Non-equilibrium processing of Ni-Si alloys at high undercooling and high cooling rates

Melt encasement (fluxing) and drop-tube techniques have been used to solidify a Ni-25 at.% Si alloy under conditions of high undercooling and high cooling rates respectively. During undercooling experiments a eutectic structure was observed, comprising alternating lamellae of single phase γ(NiSi) and Ni-rich lamellae containing of a fine (200-400 nm) dispersion of β- NiSi and α-Ni. This is contrary to the equilibrium phase diagram from which direct solidification to β-NiSi would be expected for undercoolings in excess of 53 K. Conversely, during drop-tube experiments a fine (50 nm) lamellar structure comprising alternating lamellae of the metastable phase NiSi and β-NiSi is observed. This is also thought to be the result of primary eutectic solidification. Both observations would be consistent with the formation of the high temperature form of the β-phase (β/β) being suppressed from the melt.