Twinned dendrite growth during Bridgman solidification

Abstract Twinned dendrites were successfully produced with high reproducibility in Al-4.5 wt.% Cu alloys at various growth speeds (V) of 100, 200, 500, 1000, 1500 and 2000 μm s^-1 during Bridgman solidification. Under a relatively high thermal gradient (G, around 200 K cm^-1), the growth speed condition necessary for the twinned dendrite growth has been broadened. Experimental results indicate that the increase in growth speed has substantially changed the twinned morphologies. Importantly, the isolated twinned dendrite growth was observed at the growth speeds less than 1 mm s^-1 (100, 200, and 500 μm s^-1). At higher growth speeds, the finer twinned dendrites developed the usual appearance of an ordered sequence of alternating twinned and untwinned lamellae. The G/V value is considered as the critical factor for the isolated or usual twinned dendrite growth. A detailed analysis of the melt flow confirms that the slight convection during Bridgman solidification is large enough for the twinned dendrite growth. Furthermore, a visualized way to evaluate the velocity of the in-plane twinned extension was provided and the extension velocity was typically one third of the growth speed (500 μm s^-1). Based on frequent "penetration" growth phenomena among secondary arms, a possible lateral propagation mechanism of the twinned dendrites has been hypothesized.