**3. Results**

### *3.1. As-Cast Microstructure*

The secondary phases observed in the five moulds or casting heats in as-cast condition are shown in Figure 4. The as-cast microstructure of the five moulds consisted of a γ matrix, Laves phases, Nb carbides (NbC), and smaller quantities of TiNb carbonitrides (TiNbCN). In the moulds with slow solidification rate, additional δ and γ" phases were observed in the segregated interdendritic region. Laves phases and carbides show similar colour and also morphologies when analysed by SEM; thus, for identification it is necessary to perform an EDX analysis. Therefore, for the quantification of the Laves phase area percentage included in Table 2, first the area percentage of Laves phases + carbides was determined by SEM images. Second, the area fraction of carbides was evaluated by optical microscopy, which revealed in the unetched state only the presence of carbides. Finally, as it is also explained in Section 2.3, the Laves phase area percentage was obtained by subtracting the area percentage of carbides obtained by OM from the area percentage obtained by SEM (Laves + carbides). It can be observed that, as the Si content of the alloy increased from 0.051 wt % (mould O) to 0.17 wt % (mould P), the area percentage of Laves phases increased from 2.2% to 3.5% and the Si content in the Laves phases increased from 0.28 to 1.29 wt %.

High segregation of alloying elements to the interdendritic spaces, particularly for Nb and Ti but also Mo, was observed in the as-cast state of every mould. A slight depletion in Fe and Cr was observed in those interdendritic spaces (Table 3).

The characteristics of the γ grains are depicted in Table 4. It can be observed that the grains of moulds fabricated with normal solidification rate (moulds O, E, P) showed a highly columnar grain morphology and an aspect ratio between 3.0 and 3.2, while the slowly solidified moulds presented a coarser grain size and aspect ratio below 2.0. Mean width was measured as the mean horizontal distance between grain boundaries, whereas mean length corresponded to distance along sample thickness. Morphology and grain size of γ grains in as-cast samples is shown in Figure 5.
