Interface instabilities on solidifying globulitic particles

Al-6.8 wt% Cu samples were held in the mushy zone at a fraction of solid of 50% for 5000 s and then cooled/quenched with cooling rates between 0.02 and 160 K/s. The coarse alpha-Al particles formed during the isothermal holding period grow with an unstable interface during the quenching process. Fine cells and dendrites develop on the surface of the primary alpha particles. At low cooling rates, if the solutal boundary layers of neighboured particles overlap, no solutal instabilities are seen. Measurements of the intercept length in the quenched former liquid and correlation thereof with the solutal boundary layer show a good qualitative agreement for the conditions of cell formation and ''stable'' growth, respectively. There is also a good quantitative agreement between measured and calculated cell spacings.