5 Werkstofftechnik
Filtern
Dokumenttyp
- Zeitschriftenartikel (31)
- Vortrag (1)
- Forschungsdatensatz (1)
Sprache
- Englisch (33)
Schlagworte
- Metallic glasses (6)
- Metallic glass (4)
- Nanoindentation (4)
- Plasticity (4)
- Mechanical properties (3)
- Bulk metallic glasses (2)
- Elastic microstructure (2)
- Fracture surface energy (2)
- Glass (2)
- Molecular dynamics (2)
- Shape-memory alloys (2)
- Shear bands (2)
- Steels (2)
- Transmission electron microscopy (2)
- 150 Years (1)
- 4-Dimensional scanning transmission (1)
- Adolf Martens (1)
- Advanced high strength steels (1)
- Aging (1)
- Al-Cu binary alloy system (1)
- Alloy (1)
- Bond energy (1)
- CALPHAD (1)
- Ceramics (1)
- Creation-relaxation algorithm (1)
- Cryogenic cycling (1)
- Crystal lattice (1)
- Density-based Model (1)
- Deposition microstructure (1)
- Dislocation avalanches (1)
- EXAFS (1)
- Epitaxial films (1)
- Experimental and numerical techniques (1)
- Fracture (1)
- Fracture toughness (1)
- Functional fatigue (1)
- Grain boundaries (1)
- Grain boundary engineering (1)
- Hierarchical microstructure Premartensite (1)
- High Entropy Alloys (1)
- High-entropy alloy (1)
- In situ thermal-annealing experiment (1)
- Incipient plasticity (1)
- Intermetallic (1)
- Intermittent microplasticity (1)
- Lattice distortions (1)
- Liquid metal embrittlement (1)
- Liquid-metal embrittlement (1)
- Magnetic shape memory (1)
- Material failure (1)
- Mechanical deformation (1)
- Meta material (1)
- Metallic materials (1)
- Microplastic stress (1)
- Microspine (1)
- Microstructural changes of a Fe Si alloy (1)
- Microstructural evolution (1)
- Microstructure Design (1)
- Mobility (1)
- Modeling (1)
- Nano-composite (1)
- Nanocrystalline structure (1)
- Ni-Mn-Ga (1)
- NiMaGa (1)
- Non-serrated inhomogeneous flow (1)
- Nucleation tendency (1)
- Plastic deformation (1)
- Powder processing (1)
- Rejuvenation (1)
- Relaxation (1)
- Relaxation metallic glasses (1)
- Residual strains (1)
- Resistance spot welding (1)
- Scale-dependent behavior (1)
- Segregation Engineering (1)
- Shear (1)
- Shear strain (1)
- Shear-band cavitation (1)
- Shear-band structure (1)
- Short-range order (1)
- Size effect (1)
- Slip localization (1)
- Small-scale (1)
- Split-vacancy defect complexes (1)
- Structural dynamics (1)
- Superelasticity (1)
- Thermoplastic forming (1)
- Thin films (1)
- Toughness (1)
- Transparency (1)
- Twinning (1)
- Two-photon-polymerization (1)
- Viscosity (1)
- XPCS (1)
- Yttria stabilized zirconia (1)
- electron microscopy (1)
Organisationseinheit der BAM
- 5 Werkstofftechnik (33) (entfernen)
Paper des Monats
- ja (1)
Eingeladener Vortrag
- nein (1)
The typical experimental conditions inside a transmission electron microscope (TEM), such as ultra-high vacuum, high-energy electron irradiation, and surface effects of ultrathin TEM specimens, can be the origin of unexpected microstructural changes compared with that of bulk material during in situ thermal-annealing experiments. In this paper, we report on the microstructural changes of a Fe–15%Si alloy during in situ TEM annealing, where, in its bulk form, it exhibits an ordering transformation from D03 to B2 at 650 °C. Using a heating-pot type double tilt holder with a proportional–integral–differential control system, we observed the precipitation of α-Fe both at the sample surface and inside the sample. Surface precipitates formed via surface diffusion are markedly large, several tens of nm, whereas precipitates inside the specimen, which are surrounded by Fe-poor regions, reach a maximum size of 20 nm. This unexpected microstructural evolution could be attributed to vacancies on Si sites, which are induced due to high-energy electron irradiation before heating, as well as enhanced thermal diffusion of Fe atoms.
Plastic deformation in crystals is mediated by the motion of line defects known as dislocations.
For decades, dislocation activity has been treated as a homogeneous, smooth continuous process. However, it is now recognized that plasticity can be determined by longrange correlated and intermittent collective dislocation processes, known as avalanches. Here we demonstrate in body-centered cubic Nb how the long-range and scale-free dynamics at room temperature are progressively quenched out with decreasing temperature, eventually revealing intermittency with a characteristic length scale that approaches the Burgers Vector itself. Plasticity is shown to be bimodal across the studied temperature regime, with conventional thermally-activated smooth plastic flow (‘mild’) coexisting with sporadic Bursts (‘wild’) controlled by athermal screw dislocation activity, thereby violating the classical Notion of temperature-dependent screw dislocation motion at low temperatures. An abrupt increase of the athermal avalanche component is identified at the critical temperature of the material.
Our results indicate that plasticity at any scale can be understood in terms of the coexistence of these mild and wild modes of deformation, which could help design better alloys by suppressing one of the two modes in desired temperature Windows.
How thermally activated structural excitations quantitatively mediate transport and microplasticity in a model binary glass at the microsecond timescale is revealed using atomistic simulation. These local excitations, involving a stringlike sequence of atomic displacements, admit a far-field shear-stress signature and underlie the transport of free-volume and bond geometry. Such transport is found to correspond to the Evolution of a disclination network describing the spatial connectivity of topologically distinct bonding environments, demonstrating the important role of geometrical frustration in both glass structure and its underlying dynamics.
Grain-boundary degradation via liquid-metal embrittlement (LME) is a prominent and long-standing failure process in next generation advanced high-strength steels. Here we reveal, well ahead of the crack tip, the presences of nano-scale grains of intermetallic phases in Zn-infiltrated but uncracked grain boundaries with scanning- and 4D transmission electron microscopy. Instead of the often-reported Znrich Fe-Zn intermetallics, the nano-scale phase in the uncracked infiltrated grain boundaries is identified as the G-phase, and its presence reveals the local enhancement of strain heterogeneities in the grain boundary network. Based on these observations, we argue that intermetallic phase formation is not occurring after cracking and subsequent liquid Zn infiltration but is instead one of the primary nanoscopic drivers for grain-boundary weakening and crack initiation. These findings shift the focus of LME from micro- and meso-scale crack investigations to the very early stages immediately following Zn diffusion, after which secondary phase nucleation and growth emerge as the root-cause for failure.
Brittle materials, such as oxide glasses, are usually very sensitive to flaws, giving rise to a macroscopic fracture strength that is much lower than that predicted by theory. The same applies to metallic glasses (MGs), with the important difference that these glasses can exhibit certain plastic strain prior to catas- trophic failure. Here we consider the strongest metallic alloy known, a ternary Co 55 Ta 10 B 35 MG. We show that this macroscopically brittle glass is flaw-insensitive at the micrometer scale. This discovery emerges when testing pre-cracked specimens with self-similar geometries, where the fracture stress does not de- crease with increasing pre-crack size. The fracture toughness of this ultra-strong glassy alloy is further shown to increase with increasing sample size. Both these findings deviate from our classical under- standing of fracture mechanics, and are attributed to a transition from toughness-controlled to strength- controlled fracture below a critical sample size.
We report on the mechanical properties of Cu–Nb alloys produced by combinatorial magnetron sputtering.
Depending on the composition, the microstructure is either fully amorphous (~30–65 at.% Cu), a dispersion of Cu crystallites in an amorphous matrix (~70 at.%), or a dominant crystalline phase with separated nanoscale amorphous zones (~80 at.% Cu). Nanomechanical probing of the different microstructures reveals that the hardness of the fully amorphous alloy is much higher than a rule of mixture would predict. We further demonstrate a remarkable tunability of the resistance to plastic flow, ranging from ca. 9 GPa in the amorphous regime to ca. 2 GPa in the fully crystalline regime. We rationalize these findings based on fundamental structural considerations, thereby highlighting the vast structure-property design space that this otherwise immiscible binary alloy provides.
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.
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.
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.
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.