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.

Кинетика роста зародыша из нанофазы

We present a theoretical analysis of the growth kinetics of a nucleus from a nano-sized mother phase with a restricted amount of material in the presence of the stopping effect due to the depletion of supersaturation. Such a regime is typical for GaAs nanowires grown by the vapor-liquid-solid mechanism from a Ga droplet and is observed by in situ growth monitoring inside a transmission electron microscope. An analytical solution is obtained for the time-dependent size of the nucleus and atomic concentration in a nano-phase. A principally new time-scale hierarchy is found, whereby the entire growth process id divided into 3 stages – (1) fast nucleus growth to the stopping size, (2) slow nucleus growth at the rate of refill of a mother phase, and (3) further refill of a mother phase to resume the initial supersaturation. A general criterion is formulated for observing the stopping effect depending on the size of a nano-phase.

Pertumbuhan Cangkang Kerang Mutiara Pteria penguin yang Diimplantasi yang Dibudidayakan dengan Metode Gantung di Perairan Palabusa Selat Buton

The producing half-pearls (Mabe) at the Palabusa waters of Buton Strait Southeast Sulawesi still uses simple technology, namely applying implantation with only a single nuclei, which has an impact on the large number of seeds and used space. Therefore, in order to increase the production of mabe pearls, one of effort is by planting two nucleus in a single mabe pearl shell. The test animals used were P. penguin as many as 90 oysters, with an average length of 12 cm shells, 8 cm shell height and 100-120 g individual weights. The nucleus used was made from plastic (half round) with diameter of 10 cm and height of 0.8 cm. This research was conducted for four months. The study was designed using a completely randomized design with 3 treatments and 3 replications. The treatment applied was A (a nucleus was inserted into the left shell), B (2 nuclei were inserted into the left shell), and C (a nucleus was inserted into each on the left and right shells). The results showed that all treatments did not have a significant effect on the growth of shells. Hence, the addition of two nuclei in a single pearl oyster can be used to increase cultivation production. Keywords : Pteria penguin, implantation, nucleus, growth