Chemically tailored planar defect phases in the Ta-Fe μ-phase

TL;DR

The Ta–Fe μ-phase, a topologically close-packed intermetallic, exhibits a defect landscape that shifts with Ta content. By combining EBSD and HR‑STEM experiments across 46–58 at.% Ta with DFT-derived defect energies, the study constructs a metastable defect phase diagram that links planar faults to the chemical potential via $Δμ_{Ta}$ and defect energies $ΔE^{defect}_F$. At low Ta content, basal defects and C14 TaFe$_2$ Laves lamellae dominate, while at higher Ta content, pyramidal $\{1\bar{1}02\}$ twins prevail and Laves-layer faults are suppressed; in the Ta-rich regime (58 at.% Ta) both defect types can coexist. This chemically driven defect landscape enables defect engineering strategies to tailor mechanical and functional properties of μ-phases by tuning composition, and provides a framework applicable to other complex intermetallics.

Abstract

Intermetallics often exhibit complex crystal structures, which give rise to intricate defect structures that critically influence their mechanical and functional properties. Despite studies on individual defect types, a comprehensive understanding of the defect landscape in μ-phases, a class of topologically close-packed phases, remains elusive. In this study, we investigated the planar defect structures in the Ta-Fe μ-phase across a compositional range of 46 to 58 at.% Ta using electron microscopy and density functional theory calculations. Electron backscatter diffraction and high-resolution scanning transmission electron microscopy reveal a transition from basal twin boundaries and planar faults containing C14 TaFe2 Laves phase layers at a low Ta content to pyramidal {1\bar{1}02} twins at a higher Ta content. Density functional theory calculations of defect formation energies confirm a chemical potential-driven stabilisation of Laves phase lamellae. The prevalence of pyramidal twins in Ta-rich μ-phase samples is attributed to the competitive nature of different planar defects during solidification. A defect landscape for μ-phases is proposed, illustrating the interplay between site occupancy, dislocation types and planar faults across the chemical potential space. These findings provide fundamental insights into defect engineering in structurally complex intermetallics and open pathways for optimising material properties through chemical tuning.

Figures (17)

• Figure 1: EBSD IPF maps of four different $\mu$-phase samples with target compositions of 46, 50, 54 and 58 at.% Ta, rest Fe. The unindexed (black) second phase in the 46 at.% Ta sample, identified as Laves phase, is not included in this figure. The atomic configurations of Ta$_6$Fe$_7$ and Ta$_7$Fe$_6$$\mu$-phase unit cells are also shown, where the Laves layers are highlighted in light green and the $3a$ sites in Ta$_6$Fe$_7$ are circled in red.
• Figure 2: EBSD maps where the region for the STEM lamellae positions are highlighted in white in (a) for a basal twin in the $\mu$-phase sample with 50 at.% Ta and in (b) for the pyramidal $\{1\bar{1}02\}$ twin in the $\mu$-phase sample with 54 at.% Ta. The identified basal and pyramidal twin planes are highlighted in red and orange, respectively, within the overlaid oriented unit cells of the two adjacent grains.
• Figure 3: HR-STEM EDS measurements on the 50 at.% Ta $\mu$-phase sample. (a) Overview of the STEM lamella showing several darker lines. In (b), one of them is displayed with higher magnification. The green square in (b) highlights the area measured by EDS. The EDS maps of the (c) Ta and (d) Fe distribution reveal an Fe-rich precipitate, as also summarised in the plot of molar fraction vs. position in (e).
• Figure 4: Varying numbers of TaFe$_2$ Laves phase layers in the 50 at.% Ta $\mu$-phase at different magnifications. The images show two Laves phase layers in (a) and (f), four layers in (b) and (g), five layers in (c) and (h), six layers in (d) and (i) and eight layers in (e) and (j). The atomic configurations of the TaFe$_2$ Laves phase layers are superimposed on the HAADF-STEM images. The blue spheres indicate Ta and the yellow spheres Fe atomic columns. Green markers indicate the atomic positions, illustrating an atomic displacement with an odd number of Laves layers in (h).
• Figure 5: Basal twin boundary structures in the 50 at.% Ta $\mu$-phase sample. (a) TB$_{\mathrm{CN15}}$ twin boundary (indicated by the red arrow), (b) TB$_\mathrm{K}$(2 Laves) twin boundary (indicated by the blue arrow) and (c) TB$_\mathrm{K}$(2 Laves) and TB$_{\mathrm{CN15}}$ twin boundaries next to each other. The atomic configurations of the basal twin boundary structures are superimposed on the HAADF-STEM images. The blue spheres indicate Ta and the yellow spheres Fe atomic columns.