Nanoscale gold pillars strengthened through dislocation starvation

Abstract

It has been known for more than half a century that crystals can be made stronger by introducing defects into them, i.e., by strain-hardening. As the number of defects increases, their movement and multiplication is impeded, thus strengthening the material. In the present work we show hardening by dislocation starvation, a fundamentally different strengthening mechanism based on the elimination of defects from the crystal. We demonstrate that submicrometer sized gold crystals can be 50 times stronger than their bulk counterparts due to the elimination of defects from the crystal in the course of deformation. DOI: 10.1103/PhysRevB.73.245410 PACS number͑s͒: 62.25.ϩg, 81.07.Bc, 81.16.Rf, 81.70.Bt Anyone who has ever repeatedly bent a copper wire knows that it gets progressively stronger as it becomes more deformed, through a phenomenon called strain-hardening. The strengths of cold-worked metals are known to be up to ten times greater than those of well-annealed crystals. Plasticity in metals occurs by the motion of dislocations, or line defects, which multiply in the course of plastic deformation. Impeding the motion of dislocations by introducing defects into crystals results in strengthening. Although these fundamental concepts are often assumed to be applicable to crystals of any dimensions, numerous recent studies have shown that conventional plasticity diverges at a certain length scale, with smaller samples reported to be stronger than their bulk counterparts. [1][2][3][4][5][6] Pure metals and some alloys exhibit strong size effects at the submicron scale. [1][2][3][4][5][6][7][8][9][10][11][12][13] Size effects in indentation, torsion, and bending have been understood in terms of the nonuniformity of the deformation, which sets up strain gradients leading to hardening. 7 Size effects are also found in thin films, where the strength scales inversely with film thickness and is usually attributed to the confinement of dislocations by the substrate. [8][9][10] Size effects are observed for pristine crystals, as well. 11,12 In the earliest stages of nanoindentation, for example, the crystal volume is extremely small and can be dislocation-free, requiring very large stresses to nucleate new dislocations. In addition, classic experiments on the initially dislocation-free metal whiskers indicated that whiskers with smaller diameters yielded at higher stresses. 13 In typical whiskerlike deformation behavior, initial elastic loading leads to a very high stress followed by a significant drop and continued plastic flow at low stresses. Finally, several molecular dynamics simulations 14-16 and more recent experiments on small pillars 17,18 all support the tenet that smaller is stronger. In spite of much progress on size effects research there is still no unified theory for plastic deformation at the submicron scale.We focus on size effects arising in unconstrained geometries, in the absence of strain gradients, and with nonzero initial dislocation densities. Gold nanopillars ranging in diameter between 200 nm and s...

Size effects and strength fluctuation in nanoscale plasticity

“…Plastic deformation in nanoscale volumes of materials often exhibits a stochastic, discontinuous character, in contrast to the typical smooth yield behavior in their bulk counterparts [1][2][3][4][5][6][7]. Such jerky behavior has been attributed to the instability of microscopic defect processes, such as dislocation nucleation or depinning in crystalline metals [8,9], phase transformation in semiconductors [10] and localized shear transformation in amorphous metals [11].…”

Evidence for exceptional low temperature ductility in polycrystalline magnesium processed by severe plastic deformation

“…Two micro-pillars were fabricated from the material processed through 10 turns of HPT using focused ion beam (FIB) machining [19][20][21] with the surfaces of these discs prepared using the same procedure as for EBSD characterization. These two pillars had diameters of ~1.77 and ~1.73 µm and lengths of ~6.16 and ~5.19 µm, respectively.…”

Alternating starvation of dislocations during plastic yielding in metallic nanowires

“…Recently, Uchic et al [15,16] and Greer et al [17,18] reported that the plastic deformation behavior of single-crystalline sub-micropillars is dependent on the size of the pillar, even without a deformation gradient. More recently, a ''mechanical annealing" test was used to demonstrate that dislocations can be swept out of the samples through the progressive activation and exhaustion of dislocation sources [19].…”

“…Due to the unique mechanical, thermal, electrical and optical properties, materials with nanometer-sized structure have attracted a great deal of interest as potential building blocks in nanoelectronic and nanoelectromechanical devices [1]. Many researchers have demonstrated, through both experiments and analysis, that the structure and properties of nanowires can be quite different from those of bulk materials due to the effect of the large surface to volume ratio [2][3][4][5][6][7][8][9][10][11][12][13][14].Recently, Uchic et al [15,16] and Greer et al [17,18] reported that the plastic deformation behavior of single-crystalline sub-micropillars is dependent on the size of the pillar, even without a deformation gradient. More recently, a ''mechanical annealing" test was used to demonstrate that dislocations can be swept out of the samples through the progressive activation and exhaustion of dislocation sources [19].…”