Synthesis and mechanical behavior of highly nanotwinned metals

Metals containing high densities of nanoscale growth twins have shown potential as an alternative to nanocrystalline (nc) metals. Some reports on nanotwinned (NT) Cu, for example, have revealed that high strengths, comparable to nc metals, can be produced while improving other properties such as ductility, thermal stability, and mechanical stability. However, since the synthesis of highly NT metals is not yet fully understood, most studies have been limited to only NT-Cu; therefore, it is unclear if similar trends in the mechanical performance of NT-Cu can be extended to other NT systems. In addition, the majority of the work on determining the mechanical properties and deformation mechanisms of NT metals has been produced using nano-indentation; consequently, the larger-scale deformation mechanisms (i.e. those observed using mum to mm scale mechanical testing) of NT metals is still relatively unexplored. In this study, the synthesis of a variety of NT metals was investigated using magnetron sputtering, wherein the deposition conditions were systematically changed in order to produce highly columnar grained NT Cu, Ag, and Cu-6wt%Al (CuAl). The Cu and Ag served as representatives for moderate to low stacking fault energy (SFE) pure fcc metals, respectively, while the CuAl allowed for the study of NTs in low SFE alloyed fcc systems. It was observed that the relatively low SFE of these materials facilitated NT growth during sputtering. In addition, magnetron sputtering enabled the fabrication of relatively thick (26-170 mum) free-standing foils with low initial defects, internal stresses, and dislocation densities. The mechanical performance of the NT metals was evaluated using multiple testing techniques under various conditions, including tensile tests and micro-indentation. The NT microstructures and deformation behaviors were evaluated using advanced characterization methods, including focused ion beam (FIB) and high energy microdiffraction (HEMD). Tensile results of the columnar grained NT-Cu tested at room temperature (RT) and 77K showed increases in both strength and ductility at 77K, compared to the foils tested at RT. Additionally, the deformation within the foils was observed to be primarily by shear banding at both testing temperatures. FIB and HEMD analysis led to the formulation of possible mechanisms leading to the shear band formation and the observed increase in ductility at 77K. The mechanical stability of the columnar grained NT structure was also examined under the different loading conditions. The Ag and CuAl foils were synthesized with varying volume fractions of NT columnar grains, ranging from 0% to 100% vol%. Results of tensile tests and Vickers micro-indentation for the Ag foils showed that the addition of NTs led to enhanced strengthening, similar to the NT-Cu foils; however, the ductility in the foils decreased with increasing volume fraction of NT grains. Overall, the deformation and NT stability within the 100% NT foils were consistent with the results of the NT-Cu. Conversely, the CuAl showed a general insensitivity to the presence of NTs, in which minimal differences in the mechanical properties between NT-CuAl and non-NT CuAl foils were found. Tensile tests and Vickers micro-indentation revealed similar yield strengths, hardness, and % elongation between the NT and non-NT CuAl. Plastic deformation was limited in both sets of CuAl foils, in which the samples showed semi-brittle fracture during tension tests and a general propensity for plastic instabilities and cracking.