Deformation Mechanisms in Pd Nanowhiskers
The reduced length scales inherent in nanoscale materials enable access to properties that are otherwise not achievable in bulk. The application of their novel structural and functional responses however is hindered by a lack of understanding of their mechanical behavior, which affects their assimilation into device fabrication as well as their reliability during performance. In contrast to bulk materials, nanoscale materials possess a non-negligible proportion of surface atoms, which can exert significant influence on the overall mechanical response. In addition, structures with small volumes can possess much lower defect densities, which could potentially be driven out of the volume instead of interacting and promoting traditional deformation behavior. Systematic experimental investigations will be crucial to developing the necessary understanding, although they remain challenging due to limited access to suitable test specimens and testing methodologies for directly extracting pertinent results. By employing a MEMS-based tensile testing system and a temperature-controlled cryostat configuration to test defect-free and -scarce Pd nanowhiskers, we have been able to systematically investigate some of the important deformation mechanisms in nanoscale single crystals. We first address the elastic behavior in nanoscale crystals, which is predicted to differ from bulk behavior due to the reduced coordination of surface atoms. We measured size-dependent deviations from bulk elastic behavior in nanowhiskers with diameters as small as ~30 nm. In addition to size-dependent variations in Young's modulus in the small strain limit, we measured nonlinear elasticity at strains above ~1%. In addition to providing the first measurements of higher-order elasticity in Pd, our study shows that the elasticity response in Pd nanowhiskers can be attributed to higher-order elasticity in the bulk-like core upon being biased from its equilibrium configuration due to the role of surface stresses in small volumes. Comparison of the size-independent values of δ in our nanowhiskers with studies on bulk FCC metals lends further insight into the role of length scales on both elastic and plastic mechanical behavior. We then consider incipient plasticity in nanoscale Pd nanowhiskers, which is governed not by the initial motion of pre-existing dislocations but rather the nucleation of dislocations. Whereas nucleation strengths are weakly size- and strain-rate-dependent, strong temperature dependence is uncovered, corroborating predictions that nucleation is assisted by thermal fluctuations. We measure activation volumes as small as singular atomic volumes, which explain both the ultrahigh athermal strength as well as the temperature-dependent scatter, evident in our experiments and well captured by a thermal activation model. Our experiments highlight the pronounced probabilistic nature of surface dislocation nucleation, which is crucial input to device design using nanoscale building blocks. In total, this body of work demonstrates that distinctly different processes are responsible for the deformation behavior in small volumes and underscores the importance of comprehensive characterization of material properties at the relevant length scales.
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