Ptychographic X-ray CT of ultrafine eutectic Ti-Fe-based alloy

28-09-2020

High-resolution near-field synchrotron ptychographic X-ray computed tomography carried out at ID16A reveals the ultrafine eutectic microstructures of a titanium-iron-based alloy. The material was produced by additive manufacturing, exploiting the high solidification rates of laser-based 3D printing.

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While research on additive manufacturing has initially focused on conventional alloys known from casting and forging processes, current efforts include the development of metals tailored to the specific metallurgical conditions of laser-based additive manufacturing. However, high-strength eutectic alloys have, so far, received minor attention, although previous work has shown that rapid solidification techniques can result in ultrafine microstructures with excellent mechanical performance, i.e. very high strengths (> 2.5 GPa at room temperature) together with enhanced ductility (> 10 %) [1,2]. In this work, a eutectic Ti-32.5Fe alloy was produced by high-temperature laser powder bed fusion (LPBF), exploiting the rapid solidification and the capability to produce bulk ultrafine microstructures provided by this processing technique.

Using scanning electron microscopy, two characteristic regions of interest were identified in the material: a eutectic-rich (ER) region with  fine lamellar structures in the sub-micrometre / nanometre range, which are mainly formed by β-Ti and the intermetallic phase TiFe, and dendrite-rich (DR) regions containing η-Ti4Fe2Ox dendrites that extend over several micrometres. Small cylinders from these regions were extracted by focused ion beam. The ultrafine eutectic microstructures were studied three-dimensionally using near-field synchrotron ptychographic X-ray computed tomography (NF-PXCT) at ID16A to analyse the morphology of the ER and DR regions as well as to quantify the size, distribution and mass density of the constituent phases. A colour-coded visualisation of the 3D thickness of the TiFe network (Figure 1a), the β-Ti network (Figure 1b) and the η-Ti4Fe2Ox dendrites (Figure 1c) shows characteristic regions associated with different degrees of microstructural refinement (indicated as R1-R6).

Fig1.jpg

Fig. 1: Colour-coded visualisation of 3D thickness in (a) the TiFe network (ER volume), (b) the β-Ti (ER volume) and (c) the η-Ti4Fe2Ox dendrites (DR volume). Building direction (z) is indicated by an orange arrow. Eutectic TiFe-network (R1 – R3), β-Ti network (R4-R5) and η-Ti4Fe2Ox dendrites (R6).

The analysis of the tomographs, with actual spatial resolution down to ⁓ 39 nm, revealed that the ER region contains hierarchical eutectic structures (i.e. finer eutectics coexisting besides coarser colonies). The colonies consist of eutectic structures with lamellar widths between ⁓ 90 and ⁓ 160 nm. The local mass density was calculated from the NF-PXCT images based on the grey values of the reconstructed volumes. Colour-coded mass density maps were computed for all segmented phases (TiFe, β-Ti, α-Ti and η-Ti4Fe2Ox) and are shown in Figure 2.

Fig2.jpg

Fig. 2: a-d) Mass density maps of xy-slices of NF-PXCT volume and e-h) 3D visualisations of the mass density of TiFe (a,e), β-Ti (b,f), α-Ti (c,g) and η-Ti4Fe2Ox (d,h). ER: Eutectic-rich region; DR: dendritic-rich region. Building direction (z) is indicated by orange arrow (d,h).

Sufficiently large regions within a phase (i.e. far enough from the interface to a second phase) were selected from these volumes to individually determine their mass density. The NF-PXCT investigations made it possible to quantify the 3D microstructure and the density of each phase as well as of the bulk material (⁓ 5.62 g/cm3). Moreover, a high specific compressive yield strength of ⁓ 140 MPa/(g/cm3) at 600°C for the 3D-printed Ti32.5Fe alloy was demonstrated. Thus, NF-PXCT provided unique quantitative information about the internal ultrafine architecture of the alloy, for which inter-lamellar spacings down to ⁓ 30-50 nm were achieved, revealing the potential of laser-based additive manufacturing to generate high-strength eutectic alloys with microstructures finer than in those obtained by classical rapid solidification techniques for bulk materials.

Principal publication and authors
Ultrafine eutectic Ti-Fe-based alloys processed by additive manufacturing – A new candidate for high temperature applications, J. Gussone (a), K. Bugelnig (a), P. Barriobero-Vila (a), J.C. da Silva (b,f), U. Hecht (c), C. Dresbach (d), F. Sket (e), P. Cloetens (f), A. Stark (g), N. Schell (g), J. Haubrich (a), R. Guillermo (a,h), Appl. Mater. Today 20, 100767 (2020); https://doi.org/10.1016/j.apmt.2020.100767.
(a) German Aerospace Center DLR, Institute of Materials Research, Cologne (Germany)
(b) Institut Néel, CNRS, Grenoble (France)
(c) Access e.V., Aachen (Germany)
(d) University of Applied Sciences Bonn-Rhein-Sieg, Rheinbach (Germany)
(e) IMDEA Materiales, Madrid (Spain)
(f) ESRF
(g) Helmholtz-Zentrum Geesthacht (Germany)
(h) RWTH Aachen University, Metallic Structures and Materials Systems for Aerospace Engineering (Germany)

References
[1] J. Das et al., Appl. Phys. Lett. 87, 161907 (2005).
[2] D.V. Louzguine-Luzgin, Mater. Trans. 59 1537 (2018).