Advanced Shape-memory Bulk Metallic Glass Composites
Final Report Abstract
Despite the Corona pandemic, which entailed extended periods of lockdowns and experimental downtimes, almost all intended experiments were successfully implemented. Some techniques like digital image correlation or in-situ tensile tests in a TEM had to be discarded. However, we used access to DESY to include systematic levitation experiments instead. The role of various alloying elements in CuZr-based glass formers was assessed and it was found that the thermodynamic stability of the shape-memory phase B2 CuZr can be adjusted and has to be balanced for maintaining a sufficient glass-forming ability. The structural evolution of CuZr-based glasses and of a Zr-based bulk metallic glass (Zr61Cu25Al12Ti2) on supercooling was analyzed. We can attribute the structural changes to a gradually changing population of atomic clusters and to a modified degree of structural heterogeneity. The structure of Zr61Cu25Al12Ti2 upon a very fast heat treatment (flash annealing; heating rates around 100 – 200 K/s) below its crystallization temperature, Tx, can be either relaxed or rejuvenated. The systematic analysis of high-resolution transmission electron microscope images with an autocorrelation function proves that the degree of structural ordering can be tailored in this way. The increase in the volume fraction of regions with a more crystal-like atomic packing and increased structural heterogeneity enhances plastic strain upon compressive loading. Modifying the cooling conditions by using different cooling media (water vs. salt water) has a measurable effect on the enthalpies of relaxation and crystallization. The structural differences reflect in altered yield strengths, plastic strains and a different behaviour of the serrated flow. Flash-annealing a Cu44Zr44Al8Hf2Co2 bulk metallic glass also alters its structure and with it its macroscopic deformation behaviour. Rapid heating to the supercooled liquid region clearly results in an enhanced plastic strain. Apparently, this effect is not only related to an increased amount of free volume but also to a more heterogeneous structure. Complementary molecular dynamics simulations prove that structurally different glassy regions ("hard” and "soft”) promote shear delocalization, i.e., shear band branching. Residual stresses as a result of flash-annealing are crucial for heterogeneous deformation of bulk metallic glasses. The stress fields generated during the heat treatment interact with the stress fields that accompany shear transformation zone activity. In this way, the direction of shear band propagation (its angle with respect to the loading axis) is modified. The yield strength of a monolithic Zr46Cu46Al8 bulk metallic glass reacts very sensitively to multiple nominally identical flash-anneals. Its behaviour is non-monotonic with increasing the number of consecutive anneals and more work is required to understand this unexpected behaviour. We failed to create bulk metallic glass matrix composites in Zr46Cu46Al8 via flash-annealing, which have an optimized microstructure leading to tensile plasticity. In-situ tensile testing inside a microscope, however, provided insights into the martensitic transformation within B2 CuZr crystals as well as revealed details regarding shear banding in the glassy matrix. High-resolution transmission electron microscopy proves that the number of B2 CuZr nanocrystals increases after flash-annealing to above the crystallization temperature, Tx. Not only micron-sized crystals precipitate but also crystals on a smaller length scale. The experiments conducted within this project raised additional questions, which should be tackled in the future. Even though we are optimistic that our work contributes to a more refined understanding of the interrelations between structure and mechanical properties, some aspects remain to be clarified.
Publications
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Glass-forming ability, phase formation and mechanical properties of glass-forming Cu-Hf-Zr alloys. Progress in Natural Science: Materials International, 29(5), 576-581.
Kosiba, K.; Song, Kaikai; Kühn, U.; Wang, Gang & Pauly, S.
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Modulating heterogeneity and plasticity in bulk metallic glasses: Role of interfaces on shear banding. International Journal of Plasticity, 119, 156-170.
Kosiba, K.; Şopu, D.; Scudino, S.; Zhang, L.; Bednarcik, J. & Pauly, S.
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Quantitatively determining the martensitic transformation in a CuZr-based bulk metallic glass composite. Journal of Alloys and Compounds, 782, 961-966.
Xue, Peng; Huang, Yongjiang; Pauly, Simon; Jiang, Songshan; Guo, Shu; Fan, Hongbo; Ning, Zhiliang; Cao, Fuyang & Sun, Jianfei
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Stability of the B2 CuZr phase in Cu-Zr-Al-Sc bulk metallic glass matrix composites. Journal of Alloys and Compounds, 790, 657-665.
Escher, B.; Kaban, I.; Kühn, U.; Eckert, J. & Pauly, S.
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Effect of mechanically induced structural rejuvenation on the deformation behaviour of CuZr based bulk metallic glass. Materials Science and Engineering: A, 773, 138848.
Ebner, Christian; Pauly, Simon; Eckert, Jürgen & Rentenberger, Christian
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Guiding shear bands in bulk metallic glasses using stress fields: A perspective from the activation of flow units. Physical Review B, 102(13).
Kosiba, K.; Scudino, S.; Bednarcik, J.; Bian, J.; Liu, G.; Kühn, U. & Pauly, S.
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Structural rejuvenation and relaxation of a metallic glass induced by ion irradiation. Scripta Materialia, 180, 34-39.
Sun, K.; Wang, G.; Wang, Y.W.; Chen, H.C.; Yan, L.; Pauly, S.; Wu, Y.H.; Weber, H.; Wang, Q.; Huang, B.; Jia, Y.D.; Yi, J. & Zhai, Q.J.
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A refined local structure in a metallic glass tailored via flash-annealing. Materials Characterization, 178, 111214.
Sun, K.; Xu, L.M.; Weber, H.; Wang, G.; Tseng, J.C.; Shen, J.; Wu, S.W.; Bian, X.L.; Kosiba, K.; Kühn, U. & Pauly, S.
