In situ XRD study of stress- and cooling-induced martensitic transformations in ultrafine- and nano-grained superelastic Ti-18Zr-14Nb alloy

Abstract

In this study, a superelastic Ti-18Zr-14Nb shape memory alloy was subjected to a thermomechanical treatment, combining moderate or severe cold rolling and post-deformation annealing in the 500–600 °С temperature range for 5–30 min, in order to characterize the features of nanocrystalline structure formation, stress- and cooling-induced martensitic transformations, and determine a critical grain size for these transformations. A moderate (e = 0.3) cold rolling and annealing at 500 °C (30 min) forms a predominantly polygonized nanosubgrained β-phase substructure with a small amount of α-phase. An increase in the annealing temperature to 550 °C leads to the development of the polygonization process and the growth of the β-phase nano- to submicrometer-sized subgrains. As a result of severe cold rolling (e = 3), a predominantly nanograined structure with inclusions of amorphized and nanosubgrained structures is formed. Subsequent annealing at 500 °C (5 min) forms a nanograined structure of β-phase with some amounts of α-phase. An increase in holding time at 500 °C to 15 min increases the average grain size. After annealing at 525 °C, a submicrocrystalline structure forms. The smallest Young’s modulus values and the highest total recovery strains (E = 37–39 GPa and εr = 3.0–3.5%, respectively) are observed after post-deformation annealing in the 525–550 °C temperature range. A critical average grain size for the stress-induced β→α'' martensitic transformation of this alloy at room temperature is defined as 36 ± 13 nm, while for the cooling-induced transformation, this value is almost one order of magnitude greater (~ 250 nm).