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 above findings which are unexpected from 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, these well-known overlapped diffraction peaks of multifunctional alloys can be separated accurately. The calculated results demonstrate that, in the nanoscale Nb-lean domains, the crystal parameters vary linearly with temperature along the atomic pathway of the bcc-hcp transition. This linear relationship in a wide temperature range is unusual for the first-order martensitic transformation shape memory alloys but is common for the high-order spin transition Invar alloys. Furthermore, the alloy exhibits smooth DSC curves being free of the transformation-induced heat peaks observed in shape memory alloys. This is also consistent with the proposed mechanism that the reversible transition is of high order.