TY - JOUR A1 - Maaß, Robert T1 - Beyond Serrated Flow in Bulk Metallic Glasses: What Comes Next? N2 - This manuscript is based on an oral contribution to the TMS 2020 annual meeting and is dedicated to Prof. Peter Liaw, who for decades has shown great interest in serrated plastic flow. Here we will focus on the case of bulk metallic glasses, and begin with briefly summarizing some aspects of serrated and non-serrated inhomogeneous flow—a phenomenon that has perplexed materials scientists for decades. Four directions of research are identified that emerged out of the desire to fundamentally understand the intermittent inhomogeneous flow response. These research directions gear away from the phenomenological stress–strain behavior but put the underlying shear defect into focus. Unsolved problems and future research topics are discussed. KW - Non-serrated inhomogeneous flow PY - 2020 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-513106 DO - https://doi.org/10.1007/s11661-020-05985-w SN - 1073-5623 SP - 1 EP - 11 PB - Springer Nature AN - OPUS4-51310 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Das, A. A1 - Dufresne, E.M. A1 - Maaß, Robert T1 - Structural dynamics and rejuvenation during cryogenic cycling in a Zr-based metallic glass N2 - Subjecting metallic glasses repeatedly to liquid nitrogen temperature has become a popular method to homogeneously rejuvenate the material. Here we reveal the atomic-scale structural dynamics using in- situ x-ray photon correlation spectroscopy (XPCS) during and after cryogenic cycling of a Zr-based metallic glass in two structural states (plate and ribbon). Heterogeneous structural dynamics is observed at 300 K that changes to monotonic aging at 78 K. It is found that cryogenic cycling homogenizes the relaxation time distribution. This effect is much more pronounced in the ribbon, which is the only structural state that rejuvenates upon cycling. We furthermore reveal how fast atomic-scale dynamics is correlated with longtime structural relaxation times irrespective of the structural state, and that the ribbon exhibits unexpected additional fast atomic-scale relaxation in comparison to the plate material. A structural picture emerges that points towards heterogeneities in the fictive temperature as a requirement for cryogenic energy storage. KW - Structural dynamics KW - Metallic glass KW - Relaxation KW - Rejuvenation KW - Cryogenic cycling PY - 2020 DO - https://doi.org/10.1016/j.actamat.2020.06.063 SN - 1359-6454 VL - 196 SP - 723 EP - 732 PB - Elsevier Ltd. AN - OPUS4-51311 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Honrao, S. J. A1 - Rizzardi, Q. A1 - Maaß, Robert A1 - Trinkle, D. R. A1 - Hennig, R. G. T1 - Split-vacancy defect complexes of oxygen in hcp and fcc cobalt N2 - One of the most ubiquitous and important defects in solids is oxygen. Knowledge about the solubility and diffusivity of oxygen in materials is crucial to understand a number of important technological processes, such as oxidation, corrosion, and heterogeneous catalysis. Density-functional theory calculations of the thermodynamics and kinetics of oxygen in cobalt show that oxygen diffusing into the two close-packed phases, namely α (hcp) and β (fcc), strongly interacts with vacancies.We observe the formation of oxygen split-vacancy centers (V-Oi-V) in both phases, and we show that this defect complex exhibits a similar migration energy barrier to the vacancy and oxygen interstitials. In contrast to the vacancy and oxygen interstitials, the oxygen split-vacancy centers exhibit an anisotropic strain field that couples to applied stress, making it possible to observe them through an internal friction experiment on quenched cobalt. KW - Split-vacancy defect complexes PY - 2020 DO - https://doi.org/10.1103/PhysRevMaterials.4.103608 VL - 4 IS - 10 SP - 103608-1 EP - 103608-9 PB - American Physical Society AN - OPUS4-51582 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Liu, C. A1 - Ikeda, Yuki A1 - Maaß, Robert T1 - Strain-dependent shear-band structure in a Zr-based bulk metallic glass N2 - This work presents strong evidence for structural damage accumulation as a function of shear strain admitted by shear bands in a Zr-based bulk metallic glass. Analyzing the shear-band structure of shear- band segments that experienced shear strains covering four orders of magnitude with high-angle annular dark field transmission electron microscopy (HAADF-STEM) reveals strongly scattered data with on overall trend of increasing local volume dilatation with increasing shear strain. Locally, however, a variety of trends is observed, which underlines the strong heterogeneity of structural damage in shear bands in metallic glasses. KW - Transmission electron microscopy KW - Metallic glass KW - Shear bands KW - Shear-band structure KW - Shear strain PY - 2021 DO - https://doi.org/10.1016/j.scriptamat.2020.08.030 VL - 190 SP - 75 EP - 79 PB - Elsevier Ltd. AN - OPUS4-52454 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ikeda, Yuki A1 - Mancias, J. A1 - Gan, B. A1 - Maaß, Robert T1 - Evidence of room-temperature shear-deformation in a Cu-Al intermetallic N2 - Lamellar eutectics are known to evidence plastic shear in otherwise brittle intermetallics, if the lamella spacing is small enough. Here we pursue this idea of confined plasticity in intermetallics further and demonstrate room-temperature shear-deformation in a two-phase CuAl 2 -CuAl intermetallic nano- composite. The presence of a phase with a 3-fold symmetry is also revealed after deformation. Simula- tion of transmission electron microscopy images shows this to be monoclinic CuAl. These observations are made in the deformation zone underneath locations of nanoindents, of which the force-displacement curves exhibit an unusual response of continuously increasing pop-in sizes with load. KW - Nanoindentation KW - Intermetallic KW - Nano-composite KW - Shear bands KW - Plasticity PY - 2021 DO - https://doi.org/10.1016/j.scriptamat.2020.08.033 VL - 190 SP - 126 EP - 130 PB - Elsevier Ltd. AN - OPUS4-52455 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Rizzardi, Q. A1 - Derlet, P. M. A1 - Maaß, Robert T1 - Microstructural signatures of dislocation avalanches in a high-entropy alloy N2 - Here, we trace in situ the slip-line formation and morphological signature of dislocation avalanches in a highentropy alloy with the aim of revealing their microstructural degree of localization. Correlating the intermittent microplastic events with their corresponding slip-line patterns allows defining two main event types, one of which is linked to the formation of new slip lines, whereas the other one involves reactivation of already existing slip lines. The formation of new slip lines reveals statistically larger and faster avalanches. The opposite tendency is seen for avalanches involving reactivation of already existing slip lines. The combination of both these types of events represents the highest degree of spatial avalanche delocalization that spans the entire sample, forming a group of events that determine the truncation length scale of the truncated power-law scaling. These observations link the statistics of dislocation avalanches to a microstructural observable. KW - High-entropy alloy KW - Dislocation avalanches PY - 2021 DO - https://doi.org/10.1103/PhysRevMaterials.5.043604 SN - 2475-9953 VL - 5 IS - 4 SP - 3604 PB - American Physical Society CY - College Park, MD AN - OPUS4-52458 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Maaß, Robert A1 - Derlet, P. T1 - Micro-plasticity in a fragile model binary glass N2 - Atomistic deformation simulations in the nominally elastic regime are performed for a model binary glass with strain rates as low as 10 4 /s (corresponding to 0.01 shear strain per 1 μs). A strain rate dependent elastic softening due to a micro-plasticity is observed, which is mediated by thermally-activated localized structural transformations (LSEs). A closer inspection of the atomic-scale structure indicates the material response is distinctly different for two types of local atomic environments. A system spanning iscosahe- drally coordinated substructure responds purely elastically, whereas the remaining substructure admits both elastic and microplastic evolution. This leads to a heterogeneous internal stress distribution which, upon unloading, results in negative creep and complete residual-strain recovery. A detailed structural analysis in terms of local stress, atomic displacement, and SU(2) local bonding topology shows such mi- croscopic processes can result in large changes in local stress and are more likely to occur in geomet- rically frustrated regions characterized by higher free volume and softer elastic stiffness. The thermally- activated LSE activity also mediates structural relaxation, and in this way should be distinguished from stress-driven shear transformation activity which only rejuvenates glass structure. The frequency of LSE activity, and therefore the amount of micro-plasticity, is found to be related to the degree to which the glassy state is relaxed. These insights shed atomistic light onto the structural origins that may govern re- cent experimental observations of significant structural evolution in response to elastic loading protocols. KW - Molecular dynamics KW - Bulk metallic glasses KW - Plasticity KW - Residual strains PY - 2021 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-523782 DO - https://doi.org/10.1016/j.actamat.2021.116771 VL - 209 SP - 116771 PB - Elsevier Ltd. AN - OPUS4-52378 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Fantin, Andrea A1 - Manzoni, Anna Maria A1 - Springer, H. A1 - Darvishi Kamachali, Reza A1 - Maaß, Robert T1 - Local lattice distortions and chemical short-range order in MoNbTaW N2 - Extended X-ray absorption fine structure (EXAFS) conducted on an equiatomic MoNbTaW bcc medium-entropy alloy that was annealed at 2273 K reveals unexpectedly small 1st and 2nd shell element-specific lattice distortions. An experimental size-mismatch parameter, δexp, is determined to be ca. 50% lower than the corresponding calculated value. Around W, short-range order (SRO) preferring 4d elements in the 1st and 2nd shells persists. A Nb-W ordering is found, which is reminiscent of ordering emerging at lower temperatures in the B2(Mo,W;Ta,Nb)- and B32(Nb,W)-phases. With high-temperature ordering preferences in fcc also foreshadowing low-temperature phase, these findings suggest a general feature of high-temperature SRO. KW - High Entropy Alloys KW - Short-range order KW - Lattice distortions KW - EXAFS PY - 2024 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-598164 DO - https://doi.org/10.1080/21663831.2024.2326014 SN - 2166-3831 VL - 12 IS - 5 SP - 346 EP - 354 PB - Taylor & Francis AN - OPUS4-59816 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Fareed, Adnan A1 - Rosalie, Julian M. A1 - Kar, Satyakam A1 - Fähler, Sebastian A1 - Maaß, Robert T1 - Small-scale functional fatigue of a Ni-Mn-Ga Heusler alloy N2 - Functional fatigue of shape-memory alloys is a considerable threat to the reliable service of actuation devices. Here, we demonstrate the essentially degradation-free cyclic phase-transformation behavior of Ni-Mn-Ga microcrystals up to one million stress-driven superelastic cycles. Cyclic dissipation amounts to about 1/5 of the bulk counterpart and remains unaffected during cycling, even after the introduction of dislocation structures via plastic straining. Plastic yielding and the transformation stress largely exceed the known bulk values. However, the transformation-stress is found to depend on plastic pre-straining, which suggests that the size-affected transformation stress is sensitive to the initial defect structure and that it can be tuned by a targeted introduction of dislocations. These findings demonstrate the high suitability of Ni-Mn-Ga as a robust shape-memory alloy in small-scale functional device engineering. KW - Superelasticity KW - Shape-memory alloys KW - Functional fatigue KW - Ni-Mn-Ga PY - 2024 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-600695 DO - https://doi.org/10.1016/j.actamat.2024.119988 VL - 274 SP - 1 EP - 10 PB - Elsevier B.V. AN - OPUS4-60069 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Das, A. A1 - Ott, C. A1 - Pechimuthu, Dinesh A1 - Moosavi, Robabeh A1 - Stoica, M. A1 - Derlet, P. M. A1 - Maaß, Robert T1 - Shear-band cavitation determines the shape of the stress-strain curve of metallic glasses N2 - Metallic glasses are known to have a remarkably robust yield strength, admitting Weibull moduli as high as for crystalline engineering alloys. However, their postyielding behavior is strongly varying, with large scatter in both flow stress levels and strains at failure. Using x-ray tomography, we reveal how a strain-dependent internal evolution of shear-band cavities underlies this unpredictable postyielding response.We demonstrate how macroscopic strain softening coincides with the first detection of internal shear-band cavitation. Cavity growth during plastic flow is found to follow a power law, which yields a fractal dimension and a roughness exponent in excellent agreement with self-similar surface properties obtained after fracture. These findings demonstrate how internal microcracking coexists with shear-band plasticity along the plastic part of a stress-strain curve, rationalizing the large variability of plastic flow behavior seen for metallic glasses. KW - Shear-band cavitation KW - Metallic glasses PY - 2023 DO - https://doi.org/10.1103/PhysRevMaterials.7.023602 SN - 2475-9953 VL - 7 IS - 2 SP - 1 EP - 11 PB - American Physical Society AN - OPUS4-57042 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -