Trans-twin dislocations in nanotwinned metals

The strength-controlling dislocation mechanism of a material can be illuminated by partition of the flow stress into its effective stress and back stress components. Recent experiments report a nearly constant saturated effective stress of about 100 MPa for nanotwinned Cu with a range of nanotwin thicknesses less than 100 nm [Z. Cheng et.al., Proc. Natl. Acad. Sci. U.S.A. 119 (2022) e2116808119]. This surprising result implies that the effective stress is controlled by the long dislocations spanning multiple nanotwin lamellae, termed trans-twin dislocations, rather than the commonly thought threading dislocations confined within individual nanotwin lamellae. Here we use molecular dynamics to simulate the formation and motion of trans-twin dislocations across multiple nanotwin lamellae. We show that the interconnected segments of a trans-twin dislocation can glide concertedly on the corrugated {111} slip planes in consecutive nanotwin lamellae with little resistance from coherent twin boundaries. Our results indicate that the finite resistance to slip transmission across coherent twin boundaries can set a lower limit of the effective obstacle spacing to hinder dislocation glide and thus dictate the upper limit of the saturated effective stress of nanotwinned metals. This work provides new understanding of the strength-controlling mechanism in nanotwinned metals.