Papers by Keyword: Nucleation Rate

Abstract: A simple mesoscale model has been developed for discontinuous dynamic recrystallization. Each grain is considered in turn as an inclusion, embedded in a homogeneous equivalent matrix, the properties of which are obtained by averaging over all the grains. The model includes: (i) a grain-boundary migration-equation driving the evolution of grain size via the mobility of grain boundaries, which is coupled with (ii) a single-internal-variable (dislocation density) constitutive model for strain hardening and dynamic recovery, and (iii) a nucleation equation governing the total number of grains by the nucleation of new grains. All the system variables tend to asymptotic values at large strains, in agreement with the experimentally observed steady-state regime.With some assumptions, both steady-state stress and grain-size are derived in closed forms, allowing immediate identification of the mobility of grain boundaries and the rate of nucleation. An application to Ni–Nb-pure-binary model alloys and high-purity 304L stainless steel with Nb addition is presented. More specifically on one hand, from experimental steady-state stresses and grain sizes, variations of the grain boundary mobility and the nucleation rate with niobium content are addressed in order to quantify the solute-drag effect of niobium in nickel. And on the other hand, the Derby exponents were investigated varying separately the strain rate or the temperature.

Abstract: In this paper, synchrotron X-ray radiography was used to study the grain refining performance of direct current (DC) during the solidification of Sn-50 wt.% Pb alloy. Based on the radiographs, the variations of equiaxed grain number and dendrite growth rate with time were measured and analyzed. It is shown that DC mainly affects the microstructure formation through changing the nucleation behavior and the dendrite growth rate. When DC was applied during the whole process of solidification, the nucleation was enhanced and the dendrite growth was suppressed, which resulted in significant grain refinement.

Abstract: In order to eliminating the banded structure in SAE8620H gear steel,an isothermal annealing process and its mechanism has been studied in this paper. Results showed that isothermal annealing process can effectively eliminate banded structure in SAE8620H gear steel.With isothermal temperature 640°C-a certain low temperature indeed high undercooling,and for high nucleation at this temperature resulting in the decrease of nucleation rate difference in all areas,banded structure level decreased to 1 level.When isothermal temperature decreased to 610°C and 590°C,although the undercooling is so high that various regions began to nucleate at the same time,the transition temperature is so low that alloy elements in the regions previously occupied by pearlite bands diffuse faster than other regions,thus proeutectoid ferrite grains in these regions grow faster,regenerating the banded structure .

Abstract: Recrystallization-precipitation-time-temperature (RPTT) diagrams were experimentally determined for two microalloyed steels with V and Nb, respectively, at a strain of 0.35 and a strain rate of 3.63 s^-1. From the RPTT diagrams, and applying the classic theory of nucleation, the nucleation rate was calculated for both steels. In order to determine the mentioned magnitudes, several parameters were calculated, such as: the Zeldovich factor (Z), the energy of formation of the nucleus (ΔG), the driving force for precipitation (ΔGv), the critical radius for nucleation (Rc), and the dislocation density at the start of precipitation (ρ), among others. The calculated data has made it possible to clarify the shape of precipitation start and finish curves and to plot the nucleation rate as a function of temperature. The number of precipitates was calculated by integration of the nucleation rate expression. In this way, substantial differences were established between the two types of microalloyed steels, including the final size of the V(C, N) and Nb (C, N) precipitates.

Abstract: Recrystallization-precipitation-time-temperature (RPTT) diagrams were experimentally determined for two microalloyed steels with V and Nb, respectively, at a strain of 0.35 and a strain rate of 3.63 s^-1. From the RPTT diagrams, and applying the classic theory of nucleation, the nucleation rate was calculated for both steels. In order to determine the mentioned magnitudes, several parameters were calculated, such as: the Zeldovich factor (Z), the energy of formation of the nucleus (ΔG), the driving force for precipitation (ΔGv), the critical radius for nucleation (Rc), and the dislocation density at the start of precipitation (ρ), among others. The calculated data has made it possible to clarify the shape of precipitation start and finish curves and to plot the nucleation rate as a function of temperature. The number of precipitates was calculated by integration of the nucleation rate expression. In this way, substantial differences were established between the two types of microalloyed steels, including the final size of the V(C, N) and Nb (C, N) precipitates.

Abstract: A wide variety of metallic and metal oxide nanoflowers and other exotic patterns have been fabricated using different techniques. We have created copper and cupric oxide nanoflowers using two different techniques: electro-deposition of copper in polymer and anodic alumina templates, and cytyltrimethal ammonium bromide (CTAB)-assisted hydrothermal method, respectively. Zinc oxide and manganese oxide nanoflowers have been synthesized by thermal treatment. Characterization of nanoflowers is done in the same way as for nanowires using XRD, SEM, TEM and FESEM. Scanning Electron Microscope (SEM) images record some interesting morphologies of metallic copper nanoflowers. Field Emission Scanning Electron Microscope (FESEM) has been used to determine morphology and composition of copper oxide nanoflowers. X-ray diffraction (XRD) pattern reveals the monoclinic phase of CuO in the crystallographic structure of copper oxide nanoflowers. Nanoflowers find interesting applications in industry. There is an element of random artistic design of nature, rather than science, in exotic patterns of nanoflowers fabricated in our laboratory.

Abstract: The austenitization is a solid phase transformation process accompanied by nucleation and nucleus growth controlled by long-range carbon diffusion. In the course of our work, a method was developed by which spheroidite model structures were constructed such a way that their different parameters (the size of ferrite grains, the average value of carbon concentration, the size of cementite spheroids) could be changed optionally. In addition, a nucleation model of free enthalpy base was created by which the difference between the two different places of nucleation can be distinguished on the basis of their free enthalpy. The effects of structure parameters, interface free enthalpies and temperature on the nucleation rate of austenite were investigated by cellular automaton simulations.

Abstract: Wetness is one of the important parameters of the relationship with the turbine operating efficiency and safety. Wetness is determined by the number of small water droplets in the steam, and the nucleation rate on the formation of small droplets has a direct relationship.The nucleation rate is difficult to through experiments measured. This article through numerical simulation obtained the distribution of the nucleation rate and supercooling degree.

Abstract: A computational model which is based on the classical homogenous nucleation theory was developed to analyse the alumina inclusion-nucleation process in molten steel in this paper. The idea, 'pseudomolecules', proposed by Lifeng Zhang[1,2], was cited as the basic unit of the physical process of nucleation. However, the nucleation stage was evolved to be controlled by the diffusion of pseudomolecules groups than single pseudomolecule, which is much closer to the actual situation. The differential equations of different pseudomolecules size distribution were given and calculated by computer programs using Runge-Kutta method. Some key parameters, such as supersaturation, nucleation rate, and inclusion population were calculated and compared with some others' conclusion.

Abstract: Being the first step of foaming, bubble heterogeneous nucleation of aluminum foam has remarkable effects on the final cell structure of aluminum foam. The bubble heterogeneous nucleation of foaming Al-9Si and T_iH₂ powder compact has been analyzed. The results obtained are given as follows: (1)The remaining T_iH₂ particles act as the nuclei during heterogeneous nucleation. (2)The nucleation rate increases with the increasing of T_iH₂ content, the decrease of T_iH₂ granularity, the elevation of foaming temperature, and the elongation of foaming time. (3) Only about 5‰ bubble nuclei can survive and develop to the final foam cells. (4)The influence of processing parameters on the nucleation rate descends by the following sequence: foaming temperature elevates 20°C, T_iH₂ granularity decreases from 150μm to 26μm, T_iH₂ content increases 0.5%wt., and the time for foaming extends 10s.