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Thus there is clearly a tension-compression asymmetry.Moreover, the detailed changes in slopes of the S-S curves at low strain values (shown in the inset of Fig.(d), The detailed change in scattering patterns as highlighted in the red dotted circle in (c) at different stresses marked with the same symbols shown in (a).The insets in (a) are the scattering pattern under compression loading and enlarged S-S curve at a low strain, respectively, showing the diverse elastic modulus at different stress states.
Therefore, the underlying deformation mechanism responsible for the peculiar properties of this new class of metallic materials still remains elusive.The nano-scale martensites consist of frustrated nanodomains of individual martensitic variants, which are distinctively different from the normal self-accomodating polytwinned martensitic plates.We believe that stress-induced reversible transformations of B2 to α″ and BCC to δ and switch of nanodomains of these martensites contribute to the anomalous mechanical behavior of the gum-like metals. Interestingly, the two-step superelasticity with a stress plateau at ~240 MPa, accompanied by a jump in the strain of ~2%, is observed only under tension, in clear contrast to the almost linear S-S curve obtained under compression.Ti-Nb-based Gum Metals exhibit extraordinary superelasticity with ultralow elastic modulus, superior strength and ductility, and a peculiar dislocation-free deformation behavior, most of which challenge existing theories of crystal strength.
Additionally, this kind of alloys actually displays even more anomalous mechanical properties, such as the non-linear superelastic behavior, accompanied by a pronounced tension-to-compression asymmetry, and large ductility with a low Poisson's ratio.Two main contradictory arguments exist concerning the deformation mechanisms of those alloys, i.e., formation of reversible nanodisturbance and reversible martensitic transformation.