Morphological Heredity of Intermetallic Nb5Si3 Dendrites in Hypereutectic Nb-Si Based Alloys via Non-Equilibrium Solidification

For hypereutectic Nb-Si based alloys, primary Nb5Si3 phases typically grow in a faceted mode during equilibrium or near-equilibrium solidification, which damages the ductility and toughness. To address this issue, here we artificially manipulate the growth morphology of Nb5Si3 using electron beam surface melting (EBSM) and subsequent annealing treatments. Results show that such a non-equilibrium solidification pathway enables the transition from faceted growth to non-faceted dendritic growth of Nb5Si3, along with evident microstructure refinement, generation of metastable β-Nb5Si3 phases and elimination of chemical segregation. The transformation from β-Nb5Si3 to α-Nb5Si3 and Nb solid solution (Nbss) particles is triggered by the annealing treatment at 1450 °C for 5 h. Also, we find the annealing-mediated formation of inherited Nb5Si3 dendrites that maintain the dendritic morphology of the original as-solidified β-Nb5Si3 dendrites. This work thus provides a feasible routine to obtain thermally stable and refined α-Nb5Si3 dendrites in hypereutectic Nb-Si based alloys.


Introduction
For increased efficiency and performance of advanced turbine engines, there is always a driving force to increase the inlet temperature [1]. This demand motivates the design and production of novel ultrahigh temperature materials (UHTM) that can operate at higher temperatures than current Ni-based superalloys [1,2]. As a member of UHTM family, Nb-Si based alloys, consisting of Nb solid solution (Nbss) and silicides (mainly Nb 5 Si 3 ), have been of interest due to their high melting temperatures (> 1750 °C), relatively low densities (< 7.2 g/cm 3 ) as well as excellent creep resistance with creep rates below 3×10 −8 s −1 at 1200 °C and stresses above 140 MPa [1,3,4].
The introduction of intermetallic Nb 5 Si 3 contributes largely to such property advantages, as a density reducer and a high temperature strength enhancer. But unfortunately, Nb 5 Si 3 normally causes the loss of ductility and toughness at room temperature, acting as the preferred sites for crack initiation and propagation, especially for hypereutectic Nb-Si based alloys [5,6]. Tuning the geometrical morphology and length scale of Nb 5 Si 3 can be an effective pathway to achieve better alloy property, provided that its growth behavior is well understood and suited solidification processing techniques are employed [6,7].
In conventional cast Nb-Si based alloys, Nb 5 Si 3 particles are normally distinguished by a faceted growth morphology [7][8][9], because of its high entropy of fusion and large Jackson's factor (α) of ~2.2 [10]. It thus suggests the growth of Nb 5 Si 3 is typically limited by the nucleation rate under slow-cooling and near-equilibrium solidification conditions. Recent experimental evidences demonstrates that wide microstructural variations can be achieved via rapid solidification, including morphology, phase size and volume fraction [11][12][13]. For rapidly solidified alloys, the growth mode of thermodynamics-controlled faceted phases, may be altered concomitantly, e.g., the transition from faceted to nonfaceted growth of intermetallic phases with high entropy of fusion [14,15]. The mechanism behind this morphology transition is mainly due to the change in the solid/liquid interface structures and roughness [16,17]. Previous literature [6,12,18] has reported that beneficial microstructure modification of Nb-Si based alloys is enabled by rapid solidification, resulting in improvements of oxidation resistance and fracture toughness. The non-faceted growth of Nb 5 Si 3 dendrites has been observed as result of non-equilibrium solidification [12,19], but the corresponding growth mechanism still lacks. More importantly, the thermal and morphological stabilities of Nb 5 Si 3 formed via rapid solidification remains unexplored yet, which is of particular importance for Nb-Si based alloys with potential high temperature applications.

Open Access
In this work, we shift our research focus on the phase stability and morphology of Nb 5 Si 3 upon rapid solidification and high temperature exposure. Nb 5 Si 3 dendrites are produced via electron beam surface melting (EBSM, a direct liquid/solid solidification approach) and via subsequent annealing treatment (an inherited solid/solid phase transformation approach). The morphological heredity of intermetallic Nb 5 Si 3 dendrites found here can offer a novel insight into the control of silicide morphology in Nb-Si based alloys. Note that the solidification condition of EBSM is similar to the state-of-the-art technique of additive manufacturing [20], and therefore, this work is considered as a suitability pre-evaluation of Nb-Si based alloys for electron beam additive manufacturing, and a guidance for microstructure design towards mechanical property improvement.

Materials and Methods
The Nb-22Si-24Ti-2Cr-2Al (at.%) alloys were employed in this work, fabricated by non-consumable arc-melting using high-purity staring materials of Nb (99.87 wt.%), Si (99.50 wt.%), Ti (99.76 wt.%), Cr (99.98 wt.%) and Al (99.99 wt.%). The button ingots were remelted by vacuum induction melting (VIM), in order to improve chemical homogeneity. Mechanically ground alloy disks were prepared for EBSM treatment, which was carried out using an Arcam A2XX (Arcam AB) electron beam 3D printing system with a controlled 0.2 Pa of He atmosphere. The electron beam was generated in an electron gun via a tungsten filament, which was heated to above 2000 °C. The emitted electrons were accelerated by an electrical field and focused by an electromagnetic coil. Controlled by a computer attached to the 3D printing system, the focused electron beam was selectively scanned on the Nb-Si based alloy substrate. For cracking prevention, the substrate was preheated to 1200 °C by defocused electron beam before scanning. A processing current of 4.2 mA and the zigzag scanning strategy with a scanning speed of 170 mm/s were employed. The hatch distance was 0.1 mm. Annealing treatment was performed at 1450 °C for 5 h to examine the microstructural stability, and the samples were sealed in silica tubes to avoid oxidation. Furnace cooling was utilized in this work, with an average cooling rate of approximately 10 °C/min. Phases were determined by X-ray diffraction (XRD, D/max-2500, Cu Kα) with a 2θ scanning rate of 6°/min. Backscattered electron (BSE) imaging was performed using a scanning electron microscopy (SEM, Zeiss-Merlin) with an acceleration voltage 30 kV. The elemental distributions measured using the attached energy-dispersive X-ray spectroscopy (EDS). Electron backscatter diffraction (EBSD) was performed using a Zeiss-Crossbeam XB 1540 FIB instrument with a Hikari camera operating at 15 kV. The EBSD scanning step sizes for the VIM and EBSM alloy were 1 μm and 0.1 μm, respectively. A TSL OIM package was used for data collection and analysis.
Microstructure of the annealed alloy was imaged by a scanning transmission electron microscopy (STEM, image-corrected FEI Titan Themis 80-300) with the highangle annular dark-field (HAADF) imaging mode. The acceleration voltage for HAADF-STEM imaging was 300 kV. For HAADF-STEM imaging, a probe semi-convergence angle of 17 mrad and inner and outer semi-collection angles ranging from 73 to 200 mrad were conducted. Selected area diffraction (SAD) was performed on a JEOL JEM 2100 instrument for phase identification. The simulated SAD patterns were obtained by CaRIne v3.1 software. The thin lamellae for TEM investigation, ~10 μm in width, were extracted from the Nb 5 Si 3 silicide within the annealed EBSM alloy using a dual-beam focused ion beam (FIB)/SEM instrument (FEI Helios Nanolab 600i). The standard site-specific lift-out procedure was applied for FIB-thinned lamellae [21]. The FIB-induced damage and Ga ions contamination were minimized by a final cleaning at 5 kV.

Results
According to the XRD patterns shown in Figure 1, the original VIM alloy is mainly composed of bcc-structured Nbss and tetragonal α-Nb 5 Si 3 with a D8 1 structure, and the experimental EBSM alloy primarily consists of Nbss and tetragonal β-Nb 5 Si 3 with a D8 m structure. After annealing treatment at 1450 °C for 5 h, the main phases of the EBSM include Nbss, α-Nb 5 Si 3 and β-Nb 5 Si 3 . Depending on alloy compositions and processing methods, three polymorphs of Nb 5 Si 3 have been reported, i.e., α-Nb 5 Si 3 (tI32, D8 l ), β-Nb 5 Si 3 (tI32, D8 m ) and γ-Nb 5 Si 3 (hP16, D8 8 ) [22]. This work suggests that the rapid solidification process induced by EBSM leads to the formation of β-Nb 5 Si 3 , but the subsequent annealing treatment promotes the transition from β-Nb 5 Si 3 to α-Nb 5 Si 3 that is more stable at lower temperatures [23]. Figure 2(a) displays the BSE image of the VIM alloy, and the EBSD phase map is shown in Figure 2(b). The VIM alloy without remelting shows a hypereutectic microstructure, and the primary faceted α-Nb 5 Si 3 particles (dark grey contrast) are observed, in combination with Nbss phases (light grey contrast). It is observed that α-Nb 5 Si 3 particles are embedded in the Nbss matrix. α-Nb 5 Si 3 shows a faceted polygonal morphology with a coarse size up to ~80 μm. According to EDS line scanning (Figure 2(c)) and EDS mapping (Figure 2(d)), α-Nb 5 Si 3 is enriched in Si, but depleted in Nb, Ti, Cr and Al, compared with Nbss. The black contrast region, mainly on the edge of α-Nb 5 Si 3 , contains a higher content of Ti. Within Nbss matrix, the chemical segregations of Ti and Cr are noted. Figure 3 shows the sectional microstructure of the experimental alloy subjected to EBSM treatment, and a remelted layer, ~188 μm in thickness, is produced on the surface. The remelted layer is typically generated by the overlap of molten pools, which is essentially identical to the welding and additive manufacturing processes [24,25]. Molten pool boundary is defined as the partitioning line between adjacent molten pools. The microstructure beneath the remelted layer is identical to that of the VIM alloy, suggesting no additional effect is caused by the EBSM treatment. It is evident that compared to the VIM alloy, the remelted layer demonstrates a significantly finer microstructure.
For the remelted layer via EBSM, the microstructure within molten pool is shown in Figure 4 Table 1. Combining this EDS result with XRD patterns in Figure 1, the darkgrey phase is determined to be Nb 5 Si 3 , including α-and β-forms, and the light-grey and dark phases are Nbss and Ti-oxide, respectively. The formation of Ti-oxide results from the high affinity of Ti for oxygen at elevated temperatures, and thus Ti atoms react with the residual oxygen during annealing treatment, particularly at grain boundaries and Nbss/Nb 5 Si 3 phase interfaces [26]. The silicide dendrites after annealing treatment become coarsened second dendrite arm spacing increasing from ~0.9 μm to ~2.6 μm, and most of the nanoscale silicide particles no long exist, in comparison with the as-solidified microstructure in Figure 4(b).
Noteworthily, within the Nb 5 Si 3 dendrites emerges a new dendritic silicide that shows a brighter BSE contrast, denoted as the inherited silicide in Figure 5, which, to our best knowledge, has not been reported previously. To reveal the crystal structure and identify the inherited silicide, STEM analysis has been performed, and the results are displayed in Figure 6. The SAD patterns of α-Nb5Si3  and β-Nb5Si3[-111] are obtained, which are in accordance with the simulated patterns using the CaRIne v3.1 software. Accordingly, it can thus be determined that the α-Nb 5 Si 3 dendrites are formed within original β-Nb 5 Si 3 dendrites, accompanied with the formation of Nbss particles (white contrast). This agrees with the result that both α-Nb 5 Si 3 and β-Nb 5 Si 3 are detected by XRD patterns (Figure 1). Such a phase transformation β-Nb 5 Si 3 →α-Nb 5 Si 3 + Nbss has also been reported by Tsakiropoulos et al. [27,28]. STEM-EDS has been further carried out to uncover the compositional difference between α-Nb 5 Si 3 and β-Nb 5 Si 3 , as illustrated in Figure 7. Unfortunately, owing to the detecting limit of qualitative EDS approach, a welldefined compositional difference is not obtained. Even so, after careful checking, the inherited α-Nb 5 Si 3 phase is slightly enriched in Nb and Al, and the β-Nb 5 Si 3 phase shows the trace of higher Ti content. It is in accordance with observation that inherited α-Nb 5 Si 3 displays a brighter Z-contrast than β-Nb 5 Si 3 in the STEM-HAADF image (Figures 6(a) and 7(a)).

Discussion
Schematic of microstructure evolution the experimental Nb-Si based alloy under different solidification conditions of VIM and EBSM as well as subsequent annealing treatment is shown in Figure 8, with a particular focus on the morphology of intermetallic Nb 5 Si 3 phases. The morphology change for Nb 5 Si 3 found here is associated with its growth mode. For VIM processing with a near-equilibrium solidification condition, Nb 5 Si 3 forms polyhedral morphologies, revealing a faceted solidifying Table 1 Compositions of selected positions in Figure 5 for the EBSM Nb-Si based alloy after annealing at 1450 °C for 5 h measured by SEM-EDS (at.%)  mode. In terms of EBSM processing with a rapid solidification condition, Nb 5 Si 3 dendrites are formed, displaying a non-faceted solidifying mode [14]. Namely, the transition from lateral growth (faceted) to continuous growth (non-faceted) is promoted by EBSM treatment. The growth morphology of crystalline Nb 5 Si 3 is normally determined by its intrinsic crystal structure and external solidification condition. The former dominates the equilibrium growth morphology via minimizing solid/liquid interface energy, and the latter indicated by undercooling can lead to the deviation from equilibrium morphology as determined by growth kinetics. During solidification, the solid/liquid interface, where attachment of atoms proceeds, shows a diffuse structure, and the interface roughness has been reported to determine the growth mode [29,30]. With a relatively high entropy of fusion, Nb 5 Si 3 crystal forms via the advancing of solid/ liquid interface with evident growth anisotropy along different crystal orientations [10]. Under small undercooling of VIM processing, Nb 5 Si 3 grows from atomically smooth solid/liquid interface in a lateral growth mode [10,31]. Two dimensional nucleation on liquid/solid interface for Nb 5 Si 3 is facilitated with the attachment of atoms rejected from the liquid to existing atomic steps on the growing interface [14,32]. The atomic steps formed on liquid/solid interface are generally provided by the defects in Nb 5 Si 3 crystal like dislocations. As a result, Nb 5 Si 3 in the VIM alloy displays a faceted morphology ( Figure 2).
For the rapid solidification process of EBSM, the undercooling significantly increases, and solid/liquid interface is suggested to become atomically roughened with the fraction of solid atom higher than 0.5 [15,33]. Atoms tend to add to the interface without preferred crystal orientations, and the growth of Nb 5 Si 3 crystal is in a continuous mode, instead of the lateral mode via two-dimensional nucleation. This can explain the direct formation of Nb 5 Si 3 dendrites in EBSM alloy, which is an indication of continuous growth. Such microstructural transition from faceted to non-faceted growth enabled by a larger kinetic undercooling has been observed in the  solidification of some other materials with high entropy of fusion like primary Mg 2 Si in Al-Mg 2 Si alloys [15], silicon [34,35], bismuth [17], germanium [36] and Al 2 O 3 [37], etc. It is interesting to find that the rapidly solidified β-Nb 5 Si 3 phases can transform to the α-Nb 5 Si 3 phases via annealing due to its better thermal stability at room temperatures, as indicated by the Nb-Si binary phase diagram [38]. The higher structural stability of α-Nb 5 Si 3 has also been demonstrated by first principles calculations, in terms of cohesive energy and bonding electron number per atom [39]. Considering both α-Nb 5 Si 3 and β-Nb 5 Si 3 cells contain 20 niobium atoms and 12 silicon atoms [39], the precipitation of supersaturated Nb atoms in β-Nb 5 Si 3 , as a result of rapid solidification process of EBSM, happens together with the transformation from α-Nb 5 Si 3 to β-Nb 5 Si 3 . It is worthy of note that the dendritic morphology remains ( Figure 5). This may suggest such a phase transformation takes place in an in-situ way, rather than a nucleation precipitation process, which is associated with the short diffusion distance of atoms during annealing treatment [40]. As a result, a thermally stable hypereutectic Nb-Si based alloy, distinguished by a dendrite-dominated microstructure, is expected. The morphological evolution of primary Nb 5 Si 3 found in this work can guide the artificial manipulation of the solidification processing with the aim of property optimization for Nb-Si based alloys, especially the fracture toughness at room temperature and the oxidation resistance at high temperatures.

Conclusions
Microstructures of the Nb-22Si-24Ti-2Cr-2Al (at.%) alloys prepared by VIM (near-equilibrium solidification) and EBSM (rapid solidification) techniques have been analyzed in this work, focusing on the Nb 5 Si 3 growth response to solidification conditions. The following conclusions can be drawn from the experimental results.
(1) Tetragonal α-Nb 5 Si 3 and β-Nb 5 Si 3 phases are found in the experimental VIM and EBSM alloys. The decomposition of β-Nb 5 Si 3 is facilitated by annealing at 1450 °C, resulting in α-Nb 5 Si 3 formation. (2) Primary Nb 5 Si 3 dendrites are produced via a nonequilibrium solidification of EBSM, in contrast to the primary faceted Nb 5 Si 3 particles in the VIM alloy. Significant microstructure refinement as well as a more homogeneous compositional distribution is achieved by EBSM, which is associated with its large solidification rate. (3) The microstructure becomes coarser as a result of annealing. α-Nb 5 Si 3 dendrites emerge within original as-solidified β-Nb 5 Si 3 dendrites in the annealed alloys, with a slight enrichment of Nb and Al as well as a depletion of trace Ti.