Microstructure evolution and grain refinement mechanism of rapidly solidified single-phase copper based alloys

doi:10.1016/j.jmst.2022.04.025

Abstract

Abstract:

The Cu65Ni35, Cu60Ni40 and Cu55Ni45 alloys were undercooled by fluxing method, and the rapid solidification structure with different undercoolings were also obtained. At the same time, the interface migration process during rapid solidification was photographed by high-speed photography, and the relationship between the morphological characteristics of solidification front and undercooling was analyzed. The microstructures of the three alloys were observed by metallographic microscope, and the microstructure characteristics and evolution law were systematically studied. It was found that two grain refinement events occurred in the low undercooling range and high undercooling range, respectively. The EBSD test of grain refined microstructures showed that the microstructure in the low undercooling range has a high proportion of low-angle grain boundaries and high strength textures. However, there were a large proportion of high-angle grain boundaries and a high proportion of twin grain boundaries and more randomly oriented grains in the microstructure in the high undercooling range. The TEM test of the Cu55Ni45 alloy with the maximum undercooling of 284 K showed that there were high-density dislocation networks and stacking faults in the grains. Finally, the evolution relationship between microstructure hardness and undercooling was systematically studied. It was found that the microhardness of the three alloys decreased sharply near the critical undercooling. Combined with EBSD, TEM and microhardness analysis, it was confirmed that the grain refinement under low undercooling was caused by dendrite remelting, while the grain refinement under high undercooling was caused by stress-induced recrystallization.

Key words: Rapid solidification structure, Grain refinement, Dendrite remelting, Recrystallization

Xu Xiaolong, Tang Cheng, Wang Hongfu, An Yukang, Zhao Yuhong. Microstructure evolution and grain refinement mechanism of rapidly solidified single-phase copper based alloys[J]. J. Mater. Sci. Technol., 2022, 128: 160-179.

Figures/Tables 22

Fig. 1. Schematic diagram of undercooling experimental device.

Fig. 2. Calculation diagram of alloy undercooling in solidification-recalescence curve.

Fig. 3. Microstructure of Cu65Ni35 alloy under different undercoolings.

Fig. 4. Microstructure of Cu60Ni40 alloy under different undercoolings.

Fig. 5. Microstructure of Cu55Ni45 alloy under different undercoolings.

Fig. 6. Solidification front morphology of Cu65Ni35 alloy under different undercoolings ((a) $\text{ }\!\!\Delta\!\!\text{ }T$=79 K, (b) $\text{ }\!\!\Delta\!\!\text{ }T$=165 K, (c) $\text{ }\!\!\Delta\!\!\text{ }T$=214 K).

Fig. 7. Solidification front morphology of Cu60Ni40 alloy under different undercoolings ((a) $\text{ }\!\!\Delta\!\!\text{ }T$=92 K, (b) $\text{ }\!\!\Delta\!\!\text{ }T$=174 K, (c) $\text{ }\!\!\Delta\!\!\text{ }T$=247 K).

Fig. 8. Solidification front morphology of Cu55Ni45 alloy under different undercoolings ((a) $\text{ }\!\!\Delta\!\!\text{ }T$=86 K, (b) $\text{ }\!\!\Delta\!\!\text{ }T$=161 K, (c) $\text{ }\!\!\Delta\!\!\text{ }T$=229 K).

Fig. 9. EBSD analysis of Cu65Ni35 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$=64 K ((a) Grain boundary, (b) Grain orientation of (a), (c) Polar figure of (b), (d) Grain boundary misorientation distribution).

Fig. 10. EBSD analysis of Cu60Ni40 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$= 74 K. ((a) Grain boundary, (b) Grain orientation of (a), (c) Polar figure of (b), (d) Grain boundary misorientation distribution).

Fig. 11. EBSD analysis of Cu55Ni45 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$= 70 K. ((a) Grain boundary, (b) Grain orientation of (a), (c). Polar figure of (b), (d) Grain boundary misorientation distribution).

Fig. 12. Solidification-recalescence curve of alloys. ((a) Solidification curve of Cu55Ni45 alloy, (b) Solidification curve of Cu60Ni40 alloy, (c) Solidification curve of Cu65Ni35 alloy).

Fig. 13. Dimensionless primary solid remelting fraction of Cu55Ni45 alloy.

Fig. 14. EBSD analysis of Cu65Ni35 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$= 202 K. ((a) Grain boundary, (b) Grain orientation of (a), (c) Polar figure of (b), (d) Grain boundary misorientation distribution).

Fig. 15. EBSD analysis of Cu60Ni40 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$=222 K. ((a) Grain boundary, (b) Grain orientation of (a), (c) Polar figure of (b), (d) Grain boundary misorientation distribution).

Fig. 16. EBSD analysis of Cu55Ni45 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$= 272 K ((a) Grain boundary, (b) Grain orientation of (a), (c) Polar figure of (b), (d) Grain boundary misorientation distribution).

Fig. 17. Recrystallization area and proportion of alloys. ((a) Recrystallization area and proportion of Cu65Ni35 alloy at 202 K, (b) Recrystallization area and proportion of Cu60Ni40 alloy at 222K, (c) Recrystallization area and proportion of Cu55Ni45 alloy at 272 K).

Table 1. Physical parameters used in calculation [29,39]

Parameter	Value of Cu65Ni35 alloy
heat of fusion ΔHf/kJ mol⁻¹	14090
specific heat of the liquid C_P/J⁻¹ mol⁻¹ K⁻¹	32.6
dynamic viscosity of the liquid μ/Pas	10⁻³
molar volume of the liquidVmL/m³ mol⁻¹	8.02 × 10⁻⁶
molar volume of the solid VmS/m³ mol⁻¹	7.04 × 10⁻⁶
liquidus T_L/K	1528
solute diffusivity in the liquid D_L/m² s⁻¹	4.3 × 10⁻⁹
thermal diffusivityαL/m² s⁻¹	7.5 × 10⁻⁹
atomic spacea0/m	3.0 × 10⁻¹⁰
interfacial energyσS,L/J m⁻²	0.29
speed of sound in the melt V₀/m s⁻¹	4000
equilibrium liquidus slope m_L/K (at.%)⁻¹	4.13
equilibrium solute partition coeffificient k₀	1.35
solidifification time t_f/s	0.1
size of the mushy zonea/m	0.01
gibbse-Thomson coeffificientΓ	2.86 × 10⁻⁷
solid fraction at the dendrite coherency pointfscoh	0.15
solidifification shrinkage of the primary phaseβ	0.13
atomic diffusive speed V_D/m s⁻¹	20

Fig. 18. Schematic diagram of stress accumulation of Cu65Ni35 alloy.

Fig.19. Selected area electron diffraction (SEAD) pattern and TEM bright field image of Cu55Ni45 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$= 284 K

Fig. 20. TEM bright field image of Cu55Ni45 alloy at $\text{ }\!\!\Delta\!\!\text{ }T$=284 K.

Fig. 21. Evolution of grain size and microhardness of alloy. ((a) Evolution of grain size and microhardness of Cu65Ni35 alloy, (b) Evolution of grain size and microhardness of Cu60Ni40 alloy, (c) Evolution of grain size and microhardness of Cu55Ni45 alloy).

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