The formation of regular alpha Ni- gamma (Ni31Si12) eutectic structures from undercooled Ni-25 at.% Si melts

Undercooling experiments have been performed on an Ni-25.2 at.% Si alloy using a flux encasement technique, with a maximum undercooling of 160 K having been achieved. Double recalescence was observed at all undercoolings, with crystal growth velocities being measured for the first recalescence event. These velocities were extremely low, with a maximum value of 0.018 m s-1 being recorded. At all undercoolings a eutectic structure was observed, comprising alternating lamellae of single phase γ(Ni31Si12) and Ni-rich lamellae containing of a fine (200-400 nm) dispersion of βNi3Si and αNi. This is contrary to the equilibrium phase diagram from which direct solidification to βNi3Si would be expected for undercoolings in excess of 53 K. We postulate that the direct solidification of βNi3Si from the melt is suppressed, with an α-γ eutectic being formed instead. The α appears to be supersaturated in Si and undergoes a eutectoid decomposition to α and βNi3Si, giving rise to the observed second recalescence.