Search

We report direct measurements of displacement jump velocities in deforming nano- and microcrystals. These velocities exhibit large scatter and no correlation with plastic strain, crystal size or applied stress. Statistical analysis for the displacement jump velocities shows a cubic power-law decay, with a shoulder at low velocities. Our results indicate that the collective motion of dislocations in the deforming crystals is dominated by statistical fluctuations within the internal stresses, rather than by a mobility law that depends on the externally applied stress. [ABSTRACT FROM AUTHOR]

In the past two decades, numerous relaxation or physical aging experiments of metallic glasses have revealed signatures of intermittent atomic-scale processes. Revealed via intensity cross-correlations from coherent scattering using X-ray photon correlation spectroscopy (XPCS), the observed abrupt changes in the time-domain of atomic motion does not fit the picture of gradual slowing down of relaxation times and their origin continues to remain unclear. Using a binary Lennard-Jones model glass subjected to microsecond-long isotherms, we show here that temporally and spatially heterogeneous atomic-cluster activity at different length-scales drive the emergence of highly non-monotonous intensity cross-correlations. The simulated XPCS experiments reveal a variety of time-dependent intensity-cross correlations that, depending on both the structural evolution and the q -space sampling, give detailed insights into the possible structural origins of intermittent aging measured with XPCS. [Display omitted] [ABSTRACT FROM AUTHOR]

Metallic glasses are structurally heterogeneous below a certain length scale. Here we demonstrate how elastic heterogeneities change in response to a macroscopic cyclic, but elastic, loading protocol. Fluctuations in local elastic properties are spatially resolved across the surface of a bulk sample by evaluating nano-scale contact resonances of an atomic force microscope. The findings indicate a significant increase in nano-elastic fluctuations due to loading. The distribution of these fluctuations broadens symmetrically and almost three fold, revealing how the atomically disordered structure is driven further out of equilibrium at the nano-scale. Macroscopically, the stress-driven elastic heterogeneities lead to a rejuvenation and therefore a new structural state of the bulk metallic glass, characterized by a higher energy release during heating and mechanical softening. These results show how macroscopic energy storage in metallic glasses may be linked to the development of nano-scale elastic fluctuations. [ABSTRACT FROM AUTHOR]

We report on the use of quantitative in situ microcompression experiments in a scanning electron microscope to systematically investigate the effect of self-ion irradiation damage on the full plastic response of 〈1 1 1〉 Ni. In addition to the well-known irradiation-induced increases in the yield and flow strengths with increasing dose, we measure substantial changes in plastic flow intermittency behavior, manifested as stress drops accompanying energy releases as the driven material transits critical states. At low irradiation doses, the magnitude of stress drops reduces relative to the unirradiated material and plastic slip proceeds on multiple slip systems, leading to quasi-homogeneous plastic flow. In contrast, highly irradiated specimens exhibit pronounced shear localization on parallel slip planes, which we ascribe to the onset of defect free channels normally seen in bulk irradiated materials. Our in situ testing system and approach allows for a quantitative study of the energy release and dynamics associated with defect free channel formation and subsequent localization. This study provides fundamental insight into the nature of interactions between mobile dislocations and irradiation-mediated and damage-dependent defect structures. [ABSTRACT FROM AUTHOR]

[Display omitted] • Elastic properties of a Zr-based bulk metallic glass were probed by spatially resolved nanoindentation. • An elastic microstructure is observed with fluctuations at a decorrelation length scale of 170 nm. • Spatially resolved compositional analysis excludes chemical modulations to generate the elastic fluctuations. • The elastic length scale is connected to local density fluctuations that emerge due to cooling constraints during casting. • The emerging elastic length scale suggests the formulation of a structure–property relationship for bulk metallic glasses. Metallic glasses (MGs) are known to be structurally heterogeneous at the nanometer (nm) scale. In addition, elastic property mapping has indicated the presence of at least an order-of-magnitude larger length scales, of which the origin continues to remain unknown. Here we demonstrate the existence of an elastic decorrelation length of the order of 100 nm in a Zr-based bulk MG using spatially resolved elastic property mapping via nanoindentation. Since compositional modulations, sufficiently large to account for this elastic microstructure, were not resolved by analytical scanning-transmission electron microscopy, chemical phase separation such as spinodal decomposition cannot explain their presence. Instead, we argue that the revealed long-range elastic modulations stem from structural variations affecting the local density. These emerge during solidification and are strongly influenced by the cooling constraints imposed on bulk MGs during the casting process. [ABSTRACT FROM AUTHOR]