Ultrastrong and ductile BCC high-entropy alloys with low-density via dislocation regulation and nanoprecipitates

doi:10.1016/j.jmst.2021.08.034

Abstract

Abstract:

The high strength is a typical advantage of body-centered-cubic high-entropy alloys (BCC-HEAs). However, brittleness and weak strain-hardening ability are still their Achilles’ heel. Here, extraordinary strength together with good tensile ductility are achieved in (Zr_0.5Ti_0.35Nb_0.15)_100-_xAl_x alloys (at.%, x = 10 and 20) at room temperature. Relatively low densities of less than 6 g/cm³ are exhibited in these alloys. Designing nanoprecipitates and diversifying dislocation motions are the keys to achieving such salient breakthrough. It is worth noting that the tensile strength of 1.8 GPa in (Zr_0.5Ti_0.35Nb_0.15)₈₀Al₂₀ alloy is a record-high value known in reported BCC-HEAs, as well as a tensile strain over 8%. Furthermore, the maximum strain of ∼25% in (Zr_0.5Ti_0.35Nb_0.15)₉₀Al₁₀ alloy can challenge existing limit value, and is accompanied with a tensile strength of 1.2 GPa. The current work does not only provide novel ultra-strong and tough structural materials with low density, but also sheds new light on designing BCC-HEAs with attractive performances and strain-hardening ability.

Key words: High-entropy alloys, Mechanical properties, Dislocation, Nanoprecipitates, Strain hardening

Xuehui Yan, Peter K. Liaw, Yong Zhang. Ultrastrong and ductile BCC high-entropy alloys with low-density via dislocation regulation and nanoprecipitates[J]. J. Mater. Sci. Technol., 2022, 110: 109-116.

Figures/Tables 8

Fig. 1. Composition design and structure of (Zr0.5Ti0.35Nb0.15)100-xAlx alloys. (a) Schematic diagram of composition-design philosophy. The internal XRD shows a single BCC lattice structure of as-cast (Zr0.5Ti0.35Nb0.15)100-xAlx alloys. (b) EBSD maps of the (Zr0.5Ti0.35Nb0.15)90Al10 alloy. (c) TEM bright-field image of the as-cast (Zr0.5Ti0.35Nb0.15)90Al10 alloy. The corresponding selected area electron diffraction (SAED) pattern is as an inset. (d, e) 3D APT reconstructions: atomic distributions and concentration of components.

Table 1. Nominal composition, actual composition, and theoretical densities of the (Zr0.5Ti0.35Nb0.15)100-xAlx alloy.

Nominal composition (at.%)	Actual composition (at.%)	Theoretical densities (g/cm³)	Formula:
(Zr_0.5Ti_0.35Nb_0.15)₉₀Al₁₀	Zr_45.9Ti_31.0Nb_12.7Al_10.4	5.79	$ \rho=\frac{\sum C_{\mathrm{i}} A_{\mathrm{i}}}{\sum \frac{C_{\mathrm{i}} A_{\mathrm{i}}}{\rho_{\mathrm{i}}}}$
(Zr_0.5Ti_0.35Nb_0.15)₈₀Al₂₀	Zr_41.3Ti_27.8Nb_11.4Al_19.5	5.58

Fig. 2. Compressive and tensile properties of (Zr0.5Ti0.35Nb0.15)100-xAlx alloys at room temperature. (a) Compressive engineering stress-strain curves. Morphologies of compressed Al10 and Al20 samples as insets. (b) The true tensile stress-strain curves of Al10 and Al20 alloys. (Abbreviation: yield strength (YS), tensile strength (TS), and tensile strain (ε)).

Fig. 3. Microstructure and tensile properties of recovery and recrystallization (Zr0.5Ti0.35Nb0.15)90Al10 alloys. (a) Microstructure of the recovery Al10 alloy (a-I: Orientation map, a-II: grain map). (b) Microstructure of the recrystallization Al10 alloy (b-I: recrystallization map, b-II: grain map). (c) The true tensile stress-strain curves.

Fig. 4. Deformed microstructure of the as-cast Al10 alloy. (a) TEM observation of the dislocation pattern in different tensile strains (ε): low strain of a-I, middle strain of a-II, and high strain of a-III. (b) EBSD image of tensile fracture. The regions I, II, and III correspond to a-I, a-II, and a-III, respectively. (c) A magnified view of loops and dislocation tangles. (d) A magnified view of double dislocation cross-slip.

Fig. 5. Deformed microstructure of recovery and recrystallization Al10 alloys. (a, b) Deformation microstructure in recovery and recrystallization Al10 alloy. Magnified view of short-rod dislocations is as an inset in Fig. 5(b). (c) Morphology of the interaction between nanoprecipitates and dislocations: (c-I) dislocation bypass mechanism; (c-II) dislocation-shearing mechanism. (d) Morphology and composition of spherical precipitates. (e) SAED pattern of matrix and nanoprecipitates. Matrix and precipitates are highlighted in yellow and red, respectively. (f) HRTEM image of the matrix and nano-precipitates.

Fig. 6. Discussion of strain-hardening ability. (a) Strain-hardening ability and related microstructures. (b) Schematic illustration of deformation mechanisms. [Residual dislocations: reserved after cold-rolling and recovery annealing. (Tensile dislocations: generated during the tensile deformation).

Fig. 7. Comparison of the current BCC-HEAs with existing HEAs and amorphous alloys. (a) Maps of yield strength versus tensile strain of HEAs and amorphous alloys reported previously at room temperature (Abbreviation: Yield strength (YS), Tensile strength (TS), Amorphous alloy (AM)). (b) Maps of specific strength versus density of HEAs and amorphous alloys reported previously at room temperature.

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