Abstract:

The influence of a longitudinal static magnetic field on microstructures and mechanical properties of Ni₃Al-based alloy during directional solidification at the growth speed of 25 µm/s and 100 µm/s has been experimentally investigated. Results reflected that the utilization of a 0.5 T magnetic field refines the NiAl dendrites at both speeds of growth. When applying a high magnetic field, the columnar-to-equiaxed transition (CET) occurred at growth speed of 25 µm/s and dendrite networks formed at growth speed of 100 µm/s. Tensile property results indicated that the refinement of dendrites enhanced both plasticity and ultimate tensile strength of Ni-Al alloy. The change of microstructures and mechanical properties should be attributed to the combined action of the thermoelectric magnetic convection (TEMC) in mushy zone together with the thermoelectric magnetic force (TEMF) acting on the solid. When applying a low magnetic field (0.5 T), the TEMF is too small to fragment the dendrites, and the refined dendrites is mainly due to the TEMC in the interdendritic regions. At a lower growth speed, the TEMF is supposed to strong enough to fragment the dendrites and induce the occurrence of CET under 2 or 4 T. When the growth speed increased to 100 µm/s, no obvious CET was observed, but a vertical secondary convection is induced by the circulation in the parallel plane, which promotes the growth of secondary and tertiary branches, leading to the formation of abnormally developed high order dendrites. The hierarchical dendritic structure was suggested to provide a channel for rapid crack propagation and thus degraded the mechanical properties.

Key words: Ni₃Al-based alloy, Magnetic field, Directional solidification, Columnar to equiaxed transition, Mechanical properties