Synchrotron X-ray diffraction study of the phase transformations in titanium alloys

High-resolution synchrotron X-ray diffraction was used to study the phase transformations in titanium alloys. Three titanium alloys were investigated: Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo-0.08Si and beta21s. Both room and high temperature measurements were performed. The room temperature experiments were performed to study the structure of the alloys after different heat treatments, namely as received (AR), furnace cooling (FC), water quenching (WQ) and water quenching followed by ageing. The alpha, alpha', alpha'' and beta phases were observed in different combinations depending on the heat treatment conditions and the alloy studied. A multicomponent hexagonal close packed (hcp) alpha phase, with different c and the same a lattice parameters, was detected in Ti-6Al-4V after FC. High temperature synchrotron X-ray diffraction was used for 'in situ' study of the transformations on the sample surface at elevated temperatures. The results were used to trace the kinetics of surface oxidation and the concurrent phase transformations taking place under different conditions. The influence of the temperature and oxygen content on the lattice parameters of the alpha phase was derived and new data obtained on the coefficients of thermal expansion in the different directions of the hcp alpha phase, for Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si.

Kinetics of phase transformations in a model with metastable fluid-fluid separation: A molecular dynamics study

By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid-fluid equilibrium, with a metastable fluid-fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76x10(6) MD steps. By progressively reducing the system temperature below the solid-fluid line, we first observe the metastable fluid-fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed. (C) 2002 American Institute of Physics.

Phase transformations in maraging steels

Research on the kinetics of precipitate formation and austenite reversion in maraging steels has received great attention due to their importance to steel properties. Judging from the literature in recent years, research into maraging steels has been very active, mainly extending to new types of steels, for new applications beyond the traditional strength requirements. This chapter provides an in-depth overview of the literature in this area. In addition, the kinetics of precipitate formation are analysed using the Johnson–Mehl–Avrami (JMA) theory.

Experimental and Modelling Studies of the Thermodynamics and Kinetics of Phase and Structural Transformations in a Gamma TiAl-based Alloy

This work combines microscopy, synchrotron radiation X-ray diffraction, differential scanning calorimetry and thermodynamic calculations in the characterisation of phase transformation behaviour of a Ti–46Al–1.9Cr–3Nb alloy upon continuous heating at constant rates. It has been found that the Ti–46Al–1.9Cr–3Nb alloy after being forged at 1200 °C without further treatment has a duplex microstructure consisting of fine equiaxed and lamellar ? grains with a small amount of a2 plates and particles and about 1 wt.% B2 phase. Differential scanning calorimetry revealed reproducibly several thermal effects upon heating of the as-forged alloy. These thermal effects are related to the equilibration and homogenisation of the sample, change of phase ratios between a2, ? and B2 phases in particular the increase of B2 in respect to a2 and ?, and the following five phase transformations: a2 + ? + B2 a + ? + B2, a + ? + B2 a + ?, ? + a a, a a + ß, a + ß a + ß + L. The observation of these transformations by differential scanning calorimetry is largely in agreement with literature phase diagrams and thermodynamic calculations, though care is needed to consider the different alloy compositions. Kinetics of the ? + a a phase transformation in the Ti–46Al–1.9Cr–3Nb alloy has been quantitatively derived from the calorimetry data, giving phase compositions at any point during the transformation upon continuous heating.

Differential Scanning Calorimetry Study and Computer Modeling of b a Phase Transformation in Ti-6Al-4V Alloy

The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically, using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations for different systems based on the results from DSC study. In the present work, the kinetics of the transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling–transformation (CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the Johnson–Mehl–Avrami (JMA) theory and by applying the "concept of additivity." The JMA kinetic parameters were derived. Good agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters, the transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results from the point of view of activation energy for nucleation is also presented.

Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia

The high-temperature cubic-tetragonal phase transition of pure stoichiometric zirconia is studied by molecular dynamics (MD) simulations and within the framework of the Landau theory of phase transformations. The interatomic forces are calculated using an empirical, self-consistent, orthogonal tight-binding model, which includes atomic polarizabilities up to the quadrupolar level. A first set of standard MD calculations shows that, on increasing temperature, one particular vibrational frequency softens. The temperature evolution of the free-energy surfaces around the phase transition is then studied with a second set of calculations. These combine the thermodynamic integration technique with constrained MD simulations. The results seem to support the thesis of a second-order phase transition but with unusual, very anharmonic behavior above the transition temperature.

Precipitation, microstructure and mechanical properties of low nickel maraging steel

A maraging steel with a composition of Fe–12·94Ni–1·61Al–1·01Mo–0·23Nb (wt-%) was investigated. Optical, scanning electron and transmission electron microscopy and X-ray diffraction analysis were employed to study the microstructure of the steel after different aging periods at temperatures of 450–600°C. Hardness and Charpy impact toughness of the steel were measured. The study of microstructure and mechanical properties showed that nanosized precipitates were formed homogeneously during the aging process, which resulted in high hardness. As the aging time is prolonged, precipitates grow and hardness increases. Fractography of the as forged steel has shown mixed ductile and brittle fracture and has indicated that the steel has good toughness. Relationships among heat treatment, microstructure and mechanical properties are discussed. Further experiments using tensile testing and impact testing for aged steel were carried out.

Microstructure and mechanical properties of low nickel maraging steel

Austenitization with lower temperature and intercritical annealing were introduced in the treatment of a maraging steel with a composition of Fe–12.94Ni–1.61Al–1.01Mo–0.23Nb (wt.%). Scanning electron microscopy was employed to study the microstructure after austenitization at 950 °C and intercritical annealing, followed by aging at 485 and 600 °C. X-ray diffraction (XRD) analysis was applied to evaluate the formation of retained or reverted austenite. Thermodynamic calculation was employed to calculate equilibrium phase mole fractions. Hardness and Charpy impact toughness of the steel were measured. Intercritical annealing treatments did not result in significant increase of hardness either before or after aging. The Charpy impact toughness of the alloy in aged condition was enhanced after austenitization at 950 °C. No austenite was observed in XRD. However, suspected reverted austenite was found after austenitization at 950 °C followed by aging at 600 °C for 4 h. Relationships among heat treatment, microstructure and mechanical properties are discussed.