Intersecting Stacking Faults Research Articles

Stress-induced intersecting stacking faults and shear antiphase boundary in Zr5Ge4 second phase precipitate embedded in Ge-modified Zircaloy-4

Secondary phase precipitates are vital to control mechanical, corrosion and irradiation response of the multiphase alloy systems. This work reports the mechanical response of an ordered complex intermetallic Zr5Ge4 precipitate during hot working of the experimental Zircaloy-4 modified with dilute Ge addition. Atomic scale insight is provided into the formation mechanism of straight and circular stacking faults and shear antiphase boundary in Zr5Ge4 precipitate. Two types of stacking faults, having displacement vectors 1/3[100] and 1/3[110] with small components along the c-axis, come across and form a distinct region on the (001) surface composed of a zigzag atomic arrangement different from the parent crystal. Besides this, a shear antiphase boundary along the Zr5Ge4/fcc-Zr interface is analysed and concomitant generation of nanotwins is evidenced by the streaking effect in the associated fast Fourier transform (FFT) pattern. By analysing the distortion and stacking faults in the surrounding fcc-Zr phase, comments are made on the role of external stresses in generating the aforementioned planar defects. The relative activity of two types of displacement vectors observed in Zr5Ge4 precipitate is discussed on the basis of the nature of the atomic bond.

Molecular Dynamics Study of Interfacial Micromechanical Behaviors of 6H-SiC/Al Composites under Uniaxial Tensile Deformation.

This paper investigated the micromechanical behavior of different 6H-SiC/Al systems during the uniaxial tensile loading by using molecular dynamics simulations. The results showed that the interface models responded diversely to the tensile stress when the four low-index surfaces of the Al were used as the variables of the joint surfaces. In terms of their stress-strain properties, the SiC(0001)/Al(001) models exhibited the highest tensile strength and the smallest elongation, while the other models produced certain deformations to relieve the excessive strain, thus increasing the elongation. The SiC(0001)/Al(110) models exhibited the largest elongations among all the models. From the aspect of their deformation characteristics, the SiC(0001)/Al(001) model performed almost no plastic deformation and dislocations during the tensile process. The deformation of the SiC(0001)/Al(110) model was dominated by the slip of the 1/6 <112> Shockley partial dislocations, which contributed to the intersecting stacking faults in the model. The SiC(0001)/Al(111) model produced a large number of dislocations under the tensile loading. Dislocation entanglement was also found in the model. Meanwhile, a unique defect structure consisting of three 1/6 <110> stair-rod dislocations and three stacking faults were found in the model. The plastic deformation in the SiC(0001)/Al(112) interface model was restricted by the L-C lock and was carried out along the 1/6 <110> stair-rod dislocations' direction. These results reveal the interfacial micromechanical behaviors of the 6H-SiC/Al composites and demonstrate the complexity of the deformation systems of the interfaces under stress.

Development of Low-Yield Stress Co\u2013Cr\u2013W\u2013Ni Alloy by Adding 6 Mass Pct Mn for Balloon-Expandable Stents

This is the first report presenting the development of a Co–Cr–W–Ni–Mn alloy by adding 6 mass pct Mn to ASTM F90 Co–20Cr–15W–10Ni (CCWN, mass pct) alloy for use as balloon-expandable stents with an excellent balance of mechanical properties and corrosion resistance. The effects of Mn addition on the microstructures as well as the mechanical and corrosion properties were investigated after hot forging, solution treatment, swaging, and static recrystallization. The Mn-added alloy with a grain size of ~ 20 µm (recrystallization condition: 1523 K, 150 seconds) exhibited an ultimate tensile strength of 1131 MPa, 0.2 pct proof stress of 535 MPa, and plastic elongation of 66 pct. Additionally, it exhibited higher ductility and lower yield stress while maintaining high strength compared to the ASTM F90 CCWN alloy. The formation of intersecting stacking faults was suppressed by increasing the stacking fault energy (SFE) with Mn addition, resulting in a lower yield stress. The low-yield stress is effective in suppressing stent recoil. In addition, strain-induced martensitic transformation during plastic deformation was suppressed by increasing the SFE, thereby improving the ductility. The Mn-added alloys also exhibited good corrosion resistance, similar to the ASTM F90 CCWN alloy. Mn-added Co–Cr–W–Ni alloys are suitable for use as balloon-expandable stents.

Atomic scale characterization of complex stacking faults and their configurations in cold deformed Fe42Mn38Co10Cr10 high-entropy alloy

Atomic-resolution scanning transmission electron microscopy is used to study stacking faults and their configurations formed in cold deformed samples of a face-centered cubic (FCC) Fe42Mn38Co10Cr10 (at.%) high-entropy alloy (HEA). It is found that the deformed microstructures contain at least four types of intrinsic stacking faults and two types of extrinsic stacking faults that are bounded by different partial dislocations, including Shockley, Frank and unusual 1/6<411> partials. Additionally, seventeen types of stacking fault configurations are also found, including the well-known Lomer-Cottrell lock, Hirth lock and faulted dipole, the rarely reported intrinsic-extrinsic fault bend, and thirteen hitherto unreported configurations containing different stacking faults and partial dislocations. Among these seventeen configurations, fifteen of them are constructed by conjugate stacking faults connecting at their edges, with a stair-rod partial belonging to 1/6<110>, 1/3<110> or 1/3<100> lying at every joint point of the stacking faults; and the rest two configurations have two intersecting stacking faults but contain different stair-rod dipoles at the intersection. Fourteen of these configurations are all comprised exclusively of intrinsic stacking faults, while three of them consist of a mixture of intrinsic and extrinsic stacking faults. Moreover, three out of the seventeen configurations contain surprisingly a Frank partial at one edge, in contrast to previously reported configurations that are all bounded exclusively by Shockley partials. Based on Burgers vector analysis, the formation mechanisms of the six types of stacking faults and the seventeen types of configurations are proposed and discussed.

Atomic structure and elemental segregation behavior of creep defects in a Co-Al-W-based single crystal superalloys under high temperature and low stress

With the aim of understanding the effect of creep defects in the γ′ phase on the creep resistance, the atomic structure and elemental segregation behavior of stacking faults (SFs), three types of SF interactions and anti-phase boundaries (APBs) in a Co-Al-W-based single-crystal superalloy crept at 1000 °C/137 MPa up to 1.0% strain were investigated. Both superlattice intrinsic and extrinsic stacking faults (SISF and SESF) were observed in the γ′ precipitates, and the interactions of such faults had different configurations. Both V- and T-like configurations can be composed of SISF-SISF, SISF-SESF and SESF-SESF interactions, while the X-like configuration was only observed to be composed by SISF-SISF interaction. The strengthening effect and mechanism of such SF interactions on the creep resistance are discussed based on their density and formation mechanism. W elemental segregation was present along the SISFs and SESFs. In contrast, only the γ forming element Co was enriched near the leading partial dislocations (LPDs) of the SISFs/SESFs, stair-rod dislocations and APBs. The rate limiting step of the expansion of the SFs was estimated to be the drag effect induced by the Co diffusion. In addition, the local phase transformation from the L12 to A1 structure at the intersection of SFs was mainly attributed to the Co supersaturation and aided by wetting phenomenon from the APB.

A molecular dynamics study of atomic configurations of dislocations accompanying twins in crystal growth of Si from melt

Based on the Tersoff potential, molecular dynamics simulations for the growth of Si crystals along the 〈112〉 orientation were carried out to investigate the atomic configurations of dislocations and twins. Two typical configurations were observed. One is a sandwich structure which has two twin boundaries and several rows of atoms normally arranged and is ended by a Shockley dislocation with a Burgers vector of 〈112〉/6. The other is composed of two intersecting stacking faults and a Lomer–Cottrell dislocation with a Burgers vector of 〈110〉/6. The two configurations can combine together and form complex atomic configurations in Si crystal. And the two configurations have similar formation processes. Firstly, two twin boundaries or a stacking fault forms in a {111} facet of solid–liquid interface. Then, a dislocation nucleates after the following crystal atom planes dock with each other or the second stacking fault forms. The formation of Shockley dislocation takes a longer time than that of Lomer–Cottrell dislocation due to a larger lattice mismatch () of planes.

A molecular dynamics study of the shock-induced defect microstructure in single crystal Cu

Molecular dynamics simulations of shock compression are carried out over a range of pressures (from 22GPa to 75GPa) to investigate the shock-induced defect microstructure in single crystal Cu. The results show that under increasing shock pressure, we can detect three distinct phases of the defect regimes for single crystal Cu: dislocations→stacking faults→twins. We show the evolution processes of different defect microstructures. According to the analyses of these evolution processes, the formation mechanisms of various defect microstructures are studied. When the shock pressure is less than 30GPa, the motion of dislocations is a significant mechanism in single crystal Cu under the shock compression. The motion between various dislocations and loops widely exists on the (111) close-placed plane. The interesting dislocation loops are obtained at a relatively low pressure, and a new mechanism of nucleation and development of the dislocation loop is proposed. When the shock pressureis greater than 30GPa, a large number of intersecting stacking faults are formed. On the (001) surface, stacking faults align along the [220] and [2¯20] directions at exactly 90 degrees, and they are present roughly in the same proportion. The stacking fault spacing decreases with the increase of shock pressure, and shock-induced plasticity increases as a function of shock strength. The structure of the simulation, based on the topology, matches extremely well with that found in recent transmission electron microscopy studies of single crystal Cu recovered from laser shock experiments. At a relatively high pressure (61GPa), we find that the stacking faults and twins exist together in single crystal Cu. It is shown that two types of stacking faults with vertical directions are the competitive mechanism. When one of them dominates, twins will be formed from this type of stacking fault. Thus, the directions of twins are randomly distributed in the initial stage. Rotation and combination of twins are significant mechanisms, and these mechanisms lead to the final formation of twins. At 75GPa, twins elongate along [12¯1] and [1¯2¯1¯] directions on the (1¯01) surface. The configuration and direction of the twins are in agreement with experimental results.

A molecular dynamics study of dislocation density generation and plastic relaxation during shock of single crystal Cu

The molecular dynamics simulation method is used to investigate the dependence of crystal orientation and shock wave strength on dislocation density evolution in single crystal Cu. Four different shock directions 〈100〉, 〈110〉, 〈111〉, and 〈321〉 are selected to study the role of crystal orientation on dislocation generation immediately behind the shock front and plastic relaxation as the system reaches the hydrostatic state. Dislocation density evolution is analyzed for particle velocities between the Hugoniot elastic limit (upHEL) for each orientation up to a maximum of 1.5 km/s. Generally, dislocation density increases with increasing particle velocity for all shock orientations. Plastic relaxation for shock in the 〈110〉, 〈111〉, and 〈321〉 directions is primarily due to a reduction in the Shockley partial dislocation density. In addition, plastic anisotropy between these orientations is less apparent at particle velocities above 1.1 km/s. In contrast, plastic relaxation is limited for shock in the 〈100〉 orientation. This is partially due to the emergence of sessile stair-rod dislocations with Burgers vectors of 1/3〈100〉 and 1/6〈110〉. The nucleation of 1/6〈110〉 dislocations at lower particle velocities is mainly due to the reaction between Shockley partial dislocations and twin boundaries. On the other hand, for the particle velocities above 1.1 km/s, the nucleation of 1/3〈100〉 dislocations is predominantly due to reaction between Shockley partial dislocations at stacking fault intersections. Both mechanisms promote greater dislocation densities after relaxation for shock pressures above 34 GPa compared to the other three shock orientations.

Athermal ε-martensite transformation in a Co–20Cr alloy: Effect of rapid solidification on plate nucleation

In the present work, a drastic increase in the formation of athermal ε-martensite was experimentally found after casting a Co–20Cr alloy in a water cooled Cu-mold. Under these conditions, rapid solidification was achieved with an alloy cooling rate of 278 K/s (approx.). The amount of precipitated athermal ε-martensite measured using X-ray diffraction was 97.26 vol. %. An inspection of the exhibited microstructure indicates that rapid solidification promotes appreciable dendrite grain refinement and alloy homogenization. In addition, the dendritic structure contained numerous visible striations associated with the athermal γ ↔ ε transformation. Transmission electron microscopy showed the development of an extremely large amount of stacking faults and stacking fault intersections, including ε-martensite plates. The microstructural characterization included an estimation of average ε-martensite plate dimensions by assuming an ellipsoidal plate geometry. These results where compared with predictions of critical ε-martensite embryo dimensions by using Eshelby's inclusion theory. In addition, a determination of the chemical volumetric free energy change, ΔGV using a regular solution model indicated that this term is rather small. Consequently, the energy barrier for spontaneous nucleation in the Co–20Cr is relatively large when compared with other alloy systems.

MD simulation of nanoindentation on (001) and (111) surfaces of Ag–Ni multilayers

We perform MD simulations of the nanoindentation on (001) and (111) surfaces of Ag–Ni multilayers with different modulation periods, and find that both the hardness and maximum force increase with the increase of modulation period, in agreement with the inverse Hall–Petch relation. A prismatic partial dislocation loop is observed in the Ni(111)/Ag(111) sample when the modulation period is relatively large. We also find that misfit dislocation network shows a square shape for the Ni(111)/Ag(111) interface, while a triangle shape for the Ni(001)/Ag(001) interface. The pyramidal defect zones are also observed in Ni(001)/Ag(001) sample, while the intersecting stacking faults are observed in Ni(111)/Ag(111) sample after dislocation traversing interface. The results offer insights into the nanoindentation behaviors in metallic multilayers, which should be important for clarifying strengthening mechanism in many other multilayers.

Size effects on the wave propagation and deformation pattern in copper nanobars under symmetric longitudinal impact loading

Molecular dynamics simulations were performed to study the influence of system size on wave propagation and deformation patterns in 〈1 0 0〉/{1 0 0} copper nanobars with square cross-section under symmetric longitudinal impact loading. Nanobars of longitudinal length 100a with cross-sectional edge lengths h = 10a, 20a, and 40a were impacted on both ends by flyers of size 20a × h × h, where a is the Cu unit cell length, and impact speed 500 m s−1. For reference, quasi-infinite slab samples with periodic cross-sectional edge lengths 10a and 40a were also studied. It was found that the wave propagation speed increases with increasing cross-sectional area and eventually approaches the value obtained for a quasi-infinite sample. Extensive plasticity occurs across the entire length of the nanobars, whereas the quasi-infinite samples remain in the elastic regime and exhibit a vibrating (ringing) behaviour. The deformation pattern in the nanobars is strongly dependent on the cross-sectional area. For the nanobar with h = 10a the material fully reorients from 〈1 0 0〉/{1 0 0} to 〈1 1 0〉/{1 1 1} with few stacking faults and twins. Material in the nanobar with h = 20a does not reorient completely; the local crystal deformation is mediated mainly by a partial dislocation activity leading to predominantly non-intersecting stacking faults and twins. Nanobars with h = 40a exhibit behaviour similar to that for the h = 20a case but with greater propensity for intersecting stacking faults.

Erratum: “Response to 'Comment on 'Deformation mechanisms of face-centered-cubic metal nanowires with twin boundaries'” [Appl. Phys. Lett. 93, 086102 (2008)

FIG. 1. Color online The snapshots corresponding to the first yield point of the stress-strain curves of the three wires. For the twin-free single-crystal Cu nanowire, a The nucleation of the first dislocation; b several picoseconds later after relaxation, dislocations move out of the wire, showing the dislocation starvation in the single-crystal wire. For the four-twin nanowire, c The nucleation of the first dislocations; d after relaxation a number of dislocations pile up at twin boundaries and stacking fault intersections, leading to some hardening effects after yielding. e The nucleation of first dislocation in the five-twin nanowire. The yield strain y for the twin-free wire, the four-twin wire as well as the five-twin is 0.037, 0.043, and 0.047, respectively. This clearly shows that the first dislocation emission in twinned nanowires, occur later than that of the twin-free one. APPLIED PHYSICS LETTERS 93, 249901 2008

Multiscale experimental investigations about the cyclic behavior of the 304L SS

This work follows a series of experiments carried out earlier at INSA of Rouen (Hassan, T., Taleb, L., Krishna, S., 2008. Influence of non-proportional loading paths on ratcheting responses and simulations by two recent cyclic plasticity models. Int. J. Plast. 24, 1863–1889). It investigates the elastoplastic cyclic behavior of a 304L stainless steel at room temperature. In a first step the cross path effect on ratcheting is confirmed, as well as the crucial role of the loading path non-proportionality. Strain controlled tests are also conducted for different strain amplitudes and loading paths. Cross-hardening effect appears more important when the shearing sequence is followed by the axial one. Moreover for alternating axial and shearing cycles, this phenomenon occurs after each crossing sequence leading to a very significant strain hardening, at least of the same order as the one obtained after a circular strain path. Yet, the magnitude of the observed over hardening does not necessarily seem a function of the cumulated plastic strain. The relative contributions of the isotropic and kinematic parts of the cyclic strain hardening are also very different in the axial direction. The material generally exhibits a transient period of cyclic hardening followed by cyclic softening. Under tension-compression, the importance of the cyclic strain hardening period seems to increase with the strain amplitude. Microstructural investigations have been performed in order to understand the main physical phenomena responsible for this macroscopic behavior. The additional cyclic hardening observed in cross paths may be explained by the high defect density generated: multiple slip systems, intersecting stacking faults and twins, formation of dislocation heterogeneous structures. Furthermore, it is shown that martensite nucleation takes place at the intersections between micro-shear bands or twin faults; the quantity of martensite being estimated through magnetic measurements. After the application of a magnetic field, the material anisotropy induced by the loading path is evaluated; the martensitic needles and platelets seem to develop in the axial direction.

On the determination of partial dislocation Burgers vectors in fee lattices and its application to cubic SiC films

Abstract Cubic SiC layers grown on Si(110) contain a high density of widely spread and intersecting stacking faults. Weak-beam transmission electron microscopy has been used to determine the possible Burgers vectors of both widely spread Shockley partial dislocations and sessile partial dislocations in such films. Partial Shockley dislocations that restrict non-intersecting stacking faults were characterized, and a stacking-fault energy of 0.1 mJ m−2 was determined. Edge sessile dislocations with 1/6(110) and 1/3(001) Burgers vectors were identified at the stacking-fault intersections.

Orientation dependence in molecular dynamics simulations of shocked single crystals

We use multimillion-atom molecular dynamics simulations to study shock wave propagation in fcc crystals. As shown recently, shock waves along the <100> direction form intersecting stacking faults by slippage along 111 close-packed planes at sufficiently high shock strengths. We find even more interesting behavior of shocks propagating in other low-index directions: for the <111> case, an elastic precursor separates the shock front from the slipped (plastic) region. Shock waves along the <110> direction generate a leading solitary wave train, followed (at sufficiently high shock speeds) by an elastic precursor, and then a region of complex plastic deformation.

On the mechanism of transformation of icosahedral rare-gas clusters into fcc aggregations

The experimental diffraction patterns from Kr cluster beams are compared with the diffraction functions calculated for rare-gas clusters with dimensions of 3×103 and 1×104 atoms/cluster. The experimental results are found to correlate well with the calculation if it is assumed that the clusters have a face-centered cubic structure with a constant number of intersecting stacking faults. Additional confirmation is obtained for the decisive role of the kinetic factor in the formation of the crystalline phase of the clusters. Conjectures are offered concerning the possible reasons why the densitometer traces presented by M-F. de Feraudy, G. Torchet, and B. W. van de Wall at the conference ISSPIC (1998) showed no substantial changes when the cluster size was increased by more than an order of magnitude.

Low cycle fatigue behavior of a directionally solidified cobalt base superalloy

An investigation has been made of the low cycle fatigue behavior and microstructure evolution of a directionally solidified cobalt-base superalloy at room temperature, 700 and 850 degrees C under the control of different total strain amplitudes. The results show that at room temperature the cyclic hardening of the alloy appears during the first few cycles, and then a long saturation stage begins. At 700 degrees C, the alloy exhibits a pronounced initial hardening, and a secondary hardening after a short saturation. At 850 degrees C. the alloy shows a continuous cyclic hardening until fracture. Examination by TEM indicates that the initial hardening of the alloy at room temperature is caused by the pile-ups of the stacking faults at the stacking fault intersections, while the saturation is related to the formation of the hexagonal close-packed (HCP) zones and twins. The mechanism of initial hardening at 700 degrees C is similar to that at room temperature, while the stress saturation is due to interaction obstacle to stacking-fault becoming weaker, because of thermal activation. The secondary hardening is attributed to the formation of sessile dislocation tangles. The continuous cyclic hardening at 850 degrees C is related to the interaction between the precipitates (M23C6) and dislocations. (C) 1999 Published by Elsevier Science S.A. All rights reserved.

CYCLIC DEFORMATION OF A DIRECTIONALLY SOLIDIFIED COBALT‐BASE SUPERALLOY

An investigation has been carried out on the cyclic deformation and changes in microstructure of a directionally solidified cobalt‐base superalloy. The tests are conducted at 700 °C and 850 °C in air under different total strain amplitudes. The alloy tested at 700 °C exhibits an initial hardening, a short saturation stage and an evident secondary hardening, while the alloy at 850 °C suffers continuous cyclic hardening until fracture. TEM examinations indicate that the initial hardening of the alloy at 700 °C is caused by the pile‐ups of dislocations and stacking faults at the stacking fault intersections, while the stress saturation is due to the weakening of obstacles against the dislocation movement. The secondary hardening has a contribution from the formation of sessile dislocation tangles. The early stage of continuous hardening of the alloy at 850 °C is related to the pile‐ups of dislocations and stacking faults at the intersections, and the later stage is controlled by the interaction between precipitates and dislocations.

Mechanical deformation of atomic-scale metallic contacts: Structure and mechanisms

We have simulated the mechanical deformation of atomic-scale metallic contacts under tensile strain using molecular dynamics and effective medium theory potentials. The evolution of the structure of the contacts and the underlying deformation mechanisms are described along with the calculated electronic conductance. Various defects such as intersecting stacking faults, local disorder, and vacancies are created during the deformation. Disordered regions act as weak spots that reduce the strength of the contacts. The disorder tends to anneal out again during the subsequent atomic rearrangements, but vacancies can be permanently present. The transition states and energies for slip mechanisms have been determined using the nudged elastic band method, and we find a size-dependent crossover from a dislocation-mediated slip to a homogeneous slip when the contact diameter becomes less than a few nm. We show that the results measured in a nanocontact experiment depend significantly on the elastic stiffness of the experimental apparatus. For a soft setup, some of the atomic rearrangements might not be detected, whereas others are amplified.

Analysis of partial and stair-rod dislocations by large angle convergent beam electron diffraction

It is shown how large angle convergent beam electron diffraction (LACBED) can be used to analyse the Burgers vectors of partial dislocations in Si, GaAs and CdTe. By selecting reflections in which adjoining stacking faults are out of contrast, it is demonstrated that the LACBED technique can be used to analyse Shockley and Frank partial dislocations, and stair-rod dislocations of 1 6 〈110〉 type present at stacking fault intersections. The potential of the technique for studying grain boundary and dissociated dislocations in general is discussed.