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Register a new userDislocation induced anomalous softening of solid helium
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The classical motion of gliding dislocation lines in slip planes of crystalline solid helium leads to plastic deformation even at temperatures far below the Debye temperature and can affect elastic properties. In this work we argue that the gliding of dislocations and plasticity may be the origin of many observed elastic anomalies in solid He-4, which have been argued to be connected to supersolidity. We present a dislocation motion model that describes the stress-strain $tau$-$epsilon$ curves and work hardening rate $dtau/depsilon$ of a shear experiment performed at constant strain rate $dot{epsilon}$ in solid helium. The calculated $dtau/depsilon$ exhibits strong softening with increasing temperature due to the motion of dislocations, which mimics anomalous softening of the elastic shear modulus $mu$. In the same temperature region the motion of dislocations causes dissipation with a prominent peak.
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We develop a model for the gliding of dislocations and plasticity in solid He-4. This model takes into account the Peierls barrier, multiplication and interaction of dislocations, as well as classical thermally and mechanically activated processes leading to dislocation glide. We specifically examine the dc stress-strain curve and how it is affected by temperature, strain rate, and dislocation density. As a function of temperature and shear strain, we observe plastic deformation and discuss how this may be related to the experimental observation of elastic anomalies in solid hcp He-4 that have been discussed in connection with the possibility of supersolidity or giant plasticity. Our theory gives several predictions for the dc stress strain curves, for example, the yield point and the change in the work-hardening rate and plastic dissipation peak, that can be compared directly to constant strain rate experiments and thus provide bounds on model parameters.
The current interest in compositionally complex alloys including so called high entropy alloys has caused renewed interest in the general problem of solute hardening. It has been suggested that this problem can be addressed by treating the alloy as an effective medium containing a random distribution of dilatation and compression centers representing the volumetric misfit of atoms of different species. The mean square stresses arising from such a random distribution can be calculated analytically, their spatial correlations are strongly anisotropic and exhibit long-range tails with third-order power law decay. Here we discuss implications of the anisotropic and long-range nature of the correlation functions for the pinning of dislocations of arbitrary orientation. While edge dislocations are found to follow the standard pinning paradigm, for dislocations of near screw orientation we demonstrate the co-existence of two types of pinning energy minima.
The application of stress to multiphase solid-liquid systems often results in morphological instabilities. Here we propose a solid-solid phase transformation model for roughening instability in the interface between two porous materials with different porosities under normal compression stresses. This instability is triggered by a finite jump in the free energy density across the interface, and it leads to the formation of finger-like structures aligned with the principal direction of compaction. The model is proposed as an explanation for the roughening of stylolites - irregular interfaces associated with the compaction of sedimentary rocks that fluctuate about a plane perpendicular to the principal direction of compaction.
Nucleation of a solid in solid is initiated by the appearance of distinct dynamical heterogeneities, consisting of `active particles whose trajectories show an abrupt transition from ballistic to diffusive, coincident with the discontinuous transition in microstructure from a {it twinned martensite} to {it ferrite}. The active particles exhibit intermittent jamming and flow. The nature of active particle trajectories decides the fate of the transforming solid -- on suppressing single particle diffusion, the transformation proceeds via rare string-like correlated excitations, giving rise to twinned martensitic nuclei. These string-like excitations flow along ridges in the potential energy topography set up by inactive particles. We characterize this transition using a thermodynamics in the space of trajectories in terms of a dynamical action for the active particles confined by the inactive particles. Our study brings together the physics of glass, jamming, plasticity and solid nucleation.
The results reported by Wei et al. [Phys. Rev. Lett. 124, 255502 (2020)] can be confronted with predictive, quantitative theories of negative thermal expansion (NTE) and pressure-induced softening, allowing to corroborate, or invalidate certain approaches. Motivated to corroborate the quantitative predictions of the recent Coulomb Floppy Network (CFN) microscopic theory of vibrational and thermomechanical properties of empty perovskite crystals [Tkachenko and Zaliznyak, arXiv:1908.11643 (2019)], we compared theory prediction for the mean-squared transverse displacement of the F atoms, U$_{perp}$, with that reported in Fig. 5 of Wei et al. and observed a marked discrepancy (an order-of-magnitude larger than the error bar). We then compared these results with the previously published Xray diffraction data of Greve, et al. [JACS 132, 15496 (2010)] and the neutron diffraction data of Wendt, et al. [Science Advances 5 (2019), 10.1126/sciadv.aay2748]. We found the latter two data sets to be in a good agreement with each other, as well as with the prediction of CFN theory. We thus conclude that U$_{perp}$ values reported in Fig. 5 of Wei et al. are substantially incorrect. The purpose of this Comment is twofold: (i) to caution the researchers against using the U$_{perp}$ data of Wei et al. for quantitative comparisons with theory, and (ii) to encourage Wei et al. to reconsider their analysis and obtain a reliable U$_{perp}$ data by better accounting for the beam transmission and attenuation effects.
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