Tuning thermal expansion by a continuing atomic rearrangement mechanism in a multifunctional titanium alloy

As to multifunctional titanium alloys with high strength and low elastic modulus, thermal training is crucial to tune their thermal expansion from positive to negative, resulting in a novel linear expansion which is stable in a wide temperature range. Aided by the high-order Hooke's law of elastic solids,a reversible atomic rearrangement mechanism was proposed to explain the novel findings which are unexpected from typical shape memory alloys. To confirm this continuous mechanism, a Ti-Nb based alloy, which possesses a nanoscale spongy microstructure consisting of the interpenetrated Nb-rich and Nb-lean domains produced by spinodal decomposition, was used to trace the crystal structure change by in-situ high energy synchrotron X-ray diffraction analyses. By increasing exposure time, the overlapped diffraction peaks can be separated accurately. The calculated results demonstrate that, in the nanoscale Nb-lean domains, the crystal structure parameters vary linearly with changing temperature along the atomic pathway of the bcc-hcp transition. This linear relationship in a wide temperature range is unusual for first-order martensitic shape memory alloys but is common for Invar alloys with high-order spin transitions. Furthermore, the alloy exhibits smooth DSC curves free of transformation-induced heat peaks observed in shape memory alloys, which is consistent with the proposed mechanism that the reversible transition is of high-order.

Atomic-resolution investigation of structural transformation caused by oxygen vacancy in La_(0.9)Sr_(0.1)TiO_(3+δ)titanate layer perovskite ceramics

Perovskite functional ceramics have been widely applied for thermal protection owing to their unique physical properties.However,formation of oxygen vacancies under external stimuli usually limits their performance in practical applications.Therefore,the mechanism of the effect of oxygen vacancy on the layer structure of perovskite La_(0.9)Sr_(0.1)TiO_(3+δ)was investigated by experiments and first-principles simulations.The experimental results showed that the lattice distortion occurred in oxygen-deficient environment to give a longer c-axis,along with a significant adjustment in the modes of A/B–O bond vibration,resulting in lower reflectivity.Advanced transmission electron microscopy studies revealed that oxygen vacancies induced localized atomic rearrangements via[TiO_(6)]layer movements to adapt to the lattice distortion.This eventually restructured a part of the layer interfaces by expanding the overlapping projection of atoms in the c-axial direction.The specific transformation process was described as a compendious process,while geometric phase analysis effectively clarified how oxygen vacancies can inhibit reflectivity on the layer structure.Thus,this study provides effective approaches for researching the effects of oxygen vacancy on the physical properties of orthorhombic layer perovskite structures,which may facilitate the development of perovskite-based functional devices.