Microstructural Evolution of Rapidly Solidified Ni-Cu Alloys

Abstract

This study systematically investigated the microstructural evolution of binary Ni-Cu alloys (Cu55Ni45, Cu60Ni40, and Ni65Cu35) under deep undercooling conditions. The controlled rapid solidification experiments combined with optical microscopy and electron backscatter diffraction (EBSD) analysis demonstrate that increasing undercooling (ΔT) can induce a consistent sequence of microstructural transitions: coarse dendrites, fine equiaxed grains (first refinement), oriented fine dendrites, and fine equiaxed grains (second refinement). Two distinct grain refinement events are identified, with critical undercooling thresholds (ΔT) dependent on composition: increasing Cu content increases the critical undercooling ΔT* required for the second refinement (Cu55Ni45: 227 K; Cu60Ni40: 217 K; Ni65Cu35: 200 K). The BCT (Bridgman Crystal Growth) model quantitatively elucidates this behavior, revealing a shift from solute-diffusion-dominated growth at low undercooling to thermally dominated diffusion at high undercooling (ΔT). Crucially, refined grains at high undercooling exhibit smaller sizes (10 µm) and higher uniformity than those at low undercooling (20 µm). These findings provide fundamental insights into non-equilibrium solidification mechanisms and establish a foundation for designing high-performance Ni-Cu alloys via deep undercooling processing.

Keywords

deep undercooling / Ni-Cu alloys / microstructural evolution / grain refinement / BCT model / rapid solidification

References

[1] Herlach DM. Non-equilibrium Solidification of Undercooled Metallic Melts. Metals. 2014, 4(2): 196-234 J]

[2] Mullis AM, Cochrane RF. Grain Refinement and the Stability of Dendrites Growing into Undercooled Pure Metals and Alloys. J. Appl. Phys.. 1997, 82(8): 3 783-3 790 J]

[3] Zhai W, Chang, Geng D L, et al. . Current Status and Future Prospects of Research on Solidification Process of Metal Materials. The Chinese Journal of Nonferrous Metals. 2019, 29(9): 1 953-2 008[J]

[4] Wei B, Yang C, Zhou Y. High Undercooling and Rapid Solidification of Ni-32.5%Sn Eutectic Alloy. Acta Metall. Mater.. 1991, 39(6): 1 249-1 258 J]

[5] Ahmad R, Cochrane RF, Mullis AM. The Formation of Regular αNi-γ (Ni31Si12) Eutectic Structures from Undercooled Ni-25 at.% Si Melts. Intermetallics. 2012, 22: 55-61 J]

[6] Leung KK, Chiu CP, Kui HW. Grain Refinement in Undercooled Nickel. Scripta Metal. Mater.. 1995, 32(10): 1 559-1 563 J]

[7] Schwarz M, Karma A, Eckler K, Herlach DM. Physical Mechanism of Grain Refinement in Solidification of Undercooled Melts. Phys. Rev. Lett.. 1994, 73(10): 1 380-1 383 J]

[8] Kattamis TZ. Mechanism of Establishment of Cast Microstructure During Solidification of Highly Undercooled Melts. J. Cryst. Growth. 1976, 34(2): 215-220 J]

[9] An Y, Xu X, Zhao Y, et al. . Nonequilibrium Solidification Velocity, Recalescence Degree and Grain Refinement of Highly Undercooled Ni-Based Single-phase Alloys. Journal of Alloys and Compounds. 2021, 881: 160 658 J]

[10] Sun Y, Liu X, Wang Y, et al. . Microstmctural Evolution of the Deeply Undercooled Ag-Ge Alloys with Cu Atoms Cluster Triggered Nucleation. Cailiao yu Gongcheng (Rare Metal Materials and Engineering). 2006, 35(5): 732-735[J]

[11] Jones B, Weston G. Grain Refinement in Undercooled Copper. J. Aust. Inst. Met.. 1970, 15: 3 167[J]

[12] Horvay G. The Tension Field Created by a Spherical Nucleus Freezing into Its Less Dense Undercooled Melt. Int. J. Heat Mass Transfer.. 1965, 8(2): 195-243 J]

[13] Willnecker R, Herlach DM, Feuerbacher B. Grain Refinement Induced by a Critical Crystal Growth Velocity in Undercooled Melts. Appl. Phys. Lett.. 1990, 56(4): 324-326 J]

[14] Zheng HX, Yu Y, Li JG. Microstructural Evolution of Undercooled Ni-40wt%Pb Hypermonotectic Alloy. Mater. Sci. Forum. 2005, 475: 2 651-2 654 J]

[15] Dragnevski KI, Mullis AM, Cochrane RF. The Effect of Experimental Variables on the Levels of Melt Undercooling. Materials Science and Engineering: A. 2004, 375–377: 485-487 J]

[16] Langer JS. Instabilities and Pattern Formation in Crystal Growth. Rev. Modern. Phys.. 1980, 52: 1-28 J]

[17] Kurz W, Trivedi R. Solidification Microstructure: Recent Developments and Future Directions. Acta Metal. Mater.. 1990, 38: 1-17 J]

[18] An H, Xu X. Study of Non-equilibrium Dendrite Growth in an Undercooled Alloy. Materials Science and Technology. 2020, 36(15): 1 720-1 727 J]

RIGHTS & PERMISSIONS

Wuhan University of Technology and Springer-Verlag GmbH Germany, Part of Springer Nature