First-principles thermodynamic investigation on the α phases in TiO and TiNb binary system

O and Nb are two representative alloying elements of Ti to form high-temperature and corrosion resistance α Ti alloys. The investigation on the thermodynamic characteristics of α Ti–O and Ti–Nb has attracted much attention in recent years. However, in this regard, a satisfied experimental technique or modeling scheme is still yet to be developed due to the appearance of a variety of oxides in Ti–O and the mechanical instability present in Ti–Nb. Herein, we combined first-principles calculations with the cluster expansion method to investigate the ground-state characteristics for α Ti–O and α Ti–Nb systems. The atomic bonding interactions in these two systems were first revealed based on the calculated electronic structures. Afterward, the Debye–Grüneisen model and Monte Carlo simulations were employed together to investigate the thermodynamic properties of α phases in these two systems, and the effect of vibrational entropy on the order–disorder transition temperatures of the phases in α Ti–O system was first examined. A good agreement with experimentally reported phase boundaries is obtained in the Ti–Nb system by handling the mechanical instabilities introduced by the highly distorted structures. In addition, the cluster expansion coefficients for the Ti–O and Ti–Nb system offer a good starting point to investigate the phase equilibrium in Ti–Nb–O ternary alloy. We also believe the insights provided here would be helpful for those who would like to seek an efficient scheme they are confident with to investigate the phase thermodynamic properties of other hcp Ti-based alloys

Planar fault energies in superalloys from first principles

Planar fault energies in superalloys are largely responsible for the complex deformation processes observed. In the gamma phase, the intrinsic and extrinsic stacking fault energies are thought to be responsible for controlling how easily dislocations climb around the gamma prime precipitates. In the gamma prime phase, the various planar fault energies (superlattice intrinsic and extrinsic stacking faults, complex intrinsic and extrinsic stacking faults and anti-phase boundaries on the 111 and 001 planes) control the operating shearing modes that allow dislocations or ribbons of dislocations to cut through precipitates. Whereas the effect of composition on the planar fault energies of the gamma phase are well understood, the same cannot be said of planar fault energies of the gamma prime phase. In this presentation, we will explore the effect of compositional variations on some of these planar fault energies, investigating how solute atoms change these energies on both sides of perfect stoichiometry (A3B). We will look at all transition metals, to highlight trends across the periodic table, and discuss the effects of temperature on planar fault energies. Our results show clear trends across the periodic table, indicating that gamma prime ‘stabilisers\u27 are also responsible for increasing the planar fault energies of gamma prime phase. Similar to the gamma phase, these effects appear to be primarily due to electronic structure effects rather than size effects

What is the role of rhenium in single crystal superalloys?

Rhenium plays a critical role in single-crystal superalloys –its addition to first generation alloys improves creep life by a factor of at least two, with further benefits for fatigue performance. Its use in alloys such as PWA1484, CMSX-4 and Rene N5 is now widespread, and many in this community regard Re as the “magic dust”. In this paper, the latest thinking concerning the origins of the “rhenium-effect” is presented. We start by reviewing the hypothesis that rhenium clusters represent barriers to dislocation motion. Recent atom probe tomography experiments have shown that Re may instead form a solid solution with Ni at low concentrations (< 7 at.%). Density functional theory calculations indicate that, in the solid solution, short range ordering of Re may be expected. Finally, Re has been shown to diffuse slowly in the γ-Ni phase. Calculations using a semi-analytical dislocation climb/glide model based upon the work of McLean and Dyson have been used to rationalise the composition-dependence of creep deformation in these materials. All evidence points to two important factors: (i) the preferred partitioning of Re to the γ phase, where dislocation activity preferentially occurs during the tertiary creep regime and (ii) a retardation effect on dislocation segments at γ/γ′ interfaces, which require non-conservative climb and thus an associated vacancy flux

Nickel-rhenium compound sheds light on the potency of rhenium as a strengthener in high-temperature nickel alloys

For many decades, it has been known that rhenium imparts a tremendous resistance to creep to the nickel-based high-temperature alloys colloquially known as superalloys. This effect is so pronounced that is has been dubbed "the rhenium effect." Its origins are ill-understood, even though it is so critical to the performance of these high-temperature alloys. In this paper we show that the currently known phase diagram is inaccurate, and neglects a stoichiometric compound at 20 at.% Re (Ni4Re). The presence of this precipitate at low temperatures and the short-range ordering of Re in fcc-Ni observed at higher temperatures have important ramifications for the Ni-based superalloys. The Ni4Re compound is shown to be stable by quantum mechanical high-throughput calculations at 0 K. Monte Carlo simulations show that it is thermally persistent up to ≈930 K when considering configurational entropy. The existence of this compound is investigated using extended x-ray absorption fine spectroscopy on a Ni96.62Re3.38 alloy