Size effect on tensile creep behavior of micrometer-sized single-crystal gold

In order to clarify the size effect on the tensile creep properties of micrometer-sized single-crystal Au at room temperature, we conducted long-term (up to 14 h) creep experiments for two sets of specimens of approximately 0.5 and 1.5 μm in size, and they were observed with in situ field-emission scanning electron microscopy (FESEM). We directly measured the specimen elongation from the FESEM images to eliminate the measurement error in the displacement sensor owing to thermal drift, which ensured accurate creep strain evaluation. On one hand, at high stress range, where the stress σ was close to the yield stress σY (i.e., σ/σY ≈ 1), the creep strain continuously increased, and the creep curve consisted of typical transient and steady-state creep regions for both specimens of sizes ∼0.5 and ∼1.5 μm. On the other hand, at low stress range (σ/σY≈ 0.8), the creep was mainly induced by intermittent strain bursts, which were not observed in bulk metal. Thus, the creep behavior transitioned from continuous to discrete as the applied stress decreased. The smaller specimens required higher stress to reach a strain rate on the order of 10−7-10−6 s−1, indicating that the resistance to creep deformation increased with the decrease of specimen size. The “smaller is stronger” trend presented in the long-term creep deformation in the size range of ∼0.5-1.5 μm. This finding implies that such small crystals can sustain high stress over a long period and can be used as elements of micro devices.