Transformation upon heating of electrodeposited metastable tin-nickel alloys, prepared from pyrophosphate baths, was studied by means of differential thermal analysis and X-ray diffraction. Metastable tin-nickel alloys were obtained as a single phase (NiSn) in the composition range from 40 to 55at.% Sn. Alloys containing 49-55at.% Sn were transformed exothermically into mixed or single equilibrium phases of Ni
3Sn
2+Ni
3Sn
4 or Ni
3Sn
4. Tin-nickel alloys with higher tin content had lower transformation temperature;
e.g., they were transformed at below 100°C for 55at.% Sn, whereas at 340°C for 49at.% Sn. Tin-nickel alloys containing 40-44at.% Sn were transformed to a single phase of Ni
3Sn
2 at
ca. 300°C with no distinct heat evolution, corresponding to the transformation. Tin-nickel alloys (47at.% Sn) electrodeposited from fluoride baths were transformed at 360°C into another supersaturated metastable phase. The intermediate metastable phase was decomposed into mixed equilibrium phases of Ni
3Sn
2+Ni
3Sn
4 at 600°C.
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