Growth twinning of the β(In) during lamellar growth of β(In) – In2Bi – γ(Sn) eutectic in rotating directional solidification: Cases with anisotropic phase boundary energy

In this work, we report on the growth twinning of the β(In) phase in the ternary β(In) – In₂Bi – γ(Sn) eutectic alloy observed during rotating directional solidification (RDS) experiments. Electron backscatter diffraction (EBSD) measurements were performed on the RDS samples in order to identify the crystal orientations of the phases and the crystal orientation relationships (ORs) between the phases. In two samples showing clear locking of the In₂Bi/β(In) boundary, the OR between In₂Bi and β(In) phases was the same and identified as {1‾100}^In^2Bi∥{131}^β(In) and 〈0001〉^In^2Bi∥〈101〉^β(In). This OR indicates that the energy plot of the In₂Bi/β(In) phase boundary is two-fold symmetric with two minima being potential locations for locking of the phase boundary during rotational growth in RDS. Indeed, locking occurred in the angular range corresponding to the energy minimum, near the common plane of the OR. Surprisingly, we identified a second angular range with clear locking and were able to show that it is achieved due to twinning of the β(In) phase. To our knowledge, this is the first report of growth twinning in a nonfaceted crystal associated with forcing the energetically anisotropic phase boundary to change its orientation with respect to the imposed temperature gradient. We open a discussion on the origin of the twinning in the locked grains, in which the strongly anisotropic boundary energy with forbidden orientations is present.