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

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 degrees 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 Nb5Si3. The transformation of Nb5Si3 to Nb5Si3 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 Nb-ss in the large size as cast button and only by the precipitation of Nb-ss in the cast OFZ bars. Subgrains and precipitation of Nb-ss in Nb5Si3 was observed in the small size button and suction-cast bars after the heat treatment. Subgrains formed in Nb5Si3 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 Nb-ss in Nb5Si3, but the partitioning of solutes was essential for this precipitation.