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 parameters (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 deformation 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 nanograined 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.