[110] tensile testing of single crystalline gold thin films with nanotwins: In situ TEM and XRD studies

Tensile tests of 50-nm-thick single-crystalline $(001)$ gold thin films containing nanotwins are monitored in situ with x-ray diffraction (XRD) and transmission electron microscopy (TEM). It is argued that for uniaxial tensile tests along the $[110]$ direction, the induced average stress in the films during loading hardly depends on whether they are deposited onto a polymeric substrate (for XRD) or free-standing (for TEM). When straining along this direction, XRD experiments show an impressive ≈500% increase in the twin volume for two favorably twin orientations at an applied strain of only $4.0%$. Furthermore, the twin volume increase is accompanied by a twin rotation. Submicron scale TEM observations show that the increase in volume is correlated with the glide of partial dislocations, either along the coherent twin boundaries (TB) or at the incoherent TBs, or close to microstructure imperfections. In the last case, twins coalesce and acquire a micrometer size. An intense dislocation activity occurs in these large twins, still increasing the strain concentration and resulting in crack expansion. The multiplicity of plasticity mechanisms suggests that the twinning quantification may significantly depend on the initial defect type (twins, dislocations, nanometric holes) and density. The twin rotation is also observed postmortem in TEM. However, the XRD data analysis shows that twinning is not the only plasticity mechanism: first, the yield point of the film supported on a substrate is ≈0.5%, but the twinning starts only at $1.0%$, and, second, at an applied strain of $4.0%$, the plastic strain induced by the twin is less than $0.8%$.