Effects of pre-strain on the compressive stress–strain response of Mo-alloy single-crystal micropillars

A new regime for mechanical annealing and strong sample-size strengthening in body centred cubic molybdenum

“…Similar trend was obtained, if the flow stress values were taken at a fixed strain (for example, at 5% strain, as done by some groups for other metal pillars 15 ). The relatively low data scatter of the σ f of our smaller pillars ( < ~200 nm), where mechanical annealing reduced dislocation density down to ~0, seems to be consistent with the previous observation that dislocation-free crystals tend to show smaller data scatter 17,26,27 .…”

“…Certainly, Ga solutes may escape as gas during high-temperature thermal annealing, but unlikely to do so during mechanical annealing, so the chemical implantation induced by FIB is unlikely to be cleaned away by mechanical annealing. Consequently, our mechanically annealed region is still not as structurally perfect as the high-temperature processed pillars near the melting point 4,17 , and the strong size dependence of the collapse strength is consistent with that.…”

“…σ 0 is taken to be 1.3 GPa, which is almost the same as the average yield stress for 4% pre-strained Mo pillars with size of ~1,200 nm and the flow stress for 11% pre-strained Mo pillars regardless of pillar diameter 17 . The second term, which brings in the D dependence, is the stress needed for the dislocation to execute self-multiplication by bow-out 5 , which becomes operative at HSR in lieu of the double-kinks mechanism.…”

“…First, investigations so far indicate BCC Mo pillars exhibit a relatively weak sample-size dependence at room temperature 3,15,16 . In an extreme case of Mo-alloy pillars produced by high-temperature eutectic solidification followed by chemical etching, the samples were believed to be initially dislocation free, and the flow stress was reported to be independent of the pillar size 4,17 . In focused ion beam (FIB) micro-machined pillars, α was found to be < ~0.45 for Mo by several groups 3,15,16 , considerably lower than those for FIBed FCC metallic pillars.…”

Micropillar compression of LiF [111] single crystals: Effect of size, ion irradiation and misorientation

“…It was not possible to determine the initial dislocation density in our LiF [111] pillars because they are not produced from bulk crystals, but by directional solidification of an eutectic mixture. However, the fact that these micropiUars did not display a whisker-type behavior, controlled by the nucleation of dislocations (as the one observed in the Mo micropiUars of Bei et al (2008)), indicates the existence of an initial dislocation network caused by the thermal residual stresses developed upon cooling because of the differences in the coefficients of thermal expansion of both phases in the eutectic. So, both LiF micropiUars in Nadgorny et al (2008) and in this investigation contained a network of mobile dislocations prior to testing although the actual densities are unknown.…”