Research on homogeneous nucleation and microstructure evolution of aluminium alloy melt

In this paper, based on the embedded atom method (EAM) potential, molecular dynamics simulations of the solidification process of Al–4 at.%Cu alloy is carried out. The Al–Cu alloy melt is placed at different temperatures for isothermal solidification, and each stage of the entire solidification process is tracked, including homogeneous nucleation, nucleus growth, grain coarsening and microstructure evolution. In the nucleation stage, the transition from high temperature to low temperature manifests a change from spontaneous nucleation mode to divergent nucleation mode. The critical nucleation temperature of the Al–Cu alloy is determined to be about 0.42 T m ( T m is the melting point of Al–4 at.%Cu) by calculating the nucleation rate and the crystal nucleus density. In the nucleus growth stage, two ways of growing up are observed, that is, a large crystal nucleus will absorb a smaller heterogeneous crystal nucleus, and two very close crystal nuclei will merge. In the microstructure evolution of the isothermally solidified Al–Cu alloy, it is emerged that the interior of all nanocrystalline grains are long-period stacking structure composed of face centred cubic (FCC) and hexagonal close-packed (HCP). These details provide important information for the production of Al–Cu binary alloy nano-polycrystalline products.

Numerical Simulation of Microstructure Evolution in Solidification Process of Ferritic Stainless Steel with Cellular Automaton

In order to study the basic principles of vibration-excited liquid metal nucleation technology, a coupled model to connect the temperature field calculated by ANSYS Fluent and the dendritic growth simulated by cellular automaton (CA) algorithm was proposed. A two-dimensional CA model for dendrite growth controlled by solute diffusion and local curvature effects with random zigzag capture rule was developed. The proposed model was applied to simulate the temporal evolution of solidification microstructures under different degrees of surface undercooling and vibration frequency of the crystal nucleus generator conditions. The simulation results showed that the predicted columnar dendrites regions were more developed, the ratio of interior equiaxed dendrite reduced and the size of dendrites increased with the increase of the surface undercooling degrees on the crystal nucleus generator. It was caused by a large temperature gradient formed in the melt. The columnar-to-equiaxed transition (CET) was promoted, and the refined grains and homogenized microstructure were also achieved at the high vibration frequency of the crystal nucleus generator. The influences of the different process parameters on the temperature gradient and cooling rates in the mushy zone were investigated in detail. A lower cooling intensity and a uniform temperature gradient distribution could promote nucleation and refine grains. The present research has guiding significance for the process parameter selection in the actual experimental.

Application of MXene-based materials in hybrid capacitor

MXenes, a noble category of 2D transition metal carbides or nitride-layered materials, exhibit special electroconductibility in a crystal nucleus, low-energy barriers for metal ion diffusion, satisfactory layer spaces for ion...

Toward Efficient Perovskite Solar Cells by Planar Imprint for Improved Perovskite Film Quality and Granted Bifunctional Barrier

Polycrystalline perovskite films generally have high-density defects due to the numerous crystal nucleus and randomly oriented fine grains in the film formation. These defects are commonly regarded as the source...

Features of the Use of Equilibrium State Diagrams for Description of Crystal Growth from Metastable Melts

The crystallization of metastable metal alloys is characterized by a high rate of the crystallization front, which leads to the effect of "impurity capture" and deviation from the local equilibrium near the surface of the growing crystal. To calculate the growth rate of the crystalline nuclei, a method was developed for prediction of deviation of the components’ concentration near the crystal surface from the equilibrium values. A crystal nucleus was considered to be growing from the initial multicomponent phase, due to interphase transition of the components through its surface. It became possible to distinguish the equilibrium and non-equilibrium effect of the nucleus growth rate by decomposing the molar rate of the product formation near equilibrium, as a function of the molar concentration of the components in the Taylor series and limiting with the linear members. The practical calculations were carried out for the crystallization of the amorphous alloy Fe73,5Cu1Nb3Si13,5B9 of the FINEMET type. The local deviations were investigated for the silicon concentration from the equilibrium values at the surface of the growing crystal.

The Formation and Application of Submicron Spherical BaTiO3 Particles for the Diffusion Layer of Medical Dry Films

Submicron spherical barium titanate (BaTiO3) was prepared by batch precipitation in an alkaline solution of a BaCl2–TiCl4–NaOH reaction system. The influence of various parameters on the morphology of BaTiO3 powders was investigated in this study. Spherical BaTiO3 particles can be obtained by reacting for 20 min, which was used to prepare the dry sheet of a medical dry chemical reagent. The morphology of the particles was affected by the stirring speed and the alkaline concentration; the particle size decreased as the stirring speed increased. The hydroxyl ion in the solution acts as a catalyst that can promote the formation of spherical BaTiO3. The formation mechanism of the BaTiO3 sphere is proposed to have three steps: the formation of a Ba–Ti gel and nucleation, self-combination/growth of the BaTiO3 crystal nucleus, and Ostwald ripening. In addition, it is feasible to apply the prepared BaTiO3 sphere to medical dry chemical detection reagents.