Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals

Abstract

When microscopic and macroscopic specimens of metals are subjected to cyclic loading, the creation, interaction, and accumulation of defects lead to damage, cracking, and failure. Here we demonstrate that when aluminum single crystals of submicrometer dimensions are subjected to low-amplitude cyclic deformation at room temperature, the density of preexisting dislocation lines and loops can be dramatically reduced with virtually no change of the overall sample geometry and essentially no permanent plastic strain. This "cyclic healing" of the metal crystal leads to significant strengthening through dramatic reductions in dislocation density, in distinct contrast to conventional cyclic strain hardening mechanisms arising from increases in dislocation density and interactions among defects in microcrystalline and macrocrystalline metals and alloys. Our real-time, in situ transmission electron microscopy observations of tensile tests reveal that pinned dislocation lines undergo shakedown during cyclic straining, with the extent of dislocation unpinning dependent on the amplitude, sequence, and number of strain cycles. Those unpinned mobile dislocations moving close enough to the free surface of the thin specimens as a result of such repeated straining are then further attracted to the surface by image forces that facilitate their egress from the crystal. These results point to a versatile pathway for controlled mechanical annealing and defect engineering in submicrometer-sized metal crystals, thereby obviating the need for thermal annealing or significant plastic deformation that could cause change in shape and/or dimensions of the specimen.fatigue | dislocation motion | pristine materials | yield strength | cyclic mechanical healing

Mechanically Assisted Self‐Healing of Ultrathin Gold Nanowires

“…Considering much rougher surface with more kinked sites on this nanowire sample compared with the previous case with only one necking area, it is reasonable that about 2.5 min (Figure c–f) should be taken to smooth the corrugated nanowire surface. It might also be noted that, in addition to “de‐kinking,” even those small crystalline defects (as shown in the insert of Figure a) inside the sample could be mechanically removed during the multiple manipulation processes, similar to a recently reported work …”

“…For this reason, nondestructive self‐healing appears to be an attractive solution. A few recent works demonstrated the self‐recovery in nanoscale systems where precise manual intervention is usually required, for instance, by local heating/irradiation or applying mechanical loading . However, those precise forms of external intervention are very challenging and costly, which calls for a self‐healing technique that could be activated and achieved by a relatively simple method, for instance, pure mechanical stimulations.…”

Hydrogenated vacancies lock dislocations in aluminium

“…1a, the as-prepared pillar has a high density of dislocations. Therefore, we take advantage of the ‘cyclic healing' phenomenon2728 to clean up the interior of the pillar, using the loading function shown in Fig. 1b with peak stresses of ∼330 or ∼250 MPa.…”

When ‘smaller is stronger’ no longer holds

“…In other words, intrinsically there can be a 'smaller is obviously weaker' regime, for strength controlled by surface dislocation nucleation. Other surface defects, such as surface steps [17,18], can also facilitate dislocation nucleation and reduce strength-those are additional extrinsic effects that break the 'smaller is stronger' norm and they come with large data scatter depending on sample preparation and surface conditions. Specifically, a 'smaller is weaker' trend in strength controlled by surface dislocation nucleation has been recently demonstrated by Li at al.…”