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Here we use synchrotron-based high-energy X-ray diffuse scattering (HE-XRDS), in combination with computer simulations, to explore in-situ the deformation behavior of a gum-type Ti-24Nb-4Zr-8Sn-0.10O (in weight percent) alloy (abbreviated as Ti2448 from its chemical composition).
Besides the high penetration, low absorption, great access to the reciprocal space, and high reciprocal resolution, the HE-XRDS technique can trace in-situ small changes in crystal structure, size, and elastic strain state even for nanocrystalline domains with a small volume fraction during deformation and phase transformation in bulk samples.
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.
1a) indicate completely different non-linear elastic deformation behaviors under tension and compression.
With increasing stress, an obvious reduction in modulus is observed under tension, but a slight increase in modulus is observed under compression.(a), Stress-strain (SS) curve for single crystal under tensile and compression modes.
In the “generic” case, the properties of such groups prevent the Born Rule from holding.Only when some very special non-generic conditions are met by the twinning modes of a crystalline substance is the Rule valid; the development of an elastic model is then possible by following a well-known procedure.The analysis of relevant experimental data confirms that, while basically all crystals exhibit twins, most of them do exhibit generic twinning modes for which the hypothesis of is violated.Experimental studies found no phase transformations, no high density of dislocations (even after 90% cold work), and no twinning, leading to a hypothesis of dislocation-free deformation via the growth of nanodisturbances, with essential differences from the nanodisturbance hypothesis in which the so-called ‘non-crystallographic' partial dislocations with the Burgers vectors having non-quantized magnitude exist and play a crucial role.
Therefore, the underlying deformation mechanism responsible for the peculiar properties of this new class of metallic materials still remains elusive.
Under tension the stress-induced phase transformation from B2 nanoclusters to α" is evidenced by the obvious increase in the (021) diffuse scattering signals) and, instead, ω phase starts to form at a compressive stress of ~500 MPa (as indicated by the HE-XRDS pattern displayed in the inset of Fig. After the applied compressive stress is removed, the ω phase remains (as indicated by the HE-XRDS patterns provided in the Supplementary Fig. The crystal structure of α" is determined by using high-energy X-ray diffraction (HE-XRD) by rotating the sample around .