Extrinsic Stacking Faults Research Articles

Simultaneous twinning nucleation mechanisms in an Fe–Mn–Si–Al twinning induced plasticity steel

Two simultaneous twinning nucleation mechanisms were observed in transmission electron microscopy (TEM) during low strain rate tensile deformation of a Fe–27Mn–2.5Si–3.5Al twinning induced plasticity (TWIP) steel at −40 °C. Deformation twinning took place on the conjugate (11¯1) plane through the activation of pole and three–layer mechanisms producing Shockley twinning dislocations having the same Burgers vector. Secondary twinning mechanisms were not identified. The active twinning mechanisms were influenced by the stacking fault energy (SFE) of the deformed austenite, which as estimated using electron and X-ray diffraction was ∼15 mJ/m2 and 20 mJ/m2 for 2% and 5% strains, respectively. This low SFE is believed to hinder cross–slip of twinning Shockley partials so that the stair–rod cross–slip twinning mechanism was not favored. The selection of twinning mechanisms by the microstructure is explained through critical twinning stress, the incidence of intrinsic and extrinsic stacking faults, as well as proper a2[11¯0] dislocations having a screw component that serve as suitable pole dislocations. The active twinning mechanism is plausibly not contributing markedly to the strain hardening of TWIP steels but the SFE and other hardening mechanisms are seemingly more prevalent.

Grain boundary traction signatures: Quantifying the asymmetrical dislocation emission processes under tension and compression

The disruption in crystallographic arrangement of atoms across a grain boundary interface generates local stress fields in the vicinity. Here, we reconstruct the continuum-equivalent grain boundary tractions from local atomic stresses near symmetrical-tilt 〈110〉 Ni grain boundaries. We show that the resolved shear stress contribution from the grain boundary tractions, τGB, along active slip-systems either assists or prevents the emission of dislocations, depending on its direction with respect to the resolved shear stress contribution from external loading, τext. When τGB acts in the same direction as τext, Shockley partial dislocations are readily emitted from the boundary once |τGB+τext| exceeds the critical barrier stress for shear-slip. When τGB opposes τext, the higher sustainable stresses in the grain boundary structure instead triggers: (a) emission of dislocations from the bulk, or (b) reconfiguration of the grain boundary atomic structure and subsequent emission of non-Schmid dislocations or formation of extrinsic stacking faults. Our results quantitatively explain the asymmetrical grain boundary dislocation emission processes observed in molecular dynamics (MD) simulations under applied tensile and compressive loads. The relationship between the traction signatures and periodic structural units along the grain boundary is discussed.

Evidencing extrinsic stacking faults in twinning-induced plasticity steel

In twinning induced plasticity (TWIP) steel, the question of what leads to the onset of twinning in grains whose 〈100〉 orientations are near parallel to the tensile axis at high tensile strains has not been resolved. Using aberration corrected scanning transmission microscopy we present first-hand evidence that extrinsic stacking faults serve as nuclei for twins in 〈100〉 grains and also contribute to twin growth in a low carbon TWIP steel uniaxially tensile loaded up to its ultimate tensile strength of ~1080MPa; which corresponds to a tensile true strain of 48%. The results corroborate and validate our previous Schmid factor–based predictions on the feasibility of twinning nucleation via extrinsic stacking faults in 〈100〉 grains at high tensile strains.

Substructure induced twinning in low density steel

The stacking fault energy of low density steels is sufficiently high to suppress twin formation thereby the twinning induced plasticity effect is expected to disappear. The present work devotes to address the origin of deformation twinning in a duplex low density steel. The substructure evolution (i.e. formation, refinement, size stabilization and its rotation) provides local areas holding preferred orientation for twin nucleation. Twinning is more populated in regions possessing higher Schmid factor for slip than that of twin. The observed non-Schmid behavior is attributed to the substructure induced nature of twinning and increased feasibility of extrinsic stacking fault formation.

Atomically resolved microscopy of ion implantation induced dislocation loops in 4H-SiC

During high temperature electrical activation of ion-implanted dopant species in SiC, extrinsic dislocation loops are formed on the basal planes of the SiC lattice. Investigations have suggested Si-based loops are caused in accordance with the well-known +1 model. Herein we apply aberration corrected STEM to resolve the atomic structure of these loops. It is shown that the dislocation loops formed during annealing of Al-implanted SiC consist of an extra inserted Si-C bilayer of the (0001) polar sense, which upon insertion into the lattice causes a local extrinsic stacking fault. The +1 model thus needs to be expanded for binary systems.

Ab initio molecular dynamics simulation of the effects of stacking faults on the radiation response of 3C-SiC.

In this study, an ab initio molecular dynamics method is employed to investigate how the existence of stacking faults (SFs) influences the response of SiC to low energy irradiation. It reveals that the C and Si atoms around the SFs are generally more difficult to be displaced than those in unfaulted SiC, and the corresponding threshold displacement energies for them are generally larger, indicative of enhanced radiation tolerance caused by the introduction of SFs, which agrees well with the recent experiment. As compared with the unfaulted state, more localized point defects are generated in faulted SiC. Also, the efficiency of damage production for Si recoils is generally higher than that of C recoils. The calculated potential energy increases for defect generation in SiC with intrinsic and extrinsic SFs are found to be higher than those in unfaulted SiC, due to the stronger screen-Coulomb interaction between the PKA and its neighbors. The presented results provide a fundamental insight into the underlying mechanism of displacement events in faulted SiC and will help to advance the understanding of the radiation response of SiC with and without SFs.

Structural modifications due to interface chemistry at metal-nitride interfaces.

Based on accurate first principles density functional theory (DFT) calculations, an unusual phenomenon of interfacial structural modifications, due to the interface chemistry influence is identified at two metal-nitride interfaces with strong metal-nitrogen affinity, Al/TiN {111} and Al/VN {111} interfaces. It is shown that at such interfaces, a faulted stacking structure is energetically preferred on the Al side of the interface. And both intrinsic and extrinsic stacking fault energies in the vicinity Al layers are negligibly small. However, such phenomenon does not occur in Pt/TiN and Pt/VN interfaces because of the weak Pt-N affinity. Corresponding to structural energies of metal-nitride interfaces, the linear elasticity analysis predicts characteristics of interfacial misfit dislocations at metal-nitride interfaces.

Super-X EDS Characterization of Chemical Segregation within a Superlattice Extrinsic Stacking Fault of a Ni- based Superalloy

Super-X EDS Characterization of Chemical Segregation within a Superlattice Extrinsic Stacking Fault of a Ni- based Superalloy

Nanoscopic studies of 2D‐extended defects in silicon that cause shunting of Si‐solar cells

AbstractPotential‐induced degradation (PID) can cause massive power losses in photovoltaic power plants with serially connected modules when high electrical potential between solar cells and the module front glass surface leads to shunting of Si solar cells. The shunting is reversible to a large extend through elevated temperatures and/or by application of reversed voltages. It was previously shown that PID‐related shunting (PID‐s) of the p‐n junction occurs due to Na‐decorated stacking faults in {111} planes. Defects at former PID‐shunt positions that have been thermally ‘healed' (recovered) do not show a Na decoration anymore.In this contribution we investigate stacking faults with Na decoration and with vanished Na decoration after thermal recovery. HAADF STEM imaging is used for principal investigation of the atomic structure of PID‐s defects. The Na decorated stacking fault appears as a dark region in the HAADF STEM image. By contrast, the atomic structure of the recovered stacking fault can be clearly imaged. The orientation of Si dumbbells indicates an extrinsic stacking fault that remains after recovery by Na out‐diffusion. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Extended defects in MBE‐grown CdTe‐based solar cells

AbstractExtended defects in the p ‐ZnTe/n ‐CdTe heterojunctions grown by the molecular‐beam epitaxy technique on two different substrates, GaAs and CdTe, have been investigated by deep‐level transient spectroscopy (DLTS) and transmission electron microscopy (TEM). Four hole traps, called H1 to H4, and one electron trap, called E3, have been revealed in the DLTS spectra measured for the heterojunctions grown on the GaAs substrates. The H1, H3, H4 and E3 traps have been attributed to the electronic states of dislocations on the ground of their logarithmic capture kinetics. The DLTS peaks associated with the H1 and E3 traps were not observed in the DLTS spectra measured for the heterojunction grown on the CdTe substrate. They are most likely associated with threading dislocations generated at the mismatched interface with the GaAs substrate. Cross‐sectional TEM images point out that they are dislocations of the 60°‐type. In both the types of heterojunctions the H4 trap was observed only under forward‐bias filling pulse, suggesting that this trap is associated with the CdTe/ZnTe interface. In addition, TEM images revealed also the presence of intrinsic and extrinsic stacking faults in the CdTe layers, which may considerably affect their electronic properties. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Segregation of alloying elements to intrinsic and extrinsic stacking faults in γ′-Ni3Al via first principles calculations

First principles calculations are used to investigate the segregation behaviour of Co, Cr, Re, Mo and W to intrinsic and extrinsic stacking faults in γ′-Ni3Al. It is shown that the change in stacking fault energy depends on local alloying concentration and is related to subtle changes in the electronic structure of the alloying elements and adjacent nickel atoms. The results are consistent with observed stacking fault segregation in commercial superalloys and in particular the behaviour of Co and Cr.

An effective stacking fault energy viewpoint on the formation of extended defects and their contribution to strain hardening in a Fe–Mn–Si–Al twinning-induced plasticity steel

A deformation-dependent stacking fault energy (SFE) viewpoint is invoked to interpret the low strain rate tensile deformation of a Fe–27Mn–2.5Si–3.5Al austenitic steel at room temperature by using X-ray diffraction analyses and transmission electron microscopy (TEM) observations. The effective SFE of austenite increased with strain from ∼18 to 40mJm−2, and it was reasoned that this was the result of the increase in strain energy of the stacking faults (SFs) per unit area due to its dependence on the dislocation character and density. Twinning was observed at 2% strain and confirmed to occur by a glide mechanism of a6〈121〉 Shockley partial dislocations, leading to the formation of overlapping intrinsic–extrinsic SF pairs representing a three-layer twin embryo, revealing periodic dislocation contrast in TEM. The early onset of twinning is attributed to the unusually low critical twinning stress of the steel, ∼200MPa. In spite of twinning, the stacking fault probability (Psf) of twinned austenite was remarkably low (∼10−4) at low strains, but increased moderately (to ∼10−3) up to failure strain. At the emergence of twinning, the corresponding perfect dislocation density was low (∼1014m−2) but was well above the critical dislocation density required for twinning occurrence. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character from edge to screw type. The microstructural parameters of the steel estimated in direct or indirect relation to its SFE could explain its flow stress and strain hardening behavior.

Magnesium behavior and structural defects in Mg+ ion implanted silicon carbide

As a candidate material for fusion reactor applications, silicon carbide (SiC) undergoes transmutation reactions under high-energy neutron irradiation with magnesium as the major metallic transmutant; the others include aluminum, beryllium and phosphorus in addition to helium and hydrogen gaseous species. The impact of these transmutants on SiC structural stability is currently unknown. This study uses ion implantation to introduce Mg into SiC. Multiaxial ion-channeling analysis of the as-produced damage state indicates a lower dechanneling yield observed along the 〈100〉 axis. The microstructure of the annealed sample was examined using high-resolution scanning transmission electron microscopy. The results show a high concentration of likely non-faulted tetrahedral voids and possible stacking fault tetrahedra near the damage peak. In addition to lattice distortion, dislocations and intrinsic and extrinsic stacking faults are also observed. Magnesium in 3C–SiC prefers to substitute for Si and it forms precipitates of cubic Mg2Si and tetragonal MgC2. The diffusion coefficient of Mg in 3C–SiC single crystal at 1573K has been determined to be 3.8±0.4×10−19m2/s.

Creep-induced planar defects in L12-containing Co- and CoNi-base single-crystal superalloys

Precipitate shearing mechanisms during single-crystal tensile creep of L12-containing Co- and CoNi-base alloys have been investigated. Interrupted creep tests were conducted at 900°C under vacuum at constant load, and stresses varied between 275 and 345MPa. Transmission electron microscopy diffraction contrast analysis revealed superlattice intrinsic and extrinsic stacking faults in the γ′ precipitates in the Co-base alloys. However, in alloys containing significant additions of nickel (CoNi-base alloys), antiphase boundaries formed by shearing of a/2〈110〉 dislocations on octahedral planes. The shearing mechanisms in the Co- and CoNi-base γ′ phase are compared to commercial Ni-base alloys, and the relative fault energies inferred from these observations are discussed.

Planar fault energies of copper at large strain: A density functional theory study

We present density functional theory calculations of the extrinsic stacking fault energy γesf, twin fault energy γtf, and unstable stacking fault energy γusf of copper under large strains, up to ± 10%. The calculated values of γesf, γtf, and γusf for unstrained Cu are 41.8 mJ/m2, 20.2 mJ/m2, and 163.4 mJ/m2, respectively, in good agreement with experimental data and theoretical results. Four different types of strains are applied: (i) volumetric strain; (ii) uniaxial strain perpendicular to the fault plane; (iii) uniaxial strains parallel to the fault plane; and (iv) shear strains across the fault planes. We find that γesf, γtf, and γusf are strongly dependent on the magnitude and type of strain, challenging the common conception that they are constant material properties. The predicted strong strain dependencies provide useful insight into the deformation mechanisms of copper under high pressure and shock conditions and provide essential data to improve current Cu empirical potentials.

Tensile Properties of Si Nanowires with Faulted Stacking Layers

Semiconductor nanowires (NWs) show tremendous applications in micro/nano-electro-mechanical systems. In order to fulfill their promising applications, an understanding of the mechanical properties of NWs becomes increasingly important. Based on the large-scale molecular dynamics simulations, this work investigated the tensile properties of Si NWs with different faulted stacking layers. Different faulted stacking layers were introduced around the centre of the NW by the insertion or removal of certain stacking layers, inducing twins, intrinsic stacking fault, extrinsic stacking fault, and 9R crystal structure. Stress–strain curves obtained from the tensile deformation tests reveal that the presence of faulted stacking layers has induced a considerable decrease to the yield strength while only a minor decrease to Young's modulus. The brittle fracture phenomenon is observed for all tested NWs. In particular, the formation of a monatomic chain is observed for the perfect NW, which exists for a relatively wide strain range. For the defected NW, the monatomic chain appears and lasts shorter. Additionally, all defected NWs show a fracture area near the two ends, in contrast to the perfect NW whose fracture area is adjacent to the middle. This study provides a better understanding of the mechanical properties of Si NWs with the presence of different faulted stacking layers.

Atomic configuration of irradiation-induced planar defects in 3C-SiC

The atomic configuration of irradiation-induced planar defects in single crystal 3C-SiC at high irradiation temperatures was shown in this research. A spherical aberration corrected scanning transmission electron microscope provided images of individual silicon and carbon atoms by the annular bright-field (ABF) method. Two types of irradiation-induced planar defects were observed in the ABF images including the extrinsic stacking fault loop with two offset Si-C bilayers and the intrinsic stacking fault loop with one offset Si-C bilayer. The results are in good agreement with images simulated under identical conditions.

Defects in paramagnetic Co-doped ZnO films studied by transmission electron microscopy

We study planar defects in epitaxial Co:ZnO dilute magnetic semiconductor thin films deposited on c-plane sapphire (Al2O3), as well as the Co:ZnO/Al2O3 interface, using aberration-corrected transmission electron microscopy and electron energy-loss spectroscopy. Co:ZnO samples that were deposited using pulsed laser deposition and reactive magnetron sputtering are both found to contain extrinsic stacking faults, incoherent interface structures, and compositional variations within the first 3–4 Co:ZnO layers next to the Al2O3 substrate. The stacking fault density is in the range of 1017 cm−3. We also measure the local lattice distortions around the stacking faults. It is shown that despite the relatively high density of planar defects, lattice distortions, and small compositional variation, the Co:ZnO films retain paramagnetic properties.

Thermal conductivity of Si nanowires with faulted stacking layers

Faulted stacking layers are ubiquitously observed during the crystal growth of semiconducting nanowires (NWs). In this paper, we employ the reverse non-equilibrium molecular dynamics simulation to elucidate the effect of various faulted stacking layers on the thermal conductivity (TC) of silicon (Si) NWs. We find that the stacking faults can greatly reduce the TC of the Si NW. Among the different stacking faults that are parallel to the NW's axis, the 9R polytype structure, the intrinsic and extrinsic stacking faults (iSFs and eSFs) exert more pronounced effects in the reduction of TC than the twin boundary (TB). However, for the perpendicularly aligned faulted stacking layers, the eSFs and 9R polytype structures are observed to induce a larger reduction to the TC of the NW than the TB and iSFs. For all considered NWs, the TC does not show a strong relation with the increasing number of faulted stacking layers. Our studies suggest the possibility of tuning the thermal properties of Si NWs by altering the crystal structure via the different faulted stacking layers.

TEM investigation of the formation mechanism of deformation twins in Fe–Mn–Si–Al TWIP steels

The microstructure of a Fe–Mn–Si–Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe–Mn–C TWIP steels.