THE DEEP LINK BETWEEN FCC AND BCC METALS

Sample size effect is currently a hot research topic in a world wide range [1-4]. In this talk, I will report our recent research progress on submicro-sized single crystal Ni and Mo pillars. It was found that prior to the compression tests. the < 111 > orientated single crystal face centered cubic (FCC) nickel pillars fabricated through Focused Ion Beam (FIB) contained a high density of defects. However, quite unexpectedly, the dislocation density was observed to decrease dramatically during the deformation process and, in some cases, even resulted in a dislocation-free crystal. The phenomenon, which we termed as "mechanical annealing", is the first direct observation of the dislocation starvation mechanism and sheds new light on the unusual mechanical properties associated with submicron- and nano- scale structures [2]. However, due to the dislocation core structure difference between the screw and edge dislocations in body centered cubic (BCC) metals, it was believed that BCC metals are incapable of mechanical annealing and the sample size strengthening behavior should be less that in FCC metals. By in situ compression of nanopillars inside a transmission electron microscope, we demonstrate that with the pillar diameter decreasing to hundreds of nanometers, significant mechanical annealing does occur in BCC Mo. In addition, there exists a critical size (D-C similar to 200 nm for Mo at room-temperature) below which the strengthening exponent a in Hall-Petch like regression increases dramatically to that similar to FCC metals. Thus, a new regime for size effects in BCC is discovered that converges to that of FCC, revealing deep connection in the dislocation dynamics of the two systems. We attribute the observed phenomena to the diminishing importance of lattice friction at high stresses, when the size-enhanced flow stress exceeds a single screw dislocation's lattice friction [4].