Experimental investigation of convection during vertical Bridgman growth of dilute Al-Mg alloys

Experiments were carried out to test the existence of solutal or thermosolutal convection and its effect on the solute distribution in the solid. Dilute Al-Mg binary alloys were solidified unidirectionally upward in the planar growth range and the temperature variations in the melt ahead of the solidification front were measured by differential thermal analysis (DTA). The Solidification interface was revealed by quenching and the solute distribution in the solid was analyzed. The almost flat solidification interface at the macroscopic scale indicated that thermal convection due to a radial temperature gradient was minimal. Temperature fluctuations in the melt were very small. However, the concentration profile along the axis of the crystal showed macroscopic segregation due to convection, which was more pronounced at slower growth rates and at higher concentration. The character of the convection is discussed in terms of solutal and thermosolutal convection, and a convection criterion is proposed which combines the solutal Rayleigh number, Ras, and the thermal Rayleigh number, RaT.