Al – Ti – B master alloys: structure formation in modified alloys

Abstract

This paper provides an overview of existing grain refinement mechanisms and describes the solidification kinetics typical of alloys modified with the Al – Ti – B system. Of practical interest are intermetallic compounds of the binary Al – Ti system, the state diagram of which requires further study. It was found that, with titanium concentrations being low, the behaviour of the system is influenced by peritectic reaction leading to α-grains of aluminium forming on titanium aluminide particles (TiAl3). A unique feature of the Al – Ti system is that, compared with other alloys, it has the highest limited growth rate Q, which explains the high grain refinement performance. Microparticles of titanium diboride (TiB2) that form in the liquid state of the Al – Ti – B system act as nucleation centres for the intermediate phase of TiAl3. At the same time, TiAl3 was found to display metastable properties and it dissolves as aluminium solidifies. The classical crystallization theory defines the importance of heterogeneous substrates that help grain nucleation by lowering the threshold energy. According to the heterogeneous nucleation theory, supercooling tends to increase dramatically during grain nucleation as the discrepancy rises between the crystal lattice of the solid body and that of the substrate. In terms of constitutional supercooling, it takes lower supercooling for grain nucleation on a particle. The contributing factors include a lower rate of latent heat release and low grain growth rates. As the master alloy was introduced in the melt, the authors observed reduced modification efficiency (with the concentration >1.5 %) and dropped solubility of Ti in the presence of Si. Long soaking times would lead to the master alloy gradually lose its effect. The authors also considered how the properties of master alloys can be enhanced. Thus, they looked at breaking up TiAl3 by means of plastic deformation and deagglomerating TiB2 particles by contactless mixing.