S T R U C T U R E O F M A T E R I A L S

S C I E N T I F I C H I G H L I G H T S

1 4 4 H I G H L I G H T S 2 0 2 1 I

PRINCIPAL PUBLICATION AND AUTHORS

In situ visualization of long-range defect interactions at the edge of melting, L.E. Dresselhaus-Marais (a), G. Winther (b), M. Howard (c), A. Gonzalez (c), S.R. Breckling (c), C. Yildirim (d), P.K. Cook (e), M. Kutsal (b,f), H. Simons (b), C. Detlefs (f), J.H. Eggert (a), H.F. Poulsen (b), Sci. Adv. 7, 29 (2021); https:/doi.org/10.1126/sciadv.abe8311

(a) Physics Division, Lawrence Livermore National Laboratory, California (USA) (b) Technical University of Denmark, Department of Mechanical Engineering, Kgs. Lyngby (Denmark) (c) Nevada National Security Site, North Las Vegas (USA) (d) CEA Grenoble, Grenoble (France) (e) Universität für Bodenkultur Wien, Vienna (Austria) (f) ESRF

to break down shifting from classical elastic theory to dynamics better described by statistical mechanics.

This work presents the first real-time version of dark-field X-ray microscopy (DFXM) at beamline ID06 to watch directly how dislocations evolve just at the cusp of melting (Figure 126). Melting is the phase transition we see in our everyday lives, but no unified model can currently connect the microscopic loss of order to the macroscopic transformation energy and kinetics. The role of defects is proposed to be essential to connect the scales of this multi- scale transformation; however, the relevant mesoscale experiments have never been accessible.

This study maps out the positions of 12 dislocations that define a classical boundary for 10 minutes, as the temperature is slowly heated from 97-99% of the melting temperature (Figure 127). Over this time, the known stabilising behaviour of the boundary is demonstrated as a nearby dislocation inserts into the lower of the initially separate segments, causing them to coalesce at an unprecedented 98% of the melting temperature. Following this, subtle increases to the variance of each dislocation s position cause the boundary structure to destabilise

by 99% of the melting temperature only 6oC hotter in temperature. Corroborated by force-field models, the runaway increase in the amount of dislocation motion indicates the boundary loses its inherent stability, just at the cusp of melting.

This work demonstrates an important step forward both for melting physics, and for dislocation characterisation. By presenting the first real-time DFXM with 150-nm resolution and ~150x150-μm2 field of view, the ~4 Hz movies and associated image-processing methods allow a new generation of microstructural tracking experiments. This demonstrates an important new capability at the ESRF, and offers key opportunities across many fields.

By visualising and quantifying thermally activated dynamics that were previously limited to theory, this work demonstrates a new class of bulk measurements that is now accessible with time-resolved DFXM, offering key opportunities across materials science. This work presents a large step forward for materials science, physics and related fields, as it offers a unique new way to view the intermediate scales that connect microscopic defects to the bulk properties they cause.

Fig. 127: Quantified positions of all dislocations in the tilt boundary

for the 10-minute duration of the movies described in this work,

decomposed into their climb and glide directions (corresponding to different mechanisms of motion).

The dynamics of the dislocation insertion, escape and absorption at lower temperatures reveal the

classical dislocation dynamics at an unprecedented 0.98Tm,

while the enhanced spacing and positional variance at higher temperatures demonstrates

the runaway destabilisation of the boundary, just at the cusp

of melting.