Size Effects On The Mechanical Behavior Of Small-sized Materials And The Discrete Dislocation Dynamics Mechanisms

Recently, microelectromechanical systems (MEMS) are widely used in many industrial areas, including micro-electronics, communication, medicine and manufacturing. On the one hand, these micro-systems have greatly facilitated our lives. On the other hand, they are also accompanied by a lot of unprecedented opportunities and challenges. In these micro-systems, a lot of micro-components, such as thin films, wires, micro-beams and micro-pillars, are utilized. The characteristic geometrical size of those components is at the same scale as that of micro-structures (i.e. grains). At this scale, the constraint from grains leads to significant size-dependent behavior. In addition, those micro-components are subjected to cyclic loadings at work. The coupling effect of size effect and cyclic response makes their mechanical behavior more complicated. Accordingly, the investigation on the size-dependent and cyclic behaviors of materials at the micron scale is significant for both the academia and industry. Motivated by this background, this thesis has performed some research on small-sized materials as follows:1. Two dimensional discrete dislocation dynamics (2D-DDD) on polycrystals was used to study the size-dependent behavior of polycrystalline thin films. Three types of thin films were comparatively considered, including the ones without surface treatment, with surface passivation layers (SPLs) of nanometer thickness and with surface grain refinement zones (SGRZs) consisting of nano-sized grains. The results show that the flow stress of the first thin film increases significantly with the increasing film thickness. However, the latter two films display the opposite size effects, i.e.’the smaller the stronger’. The dislocation configures in thin films reveal that these different size-dependent behaviors are originated from the competition between the exterior surface-constraint and interior grain boundary (GB)-constraint on gliding dislocations, In the first film without surface treatment, compared with the interior grains, the surface grains are easily deformed. As a result, with the increasing thickness, the decreasing percentage of surface grains results in the size effect’the bigger the stronger’. However, in the latter two films, the surface grains are difficult to deform, which is just opposite with that in the first film. Therefore, such opposite surface grains lead to the opposite size dependent behavior. It is seen that the influence of surface treatment on the mechanical behavior of small-sized materials cannot be ignored.2. Cyclic relaxation behavior severely affects the fatigue life. The cyclic relaxation behavior of ultrafine-grained thin films and the intrinsic dislocation mechanisms were explored. Three different specimens are comparatively modeled, including single crystalline film, polycrystalline ones with penetrable and impenetrable GBs. It is observed that either single crystalline film or polycrystalline one with impenetrable GBs shows weak cyclic relaxation response. However, the polycrystalline film with penetrable GBs displays remarkable cyclic relaxation behavior, although no dislocation cells and persist slip bands exit in the ultrafine-grains. By comparison, it can be concluded that this cyclic behavior is closely relate to the penetrability of GBs. Besides, the influence of the applied cyclic strain wave mode on the cyclic behavior is also discussed. The mean strain has a negligible impact on the relaxation of normalized mean stress (NMS), but the influence of strain amplitude could not be ignored, i.e. a faster relaxation of NMS is usually resulted from larger strain amplitude.3. In order to investigate the mechanical property of hollow components,3D-DDD was used to study the size-dependent behavior of hollow micro-pillars. It was found that although the yield stress increases with increasing inner diameter and decreasing outer one, actually, it is mainly controlled by the wall thickness. This is because the yield behavior is dominated by the single-arm sources, which lengths are determined by the wall thickness. In order to quantitatively depict the relationship between the wall thickness and yield stress, a single-arm source model is deduced and well fits our computational results.4. Intermittent phenomenon is disadvantageous for the measurement of mechanical property of small-sized materials. To restrain or even eliminate such phenomenon, the2D penetrable GB model was extended and integrated into3D-DDD framework to investigate the stress drops in single-and bi-crystals. Results reveal that the single-crystalline pillar displays few large stress drops. However, many small ones are seen in the bi-crystalline pillar. This difference is associated with the GB, which acts as dislocation barrier and source.