Browsing by Author "Schneider, Andreas Simon"

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Open Access
Mechanical properties of small scale BCC metal structures
(2010) Schneider, Andreas Simon; Arzt, Eduard (Prof. Dr.)
Metals with a critical dimension in the micro- and nanometer regime have demonstrated a dramatic increase in strength with decreasing sample size termed the ‘size effect’. Understanding and quantifying mechanical behavior is an essential part of creating small-scale systems with advanced properties, and is possible using highly developed experimental testing techniques such as compression of small-scale metal pillars formed using focused ion beam (FIB) machining. Although the fundamental mechanisms which govern the size effect are not totally clear, a consistent scaling relationship for the flow stress with pillar diameter has been found for several metals. The majority of these studies have focused on metals with face-centered cubic (fcc) crystal structure, however, it is expected that studying nanoscale metals with different crystal structures could give insight into the dislocation processes involved in nanoplasticity. The work presented in this dissertation deals with the mechanical characterization of body-centered cubic (bcc) nanostructures by means of small-scale compression tests. Tungsten (W), molybdenum (Mo) and niobium (Nb) [001] and [235] oriented samples were obtained by electron discharge machining from high purity single crystals. After careful mechanical and electro-polishing, compression pillars were machined with a FIB on the surfaces. Pillars with diameters ranging form 200 nm to 6 µm were tested in compression to study the influence of pillar diameter on the strength of bcc metals. In general, it was found that bcc metals have a weaker size dependence than fcc metals. Further, the strength and the size scaling of bcc metals were observed to be strongly related to their respective critical temperature to test temperature ratio (Ttest/Tc). For bcc metals with a high critical temperature high strengths and weak size dependence were measured, whereas those with a low critical temperature showed low strengths and strong size dependence. Because Ttest/Tc is a measure for the mobility of screw dislocations in bcc metals, it was concluded that the size dependent deformation of bcc pillars is likely explained by considering the temperature dependent mobility of screw dislocations. The thermally activated motion of screw dislocations at micron- and submicron scale was further investigated by testing over a wide range of loading rates. The strain rate sensitivities were close to bulk values and the calculated activation volumes were in the range of 2b³ to 9b³ corresponding to expected values for thermally activated kink-pair nucleation on screw dislocations. Additionally, compression tests were performed on pre-strained Mo pillars. It was shown that the pre-straining has no influence on the mechanical properties if the pre-strained pillars are re-shaped with the FIB prior to testing. This suggests that the defects introduced into the material by the FIB serve as dislocation sources, which control the mechanical behavior of the pre-strained pillars.