Efficient Coarse‐Grained Superplasticity of a Gigapascal Lightweight Refractory Medium Entropy Alloy

Abstract Superplastic metals that exhibit exceptional ductility (>300%) are appealing for use in high‐quality engineering components with complex shapes. However, the wide application of most superplastic alloys has been constrained due to their poor strength, the relatively long superplastic deformation period, and the complex and high‐cost grain refinement processes. Here these issues are addressed by the coarse‐grained superplasticity of high‐strength lightweight medium entropy alloy (Ti43.3V28Zr14Nb14Mo0.7, at.%) with a microstructure of ultrafine particles embedded in the body‐centered‐cubic matrix. The results demonstrate that the alloy reached a high coarse‐grained superplasticity greater than ≈440% at a high strain rate of 10−2 s−1 at 1173 K and with a gigapascal residual strength. A consecutively triggered deformation mechanism that sequences of dislocation slip, dynamic recrystallization, and grain boundary sliding in such alloy differs from conventional grain‐boundary sliding in fine‐grained materials. The present results open a pathway for highly efficient superplastic forming, broaden superplastic materials to the high‐strength field, and guide the development of new alloys.

4. Figure S3: Elemental distribution during the superplastic deformed sample at 1173 K. 5. Figure S4: Tensile performance. 6. Figure S5: The mean geometrically necessary dislocations (GND) for different areas. 7. Figure S6: The volume fraction and mean grain size of the DRX grains. 8. Figure S7: TEM Characteristics of the superplastic deformed specimen of CH-LRMEA at 1173 K. 9. Figure S8: The dislocations and subgrains observed in the superplastic deformed specimen at 1173 K. 10. Figure S9: Schematic illustrating the consecutively triggered deformation mechanism operative in the coarse-grained HC-LRMEA specimens. 11. Figure S11: EBSD characterizations and schematic illustrations of the superplastic deformation.

Activation Energy
High-temperature deformation or creep is a thermally activated process. In general, the Zener Hollomon parameter Z(σ) can describe the relationship between temperature and flow stress at each strain rate [1]. The formula of Z(σ) is given by: where Q is the activation energy, n is the stress exponent, A is a constant sensitivity to the deformation mechanism, and R is the gas constant. Q can be determined according to the following relations: (3) Figure S1a shows the plotted logσ versus 1000/T curves, and the value of Q is also calculated from the slope of the curves. The activation energy (Q) is about 234.2 to 314.6 kJ/mol with the tensile strain from 20% to 300% at 1173 K with a strain rate of 10 −2 s −1 .
According to the sluggish effect [2] in HEA, the diffusion coefficient is likely to be controlled by the slowest moving species, namely β-Ti in our HC-LRMEA. Figure S1b shows that the value of the activation energy (Q 20% ) is below Q β-Ti at Stage I. At Stage II, Q 100% is similar to Q β Ti . Finally, the value of 300% is larger than Q β-Ti at Stage III. The potential mechanisms of superplastic deformation will be discussed in the discussion section. Figure S2 presents the microstructural characteristics of the HC-LRMEA after superplastic deformation at 1173 K. A notable phenomenon pertains to the occurrence of dynamic recrystallization (DRX) at the coarse grain boundaries. Specifically, Figure S2a shows the EBSD IPF image exhibiting significant elongation of the coarse grains by about 100-200 µm due to the superplastic deformation and presence of numerous fine grains at the coarse grain boundaries, resulting from DRX. The fine grains of DRX, 1~10 m in size, are evident between the coarse grains, as illustrated in Figure S1b, and the volume fraction of the fine grains is about 16.5%. Figure S2c presents the TEM-BF image of the grain interior displaying both bright and dark phases. As shown in Figure S2d, the corresponding selected area electron diffraction along the [001] direction confirms that the bright area is the BCC matrix, while the dark area is comprised of the Zr-rich particles. Figure  In addition, the size of the Zr-rich particles increases to 500 ~1000 nm relative to the initial state, and the volume fraction of this phase also increases to approximately 40%.

Grain Boundary Segregation
The phenomenon of grain boundary segregation is observed in the current superplastic deformation at 1173 K, which is regarded as an active factor in inducing DRX at the coarsegrained boundaries. Figure S3a presents the elongated coarse grains along the tension direction. An area selected for EPMA analysis near the grain boundaries exhibits an almost uniform distribution of Ti, Nb, and Mo ( Figure S4c). However, there is a significant separation between Zr and V. In particular, large atomic size Zr elements tend to concentrate along the tension direction toward the grain boundaries. By contrast, the small atomic size V elements tend to diffuse inward of the coarse grains. Moreover, the Zr elements eventually form a Zr-rich segregation layer with a thickness of about 200 to 300 nm, or with large amounts of Zr-rich particles at the grain boundaries ( Figure S3b).
Based on thermodynamic calculations, the mixing enthalpies of V with the other two host elements (Ti, Nb) are significantly skewed to negative values, namely ΔH Ti-V =−2 kJ/mol and ΔH Nb-V =−1 kJ/mol, which suggest that V tend to combine with Ti and Nb. By contrast, the enthalpies of mixing Zr with Ti and Nb are ΔH Ti-Zr =0 kJ/mol and ΔH Nb-Zr =4 kJ/mol, respectively, which implies that Zr tends to exist independently in this Ti-V-Zr-Nb multiple systems. In addition, the atomic radii of V and Zr are 135 and 155 pm [3] , respectively, with a size mismatch of 6.9%, which was one of the main factors contributing to the separation of the two elements. In addition, lattice distortion contributed to enhancing the diffusion of Zr in the RHEA [4] . Thus, under rheological stress and temperature, the Zr elements tend to move into the high-energy zone at the grain boundary. The diffusion and segregation of Zr elements play a dual role, as they promote DRX at the coarse grain boundaries; however, they also drove GBS, which, in turn, promotes superplastic deformation, which has also been found in other high-strain-rate superplastic medium entropy alloys [5] .