Temperature-induced anomalous structural changes of Al-12wt.%Sn-4wt.%Si melt and its influence on as-cast structure

The temperature dependence of the viscosity of liquid Al-12wt.%Sn-4wt.%Si was studied with a hightemperature viscosity apparatus.Anomalous changes of viscosity of the melt were found at 1,103 K and 968 K in the cooling process,which indicates anomalous structural changes of the melt.It is calculated that the anomalous structural change is associated with an abrupt decrease of atomic clusters'size and activation energy in the melt.According to the temperature of the anomalous structural changes,melt heat treatment process(quenching from superheat to pouring temperature) was performed on Al-12wt.%Sn-4wt.%Si melt prior to pouring,aimed to keep the small atomic clusters from higher temperature to lower pouring temperature.The results suggest that relatively small atomic clusters at the pouring temperature in the melt could generate a deep under-cooling of nucleation in the subsequent solidification process,and refine the as-cast structure.After being quenched from superheating to pouring temperature,the relatively small atomic clusters,especially the Si-Si clusters in the melt will grow to equilibrium state(relatively big atomic clusters) with holding time,resulting in the prominent coarsening of the Si morphology in the as-cast structure.

Effects of individual and combined additions of phosphorus,boron and cerium on primary and eutectic silicon in an Al-30Si alloy

The modification of silicon in an Al-30Si alloy was studied using optical microscopy, electron probe micro-analysis, transmission electron microscopy and differential scanning calorimetry. It is found that phosphorus master alloys combined with boron master alloys have good modification effect on primary silicon but no evident modification effect on eutectic silicon, while boron combined with cerium has good modification effect on eutectic silicon. The results of differential scanning calorimetry show that phosphorus, boron or cerium addition and their combined addition have different undercooling effects on eutectic silicon. Many scholars thought that AlP particles were the nuclei of eutectic silicon when phosphorus was enough in the alloy. Our results show that β-（Al,Si,Fe） can still be the nucleus of plate-like entectic silicon while AlP is the nucleus of primary silicon when there is enough phosphorus in the melt. In addition, the mechanism about the modification was also discussed.

Crystallization and Corrosion Resistance of (Fe_(0.78)Si_(0.09)B_(0.13))100_(-x)Ni_x (x=0,2 and 5) Glassy Alloys

The crystallization of the （Fe0.78Si0.09B0.13）100-xNix glassy alloys （x=O, 2 and 5） has been investigated by X-ray diffraction （XRD） and differential scanning calorimeter （DSC）. Corrosion resistance analyses have been carried out using electrochemical measurements and corrosion products have been analyzed by scanning electron microscopy （SEM）. The experimental results show that the addition of Ni can promote the nucleation of c^-Fe, retard the decomposition of the metastable borides, and alter the crystallization mode of the present Fe-based glassy alloys. The lattice constant （ao^e） of c^-Fe in the annealed samples shows a decreasing trend with increasing annealing time. The Ni addition can improve the corrosion resistance of the as-quenched Fe-based glassy alloys in H2S04, NaCI and NaOH solutions. The results indicate that Ni can promote the diffusion of Si atoms during quenching and annealing processes.