2013, Song, K.K., Pauly, S., Sun, B.A, Tan, J., Stoica, M., Kühn, U., Eckert, J.

The variation of the transformation-mediated deformation behavior with microstructural changes in CuZr-based bulk metallic glass composites is investigated. With increasing crystalline volume fraction, the deformation mechanism gradually changes from a shear-banding dominated process as evidenced by a chaotic serrated flow behavior, to being governed by a martensitic transformation with a pronounced elastic-plastic stage, resulting in different plastic deformations evolving into a self-organized critical state characterized by the power-law distribution of shear avalanches. This is reflected in the stress-strain curves by a single-to-"double"-to-"triple"- double yielding transition and by different mechanical properties with different serrated flow characteristics, which are interpreted based on the microstructural evolutions and a fundamental energy theorem. Our results can assist in understanding deformation behaviors for high-performance metastable alloys.

2015, Gargarella, P., Pauly, S., Stoica, M., Vaughan, G., Afonso, C.R.M., Kühn, U., Eckert, J.

The structural evolution of Ti50Cu43Ni7 and Ti55Cu35Ni10 metallic glasses during heating was investigated by in-situ synchrotron X-ray diffraction. The width of the most intense diffraction maximum of the glassy phase decreases slightly during relaxation below the glass transition temperature. Significant structural changes only occur above the glass transition manifesting in a change in the respective peak positions. At even higher temperatures, nanocrystals of the shape memory B2-Ti(Cu,Ni) phase precipitate, and their small size hampers the occurrence of a martensitic transformation.

2018, Scudino, S., Bian, J.J., Shakur Shahabi, H., Şopu, D., Sort, J., Eckert, J., Liu, G.

A prerequisite to utilize the full potential of structural heterogeneities for improving the room-temperature plastic deformation of bulk metallic glasses (BMGs) is to understand their interaction with the mechanism of shear band formation and propagation. This task requires the ability to artificially create heterogeneous microstructures with controlled morphology and orientation. Here, we analyze the effect of the designed heterogeneities generated by imprinting on the tensile mechanical behavior of the Zr52.5Ti5Cu18Ni14.5Al10 BMG by using experimental and computational methods. The imprinted material is elastically heterogeneous and displays anisotropic mechanical properties: strength and ductility increase with increasing the loading angle between imprints and tensile direction. This behavior occurs through shear band branching and their progressive rotation. Molecular dynamics and finite element simulations indicate that shear band branching and rotation originates at the interface between the heterogeneities, where the characteristic atomistic mechanism responsible for shear banding in a homogeneous glass is perturbed.

2016, Zhang, L., Pauly, S., Tang, M.Q., Eckert, J., Zhang, H.F.

The microstructural evolution of cast Ti/Zr-based bulk metallic glass composites (BMGCs) containing β-Ti still remains ambiguous. This is why to date the strategies and alloys suitable for producing such BMGCs with precisely controllable volume fractions and crystallite sizes are still rather limited. In this work, a Ti-based BMGC containing β-Ti was developed in the Ti-Zr-Cu-Co-Be system. The glassy matrix of this BMGC possesses an exceptional glass-forming ability and as a consequence, the volume fractions as well as the composition of the β-Ti dendrites remain constant over a wide range of cooling rates. This finding can be explained in terms of a two-phase quasi-equilibrium between the supercooled liquid and β-Ti, which the system attains on cooling. The two-phase quasi-equilibrium allows predicting the crystalline and glassy volume fractions by means of the lever rule and we succeeded in reproducing these values by slight variations in the alloy composition at a fixed cooling rate. The two-phase quasi-equilibrium could be of critical importance for understanding and designing the microstructures of BMGCs containing the β-phase. Its implications on the nucleation and growth of the crystalline phase are elaborated.

2011, Ren, J.L., Chen, C., Wang, G., Mattern, N., Eckert, J.

Under compression loading, bulk metallic glasses (BMGs) irreversibly deform through shear banding manifested as a serrated flow behavior. By using a statistical analysis together with a complementary dynamical analysis of the stress-time curves during serrated flow, we characterize the distinct spatiotemporal dynamical regimes and find that the plastic dynamic behavior of a Cu50Zr45Ti5 BMG changes from chaotic to self-organized critical behavior with increasing strain rate. This plastic dynamics transition with the strain rate is interpreted in the frame of the competence between the neighboring elastic strain field forming and relaxation processes.

2015, Okulov, I.V., Soldatov, I.V., Sarmanova, M.F., Kaban, I., Gemming, T., Edström, K., Eckert, J.

Metallic glasses (MGs) inherit their amorphous structure from the liquid state, which predetermines their ability to withstand high loads approaching the theoretical limit. However, the absence of slip systems makes them very sensitive to the type of loading and extremely brittle in tension. The latter can be improved by precipitation of ductile crystals, which suppress a catastrophic propagation of shear bands in a glassy matrix. Here we report a novel approach to obtain MG-matrix composites with tensile ductility by flash Joule heating applied to Cu47.5Zr47.5Al5 (at.%) metallic glass. This homogeneous, volumetric and controllable rapid heat treatment allows achieving uniformly distributed metastable B2 CuZr crystals in the glassy matrix. It results in a significant tensile strain of 6.8±0.5%. Moreover, optimized adjustment of the heat-treatment conditions enables tuning of microstructure to achieve desired mechanical properties.

2016, Tong, X., Wang, G., Stachurski, Z.H., Bednarčík, J., Mattern, N., Zhai, Q.J., Eckert, J.

The structural evolution of a Zr64.13Cu15.75Ni10.12Al10 metallic glass is investigated in-situ by high-energy synchrotron X-ray radiation upon heating up to crystallization. The structural rearrangements on the atomic scale during the heating process are analysed as a function of temperature, focusing on shift of the peaks of the structure factor in reciprocal space and the pair distribution function and radial distribution function in real space which are correlated with atomic rearrangements and progressing nanocrystallization. Thermal expansion and contraction of the coordination shells is measured and correlated with the bulk coefficient of thermal expansion. The characteristics of the microstructure and the yield strength of the metallic glass at high temperature are discussed aiming to elucidate the correlation between the atomic arrangement and the mechanical properties.

2011, Shpak, A.P., Il’inskii, A.G., Marunyak, A.V., Slukhovskyy, O.I., Lepeeva, Yu. V., Dekhtyar, A., Kaban, I., Mattern, N., Eckert, J.

Crystallization of Fe82Si2B16 and Fe82Si4B14 metallic glasses upon heat treatment has been studied. The amorphous ribbons have been isothermally annealed at different temperatures (673, 693, 733 and 743 K) and for various times (from 15 min to 78 hours). Phase compositions and the sequence of their appearance in dependence on the annealing temperature and time have been established.

2016, Rounaghi, S.A., Eshghi, H., Scudino, S., Vyalikh, A., Vanpoucke, D.E.P., Gruner, W., Oswald, S., Rashid, A.R. Kiani, Khoshkhoo, M. Samadi, Scheler, U., Eckert, J.

Hexagonal Aluminium nitride (h-AlN) is an important wide-bandgap semiconductor material which is conventionally fabricated by high temperature carbothermal reduction of alumina under toxic ammonia atmosphere. Here we report a simple, low cost and potentially scalable mechanochemical procedure for the green synthesis of nanostructured h-AlN from a powder mixture of Aluminium and melamine precursors. A combination of experimental and theoretical techniques has been employed to provide comprehensive mechanistic insights on the reactivity of melamine, solid state metal-organic interactions and the structural transformation of Al to h-AlN under non-equilibrium ball milling conditions. The results reveal that melamine is adsorbed through the amine groups on the Aluminium surface due to the long-range van der Waals forces. The high energy provided by milling leads to the deammoniation of melamine at the initial stages followed by the polymerization and formation of a carbon nitride network, by the decomposition of the amine groups and, finally, by the subsequent diffusion of nitrogen into the Aluminium structure to form h-AlN.

2014, Tan, J., Wang, G., Liu, Z.Y., Bednarčík, J., Gao, Y.L., Zhai, Q.J., Mattern, N., Eckert, J.

A model Zr41.25Ti13.75Ni10Cu12.5Be22.5 (at.%) bulk metallic glass (BMG) is selected to explore the structural evolution on the atomic scale with decreasing temperature down to cryogenic level using high energy X-ray synchrotron radiation. We discover a close correlation between the atomic structure evolution and the strength of the BMG and find out that the activation energy increment of the concordantly atomic shifting at lower temperature is the main factor influencing the strength. Our results might provide a fundamental understanding of the atomic-scale structure evolution and may bridge the gap between the atomic-scale physics and the macro-scale fracture strength for BMGs.