Generalized stacking fault energy surface mismatch and dislocation transformation

Even though the fundamental rules governing dislocation activities have been well established in the past century, we report a phenomenon, dislocation transformation, governed by the generalized-stacking-fault energy surface mismatch (GSF mismatch for short) between two co-existing phases. By carrying out ab-initio-informed microscopic phase-field simulations, we demonstrate that the GSF mismatch between a high symmetry matrix phase and a low symmetry precipitate phase can transform an array of identical full dislocations in the matrix into an array of two different types of full dislocations when they shear through the precipitates. The precipitates serve as a passive Shockley partial source, creating new Shockley partial dislocations that are neither the ones from the dissociation of the full dislocation. This phenomenon enriches our fundamental understanding of partial dislocation nucleation and dislocation-precipitate interactions, offering additional opportunities to tailor work-hardening and twinning processes in alloys strengthened by low-symmetry precipitate phases.

Size-induced twinning in InSb semiconductor during room temperature deformation

Room-temperature deformation mechanism of InSb micro-pillars has been investigated via a multi-scale experimental approach, where micro-pillars of 2 µm and 5 µm in diameter were first fabricated by focused ion beam (FIB) milling and in situ deformed in the FIB-SEM by micro-compression using a nano-indenter equipped with a flat tip. Strain rate jumps have been performed to determine the strain rate sensitivity coefficient and the related activation volume. The activation volume is found to be of the order of 3–5 b3, considering that plasticity is mediated by Shockley partial dislocations. Transmission electron microscopy (TEM) thin foils were extracted from deformed micro-pillars via the FIB lift-out technique: TEM analysis reveals the presence of nano-twins as major mechanism of plastic deformation, involving Shockley partial dislocations. The presence of twins was never reported in previous studies on the plasticity of bulk InSb: this deformation mechanism is discussed in the context of the plasticity of small-scale samples.

Molecular dynamics study on the relationship between phase transition mechanism and loading direction of AZ31

To develop and design mg-based nanoalloys with excellent properties, it is necessary to explore the forming process. In this paper, to explore the effect of different loading directions on the phase transformation of magnesium alloy, the model of AZ31 magnesium alloy was established, the process of Uniaxial Compression (UC) of magnesium alloy in different directions was simulated, the changes of atomic position and phase structure were observed, and the phase transformation mechanism of AZ31 magnesium alloy under uniaxial compression under different loading directions was summarized. The conclusions are as follows: the stress and strain, potential energy and volume change, void evolution, phase structure change and dislocation evolution of magnesium alloy are consistent, and there is no significant difference. In the process of uniaxial compression, the phase transformation of hexagonal closely packed (HCP) → face-centered cubic (FCC) is the main, and its structure evolves into HCP → Other → FCC. Shockley partial dislocations always precede FCC stacking faults by about 4.5%, and Shockley partial dislocations surround FCC stacking faults. In this paper, the phase transformation mechanism of AZ31 magnesium alloy under uniaxial compression under different loading directions is summarized, which provides a theoretical basis for the processing and development of magnesium-based nanoalloys.

Molecular dynamics simulation of the effect of solute atoms on the compression of magnesium alloy

Magnesium alloys have a wide range of application values. To design and develop magnesium alloys with excellent mechanical properties, it is necessary to study the deformation process. In this paper, the uniaxial compression (UC) process of AZ31 magnesium alloy with different solute atom content is simulated by the molecular dynamics method. The effect of the solute atom on the uniaxial compression of magnesium alloy is investigated. It is found that solute atoms can inhibit the grain refinement of magnesium, can effectively improve the plastic strength of the alloy, can change the lattice distortion during uniaxial compression of magnesium alloy, can inhibit the generation of BCC structure, and can slow down the increase of FCC structure and dislocation density. The direction of the FCC structure diffusion is 90° to the grain boundary direction. Shockley partial dislocations are generated around the FCC structure. The direction in which the FCC structure spreads is consistent with the direction in which Shockley partial dislocations move.

Effects of temperature and FCC phase size on the deformation mechanism of pure titanium nanopillars: A molecular dynamics simulation

The mechanism of plastic deformation under tensile and compressive loading of hexagonal close-packed (HCP)/face-centered cubic (FCC) biphasic titanium (Ti) nanopillars at different temperatures (70 K, 150 K, 300 K and 400 K) and different FCC phase sizes (2 nm, 4 nm, 6 nm and 8 nm) was investigated by molecular dynamics (MD). The plastic deformation is mainly concentrated in the FCC phase during compression loading. The HCP/FCC interface is the main source of [Formula: see text] Shockley partial dislocations. As the temperature increases, the dislocation nucleation rate increases and the surface dislocation source is activated. During tensile loading, it is more likely that the Shockley partial dislocations react with each other in the FCC phase to form Lomer–Cottrell sessile dislocations and stacking fault (SF) nets. When the temperature is reduced to 70 K, tensile twins are formed at the phase interface. The plastic deformation is dominated by twins and [Formula: see text] dislocation slip occurs in the HCP phase. The effect of the FCC phase size on the plastic deformation mechanism of the nanopillar is strong. The FCC phase is transformed into the HCP phase when the FCC phase size in the nanopillar is reduced to 4 nm under compressive loading. However, twin deformation occurs at the HCP/FCC interface when the FCC phase size is reduced to 2 nm under tensile loading.

Deformation Mediated by Grain Rotation in Superhard Nanocrystalline cBN

Superhard materials such as diamond and cubic boron nitride (cBN) are becoming ever more scientifically and technologically important, and critical and fundamental knowledge about their constitutive properties and deformational mechanisms is in increasingly high demand. Although it has long been suggested by theoretical modeling that deformation of face-centered cubic superhard materials is dominated by Shockley partial dislocations and screw dislocations, there has been a glaring lack of experimental evidence. Here, we report in situ deformation experiments of nanocrystalline cBN (nc-cBN) samples at high pressures and temperatures using a deformation-DIA (D-DIA) apparatus coupled with synchrotron X-ray diffraction techniques. Intrinsic stress-strain relations have been obtained for nc-cBN for the first time, and only elastic deformation occurred up to a strain of at least 14% at room temperature (RT), demonstrating its remarkable strength, which was undoubtedly enhanced by observed microscopic features such as the Lomer-Cottrell (L-C) locks and high-angle GBs. While deformation at RT is dominated by brittle fractures and mechanic crushing induced by grain boundary twisting mediated by full dislocations, plasticity of nc-CBN at higher temperatures is controlled by grain rotation and twinning mediated by Shockley partial dislocations. At 4.0 GPa and 1200 °C, accumulated shear strain resulted in the conversion of cBN to hBN at or near twisting GBs, releasing stress and mediating deformation in the process. We demonstrate the apparent agreement between the differential micro-stress derived from peak broadening analysis and differential macro-stress deduced from lattice strain analysis.