Local structural modification and rejuvenation of a bulk metallic glass by electrons

Rejuvenation of an amorphous TiAl thin film under external tensile stress by high energy electron irradiation is observed via in-situ transmission electron microscopy (TEM). Electron beam (e-beam) irradiation results in a characteristic change of the elastic properties over time, as measured by the atomic-level elastic strain contained in the TEM diffraction pattern. Specifically, a time dependent increase/decrease of elastic strain is observed along the tensile direction, the saturation value of which correlates linearly with the preceding stress increment/decrement but shows little dependence on the e-beam condition. The low sensitivity of the saturation value to the e-beam condition indicates that the elastic strain change is induced by the structural transitions of a population (dependent on stress increment/decrement) of unstable atomic configurations to local, elastically soft areas. Classical molecular dynamics (MD) simulations including high energy electron scattering events are performed under tensile load to obtain insights into the structural modification that leads to time dependent changes in elastic strain under irradiation. The simulations reveal a change in quantities that are characteristic of structural rejuvenation, with a reduction of the local shear modulus manifesting as time dependent increase in the atomic-level elastic strain at fixed external stress. This link to the experimental data is confirmed by tracking elliptic distortions of simulated diffraction patterns calculated from MD configurations. The presented findings are highly relevant for experimental characterization of amorphous materials using TEM and give a new perspective on local structural modifications by electron irradiation.

Electron beam induced rejuvenation in a metallic glass film during in-situ TEM tensile straining

C. Ebner, J. Rajagopalan, C. Lekka, C. Rentenberger
Acta Mater. (2019) in print.
[doi: 10.1016/j.actamat.2019.09.033]

Characterization of heterogeneities in a CuZr-based bulk metallic glass processed by high-pressure torsion

Cu45Zr45A5Ag5 bulk metallic glass samples, processed by high pressure torsion (HPT) under various conditions, were characterized using synchrotron X-ray diffraction, nanoindentation, differential scanning calorimetry, atomic force and transmission electron microscopy. The experimental results clearly show that HPT modifies the amorphous structure by increasing the mean atomic volume. The level of rejuvenation, correlated with the excess mean atomic volume, is enhanced at higher shear strains as inferred from relaxation enthalpies. By mapping of structural and mechanical quantities, the strain-induced rejuvenated state is characterized on cross-sectional HPT samples on a local scale. A clear correlation both between elastic and plastic softening and between softening and excess mean atomic volume is obtained. But also the heterogeneity of the HPT induced rejuvenation is revealed, resulting in the formation of highly strain-softened regions next to less-deformed ones. A hardness drop of up to 20\% is associated with an estimated increase of the mean atomic volume of up to 0.75%. Based on synchrotron X-ray diffraction and nanoindentation measurements it is concluded that elastic fluctuations are enhanced in the rejuvenated material on different length scales down to atomic scale. Furthermore, the calculated flexibility volume and the corresponding average mean square atomic displacement is increased. The plastic response during nanoindentation indicates that HPT processing promotes a more homogeneous-like deformation.

Structural and mechanical characterization of heterogeneities in a CuZr-based bulk metallic glass processed by high pressure torsion
C. Ebner, B. Escher, C. Gammer, J. Eckert, S. Pauly, C. Rentenberger
Acta Mater. 160 (2018) 147-157.
[doi: 10.1016/j.actamat.2018.08.032]

Medium-range order in CuZrAlAg bulk metallic glasses

(and its influence on the glass-forming ability)

Fluctuation electron microscopy of bulk metallic glasses of CuZrAl(Ag) demonstrates that medium- range order is sensitive to minor compositional changes. By analyzing nanodiffraction patterns medium-range order is detected with crystal-like motifs based on the B2 CuZr structure and its distorted structures resembling the martensitic ones. This result demonstrates some structural homology between the metallic glass and its high temperature crystalline phase. The amount of medium-range order seems slightly affected with increasing Ag concentration (0, 2, 5 at.%) but the structural motifs of the medium-range ordered clusters become more diverse at the highest Ag concentration. The decrease of dominant clusters is consistent with the destabilization of the B2 structure measured by calorimetry and accounts for the increased glass-forming ability.

Influence of the Ag concentration on the medium range order in a CuZrAlAg bulk metallic glass
C. Gammer, B. Escher, C. Ebner, A. M. Minor, H. P. Karnthaler, J. Eckert, S. Pauly, C. Rentenberger
Sci. Rep. 7 (2017) 44903.
[doi: 10.1038/srep44903]

Anelasticity and structural rearrangements in metallic glasses

We used a novel diffraction-based method to extract the local, atomic-level elastic strain in nanoscale amorphous TiAl films during in situ transmission electron microscopy deformation, while simultaneously measuring the macroscopic strain. The complementary strain measurements revealed significant anelastic deformation, which was independently confirmed by strain rate experiments. Furthermore, the distribution of first nearest-neighbor distances became narrower during loading and permanent changes were observed in the atomic structure upon unloading, even in the absence of macroscopic plasticity. The results demonstrate the capability of in situ electron diffraction to probe structural rearrangements and decouple elastic and anelastic deformation in metallic glasses.

Revealing anelasticity and structural rearrangements in nanoscale metallic glass films using in-situ TEM diffraction
R. Sarkar, C. Ebner, E. Izadi, C. Rentenberger, J. Rajagopalan
Mat. Res. Lett. 5 (2017) 135-143.
[doi: 10.1080/21663831.2016.1228709]

Local strain in amorphous materials

The novel technique published in the journal "Ultramicroscopy" facitlitates to measure the atomic-level elastic strain tensor of amorphous materials by tracking geometric changes of the first diffuse ring of selected area electron diffraction patterns. An automatic procedure, which includes locating the centre and fitting an ellipse to the diffuse ring with sub-pixel precision is developed for extracting the 2-dimensional strain tensor from the SAD patterns. The Digital Micrograph Script can be downloaded for free from the corresponding Website. Using this technique, atomic-level principal strains from micrometre-sized regions of freestanding amorphous Ti0.45Al0.55 thin films were measured during in-situ TEM tensile deformation. The thin films were deformed using MEMS based testing stages that allow simultaneous measurement of the macroscopic stress and strain. The calculated atomic-level principal strains show a linear dependence on the applied stress, and good correspondence with the measured macroscopic strains. The calculated Poisson's ratio of 0.23 is reasonable for brittle metallic glasses. The technique shows also the potential to obtain localized strain profiles/maps of amorphous thin film samples.

Local, atomic-level elastic strain measurements of metallic glass thin films by electron diffraction
C. Ebner, R. Sarkar, J. Rajagopalan, C. Rentenberger
Ultramicroscopy 165, 51-58 (2016).
doi: 10.1016/j.ultramic.2016.04.004