Configurational thermodynamics of C in body-centered cubic (bcc) or tetragonal (bct) Fe is investigated combining several computational techniques. Pairwise CAC interaction energies in bcc Fe are calculated by density functional theory (DFT) and embedded atom method (EAM) potential respectively. The interaction between C atom and homogeneous strain is calculated assuming C acts as force dipoles in linear elastic medium of Fe. The CAC and C-strain interactions are input into Monte Carlo (MC) simulations to find equilibrium C configuration on octahedral interstitial sublattices (OISs) in bcc/bct Fe and corresponding thermodynamic properties. In bcc Fe, DFT-MC and EAM-MC both give a single-phase region with C distributed with disorder on all the three OISs (alpha) at high temperature, and a two-phase region with ferrite and an ordered compound (alpha''') at low temperature. The compound is Fe16C1 according to DFT or Fe16C2 according to EAM inputs, both having two OISs occupied. When a homogeneous tetragonal lattice strain is applied, the disordered phase exhibits a preferential sublattice occupation (Zener ordered alpha'), which is primarily caused by C-strain interaction and is mitigated by CAC interactions. The ordered compound may also have two (alpha''') or one (alpha '') OIS occupied. C clustering in bcc/bct Fe follows a conditional spinodal mechanism, namely long-range order being a prerequisite of spinodal decomposition. This is verified by the difference in solution thermodynamics of alpha/alpha' (ordering-type) and a'''/a '' (clusteringtype), as well as kinetic Ising model simulations which reveal a temporal sequence of short-range ordering, long-range ordering, and eventually C clustering.

As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of ferrite and austenite, leading to both high strength and large elongation. The growth of austenite during intercritical annealing is crucial for the final properties, primarily because it determines the fraction, composition, and phase stability of austenite. In the present work, the growth of austenite from as-quenched lath martensite in medium Mn steels has been simulated using the DICTRA software package. Cementite is added into the simulations based on experimental observations. Two types of systems (cells) are used, representing, respectively, (1) austenite and cementite forming apart from each other, and (2) austenite forming on the cementite/martensite interface. An interfacial dissipation energy has also been added to take into account a finite interface mobility. The simulations using the first type of setup with an addition of interfacial dissipation energy are able to reproduce the observed austenite growth in medium Mn steels reasonably well.

The energetics and atomic structures of Sigma 3[1 (1) over bar0](111) grain boundary (GB) of body-centered cubic (bcc) Ti-Mo and Ti-V alloys are investigated using density-functional-theory calculations and virtual crystal approximation. The electron density in bcc structure and the atomic displacements and excess energy of the GB are correlated to bcc-omega phase stability. Model calculations based on pairwise interplanar interactions successfully reproduce thechemical part of GB energy. The chemical GB energy can be expressed as a sum of excess pairwise interactions between bcc (111) layers, which are obtained from Gaussian elimination of the total energies of a number of periodic structures. The energy associated with the relaxation near the GB is solved by numerical minimization using the derivatives of the excess interactions. Anharmonic interlayer interactions are necessary for obtaining accurate relaxation energy and excess GB volume from model calculations. The effect of GB on vibrational spectrum is also investigated. Segregation energies of B and Y to a substitutional site on the GB plane are calculated. Preliminary results suggest that Y tends to segregate, while B tends to antisegregate.

The thermodynamics of Ti-based systems are described for beta-alpha'/alpha '' martensitic transformation and athermal omega 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 beta-stabilizing effects of Al, Sn, and Zr in ternary TieV/Nb-based alloys are well modeled for the first time. The Gibbs energy function of omega-Ti at ambient pressure is assessed. The formation temperature of athermal omega phase is assessed in some binary systems and estimated in some ternary systems based on electrical resistivity and first-principles calculations. The critical driving force for heterogeneous martensitic nucleation is modeled by solution-hardening interfacial friction using the present thermodynamic descriptions. The competition between martensite and athermal omega phase can be understood based on their transformation thermodynamic and kinetic factors. (C) 2016 Elsevier B.V. All rights reserved.

The roomerature 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.

Medium Mn steels have attracted worldwide interests recent years due to their excellent mechanical properties and low cost. These steels contain large fraction (~30%) of metastable retained austenite and exhibit good elongation due to transformation-induced plasticity (TRIP). In order to obtain the highest elongation, the mechanical stability of austenite, quantified using M_s^σ, needs to be optimized. M_s^σ is defined as the highest temperature at which martensite can form under stress without austenite yielding by slip. The present work aims to formulate a model of M_s^σ which can be used to design medium Mn steels with optimized elongation. In the present work, an Fe–0.18C–5.08Mn (mass %) steel was intercritically annealed at 650 °C. Based on tensile tests at different temperatures using a single specimen method, the M_s^σ temperatures were experimentally determined to about 0 °C regardless of intercritical annealing time between 15 min and 3 h. Microstructure observations showed that large austenite grains with a globular shape are more transformed than thin-film ones, and thus the former probably governs the determined M_s^σ. M_s^σ was further predicted at the crossing point of yielding by martensite formation and by austenite slip; the former was modeled by expanding an existing model of martensite start temperature and the latter by a constitutive model. The predicted M_s^σ showed reasonable agreement with the determined values. The model also indicated that a large and a small austenite grain have similar M_s^σ, which could partly explain why the determined M_s^σ is rather constant regardless of IA time.

Many previous studies suggest that austenite stability increases with decreasing grain size, i.e. the martensite start temperature, M_s, decreases. Therefore, the influence of small austenite grain size would be high for many modern steels containing fine austenite. Models from literature deviate severely from each other for grain sizes below 1 μm where there is a lack of experimental data. Besides, the experimental data are mostly obtained from a fully austenitic microstructure with equiaxed grains. This raises concerns about the applicability of these models to design medium Mn steels, where the austenite stability is essential for elongation via transformation-induced plasticity. The present work concerns the influence of grain size on martensitic transformation for thin-film austenite in a medium Mn steel. After IA, austenite grains exhibit two morphologies, thin-film like and globular, while the former is dominant. The globular austenite is less stable and responsible for the M_s measured by dilatometry. Similar to particles, the austenite grains are isolated and dispersed, and autocatalysis from surrounding austenite is minimized. Therefore in this work an approach to describe the transformation in small particles has been adopted, where the number fraction of partly or fully transformed austenite grains (F) is phenomenologically formulated as a function of temperature and grain size. Experimental data from the present work and from literature have been used to derive a model of M_s and grain size for thin-film austenite in medium Mn steels, using the cross-sectional area of austenite grain as a variable. The current model provides a practical and useful way of determining austenite stability from 2-dimensional micrograph, and can be used in designing medium Mn steels with optimized austenite stability.