*2.5. Optical Analysis*

Morphological analysis was carried out on cryo-fractured Charpy samples by FEI Quanta 450 FEG scanning electron microscope (SEM) (Thermo Fisher Scientific, Waltham, MA, USA). To avoid charge build up, the sample surfaces were sputtered (on a LEICA EM ACE 600 High Vacuum Sputter Coater, Wetzlar, Germany) with a thin surface layer of platinum. Image-J software was used to analyze the SEM images and to calculate the number average radius (*Rn*), the volume average radius (*Rv*) and the size distribution (*SD*) of the dispersed phase droplets. At least 150 droplets were counted to calculate *Rv, Rn* and *SD* parameters according the following Equations [70]:

$$R\_{\rm nl} = \frac{\sum\_{i} n\_{i} R\_{i}}{n\_{i}} \tag{5}$$

$$R\_{\upsilon} = \frac{\sum\_{i} n\_{i} R\_{i}^{4}}{n\_{i} R\_{i}^{3}} \tag{6}$$

$$SD = \frac{R\_v}{R\_n} \tag{7}$$

The fracture surface of the tensile specimen broken during the dilatometric tests offered reliable information about the micromechanical deformations that occurred during the tensile tests. Consequently, some specimens were cold fractured along the tensile direction. In this case the specimens were coated with a thin layer of platinum prior to microscopy to avoid charge build up.
