Xiao-Gang Lu

Thermodynamic assessment of the BaO - TiO2 quasibinary system

Abstract Experimental data from the literature on the BaO-TiO 2 quasibinary system have been evaluated to assess the thermodynamic parameters of all the stable phases by means of the CALPHAD (CALculation of PHAse Diagram) method. The two-sublattice ionic solution model was used to describe the liquid phase, while the compound energy model was applied to describe the high-temperature forms of BaTiO 3 . Other compounds were treated as stoichiometric phases. The assessment was conducted using the Thermo-calc software package and a set of parameters was given. Calculated equilibrium phase diagrams and thermochemical properties compared with the experimental data are presented. There is a good agreement between the calculations and the experimental data. Some contradictions are pointed out and discussed.

A CALPHAD Helmholtz energy approach to calculate thermodynamic and thermophysical properties of fcc Cu

A CALPHAD Helmholtz energy approach, based on the Debye–Grüneisen model, is proposed to study thermodynamic and thermophysical properties of fcc Cu. With several parameters that have physical meanings, this approach allows a consistent description of both thermodynamic properties, e.g. heat capacity and Gibbs energy, and thermophysical properties, e.g. volume, thermal expansion, bulk modulus and Poissons ratio. This method is intrinsically applicable to a large temperature and pressure ranges from 0 K upwards and from atmospheric pressure to extremely high pressures without resulting in any abnormal behavior of any properties. By taking advantage of PARROT, an optimization module in Thermo-Calc, experimental thermodynamic and thermophysical data can be assessed simultaneously so that inconsistencies among different kinds of measured properties can be detected and an optimum set of parameters can be obtained to accurately reproduce most of the experimental data. In addition, this approach leads to a straightforward way to couple CALPHAD assessments with ab initio calculations.

Experimental and Computational Study of Diffusion Mobilities for fcc Ni-Cr-Mo Alloys

Based on the available thermodynamic parameters and experimental diffusivities in the literature as well as measured in the present work, the diffusion mobilities of Ni, Cr, and Mo for fcc Ni-Cr-Mo alloys have been assessed by means of the CALPHAD method. Comprehensive comparisons between the calculated and experimental data show that an excellent agreement is obtained for the diffusion data of ternary diffusion couples, such as the interdiffusion coefficients, concentration profiles, and the diffusion path.

Calculation of the vibrational contribution to the Gibbs energy of formation for Al3Sc

Abstract The ground-state energies were calculated for the stoichiometric compound Al 3 Sc in the L1 2 structure, and its constituents (fcc-Al and hcp-Sc) using the Vienna ab initio simulation package (VASP). The enthalpy of formation for Al 3 Sc at 0 K was obtained. On the basis of the calculated equation of state (EOS) at 0 K, the vibrational contribution to the Gibbs energy of formation at finite temperatures was estimated using the Mean-Field Potential (MFP) method, and compared with the phonon spectrum calculation and the existing CALPHAD assessments. The calculated entropy of formation can serve as a reasonable start value in a CALPHAD assessment although it is not as accurate as the enthalpy of formation calculated at 0 K. Both high-temperature x-ray scattering measurements and first-principles calculation show that Al 3 Sc is almost perfectly ordered up to its melting point and thus the configurational contribution can be neglected.

In-situ analysis of retained austenite transformation in high-performance micro-alloyed TRIP steel

Microstructures and mechanical properties of Ti-V micro-alloyed TRIP (transformation-induced plasticity) steel with different compositions were investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermodynamic calculation (TC). The results indicated that the steel exhibited high ultimate tensile strength (1079 MPa), sufficient ductility (28%) and the highest product of strength and ductility (30212 MPa · %) heat treated after intercritical annealing at 800 °C for 3 min and bainitic annealing at 430 °C for 5 min. In addition, the change of volume fraction of retained austenite (VF-RA) versus tensile strain was measured using in-situ analysis by X-ray stress apparatus and micro-electronic universal testing machine. It was concluded that a-value could be used to evaluate the stability of retained austenite (S-RA) in the investigated Ti-V micro-alloyed TRIP steel. The smaller a-value indicated the higher stability of retained austenite (S-RA) and the higher mechanical properties of Ti-V micro-alloyed TRIP steel.

An animated phase diagram tutor

Although there are many ways to learn about phase diagrams, PHDT gives helpful information in the calculation and evaluation of phase diagrams and is particularly useful for teaching. The initial version now contains only six aspects, but much more will be added in the upcoming version.

First-principles investigation on diffusion mechanism of alloying elements in dilute Zr alloys

Abstract Impurity diffusion in Zr is potentially important for many applications of Zr alloys, and in particular for their use of nuclear reactor cladding. However, significant uncertainty presently exists about which elements are vacancy vs. interstitial diffusers, which can inhibit understanding and prediction of their behavior under different temperature, irradiation, and alloying conditions. Therefore, first-principles calculations based on density functional theory (DFT) have been employed to predict the temperature-dependent dilute impurity diffusion coefficients for 14 substitutional alloying elements in hexagonal closed packed (HCP) Zr. Vacancy-mediated diffusion was modeled with the eight-frequency model. Interstitial contributions to diffusion are estimated from interstitial formation and select migration energies. Formation energies for each impurity in nine high-symmetry interstitial sites were determined, including significant effects of thermal expansion. The dominant diffusion mechanism of each solute in HCP Zr was identified in terms of the calculated vacancy-mediated activation energy, lower and upper bounds of interstitial activation energy, and the formation entropy, suggesting a rough relation with the metallic radii of solutes. It is predicted that Cr, Cu, V, Zn, Mo, W, Au, Ag, Al, Nb, Ta and Ti all diffuse predominantly by an interstitial mechanism, while Hf, Zr, and Sn are likely to be predominantly vacancy-mediated diffusers at low temperature and interstitial diffusers at high temperature, although the identification of mechanisms for these elements at high-temperature is quite uncertain.

Experimental Investigation and Computer Simulation of Diffusion in Fe-Mo and Fe-Mn-Mo Alloys with Different Optimization Methods

In order to simulate the diffusional phase transformations involving the fcc and bcc phases for microalloyed steels, the diffusion mobilities for fcc and bcc Fe-Mo and Fe-Mn-Mo alloys were experimentally investigated and critically assessed. The diffusion-couple technique was employed to extract the interdiffusion coefficients in Fe-Mo and Fe-Mn-Mo alloys with the Sauer–Freise and Whittle–Green methods. Based on the present experimental interdiffsivities, the mobility parameters for the fcc and bcc phases in the Fe-Mo and Fe-Mn-Mo systems were optimized using the traditional method. Simultaneously, a direct method was developed and utilized to directly fit mobilities to the diffusion profiles rather than the diffusivities in the present work. The satisfactory description of the diffusion behavior in the Fe-Mo and Fe-Mn-Mo systems has confirmed the reliability of the direct method. Particularly, the two sets of diffusion mobilities obtained with both methods could simulate the diffusion phenomenon between the fcc and bcc phases in the Fe-Mo and Fe-Mn-Mo systems successfully.

Thermodynamic modeling of Fe-C-Mn-Si alloys

The thermodynamic database of the Fe-C-Mn-Si system has been developed in the framework of the CALPHAD approach. The sub-ternary systems have been carefully evaluated and revised based on available experimental data. A satisfactory description of the liquid phase in the Fe-C-Si system has been obtained. The C-Mn-Si system was assessed treating the liquid phase as a substitutional solution. Phase equilibria in the C-Mn-Si system, especially those involving the liquid phase, can be well described. Based on the extrapolation of the experimental data in the quaternary system, the Fe-Mn-Si system has been modified to agree well with experimental data at high temperatures. The comparison between the calculated and measured phase transformation temperatures confirms the reliability of the present quaternary database. Additionally, the solidification process of Fe-0.16C-1.5Mn-1.5 Si (wt. %) alloy was simulated by using the thermodynamic database developed.

Microstructures and mechanical properties of ferrite-based lightweight steel with different compositions

The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermodynamic calculation (TC). It was shown that the ferrite-based lightweight steels with 5 wt. % or 8 wt. % Al were basically composed of ferrite, austenite and κ-carbide. As the annealing temperature increased, the content of the austenite in the steel gradually increased, while the κ-carbide gradually decomposed and finally disappeared. The mechanical properties of the steel with 5 wt. % Al and 2 wt. % Cr, composed of ferrite and Cr7 C3 carbide at different annealing temperatures, were significantly inferior to those of others. The steel containing 5 wt. % Al, annealed at 820 °C for 50 s then rapidly cooled to 400 °C and held for 180 s, can obtain the best product of strength and elongation (PSE) of 31242 MPa · %. The austenite stability of the steel is better, and its PSE is higher. In addition, the steel with higher PSE has a more stable instantaneous strain hardening exponent (n value), which is mainly caused by the effect of transformation induced plasticity (TRIP). When the κ-carbide or Cr7C3 carbide existed in the microstructure of the steel, there was an obvious yield plateau in the tensile curve, while its PSE decreased significantly.