Formation Of Stacking Faults Research Articles

Achievement in modifying dehydrogenation kinetics for Sc-containing Mg matrix in Mg–Y–Zn alloy

To eliminate the adverse effects of coarse α-Mg phase on the hydrogen storage kinetics of Mg-rich alloys, element Sc with high solid solubility in α-Mg is introduced into Mg–Y–Zn alloy initiatively, which shows positive effects to enhance the hydrogen storage performance. The Sc-doping effectively enhances the dehydrogenation kinetics by motivating the formation of abundant stacking faults. The stacking faults provide the hydrogen atoms with diffusion channels to accelerate the mutual phase transformation of metal/hydride. With 1 at.% Sc substituting for Y, the apparent activation energy for dehydrogenation obtains a 11% reduction compared with Sc-free alloy. Besides, Sc-doping facilities the decomposition of YH3, which provides “electron transfer” effect to accelerate the decomposition of MgH2. Sc-doping affects the hydrogenation behavior by weakening the H2 sorption but ensuring the H atoms diffusion in Mg matrix. With the gradually substitution of Sc for Y, the developed dendrite arms make it difficult for hydrogen to diffuse deeper into the samples, resulting in large ratio of unhydrided Mg. It can be concluded that the quantity of defects within Mg and catalytic sites outside the Mg matrix are important for achieving rapid hydrogen absorption and desorption simultaneously.

Molecular dynamics simulation of tension and compression deformation behavior in CoCrCuFeNi high-entropy alloy: Effects of temperature and orientation

This study utilizes molecular dynamics (MD) simulations to investigate the uniaxial tensile and compressive deformation behavior of the face-centered cubic (FCC) CoCrCuFeNi high-entropy alloy (HEA) with varying orientations at different temperatures. The results indicate that the material experiences temperature-induced softening in all simulation cases. In addition, it has been observed that the mechanical properties of the CoCrCuFeNi HEA exhibit orientation dependence and tension-compression asymmetry. Analysis of the Schmid factor of the leading and trailing partial dislocations suggests that the yield asymmetry is contingent upon the leading Schmid factor asymmetry. Microstructure evolution analysis and quantitative statistics of deformation defects indicate that the plastic deformation mechanisms of uniaxial tensile deformation along the [001], [111], and [112] orientations and compressive deformation along the [110], [111], and [112] orientations can be characterized by the formation and annihilation of intrinsic stacking fault (ISF), extrinsic stacking fault (ESF), and a few twins resulting from the gliding of Shockley partial dislocations. Twins play a crucial role in compression along the [001] direction and tension along the [110] direction due to the proliferation of twin boundaries (TBs)-assisted dislocations. The interactions between stacking faults (SF) and abundant TBs cause an increase in dislocation density in the later stage of plastic deformation in the aforementioned models. A comparison with low-entropy materials and theoretical calculations reveals that alloying reduces the stacking fault energy (SFE) of the CoCrCuFeNi crystal and enhances its twinnability.

Thermal-induced decomposition and mechanical properties of AlN/CrN superlattice coating

Here, the thermal-induced decomposition and mechanical properties of AlN/CrN superlattice coating with modulation period of ∼8 nm are investigated in detail. Introduction of CrN template layers stabilizes the cubic growth of AlN layers, where the coherent AlN/CrN coating exhibits the higher hardness value of ∼24.1 GPa than CrN (∼16.9 GPa) and AlN (∼18.5 GPa) monolithic coatings. After thermal exposure, the AlN/CrN coating decomposes into stable body-centered cubic Cr and wurtzite AlN via intermediate phases of hexagonal Cr2N and cubic AlN. Especially, the formation of metastable cubic AlN is observed due to the destruction of coherent interfaces. Meanwhile, the thermal load leads to the grain deformation, which causes the change of AlN and CrN sublayers from the crossover to pinch-off to finally dissociation. The driving force of grain deformation mainly arises from the N-loss of CrN sublayers and the high-density stacking faults formation of AlN sublayers. Furthermore, the occurrence of stacking faults and h-Cr2N phase offset the softening effect caused by the thermal decomposition. Correspondingly, the hardness almost remains unchanged between 900 and 1100 °C.

Formation and propagation mechanism of complex stacking fault in 180 μm thick 4H-SiC epitaxial layers

The formation mechanism and propagation behavior of a complex stacking fault (SF) in thick silicon carbide epitaxial layers was investigated using depth-resolved section X-ray topography and ultraviolet photoluminescence (UVPL) imaging. The SF is initiated in the substrate by deflection of a threading screw dislocation onto two basal half planes creating a Frank fault. This fault propagates into the epitaxial layer and interacts with several threading mixed dislocations, which creates both Frank and Shockley SFs on the same basal plane. UVPL images using different spectral filters show various SF type conversions occurring during propagation of the complex SF.

Crack suppression in electrodeposited brass film through controlling growth stresses by co-depositing with Bi

The role of Bi in suppressing crack formation in the electrodeposited brass film is methodically examined by growing brass films from cyanide-free alkaline baths with and without Bi. Electrodeposition of the brass film with Bi produces crack-free films, whereas the brass film without Bi contains several cracks. In-situ film stress measurements by the cantilever curvature technique during the film deposition process reveal that compressive stresses are commonly generated in the film deposited with Bi, while tensile stresses are present in the brass film deposited without Bi. Both the compressive and tensile growth stresses grow with the increasing layer thickness for Bi-co-deposited and Bi-free films, respectively. Additionally, at large layer thicknesses, the growing stress in Bi-co-deposited film is highly compressive compared to the highly tensile stress developed in the Bi-free brass film. The obtained results indicate that such a change in film stress from tensile to compressive is essentially caused by grain size coarsening and stacking fault formation in the Bi-co-deposited films, because co-deposition with Bi alters the nucleation and growth of Cu-Zn grains in the electrodeposition process and the incorporation of Bi reduces the stacking fault energy of Cu-Zn alloy. This study provides a pathway to suppressing crack formation by controlling film stresses through Bi-co-deposition, thus growing crack-free brass films from environmentally friendly solutions.

Ab initio study of the effect of interstitial alloying on the intrinsic stacking fault energy of paramagnetic γ-Fe and austenitic stainless steel

Intrinsic stacking fault energy (SFE) values of γ-Fe and AISI 304 austenitic stainless steels were determined as a function of carbon and nitrogen content using ab initio calculations. In contrast to previous investigations, the analysis was conducted incorporating the paramagnetic state to account for the magnetic constitution of real austenitic stainless steels. The effect of finite temperature was partially accounted for by performing ab initio calculations at the experimental volumes at room temperature. Including paramagnetism in γ-Fe increases the SFE of non-magneticγ-Fe by ∼385 mJ.m–2. Interstitial alloying of non-magneticγ-Fe causes a linear increase in intrinsic stacking fault energy with interstitial content. In comparison, interstitial alloying of paramagneticγ-Fe increases the SFE at only about half the rate. The SFE of paramagnetic interstitial-free AISI 304 is within the range of -12 to 0 mJ.m–2 and only deviates slightly from the SFE of paramagneticγ-Fe. It follows a similar, albeit flatter linear dependency on the interstitial content compared to γ-Fe. Both γ-Fe and γ-AISI 304 were found to be metastable in their interstitial-free condition and are stabilized by interstitial alloying. The possible effect of short range ordering between interstitials and Cr on the SFE was discussed. The calculated threshold nitrogen content necessary to stabilize austenite in AISI 304 is in good agreement with experimental investigations of deformation microstructures in dependence of the nitrogen content. Finally, the calculated negative SFE values of AISI 304 were reconciled with experimentally determined positive SFE values using a recent method that accounts for the kinetics of stacking fault formation.

Effect of Segregation on Deformation Behaviour of Nanoscale CoCrCuFeNi High-Entropy Alloy

A molecular dynamics (MD) simulation method is used to investigate the effect of grain boundary (GB) segregation on the deformation behavior of bicrystals of equiatomic nanoscale CoCrCuFeNi high-entropy alloy (HEA). The deformation mechanisms during shear and tensile deformation at 300 K and 100 K are analyzed. It is revealed that upon tensile deformation, the stacking fault formation, and twinning are the main deformation mechanisms, while for the shear deformation, the main contribution to the plastic flow is realized through the GB migration. The presence of the segregation at GBs leads to the stabilization of GBs, while during the shear deformation of the nanoscale CoCrCuFeNi HEA without the segregation at GBs, GBs are subject to migration. It is found that the GB segregation can differently influence the plasticity of the nanoscale CoCrCuFeNi HEA, depending on the elemental composition of the segregation layer. In the case of copper and nickel segregations, an increase in the segregation layer size enhances the plasticity of the nanoscale CoCrCuFeNi HEA. However, an increase in the thickness of chromium segregations deteriorates the plasticity while enhancing maximum shear stress. The results obtained in this study shed light on the development of HEAs with enhanced mechanical properties via GB engineering.

Using Molecular Dynamic Simulation to Understand the Deformation Mechanism in Cu, Ni, and Equimolar Cu-Ni Polycrystalline Alloys

The grain boundaries and dislocations play an important role in understanding the deformation behavior in polycrystalline materials. In this paper, the deformation mechanism of Cu, Ni, and equimolar Cu-Ni alloy was investigated using molecular dynamic simulation. The interaction between dislocations and grain boundary motion during the deformation was monitored using the dislocation extraction algorithm. Moreover, the effect of stacking fault formation and atomic band structure on the deformation behavior was discussed. Results indicate that dislocations nucleate around the grain boundary in copper, the deformation in nickel changes from planar slip bands to wavy bands, and high density of dislocation accumulation as well as numerous kink and jog formations were observed for the equimolar Cu-Ni alloy. The highest density of the Shockley dislocation and stacking faults was formed in the equimolar Cu-Ni alloy which results in the appearance of a huge gliding stage in the stress–strain curve. The grain boundaries act as a sinking source for vacancy annihilation in Ni and Cu; however, this effect was not observed in an equimolar Cu-Ni alloy. Finally, radial distribution function was used to evaluate atom segregation in grain boundaries.

Study on the Nano-Friction Behavior of Nickel-Based Ag Film Composites Based on Molecular Dynamics

The nano-friction behavior of nickel-based Ag film composites was evaluated using molecular dynamics simulations. The mechanical properties, the surface morphology, the migration behavior of Ag atoms and the defect evolution during repeated friction were investigated. Our results show that the poor mechanical properties of the Ag film surface at the first stage of friction are related to a large amount of abrasive chip pileup. The slip channel with low shear strength formed by secondary friction significantly reduces the friction coefficient of the Ag film surface. Meanwhile, the migration of Ag atoms at the two-phase interface relies mainly on the repeated friction of the grinding ball, and the friction coefficient of the nickel surface decreases as the number of migrating atoms increases. In addition, the extension of defects inside the Ag film and atomic displacement is hindered by the two-phase interface. The defects inside the Ag film near the friction zone gradually evolve from an intrinsic stacking fault to a horizontal stacking fault as the friction proceeds. This is attributed to the horizontal layer-by-layer motion of Ag atoms, promoting the formation of horizontal stacking faults.

Atomic scale investigation of FCC → HCP reverse phase transformation in face-centered cubic zirconium

• FCC → HCP reverse phase transformation in an FCC-Zr grain along with a concomitant rotation of 70.5° with α-Zr matrix is observed. • The possible interaction among three FCC-Zr grains and a nearby secondary phase particle is discussed. • The nature and driving force behind the FCC → HCP reverse phase transformation are revealed. Mechanism of FCC → HCP reverse phase transformation in face-centered cubic zirconium (FCC-Zr) along with a concomitant 70.5° rotation of α-Zr matrix were investigated in zircaloy-4 (Zr-4) cladding tube by using transmission electron microscopy (TEM). Results showed that the interaction among a secondary phase particle (SPP) and three FCC-Zr grains resulted in the formation of cross stacking faults in SPP and exerted a drag force on minor axis of the adjacent FCC-Zr phase. Moreover, when the shear stress along [ 1 ¯ 1 ¯ 2 ¯ ] FCC-Zr direction was large enough to initiate the emission of 1 6 [ 1 ¯ 1 ¯ 2 ¯ ] Shockley partial dislocation on every other ( 11 1 ¯ ) FCC-Zr close-packed plane, the stacking sequence would change from ABC ABCA to AB ABABA viz. (0001) planes of the daughter HCP phase. Thus, FCC → HCP reverse phase transformation in FCC-Zr was presented.

Impact of Interfaces on Photoluminescence Efficiency of High-Indium-Content (In,Ga)N Quantum Wells

$(\mathrm{In},\mathrm{Ga})\mathrm{N}$-based light-emitting diodes (LEDs) are known to suffer from low electron-hole wave-function overlap due to a high piezoelectric field. Staggered $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum wells (QWs) are proposed to increase the wave-function overlap and improve the efficiency of LEDs, especially for long-wavelength emitters. In this work, we show evidence that the growth of staggered QWs by plasma-assisted molecular beam epitaxy has another beneficial effect, as it allows a reduction in the formation of defects, responsible for nonradiative Shockley-Read-Hall recombination, at the bottom interface of the QW. Staggered QWs comprised an $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ layer of an intermediate $\mathrm{In}$ content between the barrier and the QW. We show that insertion of such a layer results in a significant increase of the luminescence intensity, even if the calculated wave-function overlap drops. We study the dependence of the thickness of such an intermediate-$\mathrm{In}$-content layer on photoluminescence intensity behavior. Staggered QWs exhibit increased cathodoluminescence homogeneity that is a fingerprint of a lower density of defects, in contrast to standard QWs for which a high density of dark spots is observed in QW-emission mapping. Transmission electron microscopy of standard QWs reveals the formation of basal-plane stacking faults and voids that can result from vacancy aggregation. A stepwise increase of the $\mathrm{In}$ content in staggered QWs prevents the formation of point defects and results in an increased luminescence efficiency. The $\mathrm{In}$-composition difference between the barrier and the well is, therefore, a key parameter to control the formation of point defects in the high-$\mathrm{In}$-content QWs, influencing the luminescence efficiency. Characteristics of the cyan laser diode (LD) utilizing staggered QWs are presented.

Unexpected {[formula omitted]} pyramidal stacking faults in Zr(Fe, Cr)2 secondary phase particle induced by the C15→C14 Laves phase transformation in Zircaloy-4

The formation mechanism of (011¯1)C14 pyramidal stacking faults (SFs) with a 8.6° deflection to the SFs on (111¯)C15 planes of Zr(Fe, Cr)2 secondary phase particle (SPP) in Zircaloy-4 under cyclic deformation were investigated by using transmission electron microscopy. Results showed that local C14 domain structures can be induced into the C15 SPP when the SFs formed on (111¯)C15 close-packed planes by the emission of 16[112]C15 Shockley partial dislocations under cyclic stress effects. Moreover, when the shear stresses along [112¯]C15 direction were large enough to initiate the 16[112¯]C15 partials on every other (111)C15 planes, the C15→C14 Laves phase transformation would be accomplished, resulting in a significant reconstruction of atomic configurations of the SFs on original (111¯)C15 planes. Thus, the formation of (011¯1)C14 pyramidal SFs during C15→C14 phase transformation in SPP were presented.

Effects of trace Ca addition on microstructure and mechanical properties of as-cast Mg-Sm-Gd-based alloy

The effects of trace amounts of Ca on the microstructure and mechanical properties of as-cast Mg−Sm− Gd−Zn−Zr alloy were systematically investigated mainly by means of transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and tensile test. The results reveal that the dominant intermetallic compound in Mg−Sm−Gd−Zn−Zr alloy is Mg3RE phase with significant enrichment of Zn. The addition of Ca in Mg−Sm−Gd−Zn−Zr alloy can not only refine grains, but also promote the formation of Mg5RE phase and solute-enriched stacking faults (SESFs). The coherent interface indicates that Mg3RE phase can act as the nucleation site of Mg5RE phase. Finally, the decreased grain size and the formation of Mg5RE phase and SESFs in the Ca-modified alloy are beneficial to the enhancement of mechanical properties.

Suzuki hardening and segregation in Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloys

A Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloy was heavily cold-rolled and subsequently aged at 773 K. Aging-induced Suzuki hardening was observed. High-angle annular dark-field scanning transmission electron microscopy imaging and the corresponding energy dispersive X-ray spectroscopy elemental distribution mapping suggested the enrichment of Co atoms at stacking faults (SFs), which pinned down partial dislocations and enhanced the resistance of the alloy to plastic deformation. Furthermore, the enrichment of Co and Mo atoms at SFs was observed in a low dislocated, strain-aged specimen. The significant difference in the SF density of the specimens led to difference in segregation. The local stacking fault energy (SFE) with the segregation of Co and Mo atoms was calculated as −30.6 mJ/m2 at the aging temperature. The negative SFE did not restrict the SF expansion, indirectly resulting in the formation of wide SFs.

Creating superconductivity in WB2 through pressure-induced metastable planar defects

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, Tcof 17 K at 91 GPa. Upon further compression up to 187 GPa, the Tcgradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2 (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.

An atomistic study of the newly-developed single-phase refractory high entropy alloy of TiZrVMo: Defect accumulation and evolution under tensile deformation

For the very first time, novel refractory high entropy alloys (RHEAs) of the Ti-Zr-V-Mo system with a single bcc solid solution structure have been predicted and confirmed by molecular dynamics, resulting in twelve new compositions with a single bcc structure here. For the equiatomic RHEA of TiZrVMo specifically, the tensile deformation behavior and underlying mechanism were studied by molecular dynamic simulations. Except for the dislocation toughening and stacking fault formation, the simulation revealed that the bcc to fcc and hcp phase transformation occurred during the plastic deformation of the RHEA, which was also reversible after the tensile failure. This work provides a solid theoretical reference for the composition design and ductility understanding for the development of RHEAs in practical applications.

Stability, reversibility, and recovery of radiation-induced phase transformations in nanowires under mechanical loads

Phase transformations in nanowires can be induced via an external stimulus including mechanical loads or radiation damage. Notably, FCC nanowires exposed to an irradiation environment can undergo a size-dependent FCC-to-HCP phase transformation caused by the formation and accumulation of radiation-induced stacking faults and twinning defects. In this study, using atomistic simulations, we examine whether or not such radiation-induced phase transformation is stable and/or reversible under mechanical loads. Our results show that this type of phase transformation is stable under tension, reversible via compressive strain, and possibly subsequently recovered under tensile strain. The prerequisite conditions for the stability, reversibility, and recovery of this phase transformation are intimately related to the yielding mechanism at play and the reminiscence of residual stacking faults and twinning upon compressive loading.

Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys at 20 kelvin.

CrCoNi-based medium- and high-entropy alloys display outstanding damage tolerance, especially at cryogenic temperatures. In this study, we examined the fracture toughness values of the equiatomic CrCoNi and CrMnFeCoNi alloys at 20 kelvin (K). We found exceptionally high crack-initiation fracture toughnesses of 262 and 459 megapascal-meters½ (MPa·m½) for CrMnFeCoNi and CrCoNi, respectively; CrCoNi displayed a crack-growth toughness exceeding 540 MPa·m½ after 2.25 millimeters of stable cracking. Crack-tip deformation structures at 20 K are quite distinct from those at higher temperatures. They involve nucleation and restricted growth of stacking faults, fine nanotwins, and transformed epsilon martensite, with coherent interfaces that can promote both arrest and transmission of dislocations to generate strength and ductility. We believe that these alloys develop fracture resistance through a progressive synergy of deformation mechanisms, dislocation glide, stacking-fault formation, nanotwinning, and phase transformation, which act in concert to prolong strain hardening that simultaneously elevates strength and ductility, leading to exceptional toughness.

Synergistic strengthening of Al–SiC composites by nano-spaced SiC-nanowires and the induced high-density stacking faults

Aluminum alloys are widely used in engineering structures due to light weight and corrosion resistance but aluminum has low yielding and flow strengths. Here we reported super-strong Al-30 vol%SiC composites with a flow strength of about 1.18 GPa up to a uniform strain of 16.0%. Micromechanical tests revealed a flow strength of 0.73 GPa associated with the nano-spaced SiC nanowires strengthening, and additional flow strength of 0.45 GPa associated with high-density stacking faults (SFs) that are rarely formed and stabilized in Al due to high stacking fault energy (SFE). More surprisingly, SFs possess excellent thermal stability up to 320 °C and can be regained by thermal cooling even after they are eliminated during annealing at 600 °C. Microscopy characterizations and theoretical analysis revealed that thermal mismatch induced high stress during cooling promotes the formation of SFs, and the segregation of Si into SFs and dislocation cores enables the thermal stability of wide SFs. This work demonstrated an approach to creating high-density and thermo stable SFs in high SFE metals via microstructure-enabled thermal stress.

Stacking Fault Induced Symmetry Breaking in van der Waals Nanowires.

While traditional ferroelectrics are based on polar crystals in bulk or thin film form, two-dimensional and layered materials can support mechanisms for symmetry breaking between centrosymmetric building blocks, e.g., by creating low-symmetry interfaces in van der Waals stacks. Here, we introduce an approach toward symmetry breaking in van der Waals crystals that relies on the spontaneous incorporation of stacking faults in a nonpolar bulk layer sequence. The concept is realized in nanowires consisting of Se-rich group IV monochalcogenide (GeSe1-xSx) alloys, obtained by vapor-liquid-solid growth. The single crystalline wires adopt a layered structure in which the nonpolar A-B bulk stacking along the nanowire axis is interrupted by single-layer stacking faults with local A-A' stacking. Density functional theory explains this behavior by a reduced stacking fault formation energy in GeSe (or Se-rich GeSe1-xSx alloys). Computations demonstrate that, similar to monochalcogenide monolayers, the inserted A-layers should show a spontaneous electric polarization with a switching barrier consistent with a Curie temperature above room temperature. Second-harmonic generation signals are consistent with a variable density of stacking faults along the wires. Our results point to possible routes for designing ferroelectrics via the layer stacking in van der Waals crystals.