Effect on microstructure and plastic deformation behavior of a Zr-based amorphous alloy by cooling rate control

doi:10.1016/j.jmst.2020.12.013

Abstract

Abstract:

In this study, Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 amorphous alloys samples with the same diameter (8 mm) were prepared by using self-designed molds (viz. refractory steel, pure graphite, and copper molds) with different cooling capacities. Moreover, by eliminating the size effect, the effect of the cooling rate on the microstructure and compression deformation behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 amorphous alloys was investigated. Differentiation of the cooling curves revealed that the instantaneous cooling rates of the alloy melt at the glass transition temperature (T_g) are 45, 52, and 64 K·s^―1 for refractory steel, pure graphite, and copper molds, respectively. X-ray diffraction, differential scanning calorimetry, and high-resolution transmission electron microscopy analysis revealed that with the decrease in the cooling rate, trace icosahedral-like atomic clusters and nanocrystals appear in local areas of the amorphous alloy and that the amount of free volume decreases with the increase in the amount of icosahedra-like atomic clusters and nanocrystals. Compression test results revealed that the elastic strain, yield strength, and compressive strength of the amorphous alloy marginally change with the decrease in the cooling rate, while the plastic strain gradually increases. By fitting, the effective size of the vein-like pattern was linearly related to the enthalpy released during structural relaxation and plastic strain, indicating that at a low cooling rate, the trace nanocrystals in the amorphous alloy could not effectively improve its plasticity and that the amount of free volume mainly affects its plasticity.

Key words: Bulk amorphous alloy, Cooling rate, Compression plasticity, Free volume

Feilong Wang, Dawei Yin, Jingwang Lv, Shan Zhang, Mingzhen Ma, Xinyu Zhang, Riping Liu. Effect on microstructure and plastic deformation behavior of a Zr-based amorphous alloy by cooling rate control[J]. J. Mater. Sci. Technol., 2021, 82: 1-9.

Figures/Tables 14

Fig. 1. (a) Molds of different materials; (b, c) positions of the thermocouples in molds.

Fig. 2. Casting sample and compression sample taken from it.

Fig. 3. Cooling curves recorded in the casting process: (a) refractory steel; (b) pure graphite; (c) copper.

Table 1 Initial time and temperature recorded during the casting of different molds.

Mold material	t₁/s	T₁/K	t₂/s	T₂/K	t₃/s	T₃/K
Refractory steel	10.00	1108	10.05	1088	10.32	1063
Pure graphite	10.00	1107	10.05	1085	10.27	1056
Copper	10.00	1109	10.06	1084	10.34	1039

Fig. 4. XRD patterns and DSC curves of samples cast by different molds: (a) XRD patterns; (b) DSC curves.

Fig. 5. Compressive stress-strain curves of samples cast by different molds.

Table 2 Compressive mechanical properties of Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloy.

Mold material	σ_y (MPa)	σ_m (MPa)	ε_e (%)	ε_p (%)
Refractory steel	1900	2011	2.3	1.2
Pure graphite	2001	2073	2.5	1.7
Copper	1971	2082	2.4	2.6

Fig. 6. SEM images of the side view of samples cast by different molds: (a, d) refractory steel; (b, e) pure graphite; (c, f) copper.

Fig. 7. SEM images of the fracture morphology of samples cast by different molds: (a, d) refractory steel; (b, e) pure graphite; (c, f) copper.

Fig. 8. Fitting cooling curves of alloy melts with different molds.

Table 3 Coefficients of cooling equations for samples cast by different molds.

Mold material	T₀/K	A₁/K	A₂/K	k₁/s	k₂/s
Refractory steel	314.7	4.9 × 10⁴	876.2	2.1	12.1
Pure graphite	312.2	1.9 × 10⁵	987.1	1.6	10.9
Copper	311.1	5.1 × 10¹⁰	1691.7	0.5	7.4

Fig. 9. IFT-filtered images and FT diffraction patterns of samples cast by different molds: (a-a2) refractory steel mold; (b-b2) pure graphite mold; (c-c2) copper mold. (a-c) are the IFT-filtered images, and the enlarged images of the yellow rectangular area are shown in (a1-c1). (a3-c3) are FT diffraction patterns.

Fig. 10. Magnified DSC curves of samples cast by different molds.

Fig. 11. (a) Fitting curve for the cooling rates and enthalpies of relaxation, (b) fitting curve of enthalpies of relaxation and effective size, (c) fitting curve of enthalpies of relaxation and εp.

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