Issue 42

A. Strafella et alii, Frattura ed Integrità Strutturale, 42 (2017) 352-365; DOI: 10.3221/IGF-ESIS.42.36 358 Figure 7 : log–log plot of sscr-stress points. Table 3 : Values of Norton-power law parameters. The value of stress exponent n is indicative of the rate controlling deformation mechanism. For austenitic stainless steels stress exponents are generally in the range of 3–12 and are associated with dislocation creep [10]. This is due to the nature of austenitic stainless steels which are solid solution alloys where the precipitation takes place during creep; for this reason, they show a particular behaviour which is intermediate between those of solid solution alloys and precipitation hardened/dispersion strengthened materials. The literature data report a value of n=3–5 for pure metals and simple solid solution alloys where the main creep deformation mechanism is the dislocation creep; other major creep deformation mechanisms such as diffusion creep are characterized by n= 1, and grain boundary sliding controlled creep are identified with a value of n=2. Instead, as also reported by literature, complex alloys and precipitation hardened/dispersion strengthened materials showed much higher values of n (≈40). It means that in austenitic stainless steels, the value of n is intermediate between those of solid solution alloys and precipitation hardened/dispersion strengthened materials and it is in a range of 3–12 [7]. The experimental analysis highlighted that the 15-15Ti(Si) austenitic steel value for the stress exponent is n=4.13; this result is consistent with the expectations and confirms that the steel deforms through the dislocation creep mechanism. Based on the Norton power law, on experimental data and on calculated values of n and A , the characteristic creep curve sscr-stress (log–log plot) was plotted, enabling to simulate the creep behaviour in air at all stress values (Fig. 8). These analyses could provide some important information for the use of this material in nuclear field because only few data can be found in the literature on 15-15Ti(Si) characterization. Creep curves in lead For its particular application on nuclear field, 15-15Ti(Si) was tested in lead in order to conduct a preliminary study on its lead corrosion sensitivity. A test at 575MPa and 550°C was performed to ascertain the correct working of new cell designed and manufactured for tests in liquid metal. The three stages are clearly recognized, as showed in Fig. 9. The most important tests in lead were performed using the following parameters, deriving from tests in air: - T= 550°C - σ = 500 and 560 MPa 250 300 350 400 450 500 550 600 1E-5 1E-4 1E-3 n= 4.13 A= 5.90*10 -16 sscr-experimental points air sscr [%/h stress A n 5.90*10 -16 4.13