PSI - Issue 68

Marwa Ben Bettaieb et al. / Procedia Structural Integrity 68 (2025) 297–302 M. Ben Bettaieb et al. / Structural Integrity Procedia 00 (2025) 000–000

299

3

Fig. 1. TEM (Transmission Electron Microscopy) observations of the Nb-rich alloy at the as-received state. (a) Image in Z-contrast mode; (b) Crystal lattice image and; (c) its Fourier Transform pattern; (d,e) Two magnified images of the crystal lattice obtained from the diffraction spots by inverse Fourier transform, respectively for the nano-precipitate (indicated in yellow) and the austenitic matrix (indicated in turquoise blue). Nano-precipitates are highlighted in white in Fig. 1.a, revealing their greater atomic number Z compared to that of the matrix. This would suggest that these nano-precipitates are a Nb-rich phase. Based on TEM High Resolution microscopy combined to the Inverse Fourier Transform performed in a JEOL neoARM (Fig. 1), these nano precipitates are found to exhibit a faced centered cubic structure (fcc) with a lattice parameter a = 4.23 ± 0.15 Å, along with a cube-to-cube relationship with the austenitic matrix. These precipitates are thus identified as a non stoichiometric MX phase (M=Nb, Ti or Cr and X=N or C). Tensile tests are performed at room and at high temperature of 550 °C for the two materials. Associated mechanical properties are given in Table 2 in terms of: The 0.2% proof stress R p 0,2 , the ultimate tensile strength UTS and the maximum elongation A. The Nb-free alloy has a slightly higher tensile resistance and a superior yield strength than the Nb-rich one at both temperatures. This result can be partly related to a Hall-Petch effect (FEAUGAS and HADDOU, 2003) since the grain size of the Nb-free plate is twice smaller than the grain size of the Nb-rich alloy.

Table 2. Tensile mechanical properties of the studied materials at room and high temperature. Rolled plate Hot-forged material

Temperatures (°C)

20

550

20

550

R

! #,% (MPa)

300

163

250

127

UTS (MPa) A (%)

590

440

573

422

70

40

73

44

3. Creep behaviors and creep damages at 575 °C/310 MPa Creep tests are carried out at the temperature of 575 °C with an applied nominal stress of 310 MPa on cylindrical specimens prepared from the two studied materials. The associated creep curves are shown in Fig. 2. During the loading phase, a 14% tensile strain is applied for the Nb-rich specimen against only 8% for the rolled plate. Inversely, the creep strain to fracture developed during the constant stress phase of the test is drastically higher for the Nb-free alloy. The addition of Nb decreases significantly the minimum creep rate and extends the creep lifetime despite the reduced creep fracture strain. The fracture surfaces of the broken specimens are observed using ZEISS Sigma 300 SEM operating at 5 kV and for different magnifications. The observations are presented in Fig. 3. Different types of fracture are noticed for the two materials.