A real-time synchrotron X-ray study of primary phase nucleation and formation in hypoeutectic Al-Si alloys

This work reports the results of real-time X-ray radiography of grain refined and unrefined Al-Si alloys solidified at the SPring-8 synchrotron. The nucleation events were observed and the grain density and growth rate following nucleation were measured. Nucleation of the grain refined alloy samples occurred from the coolest to the hottest parts of the field of view in a sequence that mimicked a forward moving wave. No additional nucleation events occurred between the first nucleated grains and there was no evidence of grains being generated by fragmentation. Measurements of both grain density and growth rate show the effect of Si content and grain refiner (Al3Ti1B master alloy) additions on grain size. While the total number of grains increases in the alloys with added Al3Ti1B master alloy, the growth rates tend to be slower. Furthermore, the growth rate for all alloy compositions fluctuates between slow and fast velocities during the initial stages of growth and then tends towards a low steady state value. This decreasing trend is explained in terms of thermal and solutal field interactions between adjacent growing grains and subsequent grain impingement. These measurements result in a better understanding of the role of nucleation and solute content in influencing further nucleation and the subsequent change in the solid-liquid growth rate.