Effect of sample size on twinned microstructure development in the directionally solidified Al-4.5 wt% Cu alloy

Twinned dendrite growth of an Al-4.5 wt% Cu alloy with various sample sizes was found in Bridgman directional solidification. Under conditions of high thermal gradient and solidification rate, the effect of convection on feathery grain formation is examined as a function of sample diameter. Experimental results indicate that the increase in sample size has substantially changed the twinned morphologies. Moreover, there is a minimum of convective intensity inducing feathery grain formation in the appropriate conditions. According to the apparent "penetration" phenomena in solidification microstructures of sample diameter of only 5 mm, it is hypothesized that a new possible lateral propagation mechanism with no requirement for the nucleation of a new stacking fault.