Effects of focused ion beam induced damage on the plasticity of micropillars

The hardening effects of focused ion beam (FIB) induced damage produced during the fabrication of micropillars are examined by introducing a surface layer of nanosized obstacles into a dislocation dynamics simulation. The influence of the depth and strength of the obstacles as a function of pillar diameter is assessed parametrically. We show that for a selected set of sample sizes between 0.5 and $1.0\mu \text{m}$, the flow strength can increase by $10–20\%$, for an obstacle strength of 750 MPa, and damage depth of 100 nm. On the other hand, for sizes larger and smaller than this range, the effect of damage is negligible. Results show that the obstacles formed during the FIB milling may be expected to alter the microstructure of micropillars, however, they have a negligible effect on the observed size-strength scaling laws.

Figure 1

(Color online) Microstructure of a $D=0.75\mu \text{m}$ micropillar at 0.17% strain. (a) No damage layer. (b) Damage layer depth $t=100\text{nm}$ and precipitate strength of ${\sigma }_p=750\text{MPa}$ is used. The precipitates are not plotted for clarity. The numbers encircled reference the same dislocations in both simulations.

Figure 2

(Color online) Dislocation density versus strain for a $D=0.75\mu \text{m}$ micropillar having two precipitate strengths: (a) ${\sigma }_p=500\text{MPa}$ and (b) ${\sigma }_p=750\text{MPa}$. Both cases produce an initial increase in dislocation density, however this density persists into the steady-state regime for the larger precipitate strength.

Figure 3

(Color online) The average percentage change in the computed flow strength, for different micropillar sizes and precipitate layer depth. (a) ${\sigma }_p=500\text{MPa}$; (b) ${\sigma }_p=750\text{MPa}$. The error bars are the standard deviation of the simulated results from the average and the solid lines represent the best fit for each damage depth.

Figure 4

(Color online) A comparison of the flow strength at 0.5% strain versus micropillar size, showing results from simulations with no damage layer (Ref. 25), and the current simulation results with a layer of precipitates having strength 500 and 750 MPa. Results for all three layer depths are included.