Fig. 1. (a) Schematic of the cantilever sample geometry. (b) Schematic of the dynamic bending fatigue method to perform the fatigue tests. R is the curvature radius of the bent sample.

Fig. 2. Microstructures of as-deposited films: (a) Au film, TEM cross-sectional view, (b) Au/Ti film, TEM cross-sectional view, (c) In-plane grain size distributions of as-deposited Au and Au/Ti films, (d) HRTEM observation of the Au/Ti interface structure. Different crystallographic planes are indicated by the solid lines. The low angle grain boundary (LAGB) with a misorientation angle ～3° in the Au layer is indicated by the chain-dotted line. The Au/Ti interface is indicated by the dashed line.

Fig. 3. (a) Relation between critical failure strain amplitude (Δεc/2) and the applied loading cycles. Here, the magnitude of the error bars for Δεc/2 in Au and Au/Ti are similar under the same applied loading cycles, but they are compressed in the logarithmic coordinates, especially at larger values at the y-axis, which makes the error bars for the Au/Ti appear smaller in Fig. 3(a). (b) Relative resistance increase (R/R0) versus the applied tensile strain in the Au and Au/Ti films during the uniaxial tensile test. The deviation from the ideal electrical curve indicated by the dashed line correlates with the fracture onset during the test.

Fig. 4. SEM observations of the surface morphology after cyclic loading: (a) Au film after 105 cycles at Δε/2≈0.58 %, (b) Au/Ti film after 105 cycles at Δε/2≈0.61 %, (c) Au film after 108 cycles at Δε/2≈0.28 %, (d) Au/Ti film after 108 cycles at Δε/2≈0.35 %. The slip bands are indicated by the dashed lines. Insets in the right corner in (c) and (d) are close observations of intergranular cracks, respectively. The scale length in the insets is 100 nm.

Fig. 5. EBSD orientation maps of the cracked zones for (a) Au and (b) Au/Ti films after 105 cycles, indicating abnormal grain growth occurred in the cracked area.

Fig. 6. Cross-sectional view of film-substrate interface voids: (a) after 108 cycles in Au film, (b) after 105 cycles in Au/Ti film, (c) after 108 cycles in Au/Ti film. Here, the extrusions with similar width around the location corresponding to the critical strain were chosen to give a reliable comparison.

Fig. 7. Measured extrusion height in areas at the critical strain amplitude in Au and Au/Ti films.