Mechanisms leading to nanometer-sized materials
Deformation induced grain refinement has been revealed in pure Cu by rolling at ambient and cryogenic temperature and low temperature annealing. In addition, Zn decreases the stacking fault energy from about 55 mJ/m2 (for pure Cu) to 14 mJ/m2 (Cu-30 wt.% Zn) and enhances the grain refinement during deformation. In this project, nano-/ultrafine-grained Copper and Copper alloys will be synthesized by rolling and low temperature annealing, in order to overcome dimension limitations of specialized critical mechanical tests for strength and ductility, which are poorly understood for such NC/UFG materials. Characterization of the microstructure will be performed down to the atomic level by thorough and systematic analysis. The mechanical properties of these materials will be studied under tension, compression and torsion. In-situ deformation studies under scanning and transmission electron microscopic observation will be performed to reveal the deformation mechanisms at different length-scales. The nano-/ultrafine structures will be produced by various processing routes, i.e. rolling/annealing at IFW Dresden, ball milling at North Carolina State University and equal channel angular pressing/high pressure torsion at the University of Vienna. These structures are then investigated in order to reveal the influence of processing induced structural defects, on the ease of grain refinement in alloys with different stacking fault energy and deformation mechanisms as a function of grain size and stacking fault energy. The influence of alloy composition and processing conditions, i.e. rolling temperature, severity of deformation, on nanostructure development and final grain size will be compared in these alloys with different stacking fault energy.
Recent results on the transition of dislocation slip towards twinning mediated plastic deformation by variation of temperature and stacking fault energy of heavily deformed face-centered cubic metals are presented by V. Subramanya Sarma, et al.: "Role of stacking fault energy in strengthening due to cryo-deformation of FCC metals" in Materials Science and Engineering A (2010), in press (DOI). The transition can be obtained by a comparison of the hardness after deformation at cryogenic temperature and room temperature. The maximum at intermediate stacking fault energies indicates the transition from slip to twinning mediated deformation by lowering the deformation temperature down to 77 K.
Normalized difference in hardness of various Copper alloys deformed at cryogenic and room temperature as a function of their normalized stacking fault energy.