The effects of a strong magnetic field on the interface shape and the cellular and dendritic morphology were investigated in the directionally solidified Al-Cu, Zn-Cu and Al-Ni alloys experimentally. The results show that morphological instability of the interface, cell and dendrite has occurred during the directional solidification under the strong magnetic field. Indeed, the magnetic field caused the breakdown of a planar interface into a cellular undulation and formed an irregular shape. Especially, for the Zn-2.0 wt%Cu peritectic alloy, the wavy band-like structure appears under a strong magnetic field. Moreover, it has been found that the application of a strong magnetic field caused the cell and dendrite to twist and deflect from the solidification direction during directional solidification. The stresses in the solid near the solid/liquid interface under the strong magnetic field were analyzed, measured and numerically simulated. The magnetization force (MF) and thermoelectric magnetic force (TEMF) may be responsible for the irregularity and instability of the interface, cell and dendrite during directional solidification under a strong magnetic field. (c) 2010 Elsevier B.V. All rights reserved