Oxidation of the two-phase Nb/Nb5Si3 composite: the role of energetics, thermodynamics, segregation, and interfaces

First-principles thermodynamics and experimental study of interface oxidation in Ni/Ni3Al structures

Anti-oxidation is one of the significant properties of nickel-based superalloys useful for their potential applications in industry. However, previous research mainly focused on single-phase compounds of NiAl or Ni3Al. In the present study, first-principles density functional theory coupled with thermodynamics analysis are employed to investigate the atomistic oxidation behaviours of the Ni/Ni3Al composites systematically. An oxidation experiment with a DD6 alloy is conducted as well to further confirm the theoretical prediction. Initial surface formation energy analysis shows that the systems composed of Ni(111) and Ni3Al(100)/(111) surfaces are more stable and therefore are selected for further investigation. Thermodynamics calculations indicate that the Ni3Al phase is oxidized first, accompanied by Al-segregation on the top surfaces. This is followed by subsequent oxidation of the Ni phase. Surface oxidation diagrams with respect to the surface formation energies show that oxygen adsorption could enhance Al-segregation to the surface and Ni3Al(111) surfaces tend to be oxidized completely with slightly lower oxygen coverage. Oxidation at the interface is also investigated and the results show that oxygen atoms bind with the upper layers of the Ni3Al phase from the point of view of binding energy. The experimental results provide a reasonable explanation for the selective oxidation of Al atoms at the atomic-scale so as to form a dense anti-oxidation membrane. The present work could serve as a beneficial reference for subsequent investigations of oxidation or adsorption processes of two-phase composites.

On Nb Silicide Based Alloys: Alloy Design and Selection

The development of Nb-silicide based alloys is frustrated by the lack of composition-process-microstructure-property data for the new alloys, and by the shortage of and/or disagreement between thermodynamic data for key binary and ternary systems that are essential for designing (selecting) alloys to meet property goals. Recent publications have discussed the importance of the parameters δ (related to atomic size), Δχ (related to electronegativity) and valence electron concentration (VEC) (number of valence electrons per atom filled into the valence band) for the alloying behavior of Nb-silicide based alloys (J Alloys Compd 748 (2018) 569), their solid solutions (J Alloys Compd 708 (2017) 961), the tetragonal Nb₅Si₃ (Materials 11 (2018) 69), and hexagonal C14-NbCr₂ and cubic A15-Nb₃X phases (Materials 11 (2018) 395) and eutectics with Nbss and Nb₅Si₃ (Materials 11 (2018) 592). The parameter values were calculated using actual compositions for alloys, their phases and eutectics. This paper is about the relationships that exist between the alloy parameters δ, Δχ and VEC, and creep rate and isothermal oxidation (weight gain) and the concentrations of solute elements in the alloys. Different approaches to alloy design (selection) that use property goals and these relationships for Nb-silicide based alloys are discussed and examples of selected alloy compositions and their predicted properties are given. The alloy design methodology, which has been called NICE (Niobium Intermetallic Composite Elaboration), enables one to design (select) new alloys and to predict their creep and oxidation properties and the macrosegregation of Si in cast alloys.

Compositional phase diagram and microscopic mechanism of Ba1−xCaxZryTi1−yO3 relaxor ferroelectrics

A model of a multiphase with coexisting cubic structures is proposed to provide an atomic-scale mechanism for the relaxor behavior of lead-free BCZT relaxor ferroelectrics.

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces

Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.