Length-scale-controlled fatigue mechanisms in thin copper films

Systematic investigations of fatigue damage and dislocation structures in thin Cu films with different thicknesses (0.2–3.0 μm) and grain sizes (0.3–2.1 μm mean diameter) were carried out using focused ion beam microscopy and transmission electron microscopy. The morphologies of fatigue-induced extrusions, cracks, and dislocation structures were studied and found to be controlled by film thickness and grain size. When either of these length scales is decreased below roughly 1 μm, the typical dislocation wall and cell structures found in fatigued coarse-grained bulk materials no longer develop and are replaced by individual dislocations. Similarly, the typical surface damage of fatigued bulk metals, such as extrusions and cracks near extrusions, is gradually suppressed and replaced by damage that is localized at interfaces, such as cracks, grooves, and voids along grain and twin boundaries. This gradual transition from damage characteristic of bulk metals to damage localized at interfaces is attributed to constraints on dislocation activity at submicrometer length scales. Based on the experimental results and a theoretical analysis of extrusion formation, a mechanistic map of fatigue damage behavior is proposed that summarizes this length scale dependence.