Grain Refinement Mechanism in Undercooled Cu30Ni70

Abstract

When undercooled molten Cu30Ni70 crystallizes at an undercooling ΔT≅ 145 K (ΔT is defined as Tl – Tk where Tl is the liquidus of Cu30Ni70 and Tk is the kinetic crystallization temperature), its grain size undergoes a rapid decrease by as much as two orders of magnitude in a narrow temperature range. This phenomenon is termed grain refinement. It was found that grain refinement is brought about by multiplication of dendrites. Composition analysis of the dendrite indicates that it has the least Ni concentration at its axis. The Ni content then increases radially from the central axis. Therefore, the dendrite is unstable against melting since the melting temperature of Cu–Ni increases with Ni content. The origin of grain refinement is attributed to the remelting of these dendrites.