Formation Of Stacking Fault Tetrahedra Research Articles

Formation of Stacking Fault Tetrahedra and Diffuse Streaks along 〈111〉 in the Equiatomic Cr-Co-Ni Medium-Entropy Alloy

Formation of Stacking Fault Tetrahedra and Diffuse Streaks along 〈111〉 in the Equiatomic Cr-Co-Ni Medium-Entropy Alloy

Molecular dynamics studies on formation of stacking fault tetrahedra in FCC metals

The formation of stacking fault tetrahedra (SFT) based on the vacancy clustering mechanism and Frank loop in case of face centered cubic (FCC) metals and alloys is widely accepted, but the corresponding research for SFT formation without the involvement of Frank loop and interaction of dislocations is not well understood. In this article, we attempt to explain the formation of SFT from vacancies, voids and nanoindentation process in case of low stacking fault energy (Ag) and high stacking fault energy (Ni) FCC metals. The atomic scale processes are simulated and step wise evolution of vacancy cluster and dislocation loops to SFT are provided by molecular dynamics method. We describe in this atomistic simulation that the voids containing 13 and 43 vacancy atoms can rearrange themselves to transform to SFT without going through the Frank loop. The present work has also demonstrated the formation of SFT during nano indentation of (111) surface of Ag and Ni through interactions of Shockley partials without the involvement of vacancies or Frank loop.

Atomic origins of radiation-induced defects and the role of lamellar interfaces in radiation damage of titanium aluminide alloy irradiated with Kr-ions at elevated temperature

The irradiation microstructure of a titanium aluminide (TiAl) alloy subjected to in situ transmission electron microscope (TEM) irradiation with 1 MeV Kr ions at the elevated temperature of 873 K was investigated. Triangle and large hexagon shaped volume defects were observed within the γ-TiAl phase in the TEM images of the irradiated microstructure. High resolution TEM images and composition analyses revealed that the volume defects were vacancy-type stacking fault tetrahedra (SFTs). Molecular dynamic simulations showed that the increased diffusion coefficient at the elevated temperature promoted the movement and aggregation of vacancies, leading to the formation and growth of SFTs in the irradiated FCC γ phase. The lamellar interfaces in the irradiation microstructure were more effective for acting as strong sinks to absorb the primary point defects and defect clusters at the elevated temperature. The initial defects at the interfaces of the TiAl alloy enhanced the sink strength of the material and played an important role in refining SFTs near the lamellar interfaces.

Tensile-compression loading and pre-strain effects on the evolution of stacking fault tetrahedra, dislocation density, and free volume in crystal-amorphous thin film interface: A large-scale molecular dynamics study

Molecular dynamics (MD) simulations are carried out to study the tensile-compression cyclic loading behavior and structural changes in Al (metal)-Cu50Zr50 (metallic glass) thin film interface model (TFIM). The model comprises of 2.88 million atoms, interactions between Al, Cu and Zr atoms are modeled using EAM (Embedded Atom Method) potential. The tensile-compression test cycles are performed at a temperature of 100 K and strain rate of 109 s−1 at two pre-strains (e = 0.033 and e = 0.2). The stress-strain curves reveal that during cycle-I tensile loading, TFIM exhibits yield strength of 266 MPa and peak strength of 760 MPa. Increase in the tensile pre-strain from e = 0.033 to e = 0.2 has no significant effect on the compressive yield stress. The deformation mechanism in Al is by slip by partial dislocation nucleation. Shockley partial dislocation density (2 × 1017–4.5 × 1017 m−2) is observed to be the highest of all the other types of dislocations. Complete SFT (stacking fault tetrahedra) is formed in Al region when strained beyond e = 0.033 and during compression cycles only. The number of SFT increases from 8 during tensile to 28 during the compression cycle indicating that compressive test assists the formation of SFT. Beyond the elastic strain, the collapse of (00120) and (0 2 8 2) polyhedral network is observed in Cu50Zr50 metallic glass region at distance of 25 Å from the interface resulting in the creation of low dense regions.

Radiation-induced extreme elastic and inelastic interactions in concentrated solid solutions

One of the biggest challenges in the radiation induced defect science is to understand the complex nature of ion-atom interactions under highly extreme conditions. Here, we report the irradiation induced non-equilibrium defect formation in NiCoCr single phase concentrated solid solution alloy due to (i) the extreme inelastic and (ii) the coupled inelastic and elastic ion-atom interactions. These two conditions are achieved at 5 and 30μm penetration depths along the paths of swift heavy ions (1.542GeV Bi). In general, the irradiation induced damage consists of interstitial-type dislocation loops and vacancy-type stacking fault tetrahedra (SFT). Near the surface (~5μm) where electronic energy loss is dominating (~62.5keVnm−1), the atomic motion primarily results in the formation of SFT. A noticeable increase of dislocation loop formation is observed at 30μm near the maximum energy deposition from elastic interactions (~4.9keVnm−1), as compared to the near surface region (~0.06keVnm−1). Insights on the complex electronic and atomic correlations of extreme energy deposition and dissipation on defect dynamics and structural stability may pave the way for new design principles of radiation–tolerant structural alloys.

Effect of atomic order/disorder on vacancy clustering in concentrated NiFe alloys

Using molecular dynamics simulations, we elucidate the effect of atomic structure on vacancy clustering in ordered (L10) and random NiFe. Based on our simulations, we predict the vacancy evolution to be in complete contrast between the two systems. While large vacancy clusters, i.e., stacking fault tetrahedra (SFT) are formed in the random structure, no clustering is observed in the ordered-L10 structure. Similar simulations are performed on L10-CuAu and L10-TiAl to understand whether SFT formation is generic in L10 structures, or is specific to NiFe. Both materials show SFT formation, thereby highlighting specific defect energetics in L10 NiFe that lead to the lack of vacancy clustering. We elucidate that L10-NiFe has unique thermodynamic and kinetic defect energetics, i.e., antisite energies, vacancy sublattice preference, and directional migration energy barriers that collectively lead to the lack of vacancy clustering. Understanding such defect energetics could open avenues to prevent defect clustering in the vision towards development of radiation-tolerant concentrated alloys for nuclear reactor applications.

Cr-induced fast vacancy cluster formation and high Ni diffusion in concentrated Ni-Fe-Cr alloys

We have performed molecular dynamics (MD) simulations on pure Ni, Ni-Cr, Ni-Fe and Ni-Fe-Cr single phase concentrated solid solution systems to elucidate the kinetics of point defects and vacancy cluster formation. We find that diffusion-based vacancy clustering leads to the formation of stacking fault tetrahedra (SFT) in all of these systems. The presence of Cr leads to faster SFT formation and high Ni diffusion in Ni-Cr binary systems than in pure Ni and Ni-Fe binary systems. The same trend is observed for the ternary systems that have high Cr composition. This Cr-induced effect is due to the low Cr migration barrier that first induces vacancy diffusion and later leads to faster clustering and SFT formation. We find that the fast Ni diffusion is also Cr-induced, where the binding of two vacancies provides much lower migration barrier pathways for Ni diffusion. Due to this ‘Cr-effect’, the vacancy-vacancy pair distribution shows higher peaks in Ni-Cr and Ni-Fe-Cr systems than in pure Ni and Ni-Fe binary systems. The low migration barriers of Cr compared to Ni and Fe are confirmed by density functional theory calculations.

Formation and growth of stacking fault tetrahedra in Ni via vacancy aggregation mechanism

Using molecular dynamics simulations, the formation and growth of stacking fault tetrahedra (SFT) are captured by vacancy cluster diffusion and aggregation mechanisms in Ni. The vacancy-tetrahedron acts as a nucleation point for SFT formation. Simulations show that perfect SFT can grow to the next size perfect SFT via a vacancy aggregation mechanism. The stopping and range of ions in matter (SRIM) calculations and transmission electron microscopy (TEM) observations reveal that SFT can form farther away from the initial cascade-event locations, indicating the operation of diffusion-based vacancy-aggregation mechanism.

Closed and open-ended stacking fault tetrahedra formation along the interfaces of Cu–Al nanolayered metals

Stacking fault tetrahedra (SFTs) are volume defects that typically form by the clustering of vacancies in face-centred cubic (FCC) metals. Here, we report a dislocation-based mechanism of SFT formation initiated from the semi-coherent interfaces of Cu–Al nanoscale multilayered metals subjected to out-of-plane tension. Our molecular dynamics simulations show that Shockley partials are first emitted into the Cu interlayers from the dissociated misfit dislocations along the Cu–Al interface and interact to form SFTs above the triangular intrinsic stacking faults along the interface. Under further deformation, Shockley partials are also emitted into the Al interlayers and interact to form SFTs above the triangular FCC planes along the interface. The resulting dislocation structure comprises closed SFTs within the Cu interlayers which are tied across the Cu–Al interfaces to open-ended SFTs within the Al interlayers. This unique plastic deformation mechanism results in considerable strain hardening of the Cu–Al nanolayered metal, which achieves its highest tensile strength at a critical interlayer thickness of ~4 nm corresponding to the highest possible density of complete SFTs within the nanolayer structure.

Compression-induced stacking fault tetrahedra around He bubbles in Al

Classic molecular dynamics methods are used to simulate the uniform compression process of the fcc Al containing He bubbles. The formation of stacking fault tetrahedra (SFTs) during the collapse of He bubbles is found, and their dependence on the initial He bubble size (0.6–6 nm in diameter) is presented. Our simulations indicate only elastic deformation in the samples for the He bubble size not more than 2 nm. Instead, increasing the He bubble size, we detect several small SFTs forming on the surface of the He bubble (3 nm), as well as the two intercrossed SFTs around the He bubbles (4–6 nm). All these SFTs are observed to be stable under further compression, though there may appear some SF networks outside the SFTs (5–6 nm). Furthermore, the dynamic analysis on the SFTs shows that the yield pressure keeps a near-linear increase with the initial He bubble pressure, and the potential energy of Al atoms inside the SFTs is lower than outside because of their gliding inwards. In addition, the pressure increments of 2–6 nm He bubbles with strain are less than that of Al, which just provides the opportunity for the He bubble collapse and the SFTs formation. Note that the current work only focuses on the case that the number ratio between He atoms and Al vacancies is 1:1.

Effects of Sn on defect structures in high-speed deformed Ni–Sn alloy

Defect structures in compressed and fractured pure Ni and Ni–2 at.%Sn alloy were investigated. The dislocation cell size of high-speed compressed Ni was smaller than that of low-speed compressed Ni. In Ni–Sn, the cell structure was small and independent of compression speed. The effect of solute Sn was very strong, and Sn trapped a high density of dislocations. At the saw-tooth-like fracture tips formed by high-speed elongation, the observed dislocation density was low in both metals, but was lower in Ni than in Ni–Sn. The number of stacking fault tetrahedra (SFTs) formed at the tips was almost the same in Ni as in Ni–Sn. Because the density of the dislocations was very low despite the strong effect of solute Sn, and because the density of formed SFTs was almost the same in both metals at the saw-tooth-like fracture tips, the formation of SFTs likely involves a dislocation-free mechanism during high-speed deformation.

Molecular dynamics studies of the interaction of a/6 ⟨112⟩ Shockley dislocations with stacking fault tetrahedra in copper. Part I: Intersection of SFT by an isolated Shockley

The interactions of Shockley dislocations with stacking fault tetrahedra (SFTs) formed during twinning deformation in fcc crystals have been studied by molecular dynamics using a copper potential. The fundamental mechanisms by which a moving Shockley dislocation interacts with a SFT involve shearing the apex, formation of a residual loop around the SFT and reactions with dislocations at the tetrahedron base. The latter reactions are critical to defect stability; they lead to the formation of Shockley dislocations mobile in two faces of the parent tetrahedron making it penetrable to dislocations moving on adjacent planes during thickening of the twin. Relaxation of SFTs distorted by a homogeneous twinning shear was also studied. These results reveal that parent SFTs convert to structures containing rotated product SFTs, rows of vacancies and stacking faults on planes bounded by partial dislocations. Dislocation mechanisms involved in Shockley–SFT interactions and formation of new defects derived from twin-sheared SFTs are being analyzed.

Structural transformations of stacking fault tetrahedra upon the absorption of point defects

Mechanisms of the structural modification of stacking fault tetrahedra (SFTs) upon the absorption of point defects have been studied by the method of molecular dynamics. The sequential absorption of vacancies by a perfect SFT is accompanied by the following transformations: (i) the formation of a step on one of the SFT faces, (ii) a change in the step sign upon reaching the middle of the face, (iii) the formation of an SFT with truncated vertex, and (iv) the formation of the perfect SFT. Upon the absorption of interstitial atoms, the stages of SFT transformation follow the reverse order.

The vacancy defect clusters in polycrystalline pure aluminum induced by high-current pulsed electron beam

The specimens of polycrystalline pure aluminum were irradiated with high-current pulsed electron beam （HCPEB）. The microstructure of vacancy defect clusters has been investigated in detail by using transmission electron microscopy （TEM）. The results reveal that large numbers of vacancy cells including dislocation loop and even stacking fault tetrahedra （SFT） can be formed in the specimens of polycrystalline pure aluminum irradiated with HCPEB. For the specimen irradiated with one pulse, vacancy dislocation loops were formed in the vacancy cells. SFTs became the dominating structures after five pulses. For the specimen irradiated with ten pulses, dislocation loops were frequently present and SFTs were only formed in some local zones of vacancy cells. In the vicinity of SFT formation, dislocation-free or very low dislocation densities were observed. It is suggested that high stress and strain rate induced by rapid heating and cooling due to HCPEB irradiation could cause the shifting of whole atomic planes synchronously. This is the more probable mechanism of the formation of SFTs.

The transformation of edge dislocation dipoles in aluminium

The transformations of dipoles of dissociated edge dislocations in aluminium are investigated using molecular dynamics for dipole heights ranging from one to ten {1 1 1} interplanar distances and at temperatures up to 900 K. Three types of configurations, hollow, vertical compact and inclined dipoles, are considered and their relative stabilities compared. Hollows are unstable at finite temperature. Dipoles neither annihilate nor form stable hollow cores but transform into height- and temperature-dependent layouts including cores containing ordered free volumes, zigzag faulted dipoles and agglomerated truncated stacking fault tetrahedra (SFT). Small individual SFT are formed at the highest temperatures, attesting to short-range pipe diffusion. These rearranged configurations are strong obstacles to mobile dislocations, thence potential nucleation sites for dislocation entanglement and self-patterning under single slip. A new thermally activated mechanism of SFT formation directly from vacancy clusters resulting from local dipole pinching-off is reported.

格子間原子型転位ループ近傍での積層欠陥四面体の形成

格子間原子型転位ループ近傍での積層欠陥四面体の形成

Study of cascades damage in Ni by MD with different interatomic potentials

Molecular dynamics (MD) simulations, performed with embedded atom potentials, are used to understand the formation of defects following displacement cascades in Ni. Different empirical potentials, presenting large differences in stacking fault energy (SFE), are used. Simulations were conducted with primary knock-on (PKA) atom energies of 5–40 keV at a temperature of 10 K. Defects include, depending on the potential, individual point defects (vacancies and interstitials), dislocation loops and stacking fault tetrahedra (SFT). The results are related to TEM observations, and the mismatch between these two pictures is discussed. It appears that in a collision cascade, the formation of an SFT does not depend only on the SFE but also on other parameters such as the mobility of vacancies and self-interstitials, or the presence of replacement collision sequences. Based on these calculations we suggest that the formation of clusters of vacancies is a prerequisite to the formation of SFTs.

Formation of Stacking Fault Tetrahedra at Twin Lamellae in AISI 304 Austenitic Stainless Steel by High Current Pulse Electron Beam Irradiation

Formation of Stacking Fault Tetrahedra at Twin Lamellae in AISI 304 Austenitic Stainless Steel by High Current Pulse Electron Beam Irradiation

Formation of stacking-fault tetrahedra in aluminum irradiated with high-energy particles at low-temperatures

Stacking-fault tetrahedra (SFT) are typical vacancy clusters in fcc metals. SFT have been, however, considered to be unstable in aluminum because of its high stacking-fault energy, until the recent finding of SFT in thin aluminum foils subjected to a tensile fracture. This study confirmed that SFT form in aluminum following irradiation with high-energy particles. In electron irradiation, SFT with an average size of 2 nm formed below 203 K, while a larger irradiation intensity at lower temperature induced SFT with a larger number density. Irradiation with 60-keV Al + ions induced SFT below room temperature, although the defect yield (the ratio of the number of defect clusters to the number of incident ions) was about 10 3 , considerably smaller than that in the other fcc pure metals. With neutron irradiation below 15 K to a fluence of 2 × 10 2 1 neutrons m - 2 , SFT were not observed at room temperature. Instead, dislocation loops were observed to form and to disappear during observation with 120-kV electrons that do not cause atomic displacements. Tensile fracture of aluminum thin foil induced SFT at up to 400 K, which is close to the temperature at which SFT become unstable in isochronal annealing experiments. These results suggest that a high concentration of vacancies at lower temperature tends to cause SFT to form rather than vacancy-type dislocation loops in aluminum.

アルミニウム中の積層欠陥四面体の形成

アルミニウム中の積層欠陥四面体の形成