The Dual Role of Twinning Deformation in a Magnesium Alloy up to Very High Cycle Fatigue Regime

In this work, push–pull cyclic loading tests were conducted on a LA42 alloy up to the very high cycle regime to understand the effects of internal defects and microstructural damage on the fatigue mechanisms. The S–N curve shows a bilinear pattern with fatigue crack initiation characterized by multiple internal crack initiation sites, interior shrinkage connection, and microstructural damage induced facet formation. It is found that apart from the basal slip, the twinning deformation played dual roles in microstructural damage and fatigue crack initiation. Under larger plastic deformations, twinning improved the strain compatibility between adjacent grains relaxing matrix damage, and most of the crack initiation occurred at interior shrinkages. In contrast, twinning proliferated with long life under low strains, which enhanced the interaction between dislocation slip and twinning, promoting the formation of interior facets under shear stress. The varied roles of twinning deformation in the magnesium alloy suggest that the traditional fatigue limit does not exist during long-term cyclic loading.