THE VELOCITY OF SOLIDIFICATION OF HIGHLY UNDERCOOLED NICKEL

At large undercoolings (> 10 pct T(M)), present theories relating soli dification velocity to degree of undercooling do not agree well with r eported experimental data for the solidification velocity of nickel as a function of undercooling. The present work shows that this discrepa ncy is due to two factors. First, the majority of previously reported results overestimate the solidification velocity of nickel at large un dercoolings. Second, the scatter in experimental data is so large that a functional relationship between undercooling and velocity is not ev ident. In this study, the solidification velocity of undercooled nicke l was measured using a linear array of 38 photodiodes. The results ind icate that the velocity of the thermal field generated by the solid/li quid interface approaches a maximum velocity of 20 m s-1 at DELTAT alm ost-equal-to 10 pct T(M) (173 K) and then remains constant with increa sing undercooling. This suggests that the velocity of the solid/liquid interface, at undercoolings greater than 10 pct T(M), could be limite d by attachment kinetics at the interface.