Fine Structure in Multi-Phase Zr(8)Ni(21)-Zr(7)Ni(10)-Zr(2)Ni(7) Alloy Revealed by Transmission Electron Microscope

The microstructure of an annealed alloy with a Zr(8)Ni(21) composition was studied by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence of three phases, Zr(8)Ni(21), Zr(2)Ni(7), and Zr(7)Ni(10), was confirmed by SEM/X-ray energy dispersive spectroscopy compositional mapping and TEM electron diffraction. Distribution of the phases and their morphology can be linked to a multi-phase structure formed by a sequence of reactions: (1) L → Zr(2)Ni(7) + L’; (2) peritectic Zr(2)Ni(7) + L’ → Zr(2)Ni(7) + Zr(8)Ni(21) + L”; (3) eutectic L” → Zr(8)Ni(21) + Zr(7)Ni(10). The effect of annealing at 960 °C, which was intended to convert a cast structure into a single-phase Zr(8)Ni(21) structure, was only moderate and the resulting alloy was still multi-phased. TEM and crystallographic analysis of the Zr(2)Ni(7) phase show a high density of planar (001) defects that were explained as low-energy boundaries between rotational variants and stacking faults. The crystallographic features arise from the pseudo-hexagonal structure of Zr(2)Ni(7). This highly defective Zr(2)Ni(7) phase was identified as the source of the broad X-ray diffraction peaks at around 38.4° and 44.6° when a Cu-K was used as the radiation source.