Description
A solid loaded beyond the yield stress loses its elastic properties and becomes plastic. From a microscopic point of view, this corresponds to the condition where plastic regions become so densely packed to give rise to system-spanning structures [1]. This limit for glasses is abrupt, which makes experimental investigations challenging: Brittle glasses exhibit mechanical failure at yield and the transition is strongly dependent on the glass history and preparation protocol [2]. In this talk I present recent experimental results on atomic glasses, combining X-ray Photon Correlation Spectroscopy [3,4] with Fast Scanning Calorimetry [5]. I show that absorbed X-rays create point defects that behave as plastic regions, with their number directly controlled by the irradiation dose.
By following the atomic length-scale dynamics as a function of the exchanged momentum, it is possible to distinguish different regimes: at low defect densities, the defects behave as isolated plastic zones that induce displacements typical of an elastic solid (i.e., ballistic-like dynamics). As defect density increases, the mechanical response of the glass shifts gradually from elastic to more and more plastic behavior, characterized by non-diffusive atomic motion. Eventually, the system approaches a limit where the dynamical properties resemble the ones of a flowing system, marking the reaching of the yield point.
Fast Scanning Calorimetry evidences concurrent shift in thermodynamic properties: the yielded glasses exhibit higher enthalpy than annealed ones, with their state closely matching that of glasses instantaneously quenched from a temperature ~20% above the glass transition.
REFERENCES
1. R. Dasgupta et al., Phys. Rev. Lett. 109, 255502 (2012)
2. M. Ozawa et al., PNAS 115, 6656–6661 (2018).
3. A. Martinelli et al., Phys. Rev. X 13, 041031 (2023).
4. J. Baglioni, A. Martinelli, et al., Rep. Prog. Phys. 87, 120503 (2024).
5. A. Martinelli et al., J. Synchrotron Radiat. 31, 557-565 (2024).
