PSI - Issue 23

Tomáš Babinský et al. / Procedia Structural Integrity 23 (2019) 523–528 Babinský & Pol ák / Structural Integrity Procedia 00 (2019) 000 – 000

526

4

Fig. 1. SEM micrographs of 713LC alloy cycled at 0. 8 % at 23 °C (SE contrast) . (a) Fracture surface, (b) surface relief.

b)

a)

2

2

1st cycle 2nd cycle 3rd cycle 10th cycle 50th cycle

1st cycle 2nd cycle 3rd cycle 10th cycle 50th cycle

-2/E eff 2 · d 2 σ r /d ε r 2 ∙10 3 [MPa -1 ]

-2/E eff 2 · d 2 σ r /d ε r 2 ∙10 3 [MPa -1 ]

1,5

1,5

1

1

0,5

0,5

0

0

0

400

800

1200

1600

0

400

800

1200

1600

Fictive Stress, ε r E eff /2 [MPa]

Fictive Stress, ε r E eff /2 [MPa]

Fig. 2. Second derivatives of 713 LC alloy. (a) Tensile half-loops, (b) compressive half-loops.

4.2. Rene 41

The structure of Rene 41 shown in Fig. 3a is different from that of most superalloys as Rene 41 contains no precipitates distinguishable in SEM. Only a random distribution of carbides and few defects with the size up to 10 μm were found. The absence of precipitates is most probably caused by the material state – it was not aged. The average grain size was 15 μm. Fatigue life of the specimen was 3862 cycles. The fracture surface has mixed character as shown in Fig. 3b; both ductile fracture and cleavage facets are present. An initial hardening (first 25 cycles) was followed by a slow, gradual softening. Fig. 4 shows plot of the second derivatives of both tensile and compressive half-loops. As a consequence of an almost one-phase structure only one peak is identified. That results in uniform, gradual deformation of the matrix after the effective stress component is overcome. The drop of the peak in the first 30 cycles is similar to 713LC alloy and can be explained as the shift of the internal critical stresses to higher values.