Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations

Abstract

The enthalpy and activation energy for the transformation of the metastable form of tungsten, β-W, which has the topologically close-packed A15 structure (space group Pm3¯n), to equilibrium α-W, which is body-centered cubic (A2, space group Im3¯m), was measured using differential scanning calorimetry. The β-W films were 1 μm-thick and were prepared by sputter deposition in argon with a small amount of nitrogen. The transformation enthalpy was measured as -8.3 ± 0.4 kJ/mol (-86 ± 4 meV/atom) and the transformation activation energy as 2.2 ± 0.1 eV. The measured enthalpy was found to agree well with the difference in energies of α and β tungsten computed using density functional theory, which gave a value of -82 meV/atom for the transformation enthalpy. A calculated concerted transformation mechanism with a barrier of 0.4 eV/atom, in which all the atoms in an A15 unit cell transform into A2, was found to be inconsistent with the experimentally measured activation energy for any critical nucleus larger than two A2 unit cells. Larger calculations of eight A15 unit cells spontaneously relax to a mechanism in which part of the supercell first transforms from A15 to A2, creating a phase boundary, before the remaining A15 transforms into the A2 phase. Both calculations indicate that a nucleation and growth mechanism is favored over a concerted transformation. More consistent with the experimental activation energy was that of a calculated local transformation mechanism at the A15-A2 phase boundary, computed as 1.7 eV using molecular dynamics simulations. This calculated phase transformation mechanism involves collective rearrangements of W atoms in the disordered interface separating the A15 and A2 phases.

On‐Demand Preparation of α‐Phase‐Dominated Tungsten Films for Highly Qualified Thermal Reflectors

“…Along the film growth direction, the interface between the neighboring β/α phases clearly demonstrates the phase‐structure evolution between them. As seen, the intermixing or even transformation between β‐ and α‐W occurs at the interface area, which is owing to the atomic rearrangement at the phase interface induced by localized excitation, similar to the previous reports . In this case, β‐ and α‐W phase are balanced rivals.…”

“…As analogous to the previous studies, substrate‐bias‐induced Ar ion bombardment during the sputtering process, offering additional kinetic energy, is capable of accelerating the W adatom relaxation (see Figure ). Thus, through momentum transfer from low‐energy ion bombardment (the time‐average ion energy for the bombardment in our experiments was estimated to be less than 100 eV), excitation energy can be fueled to overcome the local energy barrier from β‐ to α‐phase ( E a , i.e., the transformation activation energy, about ≈0.7 eV), which is much larger than the free energy change ΔG (less than 0.1 eV), as schematically illustrated in Figure a.…”

Measurement of spin mixing conductance in Ni$_{81}$Fe$_{19}$/$α$-W and Ni$_{81}$Fe$_{19}$/$β$-W heterostrucutures via ferromagnetic resonance

“…We have found very little difference between the spin scattering properties (spin mixing conductance and spin diffusion length) of α-W and mixed phase (α+β)-W. The simplest interpretation is that both spin mixing conductances and spin diffusion lengths are nearly equal for the two phases. However, despite our development of an optimized technique [9][10][11] to stabilize the β phase, our control over the amounts of deposited α and β phases is less than complete, particularly near the Py/W interface.…”

“…Recently, some of us [9][10][11] have optimized a different method to stabilize the metastableβ-phase, using the introduction of N 2 gas 12 while sputtering at low power. Relatively thick (over 100 nm) monophase β-W films could be stabilized this way, when deposited on glass substrates.…”

Neural-Network-Based Path Collective Variables for Enhanced Sampling of Phase Transformations

“…TCP phases are of particular interest in high-performance materials such a Ni-base superalloys [27] as their formation significantly influences the materials properties [28]. In tungsten, the transformation between the A15 and bcc phase has recently attracted increased attention [29,30] since for applica-tions in microelectronics the formation of A15 should be avoided [31,32] whereas in spintronic devices the A15 phase is the desired one [33]. In a previous study the dynamics of the A15-bcc phase boundary in Mo was investigated using the adaptive kinetic Monte Carlo (AKMC) approach [34].…”