arXiv subjects

Harald Rösner

Publications and source records attributed to Harald Rösner.

11 recordsLinked to original sources

arXiv2024

Unveiling the Asymmetry in Density within the Shear Bands of Metallic Glasses

Plastic deformation in metallic glasses at room temperature leads to the development of shear bands due to shear localization. In many experiments, shear bands have shown local density variations along their path, with a distinct imbalance in magnitude between local densification and dilation. However, a comprehensive mechanistic understanding or theory to explain this asymmetry has been lacking until now. Here, we introduce a new model that consists of a sequential arrangement of alternating topological 'charges', generating a dipolar field. The resulting microscopic displacement field, when integrated into the deformation gradient tensor, provides an accurate analytical solution for the observed imbalances in the density variations. The implications of this method are discussed, highlighting the potential to elucidate a broader range of observations in shear bands.

cond-mat.dis-nn↗

arXiv2021

A comparison of in- and ex situ generated shear bands in metallic glass by transmission electron microscopy

Shear bands originating from in situ tensile tests of Al$_{88}$Y$_{7}$Fe$_{5}$ melt-spun ribbons conducted in a transmission electron microscope are compared with ones which had formed ex situ during cold rolling. During in situ straining, the observations of a spearhead-like shear front, a meniscus-like foil thickness reduction and no apparent shear steps to accommodate strain suggest shear band initiation by a rejuvenating shear front followed by shearing along the already softened paths. This leads to necking and subsequent failure under the reduced constraint of a 2D geometry in the thin foil and thus explains the observed lack of ductility under tension. In contrast, shear bands formed during cold rolling display distinct alternating density changes and shear off-sets. An explanation for this difference may be that in situ shear bands rip before such features could develop. Moreover, both in and ex situ experiments suggest that initiation, propagation and arrest of shear bands occur during different stages.

cond-mat.mtrl-sci↗

arXiv2021

Plasmon energy losses in shear bands of metallic glass

Shear bands resulting from plastic deformation in cold-rolled Al$_{88}$Y$_{7}$Fe$_{5}$ metallic glass were observed to display alternating density changes along their propagation direction. Electron-energy loss spectroscopy (EELS) was used to investigate the volume plasmon energy losses in and around shear bands. Energy shifts of the peak centre and changes in the peak width (FWHM) reflecting the damping were precisely determined within an accuracy of a few meV using an open source python module (Hyperspy) to fit the shapes of the plasmon and zero-loss peaks with Lorentzian functions. The maximum bulk plasmon energy shifts were calculated for the bright and dark shear band segments relative to the matrix to be about 38 and 14 meV, respectively. The damping was observed to be larger for the denser regions. The analysis presented here suggests that the changes in the plasmons are caused by two contributions: (i) Variable damping in the shear band segments due to changes in the medium-range order (MRO). This affects the static structure factor S(k), which, in turn, leads to either reduced or increased damping according to the Ziman-Baym formula. (ii) The ionic density and the effective electron mass appearing in the zero-momentum plasmon frequency formula $E_p(q=0)$ are coupled and give rise to small variations in the plasmon energy. The model predicts plasmon energy shifts in the order of meV.

cond-mat.mtrl-sci↗

arXiv2020

Impact of severe plastic deformation on the relaxation of glassy and supercooled liquid states of amorphous Pd$_{40}$Ni$_{40}$P$_{20}$

The impact of severe plastic deformation by high-pressure torsion on the relaxation of the glassy and supercooled liquid states of Pd$_{40}$Ni$_{40}$P$_{20}$ was investigated using a combination of differential scanning calorimetry, low-temperature heat capacity and fluctuation electron microscopy. The changes in the calorimetric signals due to deformation and subsequent heat treatments were analyzed and a correlation between deformation (rejuvenation) and annealing (relaxation) was found in relation to medium-range order (MRO). Moreover, a coupling between the occurrence of an exothermic peak in the supercooled liquid state and specific changes in the MRO types were identified. These findings are comprehended in a potential energy landscape scheme offering a new approach for MRO engineering of glasses.

cond-mat.mtrl-sci↗

arXiv2020

The role of minor alloying in the plasticity of bulk metallic glasses

Micro- or minor alloying of metallic glasses is of technological interest. An originally ductile Pd-based monolithic bulk metallic glass (Pd$_{40}$Ni$_{40}$P$_{20}$) was selectively manipulated by additions of Fe or Co. The alloying effects were extreme, showing either exceptional ductility upon Co addition or immediate catastrophic failure upon Fe addition when tested under uniaxial compression or 3-point bending. The amorphous structure was characterized prior to deformation with respect to its medium-range order (MRO) using variable resolution fluctuation electron microscopy (VR-FEM). We observe striking differences in the MRO between the ductile and brittle metallic glasses, with the ductile glasses exhibiting a rich structural diversity and MRO correlation lengths up to 6 nm. The MRO heterogeneity seems to enable easier shear banding and hence enhance the deformability.

cond-mat.dis-nn↗

arXiv2020

What renders bulk metallic glass ductile/brittle? -- new insight from the medium-range order

Understanding ductility or brittleness of monolithic bulk metallic glasses (BMGs) requires detailed knowledge of the amorphous structure. The medium range order (MRO) of ductile Pd$_{40}$Ni$_{40}$P$_{20}$ and brittle Zr$_{52.5}$Cu$_{17.9}$Ni$_{14.6}$Al$_{10}$Ti$_5$ (Vit105) was characterized prior to and after notched 3-point bending tests using variable-resolution fuctuation electron microscopy. Here we show the presence of a second larger MRO correlation length in the ductile material which is not present in the brittle material. A comparison with literature suggests that the larger correlation length accounts for larger shear transformation zones (STZs) which increase the heterogeneity. This enables an easier shear band formation and thus explains the difference in deformability.

cond-mat.mtrl-sci↗

arXiv2020

On the shear-affected zone of shear bands in bulk metallic glasses

Notched bars of bulk metallic glasses, Pd$_{40}$Ni$_{40}$P$_{20}$ and Zr$_{52.5}$Cu$_{17.9}$Ni$_{14.6}$Al$_{10}$Ti$_{5}$, were deformed under 3-point bending conditions, resulting in the formation of shear bands before failure. The immediate environment of shear bands was investigated in detail using fluctuation electron microscopy to extract information on the strain-induced modifications of the medium-range order (MRO) and its lateral extension. Characteristic material-independent gradients were observed for the tensile and compressive sides of the samples indicating the impact of the local stress state on the MRO. Our results reveal an upper limit of a few microns for the lateral extension of the shear-affected environments of shear bands.

cond-mat.mtrl-sci↗

arXiv2019

Grain boundary diffusion and grain boundary structures of a Ni-Cr-Fe-alloy: Evidences for grain boundary phase transformations

Grain boundary tracer diffusion of Ni, Fe and Cr was studied in a Ni-base 602CA coarse-grained alloy. A co-existence of several short-circuit contributions was distinguished at higher temperatures in Harrison's B-type regime (773-873 K), which were related to different families of high-angle grain boundaries with distinct coverages by precipitates and segregation levels as revealed by HAADF-STEM combined with EDX measurements. Annealing at 873 K for 18 hours resulted in Cr23C6-type carbides coexisting with an α-Cr-Mn-enriched phase in addition to sequential segregation layers of Al, Fe and Ni around them. Curved and hackly grain boundaries showed a high density of plate-like carbides, whereas straight grain boundaries were composed of globular carbides with similar chemical composition variations and additionally with alternating layers of Cr and Ni in between the carbides, similar to microstructures after a spinodal decomposition. At lower temperatures, discontinuous interfaces with Cr and Cr-carbide enrichment dominated and the alloy annealed at 403 K for 72 hours contained plate-like Cr23C6-type carbides surrounded by a Ni-rich layer around them. The Ni grain boundary diffusion rates at these relatively low temperatures (formally belonging to C-type kinetics) showed an anomalous character being almost temperature independent. This specific diffusion behaviour was explained by a concomitant relaxation of transformation-induced elastic strains occurring on a longer time scale with respect to grain boundary diffusion. Thermodynamic insights into the probable mechanism of decomposition at grain boundaries are provided.

cond-mat.mtrl-sci↗

arXiv2017

Shear banding in metallic glasses described by alignments of Eshelby quadrupoles

Plastic deformation of metallic glasses performed well below the glass transition temperature leads to the formation of shear bands as a result of shear localization. It is believed that shear banding originates from individual stress concentrators having quadrupolar symmetry. To elucidate the underlying mechanisms of shear band formation, microstructural investigations were carried out on sheared zones using transmission electron microscopy. Here we show evidence of a characteristic signature present in shear bands manifested in the form of sinusoidal density variations. We present an analytical solution for the observed post-deformation state derived from continuum mechanics using an alignment of quadrupolar stress field perturbations for the plastic events. Since we observe qualitatively similar features for three different types of metallic glasses that span the entire range of characteristic properties of metallic glasses, we conclude that the reported deformation behavior is generic for all metallic glasses, and thus has far-reaching consequences for the deformation behavior of amorphous solids in general.

cond-mat.mtrl-sci↗

arXiv2016

Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr Based Metallic Glasses

The interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses is investigated by a combination of high-resolution TEM imaging and molecular-dynamics computer simulations. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles, or can be blocked depending on size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results and experimental findings, a qualitative mechanism map is proposed that categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size, and distance.

cond-mat.mtrl-sci↗

arXiv2015

Quantitative measurement of density in a shear band of metallic glass monitored along its propagation direction

Quantitative density measurements from electron scattering show that shear bands in deformed Al_{88}Y_{7}Fe_{5} metallic glass exhibit alternating high and low density regions, ranging from -9 % to +6 % relative to the un-deformed matrix. Small deections of the shear band from the main propagation direction coincide with switches in density from higher to lower than the matrix and vice versa, indicating that faster and slower motion (stick-slip) occurs during the propagation. Nano-beam diffraction analyses provides clear evidence that the density changes are accompanied by structural changes suggesting that shear alters the packing of tightly-bound short- or medium-range atomic clusters. This bears a striking resemblance to the packing behavior in granular shear bands formed upon deformation of granular media.

cond-mat.mtrl-sci↗