Fine Structure in Multi-Phase Zr8Ni21-Zr7Ni10-Zr2Ni7 Alloy Revealed by Transmission Electron Microscope

The microstructure of an annealed alloy with a Zr₈Ni21 composition was studied by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence of three phases, Zr₈Ni21, Zr₂Ni₇, and Zr₇Ni10, 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 → Zr₂Ni₇ + L'; (2) peritectic Zr₂Ni₇ + L' → Zr₂Ni₇ + Zr₈Ni21 + L"; (3) eutectic L" → Zr₈Ni21 + Zr₇Ni10. The effect of annealing at 960 °C, which was intended to convert a cast structure into a single-phase Zr₈Ni21 structure, was only moderate and the resulting alloy was still multi-phased. TEM and crystallographic analysis of the Zr₂Ni₇ 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 Zr₂Ni₇. This highly defective Zr₂Ni₇ 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.