Formation Of Dislocation Substructure Research Articles

Inhibiting cracking and improving strength-ductility for laser powder bed fusion René 104 superalloy by Sc and Y microalloying

Cracking is a critical challenge for the application of additive manufacturing (AM) for producing Nickel-based superalloys with high volume fraction γ′ phase. This study systematically investigated the effects of alloying elements (Sc, Y) on the cracking susceptibility and microstructure of René 104ScY superalloy fabricated by laser powder bed fusion. The effects of Sc and Y elements on solidification behavior, microstructure evolution and crack inhibition were discussed. The results show that the nano-Al3(Sc, Y) phases in René 104ScY superalloy refine both grain and cellular structure sizes, triggering the columnar-to-equiaxed transition, and efficiently inhibiting crack generation. The formation of dislocation substructures, stacking faults and Lomer-Cottrell locks are responsible for the synergistic enhancement in strength-ductility of René 104ScY superalloy. Crack-free René 104ScY superalloy with excellent strength-ductility synergy was obtained by Sc and Y microalloying. These results provide a novel approach to developing crack-free Nickel-based superalloys for AM and offer insights for addressing high cracking susceptibility in other alloys.

Unveiling the effect of interface on torsional behavior of crystalline Al-Al90Sm10 metallic glass nanolaminates

ABSTRACT Influence of configurational design of single crystal Al-Al90Sm10 metallic glass nanolaminates on torsion deformation behaviour of Al/Al90Sm10 nanolaminate (Configuration 1) and Al90Sm10/Al nanolaminate (Configuration 2) from a structural evolution aspect have been analysed by employing Molecular Dynamics for a torsion speed of 1/600 revolution/ps. Adaptive common neighbour (a-CNA) analysis, Dislocation extraction algorithm (DXA), atomic shear strain analysis, and Voronoi Polyhedral (VP) analysis have been carried out to reveal the structural evolution in the nanolaminates specimen subjected to torque. As a consequence of dislocation density localisation under torsional loading in Al/Al90Sm10 nanolaminate high atomic strain gradient is developed in the nanolaminate specimen causing torsional buckling of the Al/Al90Sm10 nanolaminate. The localisation of dislocation density rings induces the formation of dislocation substructure in Al/Al90Sm10 nanolaminate. The crystalline/amorphous interface serves as a free surface and encourages the formation of such dislocation substructure. The collective nucleation, coalescence, and growth of shear transformation zones (STZs) leading to the formation of thick shear bands on either end of Al90Sm10/Al nanolaminate inducing an almost homogenous atomic strain gradient across the surface of the nanolaminate specimen thereby averting torsional buckling. The C/A interface serves as a nucleation site for the generation STZs in Al90Sm10/Al nanolaminate. VPs such as <0, 0, 4, 6>, <0, 3, 6, 4>, <0, 3, 6, 5> <0, 2, 8, 2> have the load bearing capacity and are resistant to fragmentation under the subjugation of torsion loading.

Dynamic recrystallization of Sn coatings on carbon-fiber-reinforced plastics during cold spray additive manufacturing

The metallization of carbon-fiber-reinforced plastic (CFRP) is a promising solution for increasing the electrical conductivity of CFRP components used in airplane applications to protect them from lightning strikes. Therefore, in this study, cold spraying was used to deposit Sn coating on an epoxy-based CFRP substrate with three different pass numbers (24, 48, and 96) at a gas temperature of 473 K, and the microstructural evolution of the deposited Sn coatings was examined. The grain size of the cold-sprayed Sn coatings gradually decreased as the pass number increased, with 96-pass deposition producing relatively homogeneous, finer-grained microstructures with an average grain size of 4.5 µm. Additionally, increasing the pass number enhanced the in-grain local misorientation of the Sn coatings, which can be correlated with the geometrically necessary dislocation density. The grain refinement observed in conjunction with the formation of dislocation substructures indicated that dynamic recrystallization occurred in the Sn coatings during the multipass cold spraying process, enabling synergistic strengthening. The dynamic recrystallization fraction increased with the pass number, eventually exceeding 60% after the 96-pass deposition. Consequently, as the pass number increased, the hardness of the deposited Sn coatings increased, resulting in more homogeneous hardness distributions along the building direction. Additionally, a thermomechanical effect originating from the low melting temperature of Sn was particularly observed. The findings of this study emphasize the critical nature of cold spray additive manufacturing for the metallization of CFRP components and strengthening of deposited metals.

Superior dynamic shear properties and deformation mechanisms in a high entropy alloy with dual heterogeneous structures

Both heterogeneous grain structure and dual nanoprecipitates were designed in a Al0.5 Cr0.9FeNi2.5V0.2 high entropy alloy (HEA), and the dynamic shear responses were investigated by hat-shaped specimens in split Hopkinson pressure bar tests. The present HEA with heterogeneous structure displays an unprecedented synergy of dynamic shear strength and ductility, as compared to the literature data for other metals and alloys. The excellent dynamic shear properties in the unaged samples could be due to the dynamical grain refinement, the dislocations hardening, and the precipitation hardening. The aged samples with a higher volume fraction of coherent L12 nanoprecipitates display even better dynamic shear properties, as compared to the unaged samples, which can be attributed to the triggered planar dislocation slip, the stored higher density of dislocations, the formation of dislocation substructure and the more pronounced precipitation hardening for postponing the occurrence of the adiabatic shear band (ASB). The high strain rate, high strain/stress magnitude, high adiabatic temperature rise, and fast-cooling process within ASB were observed to induce the dynamic recrystallization and the phase transformation from FCC phase to B2 phase, and this newly observed phase transformation phenomenon was not observed before under quasi-static deformation conditions.

Micromechanical crystal plasticity back stress evolution within FCC dislocation substructure

Experiments and theories have suggested the evolution of intragranular back stress is primarily dependent on the characteristics and development of dislocation substructure in face-centered cubic (FCC) metals and alloys. Despite this, continuum modeling of complex cyclic loading phenomena such as inelastic ratchet strain accumulation typically appeals to one of several phenomenological back stress forms with weak connections to physical mechanisms. In the current work, a micromechanically-based back stress evolution law is derived and implemented in a crystal plasticity framework for FCC metals that directly considers the evolution of dislocation substructure. The model is used in a case study of stainless steel 316L (SS316L), and good agreement is found with experiments on single- and poly-crystalline specimens subjected to monotonic, fully-reversed cyclic loading, and stress-controlled cyclic loading with mean stress. Ratcheting is attributed to dislocation substructure formation, dissolution, and stabilization within this physically-based framework.

Deformation mechanisms of CoCrFeMnNi high-entropy alloy under low-cycle-fatigue loading

Plastic deformation during low-cycle fatigue (LCF) in equiatomic face-centered cubic (FCC) CoCrFeMnNi high-entropy alloys (HEAs) is accumulated by dislocation substructure formation, which leads to crack initiation. Whilst these substructures have been reported before, little has been done to clarify their formation mechanisms and the effects of strain amplitude, cycle number and grain orientation. In this study, cyclic deformation behavior and microstructural evolution of CoCrFeMnNi were examined for two different grain sizes at room temperature. Microstructural investigations by transmission electron microscopy showed that, while the dislocation structures at low strain amplitude (0.3%) mainly consisted of planar slip bands, at higher strain amplitudes (0.5% and 0.7%), wavy-substructures including veins, walls, labyrinth and cells prevailed. Slip mode also changes from initially planar-slip to wavy-slip with cycle numbers. Dislocations in veins, walls, labyrinth and cells are found to have different Burgers vectors, suggesting that apart from wavy-slip, multiple-slip also contributes to their formation. Moreover, distinct dislocation substructure in grains is dictated more by the constraints from neighboring grains rather than by their orientation. Additionally, the formation of various dislocation structures in a single grain is also linked to the constraint effects from the neighboring grains.

Deformation Mechanisms of CoCrFeMnNi High-Entropy Alloy Under Low-Cycle-Fatigue Loading

Plastic deformation during low-cycle fatigue (LCF) in equiatomic face-centered cubic (FCC) CoCrFeMnNi high-entropy alloy (HEA) is accumulated by dislocation substructure formation, which leads to crack initiation. Whilst these substructures have been reported before, little has been done to clarify their formation mechanisms and the effects of strain amplitude, cycle number and grain orientation. In this study, cyclic deformation behavior and microstructural evolution of CoCrFeMnNi were examined for two different grain sizes at room temperature. Microstructural investigations by transmission electron microscopy showed that, while the dislocation structures at low strain amplitude (0.3%) mainly consisted of planar slip bands, at higher strain amplitudes (0.5% and 0.7%), wavy-substructures including veins, wall, labyrinth and cells prevailed. Slip mode also changes from initially planar-slip to wavy-slip with increasing cycle numbers, which defines the cyclic stress response. Dislocations in veins, walls, labyrinth and cells are found to have different Burgers vectors, suggesting that apart from wavy-slip, multiple-slip also contributes to their formation. Moreover, distinct dislocation substructure in grains is dictated more by the constraints from neighboring grains rather than by their orientation. Additionally, the formation of various dislocation structures in a single grain is also linked to the constraint effects from the neighboring grains.

Unveiling anneal hardening in dilute Al-doped AlxCoCrFeMnNi (x = 0, 0.1) high-entropy alloys

Anneal hardening has been one of the approaches to improve mechanical properties of solid solution alloys with the face-centered cubic (FCC) structure, whereby a considerable strengthening can be attained by annealing of cold-worked alloys below the recrystallization temperature (Trx). Microscopically, this hardening effect has been ascribed to several mechanisms, i.e. solute segregation to defects (dislocation and stacking fault) and short-range chemical ordering, etc. However, none of these mechanisms can well explain the anneal hardening recently observed in phase-pure and coarse-grained FCC-structured high-entropy alloys (HEAs). Here we report the observations, using high-resolution electron channeling contrast imaging and transmission electron microscopy, of profuse and stable dislocation substructures in a cold-rolled CoCrFeMnNi HEA subject to an annealing below Trx. The dislocation substructures are observed to be thermally stable up to Trx, which could arise from the chemical complexity of the high-entropy system where certain elemental diffusion retardation occurs. The microstructure feature is markedly different from that of conventional dilute solid solution alloys, in which dislocation substructures gradually vanish by recovery during annealing, leading to a strength drop. Furthermore, dilute addition of 2 at.% Al leads to a reduction in both microhardness and yield strength of the cold-rolled and subsequently annealed (≤500 °C) HEA. This Al induced softening effect, could be associated with the anisotropic formation of dislocation substructure, resulting from enhanced dislocation planar slip due to glide plane softening effect. These findings suggest that the strength of HEAs can be tailored through the anneal hardening effect from dislocation substructure strengthening.

On the mechanism of cross-slip induced dislocation substructure formation in an high-Mn steel

The underlying mechanism of dislocation substructure formation in a tensile deformed fine-grained high-Mn steel is reported using transmission electron microscopy. A cross-slip assisted dislocation truncation mechanism was revealed that formed strings of dislocation loops at early strain, which were also retained at fracture strain. Planar glide producing a Taylor lattice was also observed. The dislocation plasticity based on a cross-slip based mechanism delayed dynamic recovery in stage C to manifest an uniform strain hardening. Such a mechanism producing good combination of high strength and ductility is observed for the first time in high-Mn steel.

Correlation between Microstructures and Ductility Parameters of Cold Drawn Hyper-Eutectoid Steel Wires with Different Drawing Strains and Post-Deformation Annealing Conditions

The relationship between microstructures and ductility parameters, including reduction of area, elongation to failure, occurrence of delamination, and number of turns to failure in torsion, in hypereutectoid pearlitic steel wires was investigated. The transformed steel wires at 620 °C were successively dry-drawn to drawing strains from 0.40 to 2.38. To examine the effects of hot-dip galvanizing conditions, post-deformation annealing was performed on cold drawn steel wires (ε = 0.99, 1.59, and 2.38) with a different heating time of 30–3600 s at 500 °C in a salt bath. In cold drawn wires, elongation to failure dropped due to the formation of dislocation substructures, decreased slowly due to the increase of dislocation density, and saturated with drawing strain. During annealing, elongation to failure increased due to recovery, and saturated with annealing time. The variation of elongation to failure in cold drawn and annealed steel wires would depend on the distribution of dislocations in lamellar ferrite. The orientation of lamellar cementite and the shape of cementite particles would become an effective factor controlling number of turns to failure in torsion of cold drawn and annealed steel wires. The orientation and shape of lamellar cementite would become microstructural features controlling reduction of area of cold drawn and annealed steel wires. The density of dislocations contributed to reduction of area to some extent.

Substructure Formation, Texture, and Mechanical Behavior of Warm Rolled 2205-Type Duplex Stainless Steel

The microstructural evolution and mechanical behavior of 2205 duplex stainless steel were studied after plate warm rolling at 600 °C with 60 and 80% thickness reduction, using different microscopy techniques, X-ray diffraction, and tensile and hardness testing. The microstructures of the warm-rolled specimens consisted of flattened wavy grains. The texture evolution during rolling in the ferrite domain presented α-fiber and rotated cube components, while the austenite showed brass, copper, and cube components to a lesser extent. Intense formation of entanglement and dislocation forests characterized the microstructure. In ferrite, there was an intense formation of dislocation substructures and cell blocks. In austenite, the substructure was characterized by planar gliding and the formation of dislocation entanglements. After warm rolling, the tensile strength reached 1185 and 1328 MPa at 60 and 80% thickness reduction, respectively. Compared with the as-received steel, the warm work raised the mechanical strength level by between 65 and 72%. These results highlight the prospects for innovative routes to the industrial production of this class of duplex stainless steel, such as cold rolling suppression, considering even ductility reduction.

Decomposition Method to Detect Fatigue Damage Precursors in Thin Components Through Nonlinear Ultrasonic With Collinear Mixing Contributions

Abstract Cyclic loading of mechanical components promotes the formation of dislocation substructures in metals as precursors to crack nucleation leading to final failure of the metallic components. It is well known within the ultrasonic community that the acoustic nonlinearity parameter is a meaningful indicator of the microstructural damage accumulation. However, current nonlinear ultrasonic techniques suffer from response saturation and limited resolution after 50% fatigue life of the metallic medium. The present study investigates the feasibility of incorporating collinear wave mixing interactions into second harmonic assessments to improve the sensitivity of the nonlinear parameter to a microstructural accumulation of damage precursors (DP). To this end, a decomposition technique was explored to obtain higher harmonics from short time-domain pulses propagating through thin metallic components such as jet engine turbine blades. The results demonstrate the effectiveness of the decomposition technique to measure the acoustic nonlinearity parameter as an early and continuous indicator of fatigue damage precursors throughout the service life of critical aircraft components. A micrographic study showed a strong correlation between the nonlinearity parameter and the increase in damage precursors throughout the life of the specimens.

Theoretical and experimental measurement of intrinsic and fatigue induced material nonlinearities using Lamb wave based nonlinearity parameters

The paper presents development and application of amplitude and physics based parameters for measuring intrinsic and fatigue induced material nonlinearities of plate materials considering Lamb wave motion into them. The amplitude based parameter is derived using Lamb wave equations and expressed in terms of amplitudes of the harmonics. This parameter can be evaluated directly from the results of experiments. The physics based parameter is derived considering the fundamental dislocation theory and it depends on percent fatigue life. It is used to construct a theoretical nonlinearity curve (TNC). Both the parameters are evaluated for Al 7075-T651 at different percents of fatigue life and compared. The close agreement of results show that with reference of TNC, the amplitude based parameter obtained from a field test can be used for estimating the remnant useful life of a plate structure as formation of dislocation substructures during fatigue loading is a prestage of microcrack formation.

Nanoscale Layers Formed on the Surface of a Titanium Alloy by the Ion-Beam Mixing of Carbon with a Substrate

A complex of studies of the composition, structure, and properties of nanoscale surface layers of titanium alloy VT6 formed by the ion-beam mixing of a carbon nanofilm is conducted. It is found that ion-beam mixing in the transition layer of a film/substrate system provides conditions for the formation of titanium carbides, the content of which increases to 20 at % with an increase in the irradiation dose. After both the deposition and ion-beam mixing of a carbon film, the hyperfine surface layer of the samples mostly consists of carbon atoms in a disordered state with sp2- and sp3-hybridized C–C bonds. It is revealed that the coherent scattering region of samples decreases after ion-beam mixing; this effect can be attributed to an increase in the dislocation density and the formation of dislocation substructures. The formation of titanium carbides, a disordered carbon structure, and dislocation substructures under ion-beam mixing conditions leads to a more than twofold increase in the microhardness of the samples.

Application of Forward Modelling and Dictionary Indexing to EBSD Orientation Data as a Means of Quantifying Dislocation Substructure Formation in FCC Metals

Application of Forward Modelling and Dictionary Indexing to EBSD Orientation Data as a Means of Quantifying Dislocation Substructure Formation in FCC Metals

Dislocation Substructure Analysis in γ′-Precipitated Ni-Based Alloy by X-Ray Diffraction Combined with Positron Annihilation

The systematic changes in the dislocation density and characteristics of γ′-precipitated Ni-based model alloys that develop under cold rolling are studied as simulated deformations, to examine the fundamental dislocation behavior in terms of the dislocation substructure formation. In particular, the dislocation density is quantified through X-ray line profile analysis (XLPA), which is effective for quantifying the dislocation density and characteristics, as well as positron annihilation lifetime (PAL) measurements, which are sensitive to vacancy-type lattice defects. Similar tendencies are obtained for the strain dependency of the dislocation density analyzed using XLPA and PAL. Hence, the influence of the γ/γ′ coherent interface and γ′ precipitation on the dislocation substructure and vacancies is shown by comparing with a Ni-Cr solid solution. These results help to understand the interaction of solute atoms, vacancies, and dislocation with regard to substructure formation in Ni-based alloys.

On the evolution of dislocation cell structures in two Al-alloys (Al-5Mg and Al-11Zn) during reciprocal sliding wear at high homologous temperatures

The formation of dislocation substructures in up to 10 µm deep subsurface regions of two aluminium alloys, Al-5Mg and Al-11Zn, was investigated under conditions of high homologous temperature reciprocal sliding wear (HT-RSW). Under creep conditions, Al-5Mg shows a solid solution type of inverse primary creep. In contrast, Al-11Zn creeps obstacle controlled and exhibits normal primary creep. These two materials were subjected to reciprocal sliding wear at 200 and 300 °C for 100 and 1000 cycles. Flat polished disks were exposed to the 1 mm reciprocal movements of a spherical aluminium oxide counterbody under normal forces of 5 and 10 N at an oscillation frequency of 1 Hz. Using focused ion beam (FIB) micromachining thin electron transparent foils were prepared from the surface regions of the as received and worn material states. Transmission electron microscopy (TEM) was used to study the evolution of nano and micro grain sizes in the surface regions. Despite the different creep behavior, the two materials behave similar under conditions of reciprocal sliding wear. The results obtained in the present work show that subgrain sizes decrease with increasing numbers of wear cycles and increasing normal forces. Subgrain sizes also increase with increasing temperature. At 300 °C, dynamic recrystallization was observed in both Al-alloys. The results of the present work are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted.

The valuation of microstructural evolution in a thermo-mechanically processed transformation-twinning induced plasticity steel during strain hardening

The successive evolution of martensitic transformation, twining and dislocation substructure formation in a transformation-twinning induced plasticity steel during room temperature straining was studied in the present work. This was materialized through microstructural observations and micro-texture examinations utilizing the electron backscattered diffraction method. To evaluate the strain hardening behaviour of the thermomechanically processed steel, tensile testing procedure to different strains at ambient temperature was practiced. The results indicated that the dislocation slip, mechanical twinning, and deformation induced ɛ/α′-martensite formation were involved as the deformation mechanisms. At the early stages of deformation, the dynamic formation of dislocation substructure, strain induced ɛ-martensite and twins from austenite played the main role in the observed work hardening behaviour. Furthermore, the results demonstrated that the formation of α′-martensite was the dominant deformation mechanism at higher deformation levels. The corresponding texture analysis indicated to a double fibre texture formation, with a relatively stronger <111> at lower strains and a stronger <100> partial fibre parallel to tensile axis at higher strains. However, in the latter, the Cube, A and Goss Twin (GT)-type textures were dominated. Decreasing of the Goss and S components were attributed to the preferential transformation of austenite to α′- and ɛ-martensites, respectively. The presence of GT component even at higher strains approved the participation of deformation induced twinning as a dominant deformation mechanism up to failure.

Dislocation Substructure in the Cold-Rolled Ni-20 Mass Pct Cr Alloy Analyzed by X-ray Diffraction, Positron Annihilation Lifetime, and Transmission Electron Microscopy

The systematic change in the dislocation density and characteristics that develop under cold rolling as a simulated deformation was studied in order to examine the fundamental behavior of dislocations in terms of the dislocation substructure formation. In particular, the dislocation density was quantified by X-ray line profile analysis (XLPA), which is effective for quantifying the dislocation density and character; positron annihilation lifetime (PAL), which is sensitive to vacancy-type lattice defects; the Bailey–Hirsch equation from the hardness (Hv); and transmission electron microscopy (TEM). The strain dependency of the dislocation density analyzed by XLPA, PAL, TEM, and Hv showed a similar tendency with an increase in the dislocation. In particular, the dislocation density by XLPA had good agreement with the results of TEM at low strain levels and with PAL at high strain levels. As a result, a combination of these techniques successfully showed the behavior of the dislocation substructure.

EFFECT OF HYDROGEN ON THE LOCALIZATION OF PLASTIC DEFORMATION AND STRUCTURE OF THE ELECTROLYTICALLY SATURATED LOW-CARBON STEEL

The main features of plastic strain macrolocalization and structure of low-carbon steel after hot rolling and electrolytic hydrogen saturation in a thermostatted three-electrode electrochemical cell at a controlled constant cathode potential have been studied. Using the method of double-exposure speckle photography at different stages of strain hardening the main types and parameters of plastic flow macrolocalization (propagation rate and wavelength) have been identified. The effect of interstitial impurity of hydrogen on the change of the substructure and cementite morphology has been determined using methods of optical and electron microscopy. Attention has been given to the formation of dislocation substructures; bending-torsion of the α-phase lattice has been revealed, as evidenced by the presence of bend extinction contours. The main sources of stress field have been found to be grain and fragment boundaries.