PSI - Issue 52

Ivo Šulák et al. / Procedia Structural Integrity 52 (2024) 143–153 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 7. a) Stress exponents for bulk Alloy 400 at 830 °C and 650 °C ; b) Activation energies for bulk Alloy 400 crept at 80 MPa and 60 MPa. The strain rates are taken at the true strain of 5 %.

The five tests at 830 °C give a stress exponent n = 6.1 (Fig. 7a), whereas the tests at constant stresses of 60 MPa and 80 MPa give activation energies of 304 kJ mol⁻¹ and 360 kJ mol⁻¹, respectively ( Fig. 7b). These data have been obtained from the strain rates at 5 % true strain. Because the strain rates in the first batch highly surpass the values plausible for practical applications, the remaining tests have been carri ed out at (650 ± 50) °C. The results for five tests at 650 °C with applied stresses from 50 to 150 MPa show significantly lower steady- state creep rates between 10⁻⁸ s⁻¹ and 10⁻⁵ s⁻¹, see Fig. 8. The tests with lower applied stresses (50 and 75 MPa) also show a sharp minimum in the strain rate during the primary stage. The tertiary phase quickly ends with fracture.

Fig. 8. Creep curves for bulk Alloy 400 at 50- 150 MPa and 650 °C.

The true strains achieved span from 4 % (75 MPa) to 8 % (150 MPa). Apart from the tests at higher temperatures, the true strain achieved tends to increase with the applied stress. Also, the time to fracture is longer, from 4 hours for 150 MPa to 196 hours for 75 MPa and 1245 hours for 50 MPa. The stress exponent n = 4.6 is slightly lower (Fig. 7) than for higher temperatures. Strain rates at 5 % true strain were used. For strain rates at 2 % true strain, the stress exponent for the bulk Alloy 400 is n = 5.7 (Fig. 9a). Two more tests at 600 and 700 °C and a constant applied stress of 125 MPa allowed to obtain the activation energy – 380 kJ mol⁻¹ (Fig. 9b). These data have been obtained from the strain rates at 2 % true strain.