Exploring specimen size effects in plastic deformation of Ni₃(Al, Ta)

Abstract

In this paper we present a mechanical test methodology to explore specimen size effects in Ni3Al, where the overall test sample dimensions artificially limit the volume for substructure evolution and hence the availability of jogs/kinks along individual dislocation lines. The test methodology consists of using Focused Ion Beam milling to micromachine cylindrical compression samples that have diameters ranging from 5 to 20 microns into the surface of a bulk sample, which is followed by nanoindentation using a flat-ended tip to measure the mechanical properties of the microsamples in uniaxial compression. The initial test results show that there is a strong increase in the flow stress with decreasing sample size, although misfit between the flat indenter tip and the top surface of the compression samples complicates complete interpretation of the mechanical test results at this time.

Dislocation subgrain structures and modeling the plastic hardening of metallic single crystals

“…The theory presented here shows the Hall-Petch effect due to the formation of a dislocation subgrain structure with a grain size dependence, hence it provides a possible explanation for the Hall-Petch effect for both single crystals and polycrystals. Both Uchic et al (2004) and Greer et al (2005) also predicted a transition as the sample size decreased to a 'breakaway flow' behavior. This theory would predict that transition as the length scale at which the deformation process no longer favors the formation of dislocation structures, but instead activation of a single slip system.…”

“…It should also hold for higher order laminate structures except during the time steps in which the new laminates form. Uchic et al (2004) and Greer et al (2005) have recently worked on the deformation response of micrometer sized columns of single crystals. This work displayed that single crystals exhibit a Hall-Petch relationship, despite Hall-Petch having been found for polycrystals.…”

High temperature micropillar compression of Al/SiC nanolaminates

“…This is a small fraction of the total pillar diameter examined here (~ 1 (_im). For very small diameters in monolithic materials approaching the 50-60 nm damage depth, ion beam-induced surface damage might have an influence on the micropillar compression behavior [27]. In nanolaminates, the micropillar diameter does not influence the flow stress significantly [32], since the characteristic length scale that controls the mechanical response, the layer thickness, is much smaller than the micropillar diameter.…”

“…This novel, although already popular technique, is ideal for quantifying the stress-strain curve in compression of small volumes of material [24][25][26][27][28], and it has recently been extended to high temperatures in monolithic materials such as Si and Au [29,30]. Mechanical tests were complemented with detailed transmission electron microscopy (TEM) analysis of the deformed micropillars to elucidate the effect of temperature on the deformation mechanisms at the nanometer scale.…”

Advanced Nanomechanical Test Techniques

“…Uchic et al used the technique to demonstrate size effects on the measured strength of Ni and three Ni based alloys [41]. They demonstrated that micro-pillars of practically manageable dimensions (0.5 to 40 µm diameter) were sufficiently small to show the relationship between strength and volume based phenomena such as dislocation nucleation and movement.…”