reversal of the grain boundary migration resulting in an oscillatory motion is thus shown to be a natural consequence of the coherency strain hypothesis for the driving force if the inhibiting effect of the grain boundary curvature is taken into account.     

Computer simulation of stagnation in grain growth

The process of stagnation in grain growth has been studied by means of computer modeling of the process. Our calculations indicate that stagnation only occurs for large values of the Zener drag, for small values of the drag we find indications for a direct transition to abnormal growth.     

Computer simulation of anisotropic grain growth in ceramics

Anisotropic grain growth in ceramics has been simulated by a Monte Carlo computer model. Microstructures, similar to experimental ones, were obtained and the influence of the energy anisotropy and the number of anisotropical grains on microstructure development were studied. It was shown that a single grain with higher energy in one direction, embedded in a matrix of grains with the isotropic boundary energy, grows anisotropically with a nearly constant rate. The growth rate is linearly proportional to the energy anisotropy of the grain. The aspect ratio increases with the cube root of time. Faceted grain boundaries were simulated. Microstructures with less than 50% of anisotropic grains develop, after a short time, a bimodal grain size distribution. At this point, the mean aspect ratio of anisotropic grains reaches a maximum. The heights of the maxima are proportional to the energy anisotropy and inversely proportional to the fraction of anisotropic grains. Weighted aspect ratio distributions and their mean values agree well with experimental data.     

A study on coherency strain and precipitate morphologyvia a discrete atom method

Morphological evolution of coherent precipitates is studied by means of a discrete atom method under a plane strain condition with a purely dilatational misfit. The method is predicated upon Hookean atomic interactions and Monte Carlo diffusion and makes no assumption of a specific precipitate shape. Precipitates having elastic constants different from those of the matrix phase are treated in both isotropic and anisotropic elastic systems. Shape evolution is examined under the condition of a constant precipitate size and an isotropic interfacial energy. The results show that in general, an elastically soft precipitate tends to have an equilibrium morphology of low symmetry such as a plate, whereas a hard particle tends to take up a shape of high symmetry such as a circle. Morphological evolution proceeds through dynamic activities of coherency-induced interfacial waves whose wavelength depends upon the difference in elastic constants, precipitate geometry, anisotropy, and diffusion temperature. Coherency-induced interfacial waves seem to be responsible for the protrusions often observed along elastically hard directions in γ′ particles of Ni-base superalloys and also to be a source for fresh ledges for growthvia the ledge mechanism. For a highly nonequilibrium precipitate, first splitting followed by coalescence appears to be a common feature in achieving its equilibrium morphology. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

The influence of coherency strain on the elevated temperature tensile behavior of Ni-15Cr-AI-Ti-Mo alloys

The effect of coherency strain on elevated temperature tensile strength was examined in a model, two-phase y’-strengthened Ni-15Cr-Al-Ti-Mo alloy series. The temperature dependence of coherency strain as represented by the γ-γ’ mismatch was determined over the temperature range 25 to 800 °C. The flow stress incrementAσ _γ, due to precipitation of γ’, was found to correlate well to the magnitude of the γ-γ’ mismatch over the same temperature interval. The correlation was strongest for high misfit alloys regardless of the Antiphase Boundary Energy (APBE). The predominance of by-pass type dislocation-particle interactions in high coherency alloys confirms that strengthening is primarily due to coherency strains. Conversely, alloys with low misfit exhibit two distinct particle shear mechanisms believed to be dependent upon the relative APBE and matrix stacking fault energy of the alloy.     

不同矿物对硅酸盐细菌JXF菌株生长的抑制作用

硅酸盐细菌在不同的固体培养基和液体培养基中培养2～3d后,菌种繁殖产生的细菌数、细菌及荚膜直径、发酵液中黏度的大小顺序为:硅酸盐矿物培养基＞铝土矿培养基＞赤铁矿培养基＞阿什比基质培养基,菌种在不同的硅酸盐矿物培养基中生长特性差别不大.菌种驯化培养试验表明,不同类型矿物对细菌的生长抑制作用表现为:赤铁矿＞铝土矿＞硅酸盐矿物.摇瓶脱硅试验表明,驯化菌种的脱硅率要比未驯化菌种的平均高15%左右,不同来源的硅酸盐细菌均具有较好的脱硅能力.     

Computer simulation of chemically driven grain growth during liquid phase sintering

A simulation of solution reprecipitation during liquid phase sintering was carried out on a computer whereby compositional differences between dissolving and growing grain were assumed as the driving force and diffusion through the liquid as rate controlling mechanism. The equations developed are for the two-dimensional case, i.e. they describe the behaviour of plates and cylindrical wires separated from each other by liquid phase. Slightly different results were obtained for the two initial conditions, i.e. whether it was assumed that no solid was dissolved in the liquid initially or whether supersaturation of the liquid was assumed. In the latter case the calculated microstructural changes of the wire models are in good agreement with earlier experimental observations on W-spheres sintered in liquid Ni.     

The strain effect of dam on intrauterine incisal growth in mouse fetuses

The embryo transfer technique was carried out to examine the strain effect of dams on the intrauterine incisal growth of mouse fetuses. A mean of nine DDD/Qdj embryos was transferred to each female recipient of the four strains (DDD/Qdj, C3H/Qdjl C57BL/Qdj, and DBA/IJ Sea). The dam's weight, the litter size, the gestation period and the newborn offspring weight were recorded. The mandibular incisal size of the newborn offspring was measured two-dimensionally by means of an image analyzing system. Statistical analyses of the incisal size of the DDD/Qdj newborn offspring delivered from the four strains of recipients showed the following results: 1) There were significant interstrain differences in the dam's weight and the newborn offspring weight, but not in the litter size and the gestation period among the four strains of recipients. 2) There was an inverse relation between the litter size and the mandibular incisal size while there were direct relations between the dam's weight and the mandibular incisal size, and between the newborn offspring weight and the mandibular incisal size. 3) There was a significant strain effect of the dam on the mandibular incisal size of the newborn offspring. 4) After eliminating the effects of the litter size, the gestation period, and the newborn offspring weight on the mandibular incisal size of the DDD/Qdj newborn offspring developed in the four strains of dams, a significant interstrain difference in the adjusted mandibular incisal size was found (C3H/Qdj > DBA/IJ Sea > C57BL/Qdj > DDD/Qdj), suggesting that the uteri of the C3H/Qdj strain of dams provided a more suitable environment for the intrauterine incisal growth than those of the other three strains of dams. Thus, it is concluded that the strain effect of dam played an important role in the intrauterine incisal growth of mouse fetuses.     

Computer simulation of microstructure

The microstructure that results from a martensitic transformation is largely determined by the elastic strain that develops as martensite particles grow and interact. To study the development of microstructure, it is useful to have computer simulation models that mimic the process. One such model is a finite-element model in which the transforming body is divided into elementary cells that transform when it is energetically favorable to do so. Using the linear elastic theory, the elastic energy of an arbitrary distribution of transformed cells can be calculated, and the elastic strain field can be monitored as the transformation proceeds. In the present article, a model of this type is developed and evaluated by testing its ability to generate the preferred configurations of isolated martensite particles, which can be predicted analytically from the linear elastic theory. Both two- and three-dimensional versions of the model are used. The computer model is in good agreement with analytic theory when the latter predicts single-variant martensite particles. The three-dimensional model also generates twinned martensite in reason- able agreement with the analytic predictions when the fractions of the two variants in the particle are near 0.5. It is less successful in reproducing twinned martensites when one variant is dom- inant; however, in this case, it does produce unusual morphologies, such as “butterfly mar- tensite,” that have been observed experimentally. Neither the analytic theory nor the computer simulation predicts twinned martensites in the two-dimensional transformations considered here, revealing an inherent limitation of studies that are restricted to two dimensions.     

Computer simulation of VC

A computer simulation technique was developed on the basis of the interaction between carbide precipitation and moving γ/α interfaces for predicting both the interphase precipitation kinetics and the microstructural evolution during austenite-to-ferrite transformation. Theoretical models for the calculation of the driving and pinning forces exerted on a moving interface boundary are described. The variations of the two forces lead to a phenomenon of periodic pinning and unpinning of the interface and, in turn, the characteristic microstructure of parallel sheet particles which is often associated with the interphase precipitation. The experimental data reported for a series of V-bearing steels were analyzed using the computer simulation technique. Three un- known physical parameters,i.e., the thickness of an incoherent interface or the height of the ledge of a coherent interface, the diffusion coefficient of V at the γ/α interface, and the co- herence loss parameter of a VC nucleus, were determined. The calculated intersheet spacing, precipitate size, and precipitation start time all show a good correlation with the experimental observations.     

Computer simulation of the crystal morphology and growth rate during ultrarapid cooling of an Fe-B Melt

The solidification of a binary Fe-B melt is studied by computer simulation with regard for the temperature dependence of the diffusion coefficient and the possibility of nonequilibrium alloying-component entrapment (i.e., the dependence of the distribution coefficient on the ratio of the solidification rate V to the diffusion rate V _D). The effect of high cooling rates of the melt on the dendritic morphology is analyzed. The dependence of the dendrite-tip growth rate on the melt supercooling is obtained. At large supercoolings, a morphological transition is shown to occur; it is related to the change from a diffusion to a diffusionless dendrite growth mode.     

Protective effect of a tetracycline-sensitive strain of Escherichia coli on the growth of a tetracyclic-resistant strain in the presence of tetracycline in a coculture

The addition of tetracycline to a culture of a tetracycline-sensitive strain of Escherichia coli leads to the appearance in the medium of a chemical compound (or compounds) that in part relieve the inhibition of growth of a tetracycline-resistant strain by a high concentration of the antibiotic. An increase in the growth rate can be as high as 50%.