Size Dependence of Incipient Dislocation Plasticity inNi3Al

Abstract

Molecular dynamics simulations are carried out to study the incipient dislocation plasticity in Ni 3 Al. Dislocation nucleation is found to occur preferentially at energetic atomic clusters with larger-thanaverage relative displacements. From the simulated distribution of the atomic relative displacements, a scaling model is proposed to predict the size dependence of the incipient plasticity condition in real-sized specimens. DOI: 10.1103/PhysRevLett.94.095501 PACS numbers: 61.72.Cc, 62.20.Fe The rapid development of technologies involving microand nanomachinery calls for an urgent need to understand material behaviors at the submicron length scale. There is now rather convincing evidence showing that crystalline materials in general exhibit much higher strengths than their bulk counterparts [1,2]. In certain modes of deformation such as torsion, strain gradients are present which lead to a size-dependent hardening effect due to geometrically necessary dislocations [1]. The high strengths of small, annealed specimens, on the other hand, have been observed in early tensile experiments on annealed whisker crystals [3]. This phenomenon is attributed to the scarcity of source dislocations in very small samples, and so dislocations have to be generated from the perfect crystal environment at high stresses. Recently, Uchic et al.[2] compressed micropillars of Ni and Ni 3 Al fabricated by the focusedion-beam method and observed a marked increase of yield strength as the specimen size decreases. Incipient plasticity is also easily observable in nanoindentation on annealed, bulk crystalline materials as a load drop or displacement jump [4], and recent experiments show that the statistical distributions of the strain-burst loads in SiC [5] and Ni 3 Al [6] obey characteristics of homogeneous nucleation at room temperature. Nanoindentation experiments on annealed Fe-3%Si [4], Ni 3 Al [6,7], and Al [8] have also shown that, even if the applied stress is initially in the elastic range, yielding may occur rather suddenly after a certain waiting time, implying that these materials cannot indefinitely sustain GPa-level stresses in the elastic range [9].In this Letter, we report a theoretical study on the nucleation of defects in a highly stressed crystal at finite temperatures using molecular dynamics simulation. Homogeneous nucleation of dislocations has been studied by Li and co-workers [10,11] as a form of an unstable phonon process under a high applied stress. In the present work, we focus on temperature effects as well as the physical forms of the embryos of the nucleation process.Simulation model.-The model is set up to mimic the situation of applying homogeneous shear on a small material volume. The aim here is not to simulate directly the behavior of real, micron-sized specimens in real time, as such a task is still too big to be handled by today's computing power. Instead, the purpose of the simulation is to use a sufficiently large block to generate reliable statistical data, for the development of scaling relation...

Nanoscale gold pillars strengthened through dislocation starvation

“…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][15][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.…”

“…Fitting our gold data to this model required modification of two parameters, which are functions of the strain rate and elastic properties of the deformed material. The graphs representing predictions of these models [14][15][16] in comparison to the present experiments are shown online ͓Ref. 19͑c͔͒.…”

“…Incipient plasticity was also investigated by Zuo et al in 2005. 16 These authors also employed EAM to compute atomic fluctuations leading to dislocation nucleation in a small computational cell of Ni 3 Al subjected to homogeneous shear. It was found that atoms with relative displacements larger than 1 2 of the Shockley partial vector formed "hot zones," resulting in the creation of partial dislocation loops.…”

“…Some promising attempts have already been made to create a model explaining the presence of a characteristic length scale in mechanical deformation. [14][15][16] Transmission electron microscopy ͑TEM͒ observations of pillar cross sections before and after deformation were made in an effort to check one of the main predictions of the dislocation starvation theory. Due to the very small dimensions of the pillar specimen, conventional TEM preparation techniques could not be utilized.…”

From “Smaller is Stronger” to “Size‐Independent Strength Plateau”: Towards Measuring the Ideal Strength of Iron

“…When materials are subjected to external applied stresses, edges, corners, notches and surface roughness on free surfaces or even atomic-level surface steps can serve as dislocation nucleation sites, at a stress level significantly less than the ideal strength [18][19][20][21].…”