Compression Experiments on γ′-Nanoparticles

Abstract

The mechanical behavior of cubic γ -nanoparticles is investigated in uniaxial compression. These nanoparticles are freestanding, single crystalline and have a well-defined crystallography related to the cubic geometry (cube faces correspond to the crystallographic {100}-planes). The true values of stress and strain were measured and evaluated. With a true strength of about 3000 MPa to 5000 MPa the nanoobjects reach a significant portion of the theoretical strength. Even after a true strain of about one, no signs of particle damage are observed.

Atomistic Simulations of Compression Tests on Ni3Al Nanocubes

“…[12][13][14] Despite the differences in size, strain rate and composition, our simulation results reproduce some of the key features of the experiments, namely the lack of strain hardening which is a prerequisite for the observed large strain bursts, homogeneous deformation of the entire sample with slip steps on the surfaces rather than localized slip as in the case of FIB-irradiated γ -cubes, [13] and overall high stress levels of the order of 3-5 GPa. [12][13][14] The deformation behaviour and strain bursts observed in the experiments were attributed to the initial lack of dislocations within the γ -cubes which allows the sample to sustain large stresses close to the theoretical strength at which finally 'explosive dislocation nucleation' occurs [13] or an 'avalanche-like'; deformation process sets in [14].…”

“…However, in the experiments, the fast, highly correlated, 'laser-like, [7]' dislocation motion during pseudo-twinning would significantly reduce the possibility for their interaction with each other or with stacking faults compared with the more chaotic ODP. Thus, pseudo-twinning seems a natural alternative to the 'avalanche-like' ODP-processes proposed in [14] to explain the lack of strain hardening and the instability causing the strain burst in the closed-loop displacementcontrolled experiments of γ nanocube compression. [13] In the following we will show that pseudo-twinning is furthermore the energetically favoured initial deformation mechanism.…”

“…Deformation by pseudo-twinning has also been recently reported in MD simulations of Ni 3 Al samples subjected to high stresses. [27,30] We assume that even under the experimental conditions during nanocompression tests, [12][13][14] pseudo-twinning rather than twinning can be expected, as the latter involves thermally activated reordering processes. The critical migration barrier for this process has been determined to be ∼0.93 eV, [29] which at RT makes this process very unlikely to massively occur during the sub-second displacement bursts.…”

“…Email: erik.bitzek@fau.de Last year, two research groups used a wet-chemical process to extract 200-600 nm sized cubic precipitates from the γ /γ -microstructure of a Ni-based superalloy (CMSX-4) to perform nanocompression tests. [12][13][14] After standard heat treatment, the precipitates do not contain dislocations, and the wet-chemical route precludes the creation of defects in γ -Ni 3 Al cubes (L1 2 crystallographic structure). Contrary to samples prepared by focussed ion-beam (FIB) milling, these freestanding nanocubes are free of dislocations.…”

“…[15] Room temperature (RT) compression tests on these defect-free nanocubes show high-yield stresses up to a substantial portion of the theoretical strength, large strain bursts and homogeneous deformation for engineering strains up to 100% without signs of particle damage. [12][13][14] It is well established that RT deformation of bulk-Ni 3 Al takes place by 110 {111} superdislocations, which can include the formation of planar defects like complex stacking faults (CSF), anti-phase boundary (APB) or superintrinsic stacking fault. [16,17] Whether these superdislocations also carry the plastic deformation during the compression of the Ni 3 Al nanocubes, or whether a change in deformation mechanism like, e.g.…”