arXiv subjects

G. Wilde

Publications and source records attributed to G. Wilde.

7 recordsLinked to original sources

arXiv2021

Self-diffusion in carbon-alloyed CoCrFeMnNi high entropy alloys

Tracer diffusion of the substitutional components in (CoCrFeNiMn)$_{1-x}$C$_x$ high-entropy alloys with x = 0.002, 0.005 and 0.008 (in at. fractions) is measured at elevated temperatures from 1173 to 1373 K. Two different characteristic effects of interstitial carbon addition on substitutional diffusion in these FCC alloys are distinguished. At the highest temperature of 1373 K, alloying by C with relatively low concentrations (x = 0.002) retards diffusion of the substitutional elements with respect to those in the C-free alloy. At lower temperatures and/or higher C concentrations (x > 0.005), an enhancement of the diffusion rates of all substitutional elements is seen. A model is suggested that relates the self-diffusivities in the CoCrFeMnNi-C alloys with the lattice distortion imposed by interstitially dissolved carbon. The experimental results are interpreted in terms of a decrease of the migration barriers for vacancy-mediated diffusion due to the presence of interstitial C atoms.

cond-mat.mtrl-sci↗

arXiv2017

Radioactive isotopes reveal a non sluggish kinetics of grain boundary diffusion in high entropy alloys

High entropy alloys (HEAs) have emerged as a new class of multicomponent materials, which have potential for high temperature applications. Phase stability and creep deformation, two key selection criteria for high temperature materials, are predominantly influenced by the diffusion of constituent elements along the grain boundaries (GBs). For the first time, GB diffusion of Ni in chemically homogeneous CoCrFeNi and CoCrFeMnNi HEAs is measured by radiotracer analysis using the $^{63}$Ni isotope. Atom probe tomography confirmed the absence of elemental segregation at GBs that allowed reliable estimation of the GB width to be about 0.5 nm. Our GB diffusion measurements prove that a mere increase in number of constituent elements does not lower the diffusion rates in HEAs, but the nature of added constituents plays a more decisive role. The GB energies in both HEAs are estimated at about 0.8-0.9 J/m$^2$, they are found to increase significantly with temperature and the effect is more pronounced for the CoCrFeMnNi alloy.

cond-mat.mtrl-sci↗

arXiv2015

Shear band relaxation in a deformed bulk metallic glass

Relaxation of shear bands in a Pd40Ni40P20 bulk metallic glass was investigated by a combination of radiotracer diffusion and molecular dynamics (MD) simulations, allowing to determine for the first time the effective activation enthalpy of diffusion along shear bands in a deformed glass. The shear bands relax during annealing below the glass transition temperature and the diffusion enhancement reveals unexpectedly a non-monotonous behavior. The development of shear bands and the subsequent relaxation of stresses after switching off the shearing are characterized on microscopic to mesoscopic length scales by MD simulation subjecting the model glass to a constant strain rate. Mean-squared displacements as well as strain maps indicate that the heterogeneity, as manifested by shear bands in the systems under shear, persist after the shear is switched off. We observe a further relaxation of residual stresses that remain localized in regions where the shear band has been present before, although the system is - different from the macroscopic experiment - homogeneous with respect to the local density. These results indicate that even on a local scale one may expect strong dynamic heterogeneity in deformed glassy solids due to shear banding. The results thus suggest that plastically deformed metallic glasses present poly-amorphous systems that necessitate descriptions that are analogous to multiphase materials including the presence of heterophase interfaces.

cond-mat.mtrl-sci↗

arXiv2015

Impact of micro-alloying on the plasticity of Pd-based Bulk Metallic Glasses

Micro-alloying was performed using additions of Co and Fe to monolithic Pd40Ni40P20 bulk metallic glass to study selectively the influence on the plastic behavior in uniaxial compression and three-point bending tests. The corresponding Poisson's ratios were determined by ultrasonic measurements. The microstructure of the individual bulk metallic glasses was characterized by electron microscopy, X-ray diffraction and calorimetry. A plastic strain of 13% was found for the Co addition (1 at.%), whereas the Fe addition (0.6 at.%) led to immediate failure after reaching the elastic limit. Surprisingly, the plasticity is not reflected by the high Poisson's ratio of 0.4 since it remained unaffected by the minor alloying.

cond-mat.mtrl-sci↗

arXiv2015

Sub-micron strain analysis of local stick-slip motion of individual shear bands in a bulk metallic glass

Nanodot deposition on a side surface of a rectangular sample and digital image correlation are used to quantify the in-plane strain fields associated with the propagation of a shear band in a PdNiP bulk metallic glass,induced by rolling. Within the resolution of the method related to an average inter-dot distance of 100 nm, deformation is found to be highly localized at the shear bands, while alternating areas with a size of 100 - 400 nm with opposite local shear strains are found. This phenomenon substantiates a local stick-slip nature of shear band propagation during the metallic glass deformation, even during rolling.

cond-mat.mtrl-sci↗

arXiv2014

Wavelike fracture pattern in metallic glasses: a Kelvin-Helmholtz flow instability

We report a wavelike fracture pattern in a Zr-based bulk metallic glass that has been deformed under quasi-static uniaxial tensions at temperatures between room temperature (300 K) and liquid nitrogen temperature (77 K). We attribute this wavelike pattern to a Kelvin-Helmholtz flow instability that occurred at certain interfaces between local cracking/softening regions. The instability criterion for the pattern formation is achieved via a hydrodynamic perturbation analysis, and furthermore an instability map is built which demonstrates that the shear velocity difference on both sides of the interface is the main destabilizing factor. Finally, the characteristic instability time (the inverse of the instability growth rate) is explored by seeking the dispersion relation in the dominant (fastest) instability mode. The results increase the understanding of the flow and fracture of metallic glasses as well as the nature of their liquid structures.

cond-mat.dis-nn↗

arXiv2004

Two-Phase Equilibrium in Small Alloy Particles

The coexistence of two phases within a particle requires an interface with a significant capillary energy. We show that this entails changes in the nature of alloy phase equilibria at small size. Most notably, the eutectic points in alloy phase diagrams degenerate into intervals of composition where the alloy melts discontinuously.

cond-mat.mtrl-sci↗