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Guiding shear bands in bulk metallic glasses using stress fields : A perspective from the activation of flow units
Controlling shear band propagation is the key to obtain ductile metallic glasses. Here, we use a residual stress field to vary the direction of shear band propagation. We ascribe this behavior to the effect of the stress field on the activation of shear transformation zones (STZs) along their characteristic direction and we quantify this contribution to the energy of the process. Because of the progressively adverse orientation of the stress field, the energy stored as shear in the STZ decreases to a level where shear band propagation at alternative angles becomes energetically more favorable. © 2020 authors.
Phase formation of a biocompatible Ti-based alloy under kinetic constraints studied via in-situ high-energy X-ray diffraction
The biocompatible Ti40Cu34Pd14Zr10Sn2 bulk metallic glass was rapidly heated, also known as flash-annealed, at varying heating rates up to 1579 K/s. Thereby, the phase formation was characterized via advanced in-situ high-energy X-ray diffraction. It has been found that the evolving kinetic constraints can be used as a tool to deliberately alter the crystalline phase formation. This novel processing route permits to select phases to crystallize to a predefined fraction and, thus, to potentially design the microstructure of materials according to a specified property-profile. Consequently, flash-annealing poses a unique synthesis route to design materials with, for instance, good biomechanical compatibility.
Compositional complexity dependence of dislocation density and mechanical properties in high entropy alloy systems
This study focuses on a quantitative analysis of dislocation accumulation after cold plastic deformation and mechanical properties of FeNiCoCrMn and TiNbHfTaZr high entropy alloys (HEAs) which are single phase fcc and bcc solid solutions, respectively. In order to study the role of compositional complexity from unary to quinary compositions on dislocation accumulation and mechanical properties after plastic deformation, the single solid solution phase forming sub-alloys of the two HEAs were investigated. All studied samples revealed a large plastic deformability under cold-rotary swaging process by 85–90% area reduction without intermediate annealing. The dislocation density of all studied samples, determined by Williamson-Hall method on synchrotron X-ray diffraction patterns, were between 1014 - 1015 m−2 dependent on the alloy composition. The level of dislocation density after plastic deformation is not only affected by the number of constituent element but the lattice distortion and intrinsic properties in terms of stacking fault energy, modulus misfit, and melting point also impact the dislocation storage. The level of dislocation density determines the level of mechanical properties because of a resistance to dislocation motions. The hardness and yield compressive strength of the studied samples are proportional to the level of dislocation density.