Shuanglin Chen

Site preference of substitutional additions to triple-defect B2 intermetallic compounds

Abstract The site preferences of substitutional ternary and higher-order additions to triple-defect type B2 intermetallic compounds are studied. A simple equation that is able to predict the site preference of dilute additions is given. The only parameters needed for this equation are the standard enthalpies of formation at the equiatomic composition. General equations that are able to calculate defect concentrations and substitutional atom distributions for dilute as well as non-dilute additions are also given. These equations use bond energies of atom pairs as parameters. This study shows the site preferences of dilute additions are not always in agreement with predictions based on the direction of the solubility lobes on ternary Gibbs isotherms. The site preference of dilute additions is controlled by the relative strength of bonds formed between the component elements. The addition atoms are distributed in such a way as to favor the formation of stronger bonds over that of weaker bonds. The predictions for dilute additions to NiAl, FeAl and CoAl are compared with experimental results in the literature. There are good agreements between the predictions of the model and the experimental results. The same methodology can be applied to substitutional additions to intermetallic compounds of other crystal structures such as the important Ll 2 , structure. It is also argued that the calculation of defect concentrations and substitutional atom distributions based on this type of methodology is critical for the understanding of physical and mechanical properties of ternary and higher-order intermetallic compounds.

A generalized quasi-chemical model for ordered multi-component, multi-sublattice intermetallic compounds with anti-structure defects

A generalized quasi-chemical model was formulated for ordered multi-component, multi-sublattice intermetallic compounds with anti-structure defects. The assumptions made in formulating this model were (i) the atoms are randomly distributed on each of the sublattices, and (ii) the enthalpy term is due entirely to the interaction of the first-nearest neighbor atoms. Analytical equations were derived for the Gibbs energy and the partial Gibbs energies of the component elements. The equilibrium distributions of the atoms on the various sublattices were obtained by the minimization of the Gibbs energy. From the equilibrium distribution of the atoms, the integral and partial thermodynamic quantities of an intermetallic compound can be readily obtained. The generalized thermodynamic equations reduce to the equations reported in the literature for ordered binary intermetallic compounds exhibiting the B2, L10 and L12 structures with anti-structure defects. The generalized equations may be used to describe the thermodynamic properties of any ordered intermetallic compound with anti-structure defects. These equations may be used for calculating phase diagrams and defect concentrations in the lattices.

On the optimization of solution model parameter values of phases and the calculation of phase diagrams

Abstract Phase equilibrium data are always used in addition to thermodynamic data in the optimization of model parameter values for all the phases in a binary or higher order system. The common strategy used to carry out the optimization is to compare the differences between the partial Gibbs energy values of the component elements at equilibrium. Three drawbacks inherent in this strategy are pointed out in the present study. An improved strategy is formulated to avoid these drawbacks. In the improved strategy, the objective functions used in the optimization are direct comparisons between the model-calculated compositions and/or temperatures of heterogeneous equilibria, and the experimentally determined values. The new strategy is used to optimize the model parameter values for all the phases and subsequent calculation of the phase diagrams for at least four binary and one ternary systems. These systems are Cu-In, Al-Zn, Al-Mg, Mg-Cu and Al-Mg-Cu. The new strategy is found to be much more powerful than the existing strategy.

Simulation of Co-precipitation Kinetics of γ′ and γ″ in Superalloy 718

In this paper, we will study the co-precipitation kinetics of phases in Superalloy 718 using the simulation tool we have developed using the CALPHAD approach. This tool considers concurrent nucleation, growth and coarsening of these precipitates. Furthermore, it is directly integrated with thermodynamic calculation engine to obtain instant update of phase information, such as the composition of the matrix and the nucleation driving force for each precipitate. In addition to the average particle size, the more advanced KWN (Kampmann and Wagner Numerical) model was implemented to allow for predication of the full evolution of the particle size distribution (PSD). In this paper, we will perform virtual experiments using this tool to simulate the co-precipitation of the γ′ and γ″ phases under different heat treatment conditions. Simulation results, such as temporal evolution of volume fraction, number density, and mean size of the precipitates, as well as the final particle size distribution will be presented and discussed. The impact of δ precipitate and the initial microstructure will also be briefly discussed. These virtual experimental results can be used to understand the microstructural features of Superalloy 718 and serve as guidance for further optimization of heat treatment schedule.