FATIGUE FEATURES AND MECHANISM OF Al-7Si-0.3Mg CAST ALLOY UNDER NONPROPORTIONAL LOADINGS

Widely application of cast aluminum alloy requires an understanding of its cyclic deformation behavior which is material dependent, and it is a complex function of strain amplitude, loading path, etc. In this study, multi-axial fatigue tests were conducted on cast Al-7Si-0.3Mg alloy with the same equivalent strain amplitude of 0.22% under six multi-axial path loadings, which were proportional, circular, square, diamond, rectangle and ellipse paths. TEM was employed to investigate the dislocation structures of the fatigue failure specimens. Cyclic hardening dominates the whole fatigue process under every loading path, but the rate and extent of cyclic hardening are quite dependent on particular loading paths. The fatigue life under nonproportional loading is much lower than that under proportional loading, and it also depends on the various nonproportional loading paths. The specimen with circular path loading has the shortest life and the most severe cyclic hardening among all the loading paths. The continuously changing of direction of maximum shear-stress plane is attributed to the complicated dislocation substructures and severe stress concentration during the cyclic process. The interaction among dislocation, particle and cell boundary is the main reason for cyclic hardening. The structure and density of dislocation in fatigue failure specimens under various loading paths exhibit quite different. From double dislocation bands, multiple dislocation bands, labyrinth structure to cell structure, the dislocation mobility decreases and stress concentration degree increases. KEY WORDS Al-7Si-0.3Mg alloy, nonproportional loading, multi-axial fatigue, dislocation