Effect of melt convection on microstructure evolution of Nd-Fe-B and Ti-Al peritectic alloys

The influence of melt convection on the microstructure during solidification of peritectic Nd-Fe-B and Ti-Al alloys has been investigated. Since the magnetic and mechanical properties of these technical relevant materials depend strongly on the microstructure and especially on the volume fraction of the properitectic phase, such investigation is of growing scientific interest. On the basis of numerical simulation of melt convection modes in an inductively heated metallic melt, novel techniques for the modification of the melt convection were developed. This is the forced rotation technique and a modified floating-zone facility equipped with a special designed double coil system enabling the application of additional magnetic fields. The forced rotation technique where the crucible rotates with well defined frequencies leads in accordance to numerical simulation to a strong reduction of the melt convection in dependence on the frequency. The volume fraction of the soft magnetic α-Fe phase could be drastically reduced in comparison to a common induction melting accompanied by a simultaneous reduction of the secondary dendritic arm spacing. The floating-zone facility with the patented double coil system allows the tailoring of the melt convection in a wide range from motion at rest to strong stirring. The microstructure of the investigated Ti-Al alloy changed from dendritic to globulitic morphology under strong stirring whereas the volume fraction of the properitectic phase increases with increasing stirring. The mechanical properties show a significant increased plastic deformation of the samples solidified under strong stirring. The possible reason of the change in morphology is explained as a result of spherical growth under forced convection. The study of the influence of melt convection on the microstructure formation of peritectic alloys showed the feasibility of tailoring the microstructure and the resulting alloy properties by customized melt convection using magnetic fields.