Yingju Li

Effects of Boron Addition on Grain Refinement in TiAl-based Alloys

Solid state phase transformation characteristics of the body centred beta(Ti) into the hexagonal closed packed alpha(Ti) in Ti45Al8Nb-(0, 0.3, 0.5 and 0.8 at. pct) B alloys were investigated by heat treatment to clarify gamma/alpha(2) lamellar microstructure refinement induced by B addition. Experimental results confirmed two kinds of boron-reduced grain refinement mechanisms through refining either beta phase then alpha phase (beta-refinement) or alpha phase directly (alpha-refinement) to refine lamellar microstructure at room temperature; however, the role of alpha-refinement dominated the as-cast lamellar microstructure refinement over beta-refinement in Ti45Al8NbxB alloy. It was also found that during the alpha-refinement the convoluted flake- and plate-like borides along beta grain boundaries assisted nucleation of alpha phase, and the particle-like borides near beta grain boundaries impeded alpha phase growth.

Numerical simulation of non-dendritic structure formation in Mg-Al alloy solidified with ultrasonic field

The formation of non-dendritic structure of Mg alloy solidified with ultrasonic treatment was investigated by numerical simulation and experiment. The models of nucleation and crystal growth involved the effects of ultrasonic cavitation and acoustic streaming were built. Based on the models, the grain refinement and the microstructure change from dendrite to non-dendritic structure of a Mg-Al alloy were numerically simulated by cellular automata method. The simulation and experimental results indicated that the ultrasonic cavitation strongly contributes to the grain refinement by improving nucleation, while the acoustic streaming is mainly responsible for the formation of non-dendritic structure.