Failure properties of nanocrystalline FCC metallic ultrathin films

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Hosseinian, Ehsan
Pierron, Olivier N.
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This dissertation investigates the failure properties of nanocrystalline (NC) metallic ultrathin films. A unique MEMS-based experimental technique is presented to perform quantitative in situ transmission electron microscopy (TEM) uniaxial mechanical testing on ultrathin films. This work is the first in using a MEMS to: (1) investigate the monotonic and fatigue properties of NC ultrathin gold films. The 100-nm-thick, 1.5-micro meter-wide NC Au specimens have a tensile strength of ~1-1.13 GPa, and a total elongation to failure of ~2.8-5%. The specimens exhibit a ratcheting behavior under tension-tension cyclic loading in near stress-controlled conditions. Nanoscale fatigue cracks were observed after nearly 7000 cycles for maximum stress of ~0.8 GPa. (2) Investigate the viscoplastic behavior of NC gold thin films by performing in situ stress relaxation experiments. Upon successive relaxation segments, the power-law exponent n continuously decreases from initial large values (n from 6 to 14 at t = 0) to low values (n ~1-2) if the test lasts for several hours. The in situ TEM results show that the transient relaxation behavior under large stresses is accommodated by sustained dislocation motion, either intergranular or transgranular, only in a few larger grains that are favorably oriented for maximum shear in the main glide plane. Over time, the dislocation sources become less operative as a result of the decrease in applied stress or exhausted, likely leading to a transition to a GB-diffusion based mechanism. (3) Study crack growth behavior for two types of NC gold films with different thickness and grain size distribution. For the 100-nm-thick films, there exist both intragranular and intergranular deformation mechanisms. Based on in situ TEM observation, dislocations flow along the GBs, leading to GB sliding. The initiated sharp cracks blunt and grows slowly along the GBs during the stress relaxation segments without requiring to increase the stress even after tens of minutes. In contrast, for 30-nm-thick film, intergranular crack grows mainly via the growth of nanocracks that occurs due to GB decohesion ahead of the main crack and that eventually coalesce with the main crack. The outcome of this research significantly advances the fundamental understanding regarding the mechanical reliability of NC metallic thin films.
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