Investigation of Temperature and Feature Size Effects on Deformation of Metals by Superplastic Nanomolding

We report a novel method to introduce feature size into the prevalent deformation-mechanism map by the superplastic nanomolding technique. This new map enables various deformation mechanisms to be decoupled and allows for experimentally identifying the boundary between dislocation and diffusion dominated deformation regimes. Moreover, the proposed method provides a practical way to investigate the temperature effect on the mechanical properties of materials at small scales. As an example, the size-temperature-deformation mechanism map of gold is first determined by the proposed method. We found that the well-known Hall-Petch effect significantly weakens as the temperature increases. Besides, a transition from dislocation to diffusion dominated deformation regimes as the temperature increases is unambiguously revealed in the map, and the transition temperature is determined to be $\sim 0.54T_m$ for gold.

Figure 1

Superplastic nanomolding with crystalline metals. (a) Sketch of the experimental setup of SPNM. (b) SEM image of Bi nanowires prepared by SPNM with Bi at $285°\mathrm{C}$ ($\sim 1.02T_m$). (c) SEM image of Au nanorods prepared by SPNM with Au at about $-60°\mathrm{C}$ ($\sim 0.16T_m$).

Figure 2

Deformation mechanisms in SPNM. (a) Sketch of a metallic nanorod grows by materials creeping into a nanocavity, where the solid circles represent metallic atoms and the highlighted solid circles indicate atoms with high activity. Considering that the interested cavity size is usually far smaller than the grain size of bulk metals [25], thus the metallic nanorod can be viewed growing from one single crystal. (b) For high temperature nanomolding, the metallic nanorod grows by diffusion of atoms through either the lattice or the metal-mold interface. (c) As for LT-SPNM, the take-over deformation mechanism becomes dislocation based, then the metallic nanorod grows through dislocation nucleation and propagation. (d)–(e) Decoupling of deformation mechanisms through the introduction of the size of the mold cavity [Eqs. (2)–(4)].

Figure 3

Size-temperature-deformation map of Au. (a)–(b) SPNM with Au at $0.39T_m$ and $0.71T_m$, respectively, where molds with cavity size of $\sim 30$, 90, 300, and 500 nm were used. The forming pressure ($\overline{\sigma }$) is defined as the loading force divided by contact area. The open and solid squares stand for the success and failure of nanomolding, respectively. The dashed lines are given as guides to the eye.

Figure 4

Identification of the temperature transition from dislocation to diffusion dominated deformation in SPNM with Au. The dots are obtained from experiments, where Au discs (with similar sizes) were nanomolded under the same pressure and processing time. The aspect ratios ($L/d$) were calculated based on the measured length and diameter of Au nanorods at the center region of each sample. The error bars are defined as s.d. and obtained by measuring at least 10 nanorods in each sample. The solid lines are obtained by the least square fitting of data with the exponential function.