Fundamental Differences in Mechanical Behavior between Two Types of Crystals at the Nanoscale

We present differences in the mechanical behavior of nanoscale gold and molybdenum single crystals. A significant strength increase is observed as the size is reduced to 100 nm. Both nanocrystals exhibit discrete strain bursts during plastic deformation. We postulate that they arise from significant differences in the dislocation behavior. Dislocation starvation is the predominant mechanism of plasticity in nanoscale fcc crystals, while junction formation and hardening characterize bcc plasticity. A statistical analysis of strain bursts is performed as a function of size and compared with stochastic models.

Figure 1

(a) Au $⟨001⟩$-oriented pillar before compression with initial diameter of 241 nm. (b) Mo $⟨001⟩$-oriented pillar with initial diameter of 205 nm. (c) Midsection of a representative gold pillar after compression showing symmetric double slip. (d) Same Mo pillar after compression. All images are shown at a 52° tilt angle. Scanning electron microscope parameters: HV, high voltage; TLD, through the lens detector; WD, working distance; Acc. V, accelerating voltage.

Figure 2

Some representative stress vs strain curves for (a) fcc Au and (b) bcc Mo nanopillars. Some Mo and all Au pillars were intentionally unloaded and reloaded several times throughout the experiment. Numbers adjacent to each curve represent the initial diameter of the pillar.

Figure 3

The log-log plot of flow stress as a function of initial diameter representing the scaling laws for Mo and Au. The slope of strengthening in gold is nearly $2\times$ higher than that for molybdenum.

Figure 4

Statistical distribution of probability of a slip event of a given size vs displacement (normalized by the Burgers vector). Experimental results from this papers are compared to those previously reported as shown on the graph.