Deformation of nanocrystalline metals: The role of grain boundaries Krystyn Van Vliet, Sedina Tsikata and Subra Suresh Massachusetts Institute of Technology, Cambridge, MA USA DEFORMATION MODES AND STRESSES METHODS PURPOSE Polycrystalline bubble rafts of {111} in-plane orientation were constructed with average grain size 4 – 37 nm Polycrystals were deformed via nanocontact of a rigid indenter Variables included: Grain size GB orientation q Proximity of GBs to the loading axis Nanocrystalline metals have grain sizes < 100 nm, so the volume fraction of grain boundaries (GBs) is much greater than in conventional microcrystalline metals. Thus, GBs may significantly affect strength and deformation modes in nanocrystals. The structure of GBs in nanocrystals is clean and atomically sharp, but the role of GBs during plastic deformation is unknown. For example, the Hall-Petch relationship predicts dramatic increases in yield strength sy for decreasing grain size d: but a reverse Hall-Petch effect has been observed in several nanocrystals. Why? The Bragg-Nye bubble raft model allows visualization and quantification of how GBs contribute to plastic deformation in nanocrystals, and can estimate the strength of GBs for input for computational models. Dislocation activity always initiated at GBs or triple junctions, NEVER from the grain interior, either by GB migration or discrete emission of dislocations. Local, continuum shear stress and far-field mean stress were calculated at the onset of dislocation activity: Critical shear stress (GRAIN BOUNDARY STRENGTH): high speed/resolution camera ,where m(a, x, z) and n(a, x, z) indenter sy = so + kyd-1/2 Critical far-field mean stress (HARDNESS): ,where E*= Young’s modulus R = indenter tip radius a = contact half-width 5 nm Side view Plan view EFFECT OF GRAIN SIZE CONCLUSIONS EFFECT OF GRAIN CONFIGURATION d < 20 nm: emission from GB d > 20 nm: GB migration txz independent of d, and maximum for d = 13 nm pm strong function of grain size: For d > 7 nm, Hall-Petch strengthening For d < 7 nm, reverse Hall-Petch effect GB orientation q < 55o: GB migration; q > 55o: emission from GB txz independent of q, but maximal for q = 26o GB proximity zp Deformation mode independent of zp txz a zp, but maximum txz 30% < tcrit for perfect crystal Using the bubble raft as a 2-D analogue of close-packed metallic nanocrystals, we find: Nanocrystalline deformation originates from GBs and GB triple junctions, and proceeds via GB migration or emission of dislocations from the GB. Orientation and proximity of the GB to points of nanocontact affect both the mode and critical stresses to induce dislocation activity. A critical grain size exists below which a reverse Hall-Petch (softening) effect occurs, but a concurrent change in deformation mode is not observed. Emission of dislocation from GB triple junction a x z q pm pmmax d-1/2 Hall-Petch Reverse Hall-Petch PUBLICATIONS d = 7 nm This trend is in agreement with 3-D computational models of Cu polycrystals in tension (Schiotz et al. Nature 391, 1998).