Papers by Keyword: Nucleation

Abstract: The predictions from a grain cluster deformation texture model, GIA, are utilized to study the nucleation texture of recrystallisation of aluminium alloys. In combination with a dislocation based work hardening model, the propensity of specific grains in their granular environment for select nucleation mechanisms is investigated. Quantitative criteria for the nucleation events can be formulated. The results can be fed into a growth model of recrystallisation to predict recrystallisation textures and lend themselves to through-process modelling.

Abstract: The nucleation and development of dynamic recrystallization (DRX) in hot deformed superalloy Inconel 718 during uniaxial compression were investigated by optical microscopy and electron back-scattered diffraction (EBSD) technique. The results showed that the discontinuous dynamic recrystallization was the predominant DRX mechanism in this alloy. The variations of partial crystallographic orientations led to the individual nucleation inside the deformed grains, which implied the occurrence of local continuous dynamic recrystallization. The progressive subgrain rotation can be confirmed neither near the prior high angle grain boundaries nor within the original grains. It was found that, as the strain increased, the initial twin boundaries were gradually transformed to ordinary mobile high angle boundaries. Meanwhile, the new twin boundaries were formed inside the recrystallized grain necklaces. It was suggested that the characteristics of the twin boundaries evolution with increasing strain were associated with the transformation of initial twin boundaries as well as the generation of new ones, which resulted in the development of DRX.

Abstract: Glass ceramic was successfully synthesized using goldmine tailings as main raw material. The frit preparation and molding techniques, and the nucleation and crystallization processes were discussed. The results showed that only one crystalline phase, idiomorphic diopside, was formed in shapes of small columns and grains. The optimum temperatures for melt, sintering, nucleation and crystallization were 1350°C, 1160°C,
750°C,
and 860°C, respectively.

Abstract: Colloidal silica particles, Bimodal size distribution, Nucleation Abstract. Colloidal silica particles with bimodal size distribution have been prepared by the hydrolysis of tetraethylorthosilicate in alcoholic solutions of water and ammonia as catalyst. Experimental conditions such as concentration of NaCl, amount of water and reaction temperature were investigated to reveal the formation mechanism of the colloidal silica particles. The nucleation process of colloidal silica particles with bimodal size distribution depends on the hydrolysis rate of TEOS and the ionic strength of reaction media. The hydrolysis reaction is the rate-limiting step during the nucleation process. Nucleation involving background ions generated by TEOS hydrolysis reaction and addition of electrolyte(NaCl) is another potentially important factor for nucleation process of electrically charged clusters. A critical value of ionic strength exists in the reaction to form bimodal size distribution particles.

Abstract: The problem with zeolite crystals synthesized by conventional method is that they are extremely small of two to eight microns, To better define the structure of zeolite, scientist need to grow crystal that are 100 to 1000 times larger. In this work, Large crystal zeolite X of uniform crystal size of 50µm were grown by a continuous crystallization method from seed crystals (5-10µm) in a mother solution having a composition 3.5Na2O : Al2O3 : 2.1SiO2 : 1000H2O. In order to grow crystals of zeolite X to an appropriate size by the continuous method, the mother solution was supplied into the starting solution with various seed content (3~20wt%) in an autoclave at 90°C after 7 days, 12days, 16days, 19days and 24 days, respectively.

Abstract: Biomimetic apatite deposition behaviors and mechanical performance for as-rolled and annealed Ni-Ti plates were investigated. Apatite nucleation and growth on Ni-Ti in SBF (simulated body fluid) was not appreciably influenced by heat treatment. But, the apatite deposition rate increased slightly by NaOH surface treatment. The nodular apatite on the deposited layer is favored on a macro-scale since the surface energy of polycrystalline apatite particles can be reduced by forming nodules. The weight gain after apatite deposition for Ni-Ti (0.004 g/cm2) after 10 days were found to be smaller that that of NaOH treated Ti-6Al-4V, but it was comparable to that of non- NaOH-treated Ti-6Al-4V (0.004 g/cm2). The stress-strain responses of annealed Ni-Ti displayed the pseudoelastic behavior associated with stress-induced martensite formation with the transition stress for the martensite formation equal to 320 MPa. On the other hand the cold worked Ni-Ti displayed no appreciable pseudoelastic region and the yield stress was ~500MPa. A good biomimetic apatite formation and excellent mechanical performance of Ni-Ti suggests that Ni-Ti can be an excellent candidate material for orthopedic implants.

Abstract: The spherical anatase TiO2 nanoparticle of 50 nm in diameter was manufactured by flame method and was subsequently heat-treated to investigate the transformation behavior from anatase to rutile using TEM observation. The anatase particle was facetted at the free surface and a neck formed between the anatase particles prior to the phase transformation. This resulted in the severe lattice distortion at the region of the interface. Unfortunately, we could not find the rutile grain nucleated in the anatase particle due to very fast grain growth. All the phase boundaries observed in HRTEM images existed in the contact between anatase and rutile particles. The nucleation of rutile phase in anatase particle was suppressed at the low heat-treated temperature but the grain growth of rutile particles after the phase transformation grew very fast by the sweeping phenomena of grain boundary. It leaded to the microstructure without the rutile phase traped in anatase particle.

Abstract: Semi-solid casting (SSC) techniques have proven useful in the mass production of high integrity castings for the automotive and other industries. Recent research has shown metal matrix nanocomposite (MMNC) materials to have greatly improved properties in comparison to their base metals. However, current methods of MMNC production are costly and time consuming. Thus development of a process that combines the integrity and cost effectiveness of semi-solid casting with the property improvement of MMNCs would have the potential to greatly improve cast part quality available to engineers in a wide variety of industries. This paper presents a method of combining SSC with MMNC in a way that benefits from MMNCs’ tendency to naturally form the globular microstructure necessary for SSC. This method uses ultrasonically dispersed nanoparticles as nucleating agents to achieve globular primary grains such that fluidity is maintained even at high solid fractions. Once particle dispersion is achieved, the material needs no further processing to become a semi-solid slurry of globular primary grains as it cools. This quiescent method of slurry production, while still imposing some constraints on cooling rates, has a large process window making this process capable of industrial rates of throughput. It was found that the key factor to achieving globular microstructure is a sufficiently slow cooling rate at the onset of solidification such that particle-induced nucleation can occur. Once nucleation occurs, continued cooling is virtually unconstrained, with globular microstructure evident in quenched samples as well as samples cooled at rates as slow as 1 °C/min. This method was demonstrated in several material systems using zinc (Zn), aluminum (Al), and magnesium (Mg) alloys and nanoparticles of aluminum oxide (Al2O3), silicon carbide (SiC), and titanium oxide (TiO2). Additionally, several nucleation models are examined for applicability to nanoscale composites.

Abstract: Among semisolid forming processes, increasing attention has now been focused on the commercialization of casting processes based on rheocasting in aluminum automotive casting fields because of its advantages compared to thixoforming, in regards to the efficiency in cost and productivity, and mechanical properties. In this paper, an advanced rheocasting process is introduced, which has been developed recently at Yonsei University and is now on the way to practical uses in Korea and Japan. In this method, the semisolid slurry production is based on the nucleation method, in which the nuclei of primary
phase formed in the melt are prevented to grow into dendrites, instead they grow into fine globular particles to become semisolid slurry having rheological behavior. By increasing the number of nuclei, very fine and uniform globular microstructures can be obtained, resulting in an increase of mechanical properties. Recently, commercial rheo-diecasting machines based on the present method have been developed, and now are being used in manufacturing aluminum automotive casting parts. Furthermore, the present method is now being tested to investigate its possibility of application to various alloy systems.