Atomistic simulations reveal shape memory of fcc metal nanowires

We have previously reported the discovery of a shape memory effect (SME) in single-crystalline fcc Cu nanowires [Nano Lett. 5, 2039 (2005); J. Eng. Mater. Technol. 127, 423 (2005)]. This paper reports that the same SME also exists in single-crystalline fcc Ni and Au nanowires with lateral dimensions below 5 nm. Under tensile loading and unloading, these Cu, Ni, and Au nanowires can recover elongations of up to 50%, well beyond the recoverable strains of 5-8% typical for most bulk shape memory alloys. Results of atomistic simulations and evidences from experiments show that this phenomenon only exists at the nanometer scale and is associated with a reversible crystallographic lattice reorientation driven by the high surface-stress-induced internal stresses at the nanoscale. This understanding also explains why these metals do not show an SME at macroscopic scales. The analysis also focuses on the role of twinnability in affecting this phenomenon. It is found that fcc metals with higher tendency for twinning (such as Cu, Au, Ni) show this behavior and fcc metals with low likelihood of twinning (such as Al) do not show this behavior.