Atomistic Simulations of the Enhanced Creep Resistance and Underlying Mechanisms of Nanograined-Nanotwinned Copper

Low-excess energy twin boundary can effectively stabilize the microstructure to enhance the mechanical-thermal stability. In this work, a series of multi-temperature (300 K-800 K) creep tests at different sustained stress levels (0.2 GPa-2.0 GPa) was conducted by atomistic molecular dynamic simulations on twin-free nanograined Cu (grain size between 13.5 and 27 nm) and nanograined-nanotwinned Cu (grain size of 13.5 nm with twin thickness ranging 1.25 nm-5 nm), respectively. It is evident that the nanograined-nanotwinned structure can significantly enhance creep resistance relative to twin-free nanograined counterparts. Based on the classic Mukherjee-Bird-Dorn equation, the multi-temperature creep tests allow us to define and obtain the creep pa-rameters (e.g. activation energy, activation volume, pre-stress exponent, and grain size/twin thickness exponent) and thus further build up the formula to describe the characteristic sizes (grain size/twin thickness)-, time, stress-, and temperature-dependent creep behaviors and corresponding plastic deformation mechanisms, which are also validated via the examination of atomic configurations, statistical analyses, and the summarized creep defor-mation maps. For all measured creep mechanisms, the positive grain size exponents (0.64, 0.74, and 5.80 in three linear characteristic regions) show that refining grain has a deleterious influence on creep resistance in nano -grained Cu, whereas the corresponding negative twin thickness exponents (-0.33,-0.92, and-3.38) suggest that creep performance is effectively enhanced with the decrease of twin thickness in nanograined-nanotwinned Cu. This work deepens the understanding of creep performance in nanostructured metals via nanotwinning.