EXPERIMENTAL INVESTIGATION OF CONVECTION DURING VERTICAL BRIDGMAN GROWTH OF DILUTE AL-MG ALLOYS

Experiments were carried out to test the existence of solutal or therm osolutal convection and its effect on the solute distribution in the s olid. Dilute Al-Mg binary alloys were solidified unidirectionally upwa rd in the planar growth range and the temperature variations in the me lt ahead of the solidification front were measured by differential the rmal analysis (DTA). The Solidification interface was revealed by quen ching and the solute distribution in the solid was analyzed. The almos t flat solidification interface at the macroscopic scale indicated tha t thermal convection due to a radial temperature gradient was minimal. Temperature fluctuations in the melt were very small. However, the co ncentration profile along the axis of the crystal showed macroscopic s egregation due to convection, which was more pronounced at slower grow th rates and at higher concentration. The character of the convection is discussed in terms of solutal and thermosolutal convection, and a c onvection criterion is proposed which combines the solutal Rayleigh nu mber, Ra(s), and the thermal Rayleigh number, Ra(T).