PSI - Issue 54

Magdalena Eškinja et al. / Procedia Structural Integrity 54 (2024) 123–134 M. Eškinja et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 4. Stress-strain curve for HMoS and LMoS corresponding to the specimens in H-charged (electrochemical charging) and H-uncharged condition. 3.2. Fracture surface observation Top surface observation of H-uncharged specimens after strain test performed in air for shows central circular zone with features of ductile fracture, intermediary zone with radial marks and outermost zone comprising shear lip for both, HMoS (5-a) and LMoS (5-c). At higher magnifications fracture surfaces reveal presence of deep dimples and micro-voids characteristic for ductile fracture. For H-charged specimens none of specimens manifested necking, demonstrating a brittle fracture. The fracture surface (5-f and 5-h) consists of quasi-cleavage with serrated markings and shallow dimple patterns. This originates from enhanced mobility of dislocations, where dislocation slip is enhanced and enhanced dislocation slip due to hydrogen happens [15,16]. No indication of intergranular fracture typical for enrichment of grain boundaries with hydrogen was found. The quasi-cleavage pattern with serrated markings suggests that HE mechanism was hydrogen enhanced-localized plasticity (HELP) [4,16].

Fig. 5. SEM images showing fracture surface of the uncharged: HMoS (5 a-b) and LMoS (5 c-d), and charged: HMoS (5 e-f) and LMoS (5 g-h) specimens.