5 Werkstofftechnik
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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.
Predicting nonlinear physical aging of glasses from equilibrium relaxation via the material time
(2022)
The noncrystalline glassy state of matter plays a role in virtually all fields of materials science and offers complementary properties to those of the crystalline counterpart. The caveat of the glassy state is that it is out of equilibrium and therefore exhibits physical aging, i.e., material properties change over time. For half a century, the physical aging of glasses has been known to be described well by the material-time concept, although the existence of a material time has never been directly validated. We do this here by successfully predicting the aging of the molecular glass 4-vinyl-1,3-dioxolan-2-one from its linear relaxation behavior. This establishes the defining property of the material time. Via the fluctuation-dissipation theorem, our results imply that physical aging can be predicted from thermal-equilibrium fluctuation data, which is confirmed by computer simulations of a binary liquid mixture.
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.
Early material damage in equimolar CrMnFeCoNi in mixed oxidizing/sulfiding hot gas atmosphere
(2022)
The challenges to use more varied fuels at medium and high temperatures above 500 °C need to be addressed by tuning the materials toward a better resistance against increased corrosion. As a first step the corrosion processes need to be better understood, especially in the case of the unavoidable and highly corrosive sulfur-based gases. Herein, oxidation/sulfidation of an equimolar CrMnFeCoNi high-entropy alloy is studied at an early stage after hot gas exposure at 600 °C for 6 h in 0.5% SO2 and 99.5% Ar. The oxidation process is studied by means of X-ray diffraction, scanning and transmission electron microscopy, and supported by thermodynamic calculations. It is found that the sulfur does not enter the bulk material but interacts mainly with the fast-diffusing manganese at grain boundary triple junctions at the alloy surface. Submicrometer scaled Cr–S–O-rich phases close to the grain boundaries complete the sulfur-based phase formation. The grains are covered in different Fe-, Mn-, and Cr-based spinels and other oxides.
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.
The crystal structure of martensite in Cu-27at.%Zn-9.0 at.%Al alloy has been studied by using sphericalaberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and geometrical phase analysis (GPA) to examine possible changes in atomic rearrangements during martensitic transformation of this ternary system. Observation along [100]M zone axis is suitable for examining a chemical order of the martensite, and showed that, despite the non-stoichiometry of the alloy, atomic columns containing
Al atoms are imaged and distinguished from the others. On the other hand, observation along [010]M zone axis directly revealed that the parent and martensitic phases possess L21 and 18R (21) structures, respectively. These observations suggested that the martensite retained the local chemical order of the parent phase without shuffling before and after the transformation. GPA revealed that the interface between the two phases was coherent with tilting of the basal plane approximately 6◦ across the boundary, which makes otherwise large inclination small during the martensitic transformation.
The impact of the microstructure of Fe-16Cr-0.2C on high-temperature oxidation – sulphidation in SO2
(2021)
This study elucidates the impact of the microstructure of Fe-16Cr-0.2C on oxide layer formation at 650 ◦C in Ar-0.5 % SO2. A cold-rolled and two heat-treated states of the alloy were exposed for up to 1000 h. The samples were characterised in detail from microstructural and chemical perspectives using scanning electron microscopy (SEM), X-ray diffraction (XRD) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The microstructural modification of the alloy by heat-treatment was advantageous. It was found that Cr-carbides support chromia formation and reduce sulphidation when their area fraction is low and diameter is small.
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.