Phase structure of electrodeposited alloys

By electrodeposition it is possible to obtain alloy phases of metastable structure; these phases may be different from those reported for metallurgical equilibrium conditions or may have different compositions for the solubility limits, in some cases with very important enlargement. Typical cases are examined, discussing published results and presenting new findings. To understand the general behaviour, free energy versus composition g/x diagrams were calculated and are presented for the alloys examined in equilibrium conditions and after modification to obtain composition limits for the electrodeposited phases in stable and metastable state.The first reported case is Ni–Sn alloy, where the well known metastable phase NiSn is obtained. The second case is the electrodeposition of IB–Sn alloys from non-cyanide electrolytes. Phase structure of Zn-rich electrodeposited Zn–Ni alloys is also discussed as representative of grain size effect on phase stability.From the different cases examined, the occurrence of electrodeposited intermediate phases can be interpreted according to a local thermodynamic equilibrium, whilst long range order is difficult to observe in as-deposited layers. These phases depend on bath composition and operation conditions (temperature, stirring, direct or pulse current density) giving phases of different grain size which in turn can influence their stability.