Comparison between thermal and deformation-induced structural relaxation in atomic glasses
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Comparison between thermal and deformation-induced structural relaxation in atomic glasses Magesh Nandagopal1 and Marcel Utz1,2 of Materials Science, University of Connecticut, Storrs CT 06269, USA 2 Department of Physics, University of Connecticut, Storrs CT 06269, USA 1 Institute
ABSTRACT
Plastic yielding in glassy solids has been interpreted as a strain-biased relaxation process, or, equivalently, as a strain-induced glass transition. In the present work, the atomic motions caused by athermal plastic deformation of a binary Lennard-Jones glass are compared to thermal motion in the liquid in terms of the self part of the intermediate structure factor. We find that like at finite temperature, athermal plastic deformation leads to diffusive atomic motion at all length scales beyond about one interatomic distance, effectively promoting structural relaxation. The present approach allows to study the interplay of deformation-induced and thermal relaxation. Preliminary evidence is presented that these two processes occur independently of each other over a wide range of strain rates. INTRODUCTION
Both glasses and crystalline solids undergo plastic yielding when subjected to sufficient deviatoric stress. Whereas the phenomenon of yielding and plastic deformation can be described quantitatively in terms of the nucleation and mobility of lattice defects in the case of crystalline materials [1], the elementary processes of deformation are still not completely understood for amorphous solids. One possible viewpoint is that deviatoric stress causes a de-vitrification, analogous to the glass-to-liquid transition that occurs as the system is heated above its glass transition temperature Tg . According to this concept, the onset of plastic deformation would therefore be indicative of a stress- or strain-induced glass transition [2–5]. A closely related concept interprets plasticity as a stress-biased relaxation [6]. Recently, strong fundamental interest has arisen in the fluidization of amorphous systems by shear [7–10]. It has been realized that while glass-forming fluids behave liquid-like under shear, flow stops below a critical shear stress. The term “jamming” has been coined for this phenomenon [11]. The decay of structural correlations in simulations of a glass-forming system under shear at finite temperature has been studied in detail by Berthier and Barrat [7, 9, 12, 13]. Their results show that while structural relaxation is efficiently promoted by plastic deformation, the functional form of the relaxation curves remains essentially unaltered. These results have led to the formulation of the strain-temperature superposition principle [7]. This raises the question how deformation-induced and thermal relaxation processes superimpose at finite temperature. To address this problem, we have performed deformation simulations both at T = 0 as well as at finite temperature. This allows for a systematic comparison of purely deformation-induced relaxation to its thermal counterpart,
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and to quantify the relat
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