First-principles phase stability in the Ti-V alloy system

Abstract

Phase stability in the transition metals are mostly dictated by the bonding of the d electrons and is believed to be fairly well understood from either a canonical-band picture or a tight-binding model. These models are related and capture the fact that as one proceeds through the nonmagnetic d-transition series one encounters the hexagonal closepacked (hcp), body-centered cubic (bcc), hcp, and face-centered cubic (fcc) phases. This structural sequence depends on the gradual filling of the d band (roughly one electron per atomic number increase) that is altered when magnetism is present simply due to the spin polarization of the d band. However, recent more careful experimental and theoretical studies have shown that the aforementioned appealing models do not entirely explain the bonding and phase stability in the transition metals. First, there is a destabilization of the bcc phase due to pressure in the Group Vb metals (V, Nb, and Ta) and second, a temperature-induced stabilization of the bcc phase in the Group IVb (Ti, Zr, and Hf) metals even though at low temperatures it is strongly unstable. Here we address the latter phenomenon and study the influence of alloying titanium with its next neighbor vanadium by applying the recently developed self-consistent ab initio lattice dynamics (SCAILD) approach that has heretofore never been utilized for an alloy system.

Ab initio calculations of elastic properties of alloys with mechanical instability: Application to BCC Ti-V alloys

“…%, where C' becomes negative. This is in good agreement with the experimental data, as well as with earlier theoretical studies [16,20,43]. On the contrary, the mechanical instability of the alloys in Ti-rich region is not reproduced by the EMTO-CPA method.…”

“…The values of VCA calculations reported in Ref. [20] are marked with the yellow symbols. Blue empty circles show PAW values of Ref.…”

“…In Ref. [20] the elastic constant C' was calculated in the Ti-rich region within the virtual crystal approximation, and the mechanical instability of the bcc alloy in this region was indeed observed. In Ref.…”

“…It is therefore important to establish how different computational methods deal with the instability in simulations of elastic properties of bcc Ti alloys. The elastic moduli of Ti-V system have been studied in several experimental [16][17][18] and theoretical works [19][20][21]. In Ref.…”

Ab initiophase stability at high temperatures and pressures in the V-Cr system

“…However, only recently this approach has been expanded to deal with phase stability in an alloy system (Ti-V) [14].…”