Formation Of Prismatic Dislocation Loops Research Articles

Microstructural evolution of Ti2ZrHfV0.5Mo0.2 refractory multi-principal alloy under low-energy, high-flux He ion irradiation

The irradiation response of refractory multi-principal element alloys (RMPEAs) has gained increasing attention for the potential applications in future fusion devices. In controlled fusion reactors, plasma-facing materials have to suffer continuous low-energy high flux helium (He) ion irradiations. During high flux He irradiation, vacancy-interstitial pairs can be generated through trap mutation mechanism even without cascade collision. To simplify the model, Ti2ZrHfV0.5Mo0.2 RMPEA was irradiated with 50 eV He+ (less than the minimum He displacement energy of the alloy EdTi = 67 eV) under 1200 K to various fluences of 5 × 1024 - 1 × 1026 ions/m2, and the microstructural evolution was investigated. In the irradiated RMPEA, the intense sputtering of Ti, the growth and degradation of fuzz mainly composed of Zr, Hf elements on the surface, as well as the formation of V, Mo enriched band and high density large He cavities in the subsurface layer were observed. Although the sputtering threshold energy for He of V is similar to that of Ti (ETi = 24 eV, EV = 29 eV), V showed anomalous enrichment in the subsurface layer. Moreover, unlike the W fuzz growth process, the adatoms used to form fuzz in Ti2ZrHfV0.5Mo0.2 were derived from the Zr, Hf atoms diffusing through the vacancy-solute exchange mechanism, rather than from the atoms formed by the loop-punching mechanism. The anomalous enrichment of V atoms and the different fuzz growth mechanism were attributed to (i) different migration rates for different species of atoms in the alloy under vacancy diffusion mechanism and (ii) the formation of AB2 (A = Ti, Zr, Hf; B = V, Mo) phase blocking the continued formation of prismatic dislocation loops mainly composed of V and Mo.

Molecular dynamics study of temperature effect on deformation behavior of m-plane 4H–SiC film by nanoindentation

In this study, the effect of temperature on the nanoscale mechanical properties and deformation mechanism of m-plane 4H–SiC film was investigated by nanoindentation. Simulation results show that the hardness decreases as the temperature increases, while the pile-up events that appear in the indentation process become more obvious. The atomic displacement and strain enhance with increasing temperature, promoting the nucleation and propagation of dislocations in the main slip system, with the formation of prismatic dislocation loops moving along the two <112‾0> directions. Moreover, increasing the temperature not only increases dislocation density but also strengthens plastic deformation. This work contributes to the comprehensive understanding of temperature effect on the mechanical properties and deformation behavior of 4H–SiC film at the nanoscale when subjected to external loads.

Formation of prismatic dislocation loops during unloading in nanoindentation

The formation of prismatic dislocation loops during nanoindentation of bcc iron is simulated with discrete dislocation plasticity. Interestingly, it is not the loading phase, but the unloading phase of the test which is found to be crucial for prismatic loop formation when indenting a (001) crystal surface. In this case, prismatic loops do not form during loading, but develop as the indenter is removed, in contrast to (111) indentation where prismatic loops are nucleated almost immediately upon loading.

Formation of prismatic dislocation loops in a spherical particle embedded in a semi-infinite matrix

The introduction of prismatic dislocation loops in the interface between a misfitting spherical particle and its semi-infinite matrix has been theoretically investigated. The equilibrium position of one isolated loop has been first determined from an energy variation calculation and a shifting effect on the dislocation position relative to the particle equatorial plane has been identified due to the matrix free-surface. The case of two dislocation loops lying in the particle–matrix interface has been then discussed.

Atomistic insights into the prismatic dislocation loop on Al (1 0 0) during nanoindentation investigated by molecular dynamics

To understand the dislocation mechanism during nanoindentation on metallic materials, molecular dynamics has been widely used. It has been found that isolated prismatic dislocation loop can be formed beneath the indenter among many materials. However, there is still argument on its formation mechanism. Herein, molecular dynamics simulation has been carried on Al (1 0 0) to understand the formation mechanism of prismatic dislocation loop during nanoindentation by a rigid hemispherical diamond indenter. Based on the force-displacement curve, the generation and evolution of the crystal defects are analyzed. A new three-step mechanism for the formation of prismatic dislocation loop is proposed, and it includes three steps: (1) reactions happen between Shockley partial dislocations, and then a stair-rod forms; (2) the resultant stair-rod reacts with a Shockley partial dislocation, and then a new Shockley partial dislocation forms; (3) annihilation occurs between Shockley partial dislocations and finally a prismatic loop is released. In addition, the motion of the prismatic dislocation loop can be influenced greatly by holding the indenter at the maximum indentation depth.

Interactions between prismatic dislocation loop and coherent twin boundary under nanoindentation investigated by molecular dynamics

Despite the intense interest in coherent twin boundary (CTB) of nanotwinned materials, many details of interactions between the complex dislocation (prismatic dislocation loop (PDL)) and CTB in a single crystal remain mysteries. In this study, molecular dynamics (MD) simulation of the nanoindentation on single crystal copper with CTBs has been presented. At the initial stage of nanoindentation, indenter moves downward and PDLs form. Thompson's tetrahedron notation has been adopted to scrutinize the process of PDL's formation without cross-slip and release from the indented place. Dislocations at PDLs' side edge are stair-rod partial dislocations, including two Lomer-Cottrell locks and two Hirth Locks. Then the interactions between PDL and CTB drive the migration of CTB. As a result, a parallelogram step which is bounded by Frank partial dislocations has been left, implying the formation of twinning dislocations. When the indenter moves further, the interactions repeat and twinning dislocations multiply. Our simulation results shed light on the interactions between PDL and CTB and would provide a new insight into understanding the role CTB plays during mechanical deformation.

Initial stages of misfit stress relaxation through the formation of prismatic dislocation loops in GaN–Ga2O3 composite nanostructures

The initial stages of misfit stress relaxation through the formation of rectangular prismatic dislocation loops in model composite nanostructures have been considered. The nanostructures are either spherical or cylindrical GaN shells grown on solid or hollow β-Ga2O3 cores or planar thin GaN films on β-Ga2O3 substrates. Three characteristic configurations of prismatic dislocation loops, namely, square loops, loops elongated along the GaN/Ga2O3 interface, and loops elongated along the normal to the GaN/Ga2O3 interface, have been analyzed. The generation of prismatic dislocation loops from the interface into the bulk of the GaN shell (film), from the free surface into the GaN shell (film), and from the interface into the β-Ga2O3 core (substrate) has been investigated. It has been shown that, for the minimum known estimate of the lattice misfit (2.6%) in some of the considered nanostructures, no any prismatic dislocation loops can be generated. If the generation of prismatic dislocation loops is possible, then in all the considered nanostructures, the energetically more favorable case corresponds to prismatic dislocation loops elongated along the GaN/Ga2O3 interfaces, and the more preferred generation of prismatic dislocation loops occurs from the GaN free surface. The GaN/Ga2O3 nanostructures that are the most and least resistant to the formation of prismatic dislocation loops have been determined. It has been found that, among the considered nanostructures, the planar two-layer GaN/Ga2O3 plate is the most resistant to the generation of prismatic dislocation loops, which is explained by the action of an alternative mechanism for the relaxation of misfit stresses due to the bending of the plate. The least resistant nanostructure is the planar three-layer GaN/Ga2O3/GaN plate, in which GaN films have an identical thickness and which itself as a whole does not undergo bending. The critical thicknesses of the GaN shells (films), which must be exceeded to ensure the growth of these shells (films) so as to avoid the formation of prismatic dislocation loops, have been calculated for all the studied nanostructures and three known estimates of the lattice misfits (2.6, 4.7, and 10.1%).

The role of free surfaces on the formation of prismatic dislocation loops

Prismatic punching is a process where voids grow through the nucleation and emission of prismatic dislocation loops (PDLs). In this work we employ dislocation dynamics to determine the effect of image stresses produced by the void’s free surface on PDL formation in a face-centered cubic lattice. We find that image stresses cause PDL formation to fall into two distinct pressure regimes. In the low pressure regime, image stresses dominate dislocation cross-slip, reducing the PDL’s size and formation rate.

MeV–GeV ion induced dislocation loops in LiF crystals

Formation of prismatic dislocation loops and evolution of dislocation structure in LiF crystals irradiated with swift 238U and 36S ions of specific energy 11MeV/u at fluences up to 1013ionscm−2 has been investigated using chemical etching and AFM. It has been shown that prismatic dislocations are formed in the stage of track overlapping above threshold fluences Φ≈109Ucm−2 and Φ≈1010Scm−2. The diameter of dislocation loops reaches 600–1000nm for 238U ions and 200nm for 36S ions. The dislocations created by 238U ions are arranged in rows along the direction of ion beam, whereas 36S ions create freely distributed dislocation loops each of them being oriented along the ion beam. The role of dislocations in ion-induced nanostructuring and hardening is discussed.

Formation of Prismatic Dislocation Loop around a Spherical Inclusion Using Level Set Dislocation Dynamics

Formation of Prismatic Dislocation Loop around a Spherical Inclusion Using Level Set Dislocation Dynamics

1008 レベルセット転位動力学法を用いたプリズマティツク転位ループのモデリング(OS10-2.電子・原子・マルチシミュレーションに基づく材料特性評価(2),OS・一般セッション講演)

1008 レベルセット転位動力学法を用いたプリズマティツク転位ループのモデリング(OS10-2.電子・原子・マルチシミュレーションに基づく材料特性評価(2),OS・一般セッション講演)

Vacancy clustering and prismatic dislocation loop formation in aluminum

The formation of prismatic dislocation loops is an important factor leading to radiation damage of metals. However, the formation mechanism and the size of the smallest stable loop has remained unclear. In this Rapid Communication, we use electronic structure calculations with millions of atoms to address this problem in aluminum. Our results show that there is a cascade of larger and larger vacancy clusters with smaller and smaller energy. Further, the results show that a seven vacancy cluster on the (111) plane can collapse into a stable prismatic loop. This supports the long-standing hypothesis that vacancy clustering leads to a prismatic loop, and that these loops can be stable at extremely small sizes. Finally our results show that it is important to conduct calculations using realistic concentrations (computational cell size) to obtain physically meaningful results.

Stress relaxation in pentagonal whiskers

We analyze the mechanism of internal stress relaxation in pentagonal whiskers (PWs), which is related to the formation of prismatic dislocation loops (DLs) of subtraction in the transverse cross section of PWs. It is shown that, beginning with a certain critical PW radius, the formation of DLs becomes energetically favorable. The optimum DL radius is determined, for which the energy gain as a result of this relaxation is maximum. The dependence of the optimum DL radius on the PW size is presented.

Initial yield process around a spherical inclusion in single-crystalline aluminium

The initial yield process and the subsequent formation of prismatic dislocation loops around a spherical inclusion embedded in a single-crystalline Al matrix are studied by atomistic simulations. In conjunction with linear elastic theory, it is confirmed that the maximum shear stress is created at the inclusion–matrix interface on the {1 1 1} plane intersecting the spherical inclusion at a height of (RP: radius of spherical inclusion). The critical pressure, shear stress and strain for dislocation nucleation are then quantitatively determined. Afterwards, prismatic dislocation loops, which are constructed by four pure edge dislocations with the same Burgers vector but not on the same slip planes, are formed by energetically unstable interactions around the inclusion. Consequently, analytical considerations and atomistic simulation provide a clear explanation of experimental observations and an instructive insight into the precipitation problem.

Study of dislocation reactions and rearrangements under different loading conditions

In this dislocation dynamics study of a single crystal under a loading–unloading sequence, the role of dislocation reaction is presented. The formation of prismatic dislocation loops is observed during the loading–unloading sequence, when allowing for cross-slip. The cross-slip probability strongly affects the microstructure rearrangement.

Formation and Critical Shear Stresses of Prismatic Dislocation Loops Observed around Spherical Precipitate in Single Crystalline Aluminum and Copper Matrices (Simulations by Molecular Dynamics)

Prismatic dislocation loops have so far been observed around the misfitting particle embedded in the matrix. They are generated from the precipitate-matrix interface on which the local shear stress in the maximum. In the present study, the dislocation emissions and the subsequent formation of the prismatic dislocation loop around the inclusion embedded in the single crystalline aluminum or copper matrix are simulated by molecular dynamics. At a result, the maximum shear stress occurs at the precipitate-matrix interface on the {1 1 1} plane intersecting the spherical precipitate at the height of Rp/√2. Afterwards, the prismatic dislocation loops, which have the same gliding direction but not on the same slip planes, are attained by energetically unstable interactions around the precipitate. The representative size of the prismatic dislocation loop is found to depend on the diameter of the precipitate.

Investigation of dislocation loops associated with As–Sb nanoclusters in GaAs

A model is developed for the elastic stress relaxation via formation of prismatic dislocation loops in a vicinity of the As–Sb nanoclusters built in GaAs matrix. The model is based on the experimental investigation of the microstructure of the As–Sb nanoclusters, which can be produced in Sb-doped GaAs films by the molecular-beam epitaxy at low temperature and subsequent anneal. A strong anisotropic mismatch between the As–Sb nanoclusters and GaAs matrix has been revealed by transmission electron microscopy. This mismatch was proven to be a reason for the formation of nanoscale dislocation loops near the nanoclusters. Our theoretical model explores the elastic properties of an inclusion with uniaxial dilatation. For such inclusions, the elastic stresses and stored energy are determined in a closed analytical form. The theoretical analysis predicts a specific nonlinear dependence of the dislocation loop diameter on the cluster diameter, which fits well the experimentally observed one. It is demonstrated that both the change in the inclusion self-energy due to diminishing dilatation and the interaction between the dislocation loop and inclusion are important in the relaxation phenomena in precipitated semiconductors.

Elastic behavior of a spherical inclusion with a given uniaxial dilatation

The elastic behavior of a spherical inclusion with a uniaxial dilatation is considered. As an example, the experimental data on stressed nanoclusters in doped semiconductors (As-Sb clusters in GaAs) are presented. The fields of displacements, elastic strains, and stresses are determined for spherical inclusions with uniaxial dilatation, and the specific features of these fields are revealed. The elastic energy of a uniaxial spheroid is calculated and compared with that for a triaxial spheroid. The relaxation mechanisms for the elastic field of the inclusion associated with the formation of prismatic dislocation loops are considered.

Microstructural changes observed in a rapidly solidified Ni 3Al alloy during isothermal treatments

A series of metastable conditions arises in Ni 3Al alloys out of their rapid solidification, the most important one being the formation of a bimodal distribution of ordered domain sizes. The fine ordered domains are surrounded by elongated ones. Another characteristic is that the number of vacancies traps is higher than the equilibrium one. In this study, several tapes obtained by melt-spinning a Ni 3Al alloy with 76.5 at% Ni were annealed at 500°C for several periods from 0.5 to 120 h. These tapes were studied by means of transmission electron microscopy and X-ray diffraction. The latter technique allowed us to detect that the degree of ordering decreases in the first stages of the treatment; this occurs due to the formation of prismatic dislocation loops as a result of vacancy precipitation, this behavior is also due to the initial high long range order. Thus, the beginning of atomic movement causes the “right” sub-lattice atoms to occupy “wrong” sub-lattice positions. The structure recovers at the end of the process, followed by an increase in the degree of ordering. The vacancy precipitation was found to be heterogeneous; the region with higher density of fine ordered domains provides more sites to prismatic defects than the one with coarse domains.

Vacancy Recovery in Quenched Aluminium–Indium Alloys

The recovery of quenched-in vacancies in a number of aluminium–indium alloys based upon zone-refined aluminium has been studied by means of resistivity measurements and transmission electron microscopy. Recovery is shown to occur in three stages: (a) solute–vacancy association, (b) the formation of prismatic dislocation loops, and (c) the annealing out of the loops. The kinetics of the first stage are analysed and for low quench temperatures found to be consistent with a modified Damask and Dienes theory. Consideration is given to the possibility of solute–vacancy association occurring during quenching. From loop-density measurements the indium–vacancy binding energy is roughly estimated to be 0·27 eV.