Grain refinement mechanism in undercooled Cu 30 Ni 70

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Grain refinement mechanism in undercooled Cu30 Ni70 J. Z. Xiao,a) K. K. Leung, and H. W. Kui Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received 10 May 1996; accepted 16 December 1996)

When undercooled molten Cu30 Ni70 crystallizes at an undercooling DT > 145 K (DT is defined as Tl 2 Tk where Tl is the liquidus of Cu30 Ni70 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.

When an alloy melt is undercooled to way below its liquidus Tl , the crystallization behavior depends on the initial bulk undercooling defined as DT ­ Tl 2 Tk where Tk is the kinetic crystallization temperature. For instance, in the least undercooling regime, DT < 60 K, crystallization of undercooled Ge1 is initiated by k110l twin dendrite. For 60 < DT < 93 K, the k211l twin dendrite dominates the crystallization process. Finally, for DT > 93 K, the k100l twin free dendrite appears first during crystallization. Another example is the solidification of undercooled molten Ni. Walker2 discovered that the grain size of undercooled Ni undergoes a rapid drop, by about two orders of magnitude in a narrow temperature range, deep in the undercooling regime. This phenomenon is termed grain refinement. Later, grain refinement is also found in other systems such as metallic alloys and semiconductors.3–5 Over the years, many proposals4,6–9 have been put forward to explain the origin of grain refinement. However, none of them has direct experimental support. Recently Leung et al.10 found that grain refinement in undercooled Ni is brought about by dynamic nucleation consistent with Horvay’s proposal. The physical ground of dynamic nucleation is simple. Since molten Ni contracts on solidification, as dendrites grow into the undercooled liquid, the atoms in the liquid have to move to the dendrites for further crystal growth. At sufficient large undercooling or equivalently when

a)

Permanent address: Department of Materials Science, Huazhong University of Sciences and Technology, Wuhan, China. J. Mater. Res., Vol. 12, No. 4, Apr 1997

http://journals.cambridge.org

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the dendritic growth velocity is sufficiently rapid, the atoms in the liquid cannot respond fast enough to the volume change. The liquid is then torn apart, resulting in the formation of voids. The formation or collapse of voids can generate shock waves that are subsequently sent through the underco

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