Abstract
The lifetime of cyclically loaded devices is often limited by the fatigue resistance of their individual phases. An advanced method is presented for measuring the high-cycle fatigue behavior of materials at the micrometer scale using a nanoindenter. It is based on the cyclic deflection of focused ion beam-fabricated microcantilevers using the continuous stiffness method (CSM). In line with experimental data on bulk nanocrystalline copper, the specimens exhibit grain coarsening followed by the formation of extrusions and a fatigue strength exponent of −0.10. The method is suitable for characterizing single phases and individual components of further complex systems
Highlights
Many modern alloys consist of at least two different phases, such as ferrite and cementite in steel, or a layered architecture, such as in TiAl
Reliable cyclic methods are available for thin film testing[8,9,10] or for custom MEMS,[11] but the knowledge obtained from these specific objects cannot be extrapolated to any 3D structure
The low strain-hardening trend is consistent with the literature on equal channel angular pressing (ECAP) copper.[26,27]
Summary
Many modern alloys consist of at least two different phases, such as ferrite and cementite in steel, or a layered architecture, such as in TiAl. The top surface of the cantilevers remains in pure tension during cyclic testing, while a stress gradient reaching from tension to compression develops throughout their thickness, see Fig. 1. The low strain-hardening trend is consistent with the literature on ECAP copper.[26,27] The postmortem micrographs [Fig. 2(c)] show that the deformation is localized at the root of the cantilever, as the highest stresses are present there [see Eq (1)].
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