- Abstract One of the key factors behind the effect of pulse magneto-oscillation (PMO) on grain size is convection. According to the dendrite fragmentation model, refined equiaxed grains are formed by the increased convection that results from PMO treatment. However, when convection is weak, as in experiments within a small crucible diameter or in those conducted at low current intensities, grain size was reduced, although the columnar form was maintained. The observations support the concept that dendrite fragmentation mostly operates during solidification under relatively strong convection conditions. The cavities activation mechanism, which is related to the presence of stabilized solid embryos due to PMO treatment, operates when convection is weak.