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

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The room-temperature molar volumes of bcc (β), hcp (equilibrium α or martensitic α'), and orthorhombic (martensitic α") phases are modeled for a number of Ti-base solid solutions in the CALPHAD framework. The martensitic molar volume is continuous at the α'/α" transition composition. Hcp and orthorhombic structures are modeled separately, and the predicted martensitic structure is taken as the denser one.

A steady-state model for austenite-to-pearlite transformation in multicomponent steel is presented, including Fe, C, and eight more elements. The model considers not only classic ingredients (formation of ferrite-cementite interface, volume diffusion, boundary diffusion, and optimization of lamellar spacing) but also finite austenite-pearlite interfacial mobility that resolves some previous difficulties. A non-Arrhenius behavior of interfacial mobility is revealed from growth rate and lamellar spacing data. A smooth and physical transition between orthopearlite and parapearlite is realized by optimizing the partitioning of substitutional alloying elements between ferrite and cementite to maximize growth rate or dissipation rate while keeping carbon at equilibrium. Solute drag effect is included, which accounts for the bay in growth rate curves. Grain boundary nucleation rate is modeled as a function of chemical composition, driving force, and temperature, with consideration of grain boundary equilibrium segregation. Overall transformation kinetics is obtained from growth rate and grain boundary nucleation rate, assuming pearlite colonies only nucleate on austenite grain boundaries. Further theoretical and experimental work are suggested for generalization and improvements.

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