PSI - Issue 52

Long Jin et al. / Procedia Structural Integrity 52 (2024) 12–19 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3.4. Fracture Observation Fig. 9 presents the fracture morphologies after low cycle fatigue tests at 0.5% strain amplitude of as-received and aged specimens. It can be clearly seen the multiple crack initiation, propagation and final fracture area. In addition, the aged specimen has more compact crack propagation and a larger percentage of the final fracture area. This seems to be attributed to the shorter fatigue life after aging. The magnified views of the crack initiation area can be observed in Fig. 9c and Fig. 9d. The cracks initiated on the surface of the specimen propagate with obvious river patterns, but the river patterns after aging meander between more uneven fracture and accompanied by various visible secondary cracks in Fig. 9d. The fatigue striations in crack propagation after aging in Fig. 9f are sparser with more secondary cracks between adjacent striations, indicating higher crack propagation rate and embrittlement of aged material. It appears that for aged condition, the embrittlement of the material leads to the presence of micro-crack independently in various positions and planes, while the main crack initiating on the surface meets and merges with these micro-cracks along propagation path, leading to premature failure.

450 ° C 1000h

As-received

(a)

(b)

Crack initiation

Crack initiation

(c)

(d)

Secondary crack

(e)

(f)

3μm

3μm

Fig. 9. SEM images of overall fracture surfaces (a) , (b) , crack initiation area (c) , (d) ,and fatigue striations (e) ,(f) of as-received ( N f =6880 cycles) and aged specimens ( N f =3848 cycles) at 0.5% strain amplitude, respectively The fracture depicted in Fig. 9 was cut in half along the axial direction to study the propagation of secondary cracks. The observed SEM pictures are presented in Fig. 10. PF is polygonal ferrite, BF is bainitic ferrite and GB is grain boundary. The carbides are pinned to the grain boundaries or distributed within the grains, while the carbides seem to be disperser and coarser after aging. It is noteworthy that the cracks are both initiated around the carbides in the grain and propagate with transgranular fracture for as-received and aged specimens. This means that the incompatible deformation of the carbides as a