Microstructure and temperature effects on (sub-)micron plasticity of bcc metals

Although body centered cubic (bcc) metals resemble the most common structural materials, the dislocation mechanisms governing the size dependent mechanical properties in small dimensions are not well understood. No direct high resolution observations of the deformation processes are available, and existing studies are contradicting. Further, nothing is known about the influence of temperature and friction stress on the size dependent mechanical properties of miniaturized bcc samples. Finally, besides a pronounced temperature dependence of the mechanical properties, these materials macroscopically ehxibit a temperature dependent brittle- to-ductile transition, the occurrence of which has fatal consequences in the case of failure. However, there are no insights regarding this aspect in small dimensions. This is because worldwide there exist no equipment to perform such quantitative miniaturized tests at elevated temperatures while simultaneously permitting observation of the deformation mechanisms with high resolution, which would be required to answer such scientific problems. In this project, the temperature dependent strength and fracture properties of the bcc metals Vanadium, Chromium, and Tungsten will be investigated for sample sizes from less than 100 nm up to 10 m. Quantitative tensile tests will be performed in situ inside high resolution electron microscopes. Direct observation during sample loading down to the atomistic scale will contribute to a thorough understanding of the deformation and fracture mechanisms and related size effects in such small dimensions. To account for the temperature dependence of the investigated materials, novel heating devices for the loading equipment employed in the electron microscopes will be developed. This will, for the first time, allow to directly investigate the effect of thermal activation on the plastic deformation mechanisms of bcc metals, the effect of sample size on the temperature-dependent fracture toughness, and on the brittle-to-ductile transition in small dimensions. These quantitative in situ experiments, in conjunction with a detailed characterization of the deformed samples and accompanying computer simulations, will be used to develop new mechanism-based models capable of predicting the temperature and size dependent strength and fracture toughness of bcc specimens with dimensions in the micron or nanometer regime.

Refractory metals with body-centered cubic (bcc) crystal structure find broad usage, spanning from high-temperature application via the chemical industry to microelectronics. To improve their properties, understanding localized plasticity and the influence of interfaces is of prime concern. However, a comprehensive scientific understanding regarding the influence of size effects on crystal plasticity of bcc metals is still lacking. In this project, based on unique miniaturized in-situ experiments, the elementary deformation processes were identified, upon which novel material models were developed to close this knowledge gap. Especially the temperature-dependent, thermally activated contribution to the flow stress of bcc metals and its influence on the strength scaling behavior was examined for two typical representatives, namely Chromium and Tungsten. A realistic and more complex situation arises upon addition of interfaces to the material. Here the contribution of internal boundaries to the size effects in plasticity was examined at different length scales and for various temperatures and deformation rates. The present investigations identify microstructural mechanisms that contribute to size dependent behavior in bcc metals, as well as contributions from free surfaces. In conclusion, it was shown that at low temperatures deformation of single crystalline as well as ultra-fine grained specimens is limited by thermally activated movement of screw dislocations. Above a material specific critical temperature, the interaction between dislocations and interfaces is the rate controlling process.