Study on Anomalous Rapid Solidification of Al-35 at%Ni in Microgravity

Abstract

Experiments were carried out onboard the International Space Station using the electromagnetic levitation (EML) facility on dendritic growth in Al- 35 at%Ni under primary solidification of the NiAl(B2)-phase. The ‘‘velocity versus undercooling’’ relationship shows an anomalous behavior. At low undercoolings, DT < 250 K, the velocity unexpectedly decreases with increasing undercooling. For higher undercoolings, DT > 250 K, the crystal velocity increases with increasing undercooling following a thermodynamically consistent trend. For such anomalous behavior, a special mechanism was identified through the different crystals having scales and dendritic morphology. The scaled front consists of numerous nuclei ahead of the crystal–liquid interface that occurs in the undercooling range featuring the negative slope for the crystal growth velocity at DT < 250 K. This unusual behavior is confirmed experimentally and explained theoretically using analytical solutions for the crystal growth front with the polydisperse ensemble of crystals nucleating ahead of it.