High-temperature deformation and fracture mechanisms of an advanced heat resistant Fe-Cr-Ni alloy

In this work creep-strain test ranging from 180 to 240 MPa at 973 K, 998 K, and 1023 K were conducted to investigate the high temperature deformation and fracture mechanisms for Sanicro 25 alloy. The relationship between minimum creep rate and applied stress followed Norton's law. Apparent stress exponent values of 8, 6.5, and 5 were obtained at 973 K, 998 K, and 1073 K, respectively, with an apparent creep activation energy value of 437.3-606.1 kJ/mole illustrating that the rate-controlled creep occur in Sanicro 25 during creep. The threshold stresses were 129.5, 111.5, and 82.0 MPa at 973 K, 998 K, and 1023 K, respectively. The true creep activation energy and the true stress exponent were found to be 271.6 kJ/mole and 3, respectively. The presence of nanoscale MX and Cu precipitates were observed and theoretical calculations of threshold stresses confirmed that shearing of the nano-Cu precipitates occurred at 973 K, while the dislocations climbed up MX precipitates at 973-1023 K. The apparent stress exponent obtained for creep rupture shows that similar mechanisms operate in creep deformation and rupture behavior for Sanicro 25. A damage tolerance factor of ~5 in the alloy indicates that the microstructural degradation such as coarsening of precipitate and subgrain structure is the dominant creep damaging mechanism in the alloy.