Papers by Keyword: Heterogeneous Nucleation

Abstract: Structural modification of ternary aluminium – zinc – coper alloys influences their wear properties. In a series of studies, Al – 30 wt. % Zn – 3 wt. % Cu alloy (Al-30Zn-3Cu) has been doped with 1 wt.% Mn introduced with AlMn-based master alloys. The alloy microstructure and mechanical properties have been studied using light and SEM/EBSD microscopy, and measurements of wear resistance and dimensional changes. Reducing Cu content to 3 wt.% and doping with 1 wt.% Mn allows obtaining alloy of significantly refined grains, improved wear resistance and preserved high dimensional stability.

Abstract: The Al-5Ti-1B, Al-10Ti, Al-4B master alloys and TiB₂ powder were applied to refine the pure aluminum, respectively. The effects of the TiAl₃ phase, TiB₂ particle, and AlB₂ phase on the grain size of pure aluminum were compared. The grain refinement mechanism of the Al-5Ti-1B grain refiner was studied. The results showed that the TiAl₃ phase was an effective heterogeneous nucleus of the α-Al grain. But the TiAl₃ phase in the Al-5Ti-1B grain refiner was not the heterogeneous nucleus of the α-Al grain due to its re-melting in the Al melt. The separate TiB₂ particle or AlB₂ phase was not the heterogeneous nucleus of the α-Al grain. However, the TiB₂ coated by the TiAl₃ phase can be the effective heterogeneous nucleus of the α-Al grain. The grain refinement mechanism of the Al-5Ti-1B grain refiner can be summarized as follows: when the Al-5Ti-1B grain refiner is added into the Al melt, the TiAl₃ phases are re-melted to release the Ti atoms, while the TiB₂ particles are remaining in the Al melt. During the solidification of the Al melt, the Ti atoms are segregating on the surface of TiB₂ particles to form the TiAl₃ phases. The TiB₂ particles coated by the TiAl₃ phases then reacts with the Al melt to generate α-Al crystal nucleus.

Abstract: Poly (3-hydroxybutyrate-co-hydroxyvalerate) (PHBV) is one of the aliphatic polyesters that are completely synthesized by microorganisms. Owing to the physical and chemical properties of PHA being similar with those of polypropylene, PHBV is expected to be able to partially replace petroleum-based polymers, and to reduce the pollution of environment at the same time. However, many inherent defects, including slow crystallization rate, large-size spherulite and secondary crystallization phenomenon, restrict the development of PHBV. In the present study, PHBV/WS₂ hybrid materials where the tungsten disulphide (WS₂) acted as nucleating agent were produced, and then its accelerated crystallization effect on PHBV was evaluated. Mo’s methods were employed to describe the non-isothermal crystallization kinetics of the nucleated PHBV hybrid materials. The activation energy (ΔE) of hybrid material was calculated by Kissinger formula. It was found that the addition of low WS₂ loadings strongly increased the crystallization rate of PHBV and correspondingly the crystallization half time (t_1/2) decreased from 97s to 38s with only 1wt.% WS₂ added. Meanwhile, the crystallization temperature (T_c) increased from 81.9°C to 112 °C. The results reported here are expected to be of great interest for the practical application of PHBV.

Abstract: In this work, monodispersed silver nanoparticles with controllable size have been successfully in situ synthesized in PMMA matrix. NaHS, HCl and poly (vinyl pyrrolidone) (PVP) were used to optimize the nucleation and growth of silver nanocrystalline. UV–vis analysis and transmission electron microscopy (TEM) were used to characterize the size and dispersion of silver nanoparticles in the Ag/PMMA nanocomposites. The results show that silver nanoparticles homogeneously distribute in PMMA/DMF sol and the particle size of silver nanoparticles increase with the increasing of time. The nucleation of Ag atoms can be facilitated through the addition of a trace amount of NaHS to generate Ag₂S clusters as heterogeneous nuclei. Introducing a trace amount of Cl^- into the reaction system can effectively reduce the growth rate of the nanoparticles and thus generating more uniform silver nanoparticles in PMMA matrix.

Abstract: Experiments were carried to investigate the effect of TiC on the solidification process and microstructure of Al-Pb alloys. It is demonstrated that TiC particles are effective inoculants for the nucleation of the Pb-rich droplets during cooling an Al-Pb alloy in the miscibility gap. A model describing the kinetic behavior of TiC particles in the melt and the liquid-liquid decomposition of Al-Pb was developed. The dissolution, coarsening and precipitation processes of TiC particles as well as the microstructure evolution during the liquid-liquid phase transformation of an Al-Pb alloy were calculated. The numerical results indicate that what determines the refinement efficiency of TiC particles on the Pb-rich droplets/particles is the number density of TiC particles in the melt cooled to the binodal line temperature of the Al-Pb alloy. If the number density of TiC particles in the melt before the beginning of the liquid-liquid decomposition is high enough, the addition of TiC causes a refinement of the Pb-rich droplets/particles and promotes the formation of Al-Pb alloys with a well dispersed microstructure.

Abstract: Direct molecule dynamics (MD) simulations have also been performed to study heterogeneous nucleation and growth of iron on C₆₀ molecule. The grown mechanism of this crystallization process was explored. The results indicate that 92 iron atoms attach to C₆₀ molecule surface can form new covalent bond, forming a closed regular icosahedron. More atoms grow in layer to form bigger regular closed clathrate base on the structure of former one. As increase of atoms number, there will appear some crystal faces.

Abstract: This article reports the effect of MnCO₃ addition on the grain refinement efficiency of AZ91 magnesium alloy. The results indicate that the addition of MnCO₃ has excellent grain refining efficiency for AZ91 alloy, which is mainly attributed to the Al₄C₃ particles formed in the melt, besides Mn is indispensable to grain refinement in Al-bearing magnesium alloys. There is an optimal addition amount of 0.6% at 740 °C and the grain size is reduced from 245 to 91 μm. At the same time, the corrosion resistance performance of MnCO₃-added AZ91is improved.

Abstract: Al (OH)₃ coated CaF₂ composite powder was prepared by coating Al (OH)₃ on surface of micro-sized CaF₂ particles using heterogeneous nucleation method. Influence of Al³⁺ concentration, PH value and reaction temperature was studied. The result reveals that CaF₂ particle was uniformly coated with a layer of Al (OH)₃ and Al³⁺ concentration, pH value and reaction temperature are the main factors that affect coating result. Optimum technological conditions are Al³⁺ concentration of solution before reaction at 0.15 mol/L, pH value at 7.5 and temperature at 75 oC.

Abstract: It is reported that alloying with Cu causes the transformation from the D0₂₂ type structure of Al₃Ti into L1₂ cubic structure. Since the lattice constant of Al_2.5Cu_0.5Ti with L1₂ structure is a = 0.3927 nm, smaller disregistry value between Al and Al_2.5Cu_0.5Ti can be achieved. In this study, novel refiner for Al cast containing Al_2.5Cu_0.5Ti intermetallic compound particles with L1₂ structure is fabricated by cold pressing. It is found that the grains of Al cast are partially refined by using the novel refiner, although microstructure of Al grain is not homogeneous. In this way, the Al_2.5Cu_0.5Ti intermetallic compound particles with L1₂ structure can become favorable heterogeneous nucleation sites for Al cast.

Abstract: Specific types of non-metallic inclusions are known to act as heterogeneous nuclei for the formation of acicular ferrite, which provides excellent toughness. By increasing the amount of acicular ferrite in the microstructure, the properties of HSLA steels can be optimized significantly.Although the formation of acicular ferrite caused by heat treatments (thermomechanical treatments or welding) is quite well described in literature, there is less information to find about the formation of acicular ferrite immediately out of the liquid melt. Within the present study experiments on laboratory scale are carried out simulating the influence of cooling conditions and Ti-content on size, chemical composition and morphology of non-metallic inclusions and consequently on the amount of acicular ferrite. All experiments were carried out with a dipping test simulator enabling very well controllable cooling conditions. Optical microscopy in combination with special etching methods as well as SEM/EDS-analysis was used for microstructure and inclusion characterization.