A unique technique to perform quantitative in situ transmission electron microscopy (TEM) fatigue testing on ultrathin films and nanomaterials is demonstrated. The technique relies on a microelectromechanical system (MEMS) device to actuate a nanospecimen and measure its mechanical response. Compared to previously demonstrated MEMS-based in situ TEM techniques, the technique takes advantage of two identical capacitive sensors on each side of the specimen to measure electronically elongation (with nm resolution) and applied force (with μN resolution). Monotonic and fatigue tests were performed on nanocrystalline gold ultrathin film specimens that were manipulated and fixed onto the MEMS device without the use of a focused ion-beam microscope (and therefore, importantly, without any associated surface damage). The major advantage of the technique is its capability to use TEM imaging solely for high magnification microstructural observations while the MEMS device provides continuous tracking of the material's response, thereby expanding the capabilities of MEMS-based techniques towards more complex in situ TEM nanomechanical tests, such as fatigue tests.

This study unveils the stress relaxation transient deformation mechanisms in 100 nm-thick, nanocrystalline Au films thanks to a robust quantitative in situ TEM MEMS nanomechanical testing approach to quantify stress relaxation and to perform in situ observations of time-dependent deformation in ultrathin nanocrystalline films. The relaxation is characterized by a decrease in plastic strain rate of more than one order of magnitude over the first ∼30 minutes (from 10(-4) to less than 10(-5) s(-1)). For longer relaxation experiments, the plastic strain rate decreases down to 10(-7) s(-1) after several hours. The power-law exponent n, relating plastic strain rate and stress, continuously decreases from initial large values (n from 6 to 14 at t = 0) down to low values (n ∼ 1-2) after several hours. In situ TEM observations reveal that the relaxation behavior is initially accommodated by highly localized, sustained, intergranular and transgranular dislocation motion. Over time, the dislocation sources become less operative or exhausted, leading to a transition to grain-boundary-diffusion based mechanisms. The results also highlight a promising technique for nanoscale characterization of time-dependent deformation.

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a b s t r a c tBased on the theory of elasticity, exact analytical and numerical solutions of piezoelectric rods under static torsion are studied. In this paper, direct solution method is used. The main scope is to check the extension of validity of assumptions in previous papers that had been made based on linear distribution of electric potential through the cross section and their influences on deflection and the angle of rotation. Stress and electric induction functions are employed to obtain the exact solution of the static and electrostatic equilibrium equations under torsional loading. It is shown that previous assumptions are valid only in some types of piezoelectric materials, while in other types these assumptions lead to considerable deviations from accurate modeling. The present analytical solutions are compared with three-dimensional finite element analysis results and absolute agreements are found. At the end of this article, torsional rigidity, shape-effects on induced piezoelectric deformation and the range of valid region for linear distribution of electric potential assumption have been studied.

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