Elevated Dislocation Density Research Articles

Enhancing wear resistance of laser-clad AlCoCrFeNi high-entropy alloy via ultrasonic surface rolling extrusion

The AlCoCrFeNi high-entropy alloy (HEA) coating was synthesized by laser cladding and subsequently processed by ultrasonic surface rolling extrusion (USRE) with varying static loads of 100 N, 200 N, and 300 N. This study aimed to explore the effects of static load on the microstructure, mechanical properties, and tribological characteristics of the coatings. The findings indicate that increased static load results in the precipitation of martensitic phases, grain refinement, enhanced microhardness, and an elevated dislocation density. Specifically, after USRE with 300 N static load, the coating's microhardness reaches 753 HV, 1.4 times greater than the un-USRE coating. This enhancement is attributed to the synergistic effect of martensite phase transformation, grain refinement, and dislocation strengthening. The volume wear rate of the coating decreases to 1.31 × 10−4 mm3·N−1·m−1 following USRE at 300 N, demonstrating superior resistance to abrasive wear due to the high microhardness. Besides, the high-density dislocations facilitate rapid migration and diffusion of oxygen elements, thereby enhancing the oxide layer's protective properties and improving the coating's resistance to oxidation wear.

Investigation of GH3625 alloy: Contrasting microstructures, mechanical properties, and corrosion performance via conventional and extreme high speed laser material deposition

The GH3625 nickel alloy, known for its exceptional mechanical properties and corrosion resistance, has been widely adopted. Nonetheless, the current additive manufacturing (AM) of GH3625 alloy presents compromised strength and significant segregation, thus limiting its potential applications. To address these challenges, the utilization of the extreme high-speed laser material deposition (EHLA) process with its rapid cooling capabilities has been investigated, aiming to enhance grain refinement and suppress segregation. In this study, a comparative analysis was conducted between GH3625 nickel alloy sample fabricated by conventional laser material deposition (LMD) and the EHLA technique. The investigation covered thermal processes, microstructural evolution, mechanical properties, and corrosion resistance. The EHLA-fabricated specimen exhibits a refined microstructure and elevated dislocation density attributed to the rapid solidification process. This leads to an impressive ultimate tensile strength of 1005 MPa, surpassing the LMD-deposited counterpart by 20 %. The Clyne-Kurz model outperforms the classic Scheil model in evaluating AM processes sensitive to cooling rates. Notably, the EHLA sample demonstrates reduced Nb segregation compared to the LMD sample. Furthermore, the formation of the precipitated Laves phase is reduced, and the concentrations of Nb and Mo within the dendritic structure are increased in the EHLA sample. Consequently, enhancements in pitting corrosion resistance and the protective effectiveness of passive films are observed.

23.4% monolithic epitaxial GaAsP/Si tandem solar cells and quantification of losses from threading dislocations

A 2-terminal, dual-junction, epitaxially integrated, GaAsP/Si tandem solar cell with an 3rd party certified efficiency of 23.4 % was fabricated via MOCVD growth on an ex-situ produced Si sub cell. The drastic efficiency improvement over the authors previous peer-reviewed demonstration of such a device architecture is examined. Critical advancements in top cell design to maximize short wavelength response were critical in enabling improved top cell response. An in-depth analysis of this champion tandem cell has identified key loss mechanisms which elucidate the pathway for further efficiency gains. First, voltage dependent collection efficiency in the GaAsP top cell is the primary cause of fill factor losses currently limiting efficiency. Analysis of spectrally resolved I–V measurements and analytical device modeling and indicate poor diffusion length due to elevated dislocation densities as the likely cause for the voltage dependent collection efficiency. Second, modeling for the GaAs0.75P0.25 top cell, using experimental data at multiple dislocation densities, provides quantitative understanding of the current and voltage losses associated with threading dislocations providing a clear efficiency pathway with reduction in dislocation density. Lastly Si subcell modeling identifies the pathway for further Si subcell advances over the present, simplistic design, which has yet to employ the known benefits of rear surface texture or dielectric passivation.

Effect of microstructural anisotropy on severe plastic deformation during material removal at micrometer length-scales

The emergence of shear inhomogeneity across grain boundaries during severe shear deformation in Cu at the micrometer length-scales is demonstrated through in situ imaging inside a scanning electron microscope. The deformation mechanics during machining is found to be orientation dependent. Microstructure refinement from severe plastic deformation in confined volumes is entirely suppressed in certain orientations but results in a characteristic ultrafine structure in others. Rampant ductile fracture is found to nucleate at some grain boundaries due to elevated dislocation densities, and this failure mechanism is imprinted on the freshly generated surface, which limits the precision of machining-based processes (e.g., diamond turning) in nanometrically-smooth roughness profiles.

Investigation of Rear-Emitter GaAs0.75P0.25 Top Cells for Application to III–V/Si Tandem Photovoltaics

A rear-emitter (rear-junction) n -on- p + device design was investigated for use in metamorphic monolithic III–V/Si tandem solar cells as an alternative to the traditional front-emitter (front-junction) n +-on- p design for potentially greater resistance to threading dislocation-induced performance degradation. A comparison of MOCVD-grown rear- versus front-emitter 1.7-eV bandgap GaAs0.75P0.25 top cell isotypes demonstrated as 30-mV advantage in W OC for the rear-emitter design. This reduction in W OC was determined to be nearly equally caused by a reduction in junction recombination current as well as reduced reverse saturation current from improved quasi-neutral region transport. These results suggest that the rear-emitter design may indeed be a promising pathway for application to metamorphic cells, including III–V/Si tandems, where achieving maximum voltage output is often hindered by elevated dislocation densities. However, further optimization of short-circuit current collection is needed to overcome suboptimal collection probability profiles that could limit the efficacy of such structures.

A comparative study on microstructure and mechanical properties between friction and laser beam welded titanium tubes

The microstructure and mechanical properties of friction and laser beam welded Grade 2 titanium tubes (wall thickness of 3.9 mm and outer diameter of 60 mm) are compared in the present investigation. Friction welding (FW) was conducted on a vertical friction processing system. Laser beam welding (LBW) was accomplished by utilizing a 4 KW Nd:YAGlaser. The microstructure evolution was observed by optical microscopy and advanced techniques such as TEM and EBSD. The friction welded joints showed fine grains due to dynamic recrystallization. A variation in grain size towards the outer circumference of the tube was observed due to flash formation. The laser beam welded joints displayed large grains with irregular boundaries. Both joints exhibited an elevated dislocation density due to severe plastic deformation and thermal stresses respectively. Strengthening of the joint area was recorded in both joints albeit due to different strengthening mechanisms. The failure location, during mechanical testing, occurred in the base metal for both processes. Overall when compared, LBW is probably the preferred process to join these tubes due to its higher welding speed and therefore reduced production cycle.

Indentation response of nanoporous gold from atomistic simulations

Abstract

Materials response to glancing incidence femtosecond laser ablation

A femtosecond Ti:sapphire laser was used to ablate samples of copper, strontium titanate (STO), a nickel alloy René 88DT (R88), {111}-oriented single crystal silicon, and gallium nitride (GaN) in situ in a focused ion beam scanning electron microscope (FIB-SEM). The laser beam was scanned parallel to the specimen surface, which resulted in laser ablation using the tail of the Gaussian beam distribution, near the ablation threshold for each of the materials. Transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were utilized to investigate damage in the bulk and at the surface of the laser ablated samples in cross-sections that were extracted by FIB-SEM. In contrast to normal incidence, post-ablation damage in the glancing incidence configuration was extremely limited across a wide range of laser pulse energies. Elevated dislocation densities were observed within 150–200 nm of the ablated surface in the Cu, STO, and R88 samples. An amorphized Si layer as thin as 30–50 nm was observed with no dislocations near the surface or in the bulk. Gallium nitride exhibited exceptional damage resistance to femtosecond laser irradiation, whereby no laser-induced dislocations or amorphization near the ablated surface was observed. For materials where there is surface damage following laser ablation, we show that a subsequent machining step with a Ga+ FIB beam located in the same chamber can remove this damage in a short period of time.

Localization of submicron inclusion re-equilibration at healed fractures in host garnet

Microstructures in Permian inclusion-bearing metapegmatite garnets from the Koralpe (Eastern Alps, Austria) reveal re-equilibration by coarsening of abundant submicron-sized inclusions (1 μm–2 nm diameter) at the site of healed brittle cracks. The microstructures developed during Cretaceous eclogite-facies deformation and the related overprinting of the host–inclusion system. Trails of coarsened inclusions (1–10 μm diameter) crosscut the garnet, defining traces of former fractures with occasional en-echelon overlaps. Trails are flanked by 10- to 100-μm-wide ‘bleaching zones’ characterized by the absence of ≤1-μm-sized inclusions in optical and SE images. FEG-microprobe data show that trails and bleaching zones can form isochemically, although some trails exhibit non-isochemical coarsening. Cross-correlation-based EBSD analysis reveals garnet lattice rotation of up to 0.45°, spatially correlated with bleaching zones. The garnet lattice in the center of trails is misoriented around different axes with respect to the lattice either side of the trail. Elevated dislocation density within bleaching zones is confirmed by TEM observations. Dislocations represent a plastic wake formed by crystal plastic deformation at the crack tip. Fracture enhanced diffusion rates in the lattice adjacent to crack planes by introducing dislocations, priming these areas to behave differently to the bulk of the garnet during Cretaceous metamorphism and facilitating localized coarsening of inclusions. Diffusion within the bleaching zone was enhanced by a minimum factor of 102. The partially closed host–inclusion system records the influence of deformation mechanisms on re-equilibration and contributes to understanding of the interaction between deformation and chemical reaction during metamorphism.

Long range internal stresses in single-phase crystalline materials

Backstresses, or long range internal stresses (LRISs), have been suggested by many to exist in plastically deformed crystalline materials. Elevated stresses may be present in regions of elevated dislocation density or dislocation heterogeneities in the deformed microstructures. The heterogeneities include dislocation pile-ups, edge dislocation dipole bundles and cell walls in monotonically and cyclically deformed materials. The existence of LRIS is especially important for the understanding of cyclic deformation and monotonic deformation. Theories and supporting experiments for assessing LRIS will all be discussed in this review. This review includes several new developments over the past few years.

Defect evolution and interplay in n-type InN

The nature and interplay of intrinsic point and extended defects in n-type Si-doped InN epilayers with free carrier concentrations up to 6.6×1020 cm−3 are studied using positron annihilation spectroscopy and transmission electron microscopy and compared to results from undoped irradiated films. In as-grown Si-doped samples, mixed In-N vacancy complexes (VIn-VN) are the dominant III-sublattice related vacancy defects. An increase in the number of VN in these complexes toward the interface suggests high concentrations of additional isolated VN and VN-clusters near the GaN buffer layer and coincides with elevated dislocation densities in that area.

Backstress, the Bauschinger Effect and Cyclic Deformation

Backstresses or long range internal stresses (LRIS) in the past have been suggested by many to exist in plastically deformed crystalline materials. Elevated stresses can be present in regions of elevated dislocation density or dislocation heterogeneities in the deformed microstructures. The heterogeneities include edge dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials and cell and subgrain walls in monotonically deformed materials. The existence of long range internal stress is especially important for the understanding of cyclic deformation and also monotonic deformation. X-ray microbeam diffraction experiments performed by the authors using synchrotron x-ray microbeams determined the elastic strains within the cell interiors. The studies were performed using, oriented, monotonically deformed Cu single crystals. The results demonstrate that small long-range internal stresses are present in cell interiors. These LRIS vary substantially from cell to cell as 0 % to 50 % of the applied stress. The results are related to the Bauschinger effect, often explained in terms of LRIS.

Thermomechanical treatment of austempered ductile iron

The production of lightweight ferrous castings with increased strength properties became unavoidable to face the serious challenge of lighter aluminium and magnesium castings. The relatively new ferrous casting alloy austempered ductile iron (ADI) offers promising strength prospects, whereas the thermomechanical treatment of ductile iron may suggest a new route for production of thin wall products. This work aims at studying the influence of thermomechanical treatment, either by ausforming just after quenching and before the onset of austempering reaction or either by cold rolling (CR) after austempering. In the first part of this work, ausforming of ADI up to 25% reduction in height during a rolling operation was found to add a mechanical processing component to the conventional ADI heat treatment, thus increasing the rate of ausferrite formation and leading to a much finer and more homogeneous ausferrite product. The kinetics of ausferrite formation was studied using both metallographic and X-ray diffraction (XRD) techniques. The effect of ausforming on the strength values was quite dramatic (up to 70 and 50% increase in the yield and ultimate strength respectively). A mechanism involving both a refined microstructural scale and an elevated dislocation density was suggested. Nickel is added to ADI to increase hardenability of thick section castings, and ausforming to higher degrees of deformation is necessary to alleviate the deleterious effect of alloy segregation on ductility. In the second part of this work, the influence of CR on the mechanical properties and structural characteristics of ADI was investigated. The variation in properties was related to the amount of retained austenite (γr) and its mechanically induced transformation. In the course of tensile deformation of ADI, transformation induced plasticity (TRIP) takes place, indicated by the increase in the instantaneous value of strain hardening exponent with tensile strain. The amount of retained austenite was found to decrease due to partial transformation of γr to martensite under the CR strain. Such strain induced transformation resulted in higher amounts of mechanically generated martensite. The strength and hardness properties were therefore increased, while ductility and impact toughness decreased with increasing CR reduction.

HgCdTe/Si materials for long wavelength infrared detectors

The heteroepitaxial growth of HgCdTe on large-area Si substrates is an enabling technology leading to the production of low-cost, large-format infrared focal plane arrays (FPAs). This approach will allow HgCdTe FPA technology to be scaled beyond the limitations of bulk CdZnTe substrates. We have already achieved excellent mid-wavelength infrared (MWIR) and short wavelength infrared (SWIR) detector and FPA results using HgCdTe grown on 4-in. Si substrates using molecular beam epitaxy (MBE), and this work was focused on extending these results into the long wavelength infrared (LWIR) spectral regime. A series of nine p-on-n LWIR HgCdTe double-layer heterojunction (DLHJ) detector structures were grown on 4-in. Si substrates. The HgCdTe composition uniformity was very good over the entire 4-in. wafer with a typical maximum nonuniformity of 2.2% at the very edge of the wafer; run-to-run composition reproducibility, realized with real-time feedback control using spectroscopic ellipsometry, was also very good. Both secondary ion mass spectrometry (SIMS) and Hall-effect measurements showed well-behaved doping and majority carrier properties, respectively. Preliminary detector results were promising for this initial work and good broad-band spectral response was demonstrated; 61% quantum efficiency was measured, which is very good compared to a maximum allowed value of 70% for a non-antireflection-coated Si surface. The R0A products for HgCdTe/Si detectors in the 9.6-µm and 12-µm cutoff range were at least one order of magnitude below typical results for detectors fabricated on bulk CdZnTe substrates. This lower performance was attributed to an elevated dislocation density, which is in the mid-106 cm−2 range. The dislocation density in HgCdTe/Si needs to be reduced to <106 cm−2 to make high-performance LWIR detectors, and multiple approaches are being tried across the infrared community to achieve this result because the technological payoff is significant.

An Experimental and Computational Study of the Elastic-Plastic Transition in Thin Films

ABSTRACTNanoindentation studies of thin metal films have provided insight into the mechanisms of plasticity in small volumes, showing a strong dependence on the film thickness and grain size. It has been previously shown that an increased dislocation density can be manifested as an increase in the hardness or flow resistance of a material, as described by the Taylor relation [1]. However, when the indentation is confined to very small displacements, the observation can be quite the opposite; an elevated dislocation density can provide an easy mechanism for plasticity at relatively small loads, as contrasted with observations of near-theoretical shear stresses required to initiate dislocation activity in low-dislocation density materials. Experimental observations of the evolution of hardness with displacement show initially soft behavior in small-grained films and initially hard behavior in large-grained films. Furthermore, the small-grained films show immediate hardening, while the large grained films show the ‘softening’ indentation size effect (ISE) associated with strain gradient plasticity. Rationale for such behavior has been based on the availability of dislocation sources at the grain boundary for initiating plasticity. Embedded atom method (EAM) simulations of the initial stages of indentation substantiate this theory; the indentation response varies as expected when the proximity of the indenter to a Σ79 grain boundary is varied.

TEM Observations of Defects in Ge‐Doped LPE GaAs and Al0.4Ga0.6As

The formation of structural defects in degenerate Ge‐doped LPE and films was investigated. At growth initiation temperatures of 780°C, Ge doping does not result in structural defects even in films grown from solutions containing ∼50 a/o Ge and even after heat‐treatment at 720° and 820°C. On the other hand, films grown from heavily Ge‐doped solutions below the decomposition temperature of (737°C) contained defects after a 15 hr heat‐treatment at the growth temperature. These defects consisted of dislocation clusters and were only observed in films grown on substrates with elevated dislocation densities. It is concluded that the observed defects did not nucleate from the threading dislocations, but from or particles decorating the dislocations.

Shock effects in olivine and implications for Hugoniot data

Deformation produced in single‐crystal olivine samples by shock wave stresses ranging between 16 and 75 GPa are documented by microscopy and spectroscopy. No evidence was found for phase transformation throughout the ‘mixed‐phase’ region (about 25 to 70 GPa) contrary to expectations based on the current interpretations of Hugoniot data. Samples retrieved from peak pressures above 20 GPa exhibit extensive plastic strain; a distinctive microundulatory extinction (mosaicism) is associated with elevated dislocation densities. Shock effects in olivine are relatively minor compared to those typical of other silicates: only traces of diaplectic glass are found at pressures above about 50 GPa, and there is no evidence of melting, recrystallization, or recovery. Although civilian (reconstructive) phase transformation has been previously inferred from the Hugoniot data for olivine, this textural evidence is inconsistent with such a process. Calculations of transformation rates for olivine similarly indicate that reconstructive phase transformation is very unlikely under experimental shock conditions and that the Hugoniot data represent highly nonequilibrium states. A new, crystallographically based model is presented for the behavior of olivine under shock: it successfully reproduces the Hugoniot data and is in accord with all available observations. According to this model, olivine is compressed into a slighlty randomized, olivinelike structure at shock pressures above about 20–30 GPa and does not undergo a polymorphic transformation or an increase in cation coordination number. Some evidence indicates that many other silicates may behave similarly and remain untransformed in shock experiments. The results of this study suggest that the current interpretations of shock wave data for silicates and their geophysical applications may need reappraisal.

The susceptibility of austenitic stainless steel to intercrystalline corrosion

1. The dislocation arrays affect the precipitation of carbides in steel Kh18N10T and also its susceptibility to ICC. 2. Intercrystalline corrosion of steel Kh18N10T in nitric acid is due to the elevated dislocation density in the grain boundaries after tempering at 600–650° and is not the result of improvement of the boundaries in chromium. 3. Deformation of steel Kh18N10T (up to 30%) increases its resistance to ICC in tests by the AM and D methods. 4. Carbon can be more completely combined in titanium carbide by a combined treatment consisting of deformation, heating at 600° for 10 h, and stabilizing at 900° for 8 h. This treatment is recommended to increase the resistance to ICC for parts operating at 500–650°.

The mechanism of accelerated nitrogen diffusion in iron

1. Accelerated diffusion during ion nitriding is due to the more rapid creation of a surface layer with the limit nitrogen concentration and elevated dislocation density. 2. A preeminence in the rate of formation of a layer with the limit nitrogen concentration in the surface of iron samples during ion and gas nitriding at 550°C was observed only in the first 10 min. 3. Under the influence of ion bombardment the self-diffusion coefficient of nickel at 700°C increases 5.8 times. 4. We observed an elevated dislocation density and characteristic direction of the dislocations occurring in iron foil under the influence of ion bombardment at a depth of 0.05 mm.