Mechanical behavior of crystals is dictated by dislocation motion in response to applied force. While it is extremelyuddifficult to directly observe the motion of individual dislocations, several correlations can be made between theudmicroscopic stress-strain behavior and dislocation activity. Here, we present for the first time the differences observedudbetween mechanical behavior in two fundamental types of crystals: face-centered cubic, fcc (Au, Cu, AI, Ni, etc.) andudbody-centered cubic, bcc (W, Cr, Mo, Nb, etc.) with sub-micron dimensions subjected to in-situ micro-compression inudSEM chamber. In a striking deviation from classical mechanics, there is a significant increase in strength as crystal sizeudis reduced to 100nm; however in gold crystals (fcc) the highest strength achieved represents 44% of its theoreticaludstrength while in molybdenum crystals (bcc) it is only 7%. Moreover, unlike in bulk where plasticity commences in audsmooth fashion, both nano-crystals exhibit numerous discrete strain bursts during plastic deformation. These remarkableuddifferences in mechanical response of fcc and bcc crystals to uniaxial micro-compression challenge the applicability ofudconventional strain-hardening to nano-scale crystals. We postulate that they arise from significant differences inuddislocation behavior between fcc and bcc crystals at nanoscale and serve as the fundamental reason for the observeduddifferences in their plastic deformation. Namely, dislocation starvation is the predominant mechanism of plasticity inudnano-scale fcc crystals while junction formation and subsequent hardening characterize bcc plasticity, as confirmed byudthe microstructural electron microscopy. Experimentally obtained stress-strain curves together with video frames duringuddeformation and cross-sectional TEM analysis are presented, and a statistical analysis of avalanche-like strain bursts isudperformed for both crystals and compared with stochastic models.
展开▼
