PSI - Issue 51

Lucia Pastierovičová et al. / Procedia Structural Integrity 51 (2023) 135 – 140 L. Pastierovi č ová et al. / Structural Integrity Procedia 00 (2022) 000–000

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Regarding the character of the curves of individual alloys, the results of corrosion fatigue tests correlate with the fatigue tests carried out on experimental alloys without the influence of the corrosive environment, see Fig.3. The highest corrosion fatigue life is in the alloys with the lowest Fe content.

Fig. 3. S-N curves for A357.0-T6 in 3.5% NaCl solution.

Since it is not possible to present all the results of fractographic analysis after fatigue testing in this research work, only representative samples were selected. A macroscopic view of the fracture surface documented in Fig. 3 is used to identify the crack nucleation sites. Surface and subsurface casting defects - primarily pores, were identified as crack initiation sites. The fatigue fracture surface is macroscopically relatively smooth and characterized by visible signs of crack propagations. The final fracture is significantly coarser and the surface roughness is higher. With increasing stress amplitude and increasing Fe in the alloy, the number of crack initiation sites increased too, see Fig. 4c.

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Fig. 4. Macroscopic view of fatigue fracture surfaces of alloy A (0.128 % Fe); (a) σ a = 68 MPa; (b) σ a = 78 MPa;(c) σ a = 88 MPa, SEM.

Microfractographic evaluation of the fracture surfaces of alloys with different iron contents did not show significant changes in the micromechanisms of failure as a function of the magnitude of the stress amplitude. The area of the casting defects is characterized by transcrystalline fatigue fracture of the Al matrix with smooth areas identified as Fe rich and Mg-rich intermetallic phases or Si particles, see Fig. 5a.