On the binary phases ~YNi₄ and Y₂Ni₇

Chem. Met. Alloys 11 (2018) 92-99

https://doi.org/10.30970/cma11.0375

Volodymyr BABIZHETSKYY, Oksana MYAKUSH, Volodymyr LEVYTSKYI, Bogdan KOTUR, Viola DUPPEL, Lorentz KIENLE

Y–Ni alloys in the composition range 79-81 at.% Ni, annealed at 600°C and 800°C, were investigated by complementary analytical techniques providing chemical and structural information. In contradiction to former reports, the ~YNi₄ phase was not observed at these temperatures. The samples annealed at 600°C consisted of two phases: the rhombohedral HT β‑Y₂Ni₇ phase (structure type β‑Gd₂Co₇, space group R-3m) and YNi₅ (structure type CaCu₅, space group P6/mmm). At 800°C the hexagonal LT form α‑Y₂Ni₇ (structure type Ce₂Ni₇ type, space group P6₃/mmc) was found to coexist with HT β-Y₂Ni₇. The transformation β‑Y₂Ni₇ ↔ α‑Y₂Ni₇ is very sluggish and increasing the annealing time to five months increased the amount of α-Y₂Ni₇ only up to ~23%. The transformation β‑Y₂Ni₇ ↔ α‑Y₂Ni₇ occurs at T > 800°C, but a more precise determination was not possible. The ~YNi₄ phase exists in a very narrow temperature range: it forms by the peritectic reaction YNi₅ + L ↔ ~YNi₄ at 1338±2°C and decomposes by the metatectic reaction ~YNi₄ ↔ L + YNi₅ at 1328±2°C. The section YNi₅–Y₂Ni₇ of the phase diagram of the binary Y−Ni system was revised consequently.

PED [100] zone axis patterns recorded on single crystalline areas of Y₂Ni₇ in the Y₂₀Ni₈₀ sample annealed at 600°C and HRTEM micrograph with inserted multislice contrast simulations.

Keywords

Intermetallics / Phase diagram / Crystal structure / X-ray diffraction