Coupled Strengthening Effects by Lattice Distortion, Local Chemical Ordering, and Nanoprecipitates in Medium-Entropy Alloys

Extraordinary mechanical properties can be achieved in high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) with nanoprecipitates. In the present study, the extra coupled strengthening effects by lattice distortion, local chemical ordering, and nanoprecipitates in the HEAs and MEAs with nanoprecipitates have been systematically investigated by large-scale molecular dynamics simulations. The moving of the dislocation can be slowed down, and the dislocation line shows a wavy configuration due to lattice distortion and local chemical ordering, resulting in strengthening. The degree of the wavy configuration increases and the sliding velocity of the dislocation decreases with increasing degrees of local chemical ordering. It is clearly indicated that the dislocation moves via nanoscale segment detrapping mechanism due to the effects of lattice distortion and local chemical ordering, resulting in roughened dislocation pathways for strengthening. The activated nanoscale segments are observed to be easier to detrap from the regions with stronger Co-Cr local chemical ordering and then propagate into the regions without such chemical ordering. These moving characteristics of the dislocation can delay the unpinning process from nanoprecipitates; thus, extra coupled strengthening effect has been revealed in the HEAs and MEAs with nanoprecipitates compared to pure Orowan’s strengthening.

Dislocation Topological Evolution and Energy Analysis in Misfit Hardening of Spherical Precipitate by the Parametric Dislocation Dynamics Simulation

Interaction of a single dislocation line and a misfit spherical precipitate has been simulated by the Parametric Dislocation Dynamics (PDD) method in this research. The internal stress inside the precipitate is deduced from Eshelby’s inclusion theory, the stress of the dislocation line and outside the precipitate is calculated by Green’s function. The influence of different relative heights of the primary slip plane on dislocation evolution is investigated, while the cross-slip mechanism and annihilation reaction are considered. The simulation results show three kinds of dislocation topological evolution: loop-forming (Orowan loop or prismatic loop), helix-forming, and gradual unpinning. The dislocation nodal force and the velocity vectors are visualized to study dislocation motion tendency. According to the stress–strain curve and the energy curves associated with the dislocation motion, the pinning stress level is strongly influenced by the topological change of dislocation as well as the relative heights of the primary slip plane.

Ge/Si Multilayer Epitaxy and Removal of Dislocations from Ge-nanosheet-channel MOSFETs

Horizontally stacked pure-Ge-nanosheet gate-all-around field-effect transistors (GAA FETs) were developed in this study. Large lattice mismatch Ge/Si multilayers were intentionally grown as the starting material rather than Ge/GeSi multilayers to acquire the benefits of the considerable difference in material properties of Ge and Si for realising selective etching. Flat Ge/Si multilayers were grown at a low temperature to preclude island growth. The shape of Ge nanosheets was almost retained after etching owing to the excellent selectivity. Additionally, dislocations were observed in suspended Ge nanosheets because of the absence of a Ge/Si interface and the disappearance of the dislocation-line tension force owing to the elongation of misfit dislocation at the interface. Forming gas annealing of the suspended Ge nanosheets resulted in a significant increase in the glide force compared to the dislocation-line tension force; the dislocations were easily removed because of this condition and the small size of the nanosheets. Based on this structure, a new mechanism of dislocation removal from suspended Ge nanosheet structures by annealing was described, which resulted in the structures exhibiting excellent gate control and electrical properties.

Shape of dislocation line in stochastic shear stress field

The shape of the dislocation line in the stochastic shear stress field in the glide plane was studied using the method of discrete dislocation dynamics. Stochastic shear stresses can occur due to the distortion of the crystal lattice. Such distortion may exist, for example, in a solid solution. Different atoms in a solid solution induce atomic size misfit and elastic modulus misfit into crystal lattice. These misfits result in crystal lattice distortions which varies spatially. The distortions are the origin of internal stresses in the lattice. Such internal stress in certain location has stochastic value normally distributed. The particular case of such stresses is shear stress distribution in the glide plane. The special method was developed to model such stress distribution. The stochastic shear stress field results in movement of different segments of dislocation line to form its equilibrium shape. The important characteristic parameters of the equilibrium shape can be measured by numerical methods. This shape also includes a "long-wavelength" component that has a non-zero amplitude and was formed without thermal activation. The shape of the dislocation line determines to some extent the yield strength of the material. Thus, the study of dislocation line shape and modeling its formation in the field of stochastic shear stresses can help to determine the yield strength of multicomponent alloys, especially multi-principal element alloys. Keywords: dislocation, discrete dislocation dynamics, shear stresses.

Generalized Susceptibility of Mixed Dislocation in Ferroelastics near Structural Phase Transition

Bending vibrations of a mixed dislocation in ferroelastics near structural phase transition were considered. It was assumed that the dislocation line performs small bending vibrations near equilibrium position. Complete system of equations describing the vibrations of a mixed dislocation near the structural phase transition is written. Based on these set of equations describing the vibrations of a crystal with a dislocation near the structural phase transition, written equations for dynamics of the mixed dislocation in linear approximation of dislocation displacement. Fourier transform of these equations is satisfied. Expression for Peach-Kohler force acting on the dislocation is obtained, and linear response function (generalized susceptibility) of the mixed dislocation in ferroelastics is found.

Phase-Field Modeling of Chemoelastic Binodal/Spinodal Relations and Solute Segregation to Defects in Binary Alloys

Microscopic phase-field chemomechanics (MPFCM) is employed in the current work to model solute segregation, dislocation-solute interaction, spinodal decomposition, and precipitate formation, at straight dislocations and configurations of these in a model binary solid alloy. In particular, (i) a single static edge dipole, (ii) arrays of static dipoles forming low-angle tilt (edge) and twist (screw) grain boundaries, as well as at (iii) a moving (gliding) edge dipole, are considered. In the first part of the work, MPFCM is formulated for such an alloy. Central here is the MPFCM model for the alloy free energy, which includes chemical, dislocation, and lattice (elastic), contributions. The solute concentration-dependence of the latter due to solute lattice misfit results in a strong elastic influence on the binodal (i.e., coexistence) and spinodal behavior of the alloy. In addition, MPFCM-based modeling of energy storage couples the thermodynamic forces driving (Cottrell and Suzuki) solute segregation, precipitate formation and dislocation glide. As implied by the simulation results for edge dislocation dipoles and their configurations, there is a competition between (i) Cottrell segregation to dislocations resulting in a uniform solute distribution along the line, and (ii) destabilization of this distribution due to low-dimensional spinodal decomposition when the segregated solute content at the line exceeds the spinodal value locally, i.e., at and along the dislocation line. Due to the completely different stress field of the screw dislocation configuration in the twist boundary, the segregated solute distribution is immediately unstable and decomposes into precipitates from the start.

The Study of the Structural, Morphological, and Mechanical Characteristics of the Laser-Irradiated Aluminium Targets

Aluminium samples are irradiated using a continuous-wave diode laser in a laboratory environment to study the effect on its surface, structural, and mechanical properties. The exposed samples are investigated by scanning electron microscopy and x-ray diffractometer for the surface and structural morphology, respectively. The scanning electron microscopic analysis unveils the realization of micrometer grain size, exfoliational sputtering, and crater production. The diffractometric x-ray analysis reveals the grain size, d-spacing, and dislocation line density of the targeted samples. The hardness of the samples as a function of exposure time is investigated using the micro Vickers hardness tester to perceive the mechanical properties. An increase in micro-hardness is observed with the increase in the exposure time.