arXiv subjects

Robert Maass

Publications and source records attributed to Robert Maass.

3 recordsLinked to original sources

arXiv2019

Emergent structural length scales in a model binary glass --- the micro-second molecular dynamics time-scale regime

It is now possible to routinely perform atomistic simulations at the microsecond timescale. In the present work, we exploit this for a model binary Lennard-Jones glass to study structural relaxation at a timescale spanning up to 80 microseconds. It is found that at these longer time-scales, significant mobility and relaxation occurs, demonstrating a reproducible relaxation trajectory towards a class of asymptotic structural states which are strongly heterogeneous. These structures are discussed both in terms of the ideal glass state and a production route towards novel amorphous crystalline nano-composite micro-structures with strong interface stability.

cond-mat.mtrl-sci↗

arXiv2018

Shapes and velocity relaxation of dislocation avalanches in Au and Nb single crystals

We capture the spatiotemporal velocity dynamics of dislocation avalanches in face-centered cubic (FCC) gold and body-centered cubic (BCC) niobium crystals by compression testing of cylindrical microcrystals. In niobium, avalanche peak-velocities are over one order of magnitude lower, have distinctly rougher avalanche shapes, and relax more slowly to zero velocity than in gold. The avalanche dynamics (including averaged avalanche shapes) can be described reasonably well by mean-field predictions for avalanches near the depinning transition in the case of gold, but not for niobium. A detailed analysis shows consistent deviations (non-trivial exponents) from the predicted functional forms for both gold and niobium if the exponent that describes the velocity decay of the shape function is treated as a free parameter. While the avalanche size statistics and related scaling exponents are similar, these noticeable differences in the dislocation-avalanche dynamics demonstrate material-specific variations not predicted by "universal" behavior. The marked difference in avalanche shapes is discussed in terms of edge- and screw-dislocation mobility of an operating spiral arm source.

cond-mat.mtrl-sci↗

arXiv2014

A probabilistic explanation for the size-effect in crystal plasticity

In this work, the well known power-law relation between strength and sample size, $d^{-n}$, is derived from the knowledge that a dislocation network exhibits scale-free behaviour and the extreme value statistical properties of an arbitrary distribution of critical stresses. This approach yields $n=(τ+1)/(α+1)$, where $α$ reflects the leading order algebraic exponent of the low stress regime of the critical stress distribution and $τ$ is the scaling exponent for intermittent plastic strain activity. This quite general derivation supports the experimental observation that the size effect paradigm is applicable to a wide range of materials, differing in crystal structure, internal microstructure and external sample geometry.

cond-mat.mtrl-sci↗