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. 2018 Jun 7;11(6):967.
doi: 10.3390/ma11060967.

Type of Primary Nb₅Si₃ and Precipitation of Nb ss in αNb₅Si₃ in a Nb-8.3Ti-21.1Si-5.4Mo-4W-0.7Hf (At.%) Near Eutectic Nb-Silicide-Based Alloy

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Free PMC article

Type of Primary Nb₅Si₃ and Precipitation of Nb ss in αNb₅Si₃ in a Nb-8.3Ti-21.1Si-5.4Mo-4W-0.7Hf (At.%) Near Eutectic Nb-Silicide-Based Alloy

Conor McCaughey et al. Materials (Basel). .
Free PMC article

Abstract

The Nb-silicide-based alloy of near eutectic composition (at.%) Nb-21.1Si-8.3Ti-5.4Mo-4W-0.7Hf (alloy CM1) was studied in the cast and heat-treated (1500 °C/100 h) conditions. The alloy was produced in the form of buttons and bars using three different methods, namely arc-melting, arc-melting and suction casting, and optical floating zone (OFZ) melting. In the former two cases the alloy solidified in water-cooled copper crucibles. Buttons and suction-cast bars of different size, respectively of 10 g and 600 g weight and 6 mm and 8 mm diameter, were produced. The OFZ bars were grown at three different growth rates of 12, 60 and 150 mm/h. It was confirmed that the type of Nb₅Si₃ formed in the cast microstructures depended on the solidification conditions. The primary phase in the alloy CM1 was the βNb₅Si₃. The transformation of βNb₅Si₃ to αNb₅Si₃ had occurred in the as cast large size button and the OFZ bars grown at the three different growth rates, and after the heat treatment of the small size button and the suction-cast bars of the alloy. This transformation was accompanied by subgrain formation in Nb₅Si₃ and the precipitation of Nbss in the large size as cast button and only by the precipitation of Nbss in the cast OFZ bars. Subgrains and precipitation of Nbss in αNb₅Si₃ was observed in the small size button and suction-cast bars after the heat treatment. Subgrains formed in αNb₅Si₃ after the heat treatment of the OFZ bars. The partitioning of solutes and in particular of Mo and Ti was key to this phase transformation. Subgrain formation was not necessary for precipitation of Nbss in αNb₅Si₃, but the partitioning of solutes was essential for this precipitation.

Keywords: Nb-silicide-based alloys; intermetallics; phase equilibria; solidification.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
As cast alloy CM1 (a) CM1-10g button, (b) CM1-6mm (bulk) and (ce) CM1-8mm suctions cast bars, (c) bulk, (d) and (e) from area of high cooling rate and 18.6 < Si < 19.2 at.%. Rectangles show lamellar microstructures. The lamellar microstructure resulting from the eutectoid transformation of Nb3Si is shown in the right hand side of (e).
Figure 1
Figure 1
As cast alloy CM1 (a) CM1-10g button, (b) CM1-6mm (bulk) and (ce) CM1-8mm suctions cast bars, (c) bulk, (d) and (e) from area of high cooling rate and 18.6 < Si < 19.2 at.%. Rectangles show lamellar microstructures. The lamellar microstructure resulting from the eutectoid transformation of Nb3Si is shown in the right hand side of (e).
Figure 2
Figure 2
Alloy CM1-600g button (a) and (b) as cast, (c) and (d) heat treated. The rectangles show lamellar microstructure. For (b) and (d) see text.
Figure 3
Figure 3
BSE images of CM1-OFZ processed at 150 mm/h, 60 mm/h and 12 mm/h, respectively from top to bottom. Left hand images show the microstructure at the edge of the bar and right hand images show the bulk microstructure. The arrow indicates the growth direction. Rectangles show lamellar microstructure.
Figure 4
Figure 4
Backscattered electron images showing regions of light contrast, including precipitates, within the silicide phase in CM1-OFZ for the growth rates of (a) 150, (b) 60, (c) 12 and (d) 150, (e,f) 12 mm/h.
Figure 5
Figure 5
Heat-treated alloy CM1 (a), (b) and (g) 10 g button, (c) and (d) 6 mm diameter suction-cast bar, (e) and (f) 8 mm diameter suction-cast bar, (c) to (f) images from bulk of bars.
Figure 5
Figure 5
Heat-treated alloy CM1 (a), (b) and (g) 10 g button, (c) and (d) 6 mm diameter suction-cast bar, (e) and (f) 8 mm diameter suction-cast bar, (c) to (f) images from bulk of bars.
Figure 6
Figure 6
BSE images of heat-treated CM1-OFZ showing precipitation of Nbss and sub-grains in Nb5Si3 (a) 60 mm/h, (b) 150 mm/h.
Figure 7
Figure 7
Schematic diagram of the skewed coupled zone of the Nb + βNb5Si3 metastable eutectic. Red continuous lines show the liquidus of Nb and βNb5Si3 and dashed red lines the growth temperatures Ti (i = Nb, βNb5Si3) dendrites at growth rate V2. The black horizontal line is the eutectic temperature TEu. Drawing based on [37].
Figure 8
Figure 8
Schematic diagram showing the geometry considered by Louchev et al. [54].
Figure 9
Figure 9
Schematic diagram showing concentration and temperature gradients ahead of an S/L interface.

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