Directionally-solidified dendrite morphology with eight secondary arms in an FCC ordered phase alloy

We found a new dendritic solidification morphology with secondary arms growing in eight directions rather than the 〈1 0 0〉 directions expected for a face-centered cubic (FCC) alloy. A bulk single crystal of a Ni-base γ′ single-phase alloy was cast by withdrawing method in a directional solidification furnace. The solidification structure was observed using scanning electron microscopy and analyzed using electron back-scatter diffraction. These results demonstrated that the growth direction of the primary dendrites was [0 0 1], which is known to be the preferred growth direction for FCC metals and alloys. However, the primary dendrite had eight secondary arms growing in the high-Miller-index directions between 〈1 0 0〉 and 〈1 1 0〉. The main factor behind this new dendrite morphology is likely to be the anisotropic atomic stacking of L12 ordered phase, which is the primary phase of the investigated alloy.