Transition of creep mechanism in nanocrystalline metals

Understanding creep mechanisms with atomistic details is of great importance to achieve the mechanical and thermodynamical stabilities of nanocrystalline (NC) metals over a wide temperature range. Here we report a molecular dynamics analysis of creep in NC copper dominated by competing deformation mechanisms. We found the dominating creep mechanism transits from grain boundary (GB) diffusion to GB sliding, and then dislocation nucleation with increasing stress. The derived stress exponent, small activation volume of $0.1-10b^3$, and grain size exponent all agree quantitatively with experimental values. We proposed a stress-temperature deformation map in NC metals accommodated by the competition among different stress-driven, thermally activated processes. The model is general to answer the question why deformation mechanism transits with stress in NC metals.

Figure 1

(a) Simulation model of the NC copper after thermal relaxation. Atoms are colored by centrosymmetry parameter,[22] which distinguishes the atoms in grain boundaries from those in a perfect face-centered cubic (fcc) lattice. The calculated creep curves at varying (b) stress and (c) temperature. The creep curves increase monotonically with increasing stress and temperature, respectively. (d) The natural logarithm plot of the determined steady-state creep rate $\ln \dot{\varepsilon }$ against stress and inverse temperature.

Figure 2

(a) Log-log plot of steady-state creep rate against stress. The experimental data from P. Huang et al. (Ref.17) are normalized according to our simulation conditions (Ref.25). (b) Log-log plot of creep rate against grain size at stress of 664 MPa and 1.66 GPa. The straight lines are linear fittings. The snapshots show (c) initial configuration before loadings, (d) 66 MPa, 300 ps, $\varepsilon =0.23\%$, (e) 166 MPa, 1000 ps, $\varepsilon =0.95\%$, and (f) 1.66 GPa, 30 ps, $\varepsilon =3.28\%$, respectively. (d) to (f) are within steady-state creep. Atoms are colored by centrosymmetry parameter,[22] which distinguishes those atoms in GBs and stacking faults. Detailed deformation mechanisms can be found in Movie 1(a)–1(c) in the Supplemental Material (Ref.18).

Figure 3

Dislocation nucleation from grain boundary and the left stacking fault during creep deformation at (1.66 GPa, 480 K), and 74 picoseconds. The grain size is 10.7 nm. Atoms are colored by centrosymmetry parameter,[22] while the perfect fcc atoms are not shown for clarity.

Figure 4

Natural logarithm plot of creep rate versus stress, and the red curve is the polynomial fit. The inset shows the activation volume as a function of stress.

Figure 5

(a) The proposed deformation map in the temperature-stress space for nanocrystalline copper showing competition among diffusion, grain boundary sliding, and dislocation nucleation creep. (b) $-\ln \dot{\varepsilon }$ versus stress for creep governed by three individual mechanisms, the present MD results is plotted for comparison. The deformation mechanism transition is driven by the competition among these stress-driven, rate controlling processes. Region I, II, III define the stress range where GB diffusion, sliding, and dislocation nucleation dominate creep, respectively.