Effects of annealing on Zr8Ni19X2 (X = Ni, Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf): Structural characteristics

The effects of an 8-h annealing treatment at 960 °C on the phase structures of a series of partial-Ni substituted Zr8Ni21 alloys, potential metal hydride candidates for high-rate nickel/metal hydride batteries, were studied. The substitution elements included Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf. Only the main phase of the annealed Sn-substitution remained Zr8Ni21-structured while those of other substitutions turned into Zr7Ni10 or Zr2Ni7. Similar to the preference of C14/C15 main phase in AB2 alloys, the number of outer-shell electrons played an important role in determining the main phase of the A8B21 alloys. As the number of outer-shell electrons of the substituting element increased, the main phase evolved from tetragonal Zr7Ni10 (Mg, Al, and Sc), to orthorhombic Zr7Ni10 (La and Hf), cubic Zr2Ni7 (V, Mn, Co, and Sn (before annealing)), and finally triclinic Zr8Ni21 (Ni and Sn (after annealing)). After annealing, the abundance of the predominant phase increased, and the number of secondary phases diminished. The only exception was the Sn-substituted alloy, where the major phase transformed from Zr2Ni7 to Zr8Ni21 (after annealing). In this study, several alloys showed an X-ray diffraction peak at around 38.4°, which cannot be correlated to any phase in the database. The stoichiometry of this phase is close to Zr2Ni7 according to X-ray energy dispersive spectroscopy.

► Structures of modified Zr8Ni21 alloys before and after annealing were studied.
► The modifying elements include Ni, Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf.
► Main phase evolves from Zr7Ni10 → Zr2Ni7 → Zr8Ni21 as average electron density rises.
► Annealing generally increases the amount of main phase and reduces secondary phase.