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- Skripnyak, Natalia, 1991-, et al.
(författare)
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Mixing enthalpies of alloys with dynamical instability : bcc Ti-V system
- 2020
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 188, s. 145-154
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Tidskriftsartikel (refereegranskat)abstract
- Enthalpy of mixing is among the key materials parameters to determine phase stability and phase transformations in solid solutions. The possibility to predict it from first principles in the framework of the density functional theory is one of the corner stones of the modern materials modeling and the future data-driven materials design. Here we have considered body-centered cubic (bcc) Ti-V alloys, a system with high potential for aerospace, automotive biomedical and energy applications, which is known to exhibit the dynamical instability of the crystal lattice for Ti-rich alloys at low temperature. We have calculated the mixing enthalpies ΔH of bcc Ti-V alloys in the whole interval of concentration at high temperature using ab initio molecular dynamics (AIMD) simulations. A comparison with state-of-the-art static calculations at temperature 0 K shows drastic difference between the two methods: while AIMD predicts positive values of ΔH in the whole range of concentrations, the static zero-temperature simulations result in negative values of ΔH for Ti-rich alloys. We have measured the mixing enthalpy of bcc Ti-V alloys experimentally at 1073 K using an isoperibol high temperature Tian-Calvet calorimeter and found that the enthalpies are positive, in agreement with our finite temperature AIMD calculations. We attribute the failure of the standard static calculations of ΔH to lattice distortions associated with the dynamical instability of bcc Ti-V alloys at zero temperature and argue that the effect should be generally important in theoretical predictions of thermodynamic properties, especially for systems with dynamical instability.
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| 2. |
- Skripnyak, Natalia, 1991-
(författare)
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Theoretical description of Ti and Ti alloys from first principles
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modern world is known for many advanced technologies and solutions to complex problems. Technical progress runs at high speed. In order to most effectively use materials, given to us by Nature, it is important to know their properties. To do laboratory experiments is often too expensive and time consuming. Therefore, it is very important to possess the knowledge and capabilities of studying materials properties without actual experiments. I use different methods based on the laws of Quantum mechanics to conduct my investigations. In this work I studied from first principles properties of titanium and titanium alloys that are of potential interest for various applications. Titanium was chosen because of its unique properties, which are both useful and reveal interesting physics. First, I investigated elastic properties using density functional theory (DFT) in different implementations, such as the projector augmented wave (PAW) and the exact muffin-tin orbitals (EMTO) methods. The single crystal’s elastic constants Cαβ of pure Ti, Ti-V, and Ti-Ni-Al alloys were obtained by calculating the total energy as a function of appropriate strains or stress-strain relations. Disordered substitutional alloys were modeled using a special quasi-random structure (SQS) technique combined with PAW as well as the coherent potential approximation (CPA) combined with EMTO. The concentration dependence of Cαβ and also the family of material characteristics, such as Young’s modulus E, bulk modulus B, shear modulus G, Cauchy pressure Pc, Pugh’s coefficient k, and Poisson’s coefficient ν for the TiV system were estimated and discussed. The elastic properties of alloys in the Ni-Al- Ti system were also calculated and analyzed, as well as the temperaturedependent elastic constants of pure Ti. The influence of the amount of V on the mechanical phases stability of body-centered cubic (bcc) Ti-V alloys was studied. It was found that Ti-rich Ti-V alloys are mechanically unstable in the bcc phase, but at higher concentration of V in the system the mechanical stability is increased. It was found that the Ni-Al-Ti system is mechanically stable in accordance with the requirements of mechanical stability for a cubic crystal. The first-principles calculations yielded solution enthalpies for B2 and bcc solid solution alloys. The enthalpies of bcc Ti-V alloys were calculated from first principles at 0 and 1300 K as a function of concentration using static and molecular dynamics simulations. The enthalpy curves for the B2 Ti-V alloys were described as a function of the V concentration by using the calculated solution enthalpies. The enthalpies of the β-phase Ti-V alloys decrease with increasing concentration of vanadium in the range from 0 to 1. Next, selfdiffusion in pure Ti was studied at high temperature using classical and ab initio molecular dynamics. We reveled a physical mechanism entailing a rapid collective movement of numerous (from two to dozens) neighboring titanium atoms along tangled closed-loop paths in defectfree crystal regions. Further, we addressed the effect of atomic relaxations on the formation enthalpy and the size of the tetra and octa voids in the body-centered cubic (bcc) high entropy alloys (HEA), where one of the principal elements is Ti. These are the alloys with 5 different components in equal proportions, which recently become the objects of extensive research due to their interesting properties, such as, for example, combined toughness and plasticity as well as corrosion resistance. We found that the relaxations are crucial and can change the energetically preferable distribution of elements in the periodic bcc lattice from segregated to random-alloy-like. The tetra and octa voids in HEAs can accommodate interstitial impurities that can be of interest to improve the alloy properties. We found that the distribution of void volumes due to atomic relaxations can be described by a set of Gaussians, whose number depends on the type of the void and the atomic distribution (random vs segregated). It could also be important that the largest volumes of the voids due to atomic relaxations are increased by nearly 25%.
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