D. Pan

Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses

We report experimental characterization of shear transformation zones (STZs) for plastic flow of bulk metallic glasses (BMGs) based on a newly developed cooperative shearing model [Johnson WL, Samwer K (2005) A universal criterion for plastic yielding of metallic glasses with a (T/Tg)2/3 temperature dependence. Phys Rev Lett 95: 195501]. The good agreement between experimental measurements and theoretical predictions in the STZ volumes provides compelling evidence that the plastic flow of metallic glasses occurs through cooperative shearing of unstable STZs activated by shear stresses. Moreover, the ductility of BMGs was found to intrinsically correlate with their STZ volumes. The experiments presented herein pave a way to gain a quantitative insight into the atomic-scale mechanisms of BMG mechanical behavior.

Strengthening and softening of nanocrystalline nickel during multistep nanoindentation

Multistep load-unload nanoindentation was employed to address the effect of deformation-induced microstructural evolution on mechanical behavior of nanocrystalline Ni. Deformation discontinuity was deliberately introduced by unloading-reloading during nanoindentation testing, which allows us to examine the influence of microstructural evolution on the successive deformation. Strain strengthening/softening of nanocrystalline nickel, associated with the transition of deformation behavior from dislocation activity at high loading rates to a grain-boundary-mediated process at low loading rates, was uncovered by means of this experimental methodology.

Correlation between structural relaxation and shear transformation zone volume of a bulk metallic glass

The effect of structural relaxation on shear transformation zones (STZs) in a Zr70Ni16Cu6Al8 glassy alloy is evaluated. Upon annealing, the measured STZ size dramatically decreases with moderate augment of mass density caused by the increase of icosahedra short-range orders. The greater atomic packing density gives rise to involvement of lesser atoms in the formation of STZs and thereby degradation of ductility. This study demonstrates that STZ volume is a key parameter reflecting the intrinsic relationship between atomic structure and mechanical properties of metallic glasses.

Temperature-induced anomalous brittle-to-ductile transition of bulk metallic glasses

We report an anomalous brittle-to-ductile transition (BTDT) in Au-based bulk metallic glasses (BMGs). Despite of brittle failure without noticeable plastic strain under uniaxial compression at room temperature, the Au-based BMGs exhibit remarkable plasticity at cryogenic temperatures, which is opposite to the BTDT in crystalline materials. This anomalous transition originates from the strong temperature dependence of the fundamental deformation units, viz. shear transformation zones, giving rise to both superior plasticity and high strength of BMGs for low-temperature applications.

Effect of doping and counterdoping on high-pressure phase transitions of silicon

The influence of dopants and counterdopants on high-pressure phase transitions of silicon was investigated by high-pressure Raman microscopy. A small amount of dopants were found to dramatically influence the high pressure stability of silicon. The combination of doping and counterdoping provides an effective way to manipulate the critical pressures of the phase transitions, which offers unique insights on atomic mechanisms of high pressure phase transitions of Si.

Microstructural Origin of Superior Compressive Ductility of a Nanocrystalline Metal

Ultra-large compressive plasticity at room temperature has recently been observed in electrodeposited nanocrystalline nickel (nc-Ni) under micro-scale compression (Pan, Kuwano, Fujita and Chen: Nano Lett. Vol. 7 (2007), p. 2108). With aid of a TEM sample preparation technique employing focused ion beam (FIB), TEM observations on deformed nc-Ni evidenced deformation-induced microstructural evolution of nc-Ni at a variety of strain levels: Whilst the deformation increases, substantial grain growth is uncovered in the nc-Ni. No apparent ex situ evidence of intragranular dislocation activities is found in the deformed sample. As thermal diffusion plays an insignificant role in the deformation in nc-Ni at room temperature (~0.17Tm), this premium plasticity is achieved in accommodation with the grain-boundary-mediated deformation, with assistance of extensive grain growth that is mainly driven by high stresses at steady plastic flow.