Issue 22

A. Brotzu et alii, Frattura ed Integrità Strutturale, 22 (2012) 20-25; DOI: 10.3221/IGF-ESIS.22.03 22 R ESULTS AND DISCUSSION ngots produced by centrifugal casting were analysed by means of optical and electron microscopy (SEM / EDS) in order to evaluate their composition and structure. Analyses revealed that the different castings have a very similar composition and that are characterized by the presence of shrinkage microporosity. The alloy microstructure is fully lamellar (Fig. 2) and it is characterized by grains of alternate α 2 / γ lamellae as shown by the XRD pattern reported in Fig. 3. SEM analysis highlighted also the presence of a very small quantity of a bright phase in the backscattered image that is a chromium and molybdenum rich phase. Figure 2 : Optical micrograph showing the alloys’ microstructure. Figure 3 : XRD pattern showing the alloy composition. The studied alloys show a brittle behaviour during tensile tests with a fracture surface inclined at 45° to the tensile axis. The stress increases continuously without any appreciable plastic deformation up to the maximum stress. The maximum measured value of the ultimate tensile strength was 435.9 MPa. The morphological analysis of the tensile test fracture surfaces points out that the fracture propagates following an integranular path (Fig. 4). Presence of dimples was never observed. The fracture surface is characterized by a lamellar morphology that closely reminds the lamellar microstructure shown in Fig. 2. Hardness tests carried out on those alloys gave hardness values ranging from 436 HV to 510 HV. I