PSI - Issue 23

Jaroslav Polák et al. / Procedia Structural Integrity 23 (2019) 275–280 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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(a)

(b)

Fig. 1: Cyclic hardening/softening curves in constant total strain amplitude cycling at temperature 700 °C ; (a) cycling without dwells (b) cycling with 10 min tensile dwells.

Fig. 2: Manson-Coffin fatigue life curves at 700 °C for constant strain rate cycling (2 × 10 -3 s -1 ) and cycling with 10 min tensile dwell.

In order to find the source of the lower resistance of the material to high temperature cyclic loading with dwells we have inspected the surface of the specimens cycled at 700 °C without dwells and with dwells. Originally polished surface is during high temperature cyclic straining highly oxidized. Fig. 3a shows the surface of the specimen cycled without dwells until fracture and Fig. 3b the surface of the specimen cycled to fracture with dwells in maximum tension. The surface is in both cased covered by a thin layer of the oxide. Grain boundaries are oxidized much more than interior of the grains and the oxide is protruding on the surface. The extruded oxide in grain boundaries perpendicular to the loading axis is mostly cracked. Some grain boundary cracks in the specimen cycled with dwells are more open than those in cycling without dwells, some cracks running in the bulk at an angle around 45 degrees start sliding and look like wedge cracks. The oxide is cracked perpendicular to the loading axis also in locations outside the grain boundaries.