*3.2. Microstructure after Heat Treatment and before Welding Trials*

Heat treatment did not modify the as-cast grain size and morphology observed in Figure 5, however, segregation and amount of Laves phase were significantly reduced in comparison with as-cast condition as can be concluded by comparing Tables 3 and 5. The characteristics of the Laves phases after heat treatment (HIP+S) are shown in Table 2, together with the ones of as-cast samples. No Laves phases were detected in the low and standard Si-bearing alloys (moulds O and E), but small area percentages between 0.14 and 0.35% remained in high-Si alloy (mould P) and slowly solidified alloy with (mould NP) and without pre-HIP (mould N) heat treatments. It is also worthwhile mentioning that after heat treatment, the Laves phase composition was enriched in Mo and Si.

(**e**)

**Figure 4.** Secondary phases present in the microstructure of different moulds in the as-cast state: (**a**) O, (**b**) E, (**c**) P, and (**d**) NP; (**e**) NP: magnification of (**d**) showing the presence of Laves, δ phase, and γ" in the interdendritic region.


**Table 2.** Area percentage and chemical composition of Laves phases (in wt %) as-cast and after heat treatment obtained by / EDX) analysis (\* Laves phase was not detected).

**Table 3.** Segregation ratio (SR) of alloying elements in the as-cast state.


**Table 4.** Grain size (mean with and length with standard deviation) in the as-cast state.

