Strukturelle Inhomogenitäten metallischer Massivgläser
Zusammenfassung der Projektergebnisse
The most important findings, conclusions and outcomes of our work are: An unprecedented systematic and comprehensive approach was chosen in this project to study the structure-property relationship in a family of CuZr-based bulk metallic glasses. The fundamental questions of our proposal are still intensely investigated to date and, nonetheless, no such systematic investigation has been reported so far. Especially the combination of methods providing average structural information (high-energy X-ray diffraction) with local probes (fluctuation electron microscopy) is helpful for gaining a better understanding of how mechanical response and structure are intertwined in metallic glasses. - Within this project we reacted to technological and scientific advances and included processes such as flash-annealing and cryogenic thermal cycling in order to modify the structure and, with it, the mechanical properties of monolithic metallic glasses. - The as-cast state is already subjected to a significant structural heterogeneity. The scatter of the data makes it very difficult to detect subtle structural changes after mild pre-treatments (elastic predeformation and cryogenic thermal cycling). The degree of heterogeneity in the as-cast state seems to be pivotal for the extent of structural rejuvenation after the different pre-treatments. These aspects are not sufficiently considered in the community. - We qualitatively discuss our findings in terms of the potential energy landscape and the effects the different pre-treatments have on the details of it. - Fluctuation electron microscopy is capable of detecting small differences in the structure of metallic glasses: The medium-range order of CuZr-based glasses 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. Flash-annealing also alters the medium-range order. Based on the pair persistence model the correlation length related to medium range order was derived that shows a continuous increase with increasing annealing temperature. - Codes were developed for analyzing electron and X-ray diffraction patterns, which became necessary because of the tremendous amount of data. These codes are available to other colleagues. Also, a dedicated sample holder was developed for the synchrotron measurements. - Unfortunately, we did not have the desired detector available during on beamtime at the ESRF, which made the direction-dependent analysis of atomic strains impossible. Because we measured a large number of samples, it was not possible to repeat all measurements at later beamtimes. - Plastic deformation increases the heterogeneity of the glasses owing to the generation of shear bands and residual stresses. This affects the activation energies of shear transformations, which are the starting points for irreversible flow. In this way, strain delocalization can be promoted. - High-pressure torsion induces a rejuvenated amorphous structure characterized by an increased average free volume compared to the as-cast material. - A clear correlation both between elastic and plastic softening and between softening and excess mean atomic volume is obtained for glasses deformed by high-pressure torsion. The work shows that the change of Young’s modulus and hardness due to structural rejuvenation follows the same relationship as for alloying. On the basis of nanoindentation measurements, the measured correlation was used to evaluate the excess mean atomic volume, flexibility volume and the corresponding average mean square atomic displacement on the micron-scale. - The atomic strains in bulk metallic glass matrix composites were analyzed by means of diffraction data for both phases simultaneously for the first time. The crystalline volume fraction and distribution of crystals have a pronounced effect on the respective atomic strains. - The deformation-induced martensitic transformation in the B2 CuZr crystals of the composites exhibits an orientation dependence. - The border-crossing expertise, exchange of samples and the intensive collaboration with our Austrian partner was the cornerstone of this project.
Projektbezogene Publikationen (Auswahl)
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Microstructural evolution and mechanical behaviour of metastable Cu-Zr-Co alloys. J. Mater. Sci. Techn. 30, 584 (2014)
Pub1 S. Pauly, K. Kosiba, P. Gargarella, B. Escher, K.K. Song, G. Wang, U. Kühn and J. Eckert
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Atomic structure and formation of CuZrAl bulk metallic glasses and composites. Acta Mater. 100, 369 (2015)
I. Kaban, P. Jovari, B. Escher, D.T. Tran, G. Svensson, M.A. Webb, T.Z. Regier, V. Kokotin, B. Beuneu, T. Gemming, J. Eckert
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Structural and mechanical characterization of heterogeneities in a CuZr-based bulk metallic glass processed by high pressure torsion. Acta Mater. 160, 147 (2018)
C. Ebner, B. Escher, C. Gammer, J. Eckert, S. Pauly, C. Rentenberger
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Rentenberger and S. Pauly: Influence of Ag and Co additions on glass-forming ability, thermal and mechanical properties of Cu-Zr-Al bulk metallic glasses. Mater. Sci. Eng. A 673, 90 (2016)
B. Escher, U. Kühn, J. Eckert, C.
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Influence of the Ag concentration on the medium-range order in a CuZrAlAg bulk metallic glass. Sci. Rep. 7, 44903 (2017)
Gammer, B. Escher, C. Ebner, S. Pauly, J. Eckert and C. Rentenberger
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Modulating heterogeneity and plasticity in bulk metallic glasses: Role of interfaces on shear banding. Int. J. Plast. 119 (2019) 156
K. Kosiba, D. Sopu, S. Scudino, L. Zhang, J. Bednarcik and S. Pauly
