Computational thermodynamics and kinetics of displacive transformations in titanium-based alloys

Abstract

The thermodynamics of Ti-based systems are described for β-α'/α‖ martensitic transformation and athermal ω formation at low temperatures. The new descriptions can better represent the relationship between the partitionless equilibrium temperature and the measured martensite-formation/reversion temperatures. The anomalous β-stabilizing effects of Al, Sn, and Zr in ternary Ti-V/Nb-based alloys are well modeled for the first time. The Gibbs energy function of ω-Ti at ambient pressure is assessed. The formation temperature of athermal ω phase is assessed in some binary systems and estimated in some ternary systems based on electrical

Routes to control diffusive pathways and thermal expansion in Ti-alloys

“…1b and hypothesized early on 28 , lacks definite experimental verification and remains speculative. We note that the underlying TiGen database 24 was optimized using low-temperature data and, as the database authors remark, may not be accurate at high temperature. Therefore, we base the following treatment on the thermodynamical Ti-Nb assessment by Zhang, Liu and Jin ( 22 , Fig.…”

“…The quest for a faithful thermodynamic description for Ti-Nb. Based on the recent assessment by Yan and Olson 24 we calculated the pertaining phase diagram (see Methods for details) and compare the results to the assessment by Zhang, Liu and Jin 22 in Fig. 1.…”

“…2c consequently represent educated guesses as detailed below. For purposes of calculating T 0 β-α″ the Gibbs free energy of α″ is, for the lack of a more suitable formulation, commonly approximated through α″ G ≅ α G ( 24,22 and Fig. 1).…”

“…Thermodynamic calculations served two purposes: (i) examine and validate experimentally observed transformation pathways against the system's thermodynamics in terms of the www.nature.com/scientificreports www.nature.com/scientificreports/ participating phases' free enthalpy curves; (ii) derive the Ti-Nb phase diagram from the recently published TiGen database 24 . Within the current scope two differing thermodynamic descriptions were employed; that by Zhang, Liu and Jin from 2001 22 and that by Yan and Olson (TiGen database) from 2016 24 . Each phase (indicated by φ) is modelled as a disordered substitutional solid solution with its molar Gibbs free energy formulated as…”

“…For ω G Ti below 298.15 K Yan and Olson used an expression different from Eq. (1) formulated independently of ω G Ti 24 . Chemical interactions between Ti and Nb are accounted for by the excess Gibbs free energy expressed as a Redlich-Kister polynomial (last 2 terms in Eq.…”