Revealing the Maximum Strength in Nanotwinned Copper

Abstract

The strength of polycrystalline materials increases with decreasing grain size. Below a critical size, smaller grains might lead to softening, as suggested by atomistic simulations. The strongest size should arise at a transition in deformation mechanism from lattice dislocation activities to grain boundary-related processes. We investigated the maximum strength of nanotwinned copper samples with different twin thicknesses. We found that the strength increases with decreasing twin thickness, reaching a maximum at 15 nanometers, followed by a softening at smaller values that is accompanied by enhanced strain hardening and tensile ductility. The strongest twin thickness originates from a transition in the yielding mechanism from the slip transfer across twin boundaries to the activity of preexisting easy dislocation sources.

The Impact of Organic Additives on Copper Trench Microstructure

“…These microstructural characteristics are desirable, and believed to be essential in reducing thermo-mechanical stresses in multi-material systems such as microelectronic interconnects without degrading electrical properties of the material. 6,7,9,28,29 …”

“Bulk” Nanocrystalline Metals: Review of the Current State of the Art and Future Opportunities for Copper and Copper Alloys

“…The line is used to demarcate the ductility versus strength trade-off for most nanocrystalline Cu studies in the literature 121,[135][136][137][138][139] (versus coarse-grain copper 140,141 ). However, several studies used microstructure design and consolidation to push the strength and ductility beyond this limit 134,140,143,144 Despite the ability to push the strength and ductility, these nanocrystalline and highly twinned microstructures still have problems with either thermal stability and/or scalability to bulk dimensions due to the processing method used.…”

“…The solid line shows the general trend between ductility and strength. However, several recent advances in microstructure design and consolidation are pushing the strength and ductility beyond this limit, e.g., bimodal nanocrystalline/coarse-grained Cu, 142 nanotwinned Cu, 143 ''artifact-free'' nanocrystalline Cu, 144 and cryomilled in situ consolidation of Cu. 134 Fig.…”

High-strength and thermally stable bulk nanolayered composites due to twin-induced interfaces

“…One important example of a successful application of grain boundary engineering 24 is the fabrication of nanotwinned copper, in which the high-strength microstructure contains nanotwins separated by finely spaced twin boundaries. Achieving peak strength relies upon stability of coherent twin boundaries and their ability to act as either effective barriers to dislocation motion or sources for dislocations [25][26][27] . The high stability of the coherent twin boundary also makes the nanotwinned structure thermally stable down to B10 nm in twin width and up to 400°C (ref.…”