Crystal structure, thermodynamics, magnetics and disorder properties of Be–Fe–Al intermetallics

Abstract

The elastic and magnetic properties, thermodynamical stability, deviation from stoichiometry and order/disorder transformations of phases that are relevant to Be alloys were investigated using density functional theory simulations coupled with phonon density of states calculations to capture temperature effects. A novel structure and composition were identified for the Be-Fe binary ε phase. In absence of Al, FeBe 5 is predicted to form at equilibrium above ∼1100 K, while the ε phase is stable only below ∼1500 K, and FeBe 2 is stable at all temperatures below melting. Small additions of Al are found to stabilise FeBe 5 over FeBe 2 and ε, while at high Al content, AlFeBe 4 is predicted to form. Deviations from stoichiometric compositions are also considered and found to be important in the case of FeBe 5 and ε. The propensity for disordered vs ordered structures is also important for AlFeBe 4 (which exhibits complete Al-Fe disordered at all temperatures) and FeBe 5 (which exhibits an order-disorder transition at ∼950 K).

Resolving the structure of TiBe12

“…Zero-point energy and temperature contributions to the Helmholtz free energy were obtained by integrating the phonon DOS, following the harmonic and quasi-harmonic approximations, as outlined in Burr et al (2015). Fig.…”

Solubility and partitioning of impurities in Be alloys

“…As discussed in previous work [4], Fe is soluble in Be metal (i.e. a small negative solution energy), but also forms (more favourably) three intermetallic phases: ε-Fe 2+x Be 17−x , δ-FeBe 5 and ζ-FeBe 2 , with the latter two commonly observed in commercial alloys.…”

“…In our previous work [4], we showed that lattice disorder plays an important role for AlFeBe 4 and -at high temperatures -for FeBe 5 . Calculating the energy associated with point defects within disordered phases is computationally impractical, hence the current work assumed only the ordered form of AlFeBe 4 and FeBe 5 .…”

“…This is in agreement with observations by Rooksby and Green [18] that in some Fe-containing Be alloy samples, Si was detected by chemical analysis but no elemental Si was found via XRD analysis. Scavenging of Si impurities by careful alloying additions of Fe may then be beneficial for the mechanical properties of the alloy [4].…”

Simulations of threshold displacement in beryllium

“…[3][4][5] More recently, efforts have been undertaken to evaluate its performance under fusion conditions both experimentally [6][7][8] and through modelling, whereby the fundamental defect processes, such as the interaction of interstitials, vacancies, impurities, and radiogenic H and He, have been characterized. [9][10][11][12] One issue that modelling has yet to investigate is the threshold displacement energy (E D ) of Be, which is a parameter used in estimations of how many point defects will be created during irradiation. This value may be fed into stochastic Monte-Carlo simulations and approaches based on derivatives of the Kinchin-Pease model, to evaluate the longterm irradiation response of materials.…”