Researchers Show that Metals Can Recover after Plastic Deformation

MechSE researchers showed, for the first time, that plastic deformation in nanocrystalline metal films can be highly reversible. This surprising finding challenges the widely accepted view that plastic deformation in metals is irrecoverable, and raises the possibility of designing and manufacturing metal components that may recover or heal themselves after being deformed or dented. Graduate students Jagannathan Rajagopalan and Jong H. Han, and Professor and Willett Faculty Scholar Taher Saif shared the details in the March 30 issue of Science.

Their experiments showed that nanocrystalline aluminum and gold thin films with a grain size of 50 to 65 nanometers recovered 50 to 100 percent of plastic deformation after unloading under macroscopically stress-free conditions. Their recovery was time dependent, thermally activated and involved a distribution of activation energies. What's more, after strain recovery was complete, the films showed no effect of previous deformation when they were loaded again. This behavior differed markedly from bulk corse-grained metals, which exhibited a pronounced residual hardening (increase in yield stress) after being deformed plastically.

The researchers have proposed that the strain recovery they observed could be due to variations in microstructure, such as differences in the size and orientation of individual grains, that create non-uniform local stress fields during deformation. This could cause relatively larger, or favorably oriented, grains to deform plastically and smaller, or unfavorably oriented, grains to accommodate the strain elastically. Upon unloading, the elastically deformed grains might reduce their strain energy by inducing reverse plasticity in the larger grains, which would lead to time-dependent strain recovery.

The effect of non-uniform microstructures on the deformation behavior of metals under macroscopically uniform loading conditions is a relatively unexplored paradigm. The MechSE researchers' groundbreaking experimental results suggest that incorporating the effect of microstructural variations is vital to understanding the mechanical behavior of metals--particularly at the nanoscale level.

To learn more, read the full article in Science. You can also view the media version in Science Daily.