Texture changes in the hcp → bcc → hcp transformation of zirconium studied in situ by neutron diffraction

The crystallographic texture of hot-rolled polycrystalline zirconium has been studied below and above the hcp-bcc transition temperature with HIPPO, the new time-of-flight neutron diffractometer at Los Alamos Neutron Science Center, making use of the multidetector capabilities and a vacuum furnace. Incomplete pole figures were extracted from diffraction spectra to determine the orientation distribution function and recalculate complete pole figures in situ at various temperatures. The texture analysis reveals that the orientation of grains in the new high-temperature (bcc) phase is related to the texture of the low-temperature (hcp) phase by Burgers relation, but with both an orientation selection and a symmetry variant selection. The cubic transformation texture is best explained if we assume preferential nucleation of the bcc phase in the hcp grain orientations that are most subject to mechanical twinning. After cooling, the new hcp texture closely resembles the original texture. Thermal cycling repeats this process with slight strengthening of the texture. The hexagonal transformation texture (after cooling) may be caused by nucleation and growth of untransformed domains or through variant selection by stresses imposed by neighboring grains.