Fig. 6: On the mechanism of superelasticity of Nb6-HEA.

a–c Room-temperature cyclic compressive stress-strain curves of Nb6-HEA micropillars which consist of single matrix phase (a), mixture of matrix and Cu-rich phases (b), mixture of matrix and β-Nb phases (c), and “1, 2, 3, 4, 5” represent the number of loading-unloading cycles. The inset scanning electron microscope backscattered electron (SEM-BSE) images (scale bar, 5 μm) show the selected areas for the corresponding micropillars fabricated by the focused ion beam (FIB) milling, which are indicated by the yellow crosshairs. The inset scanning electron microscope secondary electron (SEM-SE) images (scale bar, 1 μm) show the morphology of the corresponding micropillars. d ε_E + ε_SE and elastic energy E_elastic as a function of applied strain for the “Matrix” micropillar and the “Matrix + Cu-rich” micropillar. e A comparison of superelastic stress σ_C between the “Matrix” micropillar, the “Matrix + Cu-rich” micropillar and previously-reported SMA micropillars^{32,55,56,57,58}. The inset shows the loading-unloading stress-strain curves of the “Matrix + Cu-rich” micropillar and the Ti-50.8Ni micropillar³² by applying strain of about 4.7%. f Compressive loading-unloading stress-strain curves of the “Matrix + Cu-rich” micropillar from cycle 6 to cycle 9, and the inset reveals that E_elastic for “Matrix + Cu-rich” micropillar is 6 times of that for Ti-50.8Ni micropillar³². g–i In-situ heating TEM dark-field images (DFIs, scale bar, 50 nm) taken at 123 K (g), 223 K (h), 333 K (i), which are obtained by capturing one 1/2 spot belong to the B19’ structure (indicated by yellow circles), and the insets show the corresponding TEM electron diffraction patterns with [111]-B2 zone axes taken at 123 K, 223 K, 333 K respectively (scale bar, 2 nm^-1), and intensity profiles recorded along the dotted white lines in corresponding diffraction patterns.