Abstract text (incl. figure legends and references)

Achieving both temporal and spatial resolution in an analysis opens new ways of investigating materials. Electron Correlation Microscopy (ECM) is a rather new method proposed by He et al. [1] yielding temporally resolved data on structural rearrangements in spatially well-resolved transmission electron microscopy (TEM). This opens up the possibility of observing atomic processes that govern basic mechanisms and the unique properties of amorphous metals.

In the present study, the focus lies on amorphous PdNiP. Pre-treatments, such as annealing and treatments of thermo-mechanical kind, significantly influence the local structure, kinetic stability, and mechanical properties of this bulk metallic glass. Thus, it is of great interest to study the dynamics in the material on the atomic scale after treatment to shed light on the origin of the property changes.

For these investigations, Pd₄₀Ni₄₀P₂₀ samples are produced via copper-mould casting. Basic characterizations of the material are performed, including verifying the amorphous state and the composition. Subsequently, samples are pre-treated thermally in a differential scanning calorimeter (DSC) by implementing an annealing routine below the glass transition temperature. The atomic dynamics of different thermally treated sample states are investigated at room temperature using ECM. This technique is based on investigating the speckle contrasts in dark-field images over time. By that, atomic rearrangements and local dynamics can be studied with nanometer spatial resolution [1,2]. The schematic of an ECM measurement and the corresponding analysis are shown in Fig. 1.

The obtained time series of dark-field images from ECM measurements need to be corrected for drift before speckle contrasts can be analysed. The drift-correction method that we found to work for all our data sets consists of three steps including a contrast enhancement step for more precise drift measurements. From changing speckle contrasts over time an autocorrelation function g₂(t) can be calculated. By fitting a Kohlrausch-Williams-Watts function (KWW) to g₂(t) the characteristic relaxation times τ are found. We analysed partial data stacks to evaluate the resilience of the obtained relaxation times and compared calculated autocorrelation functions of different sample states. The obtained relaxation times for a PdNiP as-cast state depicted in Fig. 2 are a measure of the local dynamics within the time spectrum accessible by the method.

To conclude, the effect of thermal pre-treatments on ECM evaluation parameters of amorphous PdNiP is discussed. Such investigations of atomic mobilities give further insight into the amorphous phase as well as into the local relaxation dynamics of a bulk metallic glass.

[1] L. He at al., Microsc. and Microanal. 21 (2015) 1026-1033.
[2] K. Spangenberg et al., Adv. Funct. Mater. 31 (2021) 2103742.
[3] P. Zhang et al., Nat. Commun. (2018) 9:1129.

Funding by DFG is acknowledged.

Fig. 1: Schematic setup for ECM measurement (left) and data analysis (right), adapted from [3]. Speckle contrast changes in dark-field images over time are correlated and fitted by a stretched exponential function (KWW).
Fig. 2: Relaxation times τ for as-cast PdNiP as obtained after fitting a KWW function to the correlated speckle contrast changes over a measurement time of 20000s.