Abstract
The in-situ anomalous small-angle X-ray scattering (ASAXS) technique was used to investigate the strain glass transition (SGT) in as-quenched Ti48.7Ni51.3 shape memory alloy during a thermal cycle of 30 °C to the SGT temperature Tg (−50 °C) and then to 30 °C again. The Ni atoms play a critical role as point defects in the SGT mechanism and are very difficult to characterize using conventional tools. ASAXS identified the distribution of Ni atoms in nanodomains, which have a disk-like core–shell configuration with a Ni-rich shell and a highly Ni-rich core. Moreover, the morphological evolution, growth and shrinkage of the highly Ni-rich core domains during the thermal cycle through Tg are demonstrated. The enhancement and reversible behavior of the local lamellar ordering arrangement of nanodomains during the SGT process at Tg are revealed. The structural evolution and local ordering arrangement of nanodomains can play a role in hindering martensitic transformation. The ASAXS results provide new knowledge about the SGT beyond that from current simulation works. However, this corresponding structure of the nanodomains was destroyed when the specimen was heated to 250 °C.
Highlights
The in-situ anomalous small-angle X-ray scattering (ASAXS) technique was used to investigate the strain glass transition (SGT) in as-quenched Ti48.7Ni51.3 shape memory alloy during a thermal cycle of 30 °C to the SGT temperature Tg (−50 °C) and to 30 °C again
Unlike the 1D ASAXS profiles without the structure peaks measured at 8226 eV, the structure peaks in the profiles measured at 8000 eV were apparently induced by Ni atoms in the nanodomains and provide evidence of Ni-rich nanodomains
Past simulations and phenomenological models proposed that the induced short-range ordered (SRO) strain in the glass state was due to the statically-disordered strain domains in the bulk material, based on the assumption that randomly-distributed Ni point defects were fixed in position during strain glass transition
Summary
The in-situ anomalous small-angle X-ray scattering (ASAXS) technique was used to investigate the strain glass transition (SGT) in as-quenched Ti48.7Ni51.3 shape memory alloy during a thermal cycle of 30 °C to the SGT temperature Tg (−50 °C) and to 30 °C again. According to the existing phenomenological models and simulations of strain glass transition[3,6,9,16], point defects (excessive dopant Ni atoms), or most probably defect pairs randomly distributed in the whole sample, form locally-ordered nano-strain domains and induce SRO strain at T < Tg to effectively inhibit the LRO or martensitic transformation[2,3]. These simulations suggest non-compositional transformation and fixed doped defects during strain glass transition. The real spatial distribution at the large scale of the strain nanodomains at RT and the way the frozen nanodomains lead to the SRO strain in bulk glass when T decreases to Tg remain experimentally unknown due to the limitations of TEM and sampling
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