Shuffle-nanodomain regulated strain glass transition in Ti-24Nb-4Zr-8Sn alloy

The unprecedented properties of multi-functional metastable beta-Ti alloys, including superelasticity over a wide temperature range, ultra-low modulus, and Invar and Elinvar anomalies, have attracted a great deal of attention. Persistent research efforts have been made towards the understanding of the origins of these unique properties. In this article we report a novel shuffle-nanodomain regulated strain glass transition in a metastable beta-Ti alloy, Ti-24Nb-4Zr-8Sn (wt.%, Ti2448), which could be the dominant transformation pathway that offers these unique properties. Using the ex-situ aberration-corrected scanning transmission electron microscopy and in-situ cooling transmission electron microscopy, we find that randomly distributed {011}(0 (1) over bar1)(beta) O ' phase (orthorhombic, shuffle only) nanodomains embedded in the beta phase (BCC) matrix at room temperature transform to alpha '' phase (orthorhombic) with a continuous increase in the amount of {2 (1) over bar1}((11) over bar1)(beta) shear upon cooling or loading. Crystallographic analysis shows that the shuffle of the O ' phase will restrain the twelve possible shears that transform a BCC lattice to alpha '' martensite to only two. Thus, the randomly distributed O ' nanodomains prevent the formation of long-range-ordered, self-accommodating transformation-strain domain patterns seen in normal martensitic transformations and suppress completely the sharp first-order, auto-catalytic and avalanche-like martensitic transformation into a high-order-like (continuous) strain glass transition. Such a continuous beta -> O ' ->alpha '' strain glass transition has been confirmed by dynamic mechanical analysis, resistivity and differential scanning calorimetric measurement. This unique transition pathway allows us to offer new insights into the unique properties found in this alloy. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.