Increase In Dislocation Density Research Articles

Grain size dependent deformation microstructure evolution and work-hardening in CoCrNi medium entropy alloy

This study clarified the grain size dependence of the deformation microstructure evolution and work-hardening behavior in CoCrNi medium entropy alloy. We fabricated fully recrystallized specimens with coarse-grained (CG) and ultrafine-grained (UFG) specimens by severe plastic deformation and subsequent annealing processes. Tensile deformation was applied to the specimens at room temperature. The UFG specimen exhibited both high strength and high ductility compared to conventional UFG metals due to the high work-hardening ability. In the CG specimen, three distinct types of deformation microstructures consisting of dislocations and deformation twins developed depending on grain orientations, similar to the single-crystalline specimens. In the UFG specimen, widely extended stacking faults and randomly-tangled dislocations were found to coexist in most grains. Deformation twins were found to nucleate without evidence of dislocation reactions regardless of grain orientations, implying abnormal nucleation mechanisms of deformation twins in the UFG specimen. Dislocation densities quantified by in-situ synchrotron XRD measurements during tensile deformation were higher in the UFG specimen than those in the CG specimen and conventional UFG metals. Our analysis showed that the work-hardening behavior of the specimens was primarily controlled by increases in dislocation density as well as the introduction of planar defects during deformation. Through comparisons with the CG specimen and conventional UFG metals, we concluded that the excellent work-hardening ability of the UFG specimen was mainly due to the evolution of unique deformation microstructures and rapid increase in dislocation density, which could be due to inhibited dynamic recovery in the MEA.

The Effect of Heating Rate on the Microstructure Evolution and Hardness of Heterogeneous Manganese Steel

The use of a rapid heating method to achieve heterogeneity of Mn in medium-manganese steel and improve its comprehensive performance has been widely studied and these techniques have been widely applied. However, the heating rate (from α to γ) has not received sufficient attention with respect to its microstructure-evolution mechanism. In this study, the effect of heating rate on the microstructure evolution and hardness of heterogeneous medium-manganese steel was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and DICTRA simulation. The results showed that the Mn distribution was heterogeneous in the initial microstructure of pearlite due to strong partitioning of Mn between ferrite and cementite. At low heating rates (<10 °C/s), the heterogeneity of Mn distribution was diminished to some extent due to the long-distance diffusion of Mn in high-temperature austenite. Contrastingly, at high heating rates, the initial heterogeneity of the Mn element could be largely preserved due to insufficient diffusion of Mn, which resulted in more ghost pearlite (GP: pearlite-like microstructure with film martensite/RA). Moreover, the high heating rate not only refines the prior austenite grain but also increases the total RA content, which is mainly composed of additional film RA. As the heating rate increases, the hardness gradually increases from 628.1 HV to 663.3 HV, due to grain refinement and increased dislocation density. Dynamic simulations have also demonstrated a strong correlation between this interesting microstructure and the non-equilibrium diffusion of Mn.

Microstructural Evolution and Mechanical Properties of Extruded AZ80 Magnesium Alloy during Room Temperature Multidirectional Forging Based on Twin Deformation Mode.

This study investigates the preparation of ultrahigh-strength AZ80 magnesium alloy bulks using room temperature multidirectional forging (MDF) at different strain rates. The focus is on elucidating the effects of multidirectional loading and strain rates on grain refinement and the subsequent impact on the mechanical properties of the AZ80 alloy. Unlike hot deformation, the alloy subjected to room temperature MDF exhibits a lamellar twinned structure with multi-scale interactions. The key to achieving effective room temperature MDF of the alloy lies in combining multidirectional loading with small forging strains per pass (6%). This approach not only maximizes the activation of twinning to accommodate deformation but ensures sufficient grain refinement. Microstructural analysis reveals that the evolution of the grain structure in the alloy during deformation results from the competition between {101¯2} twinning or twinning variant interactions and detwinning. Increasing the forging rate effectively activates more twin variants, and additional deformation passes significantly enhance twin interaction levels and dislocation density. Furthermore, at a higher strain rate, more pronounced dislocation accumulation facilitates the transformation of twin structures into high-angle grain boundaries, promoting texture dispersion and suppressing detwinning. The primary strengthening mechanisms in room temperature MDF samples are grain refinement and dislocation strengthening. While increased dislocation density raises yield strength, it reduces post-yield work hardening capacity. After two passes of MDF at a higher strain rate, the alloy achieves an optimal balance of strength and ductility, with a tensile strength of 462 MPa and an elongation of 5.1%, significantly outperforming hot-deformed magnesium alloys.

Mechanisms of laser non-uniform shock peening on the mechanical properties of SUS 304 stainless steel weld joints

The technique of Laser Non-Uniform Shock Peening (LNUSP) is proposed based on the residual stress distribution in welded joints and the shock wave peak pressure of SUS 304 stainless steel. The study investigated the effects of LNUSP on the mechanical properties of laser beam oscillation welded (LWW) joints of SUS 304 stainless steel. The results indicate that LNUSP induced residual compressive stress with an affected layer depth exceeding 0.9 mm, leading to a more uniform residual stress distribution. LNUSP also induced martensitic phase transformation, altered the grain structure orientation, formed parallel dislocation arrays that refined the grains, increased dislocation density leading to dislocation tangles, and induced mechanical twins of 20–70 nm. Furthermore, LNUSP significantly enhanced microhardness and promoted a more uniform hardness distribution, favoring a better balance between strength and ductility. LNUSP notably increased the yield strength (YS) and ultimate tensile strength (UTS) by 49.4% and 5.1%, respectively, compared to the original samples. The LNUSP-treated samples demonstrated superior strength and plasticity, attributed to the uniformity of internal plastic deformation.

Enhancing high-temperature strength in Al-Si alloys: The critical role of vanadium

Al-Si alloys are widely utilized in casting applications across various industries due to their favorable properties. This study addresses the imperative for improving the high-temperature strength of these alloys to expand their applicability. The stability of the microstructure of such alloys at elevated temperatures is closely associated with the diffusivity of their constituent elements. We explored the role of vanadium (V), which is known for its low diffusivity within the aluminum matrix, in enhancing the high-temperature strength of the Al-7Si alloy. The research evaluated mechanical properties, thermal stability, and fractographic characteristics of alloys. Microstructural analysis was conducted using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD), with a focus on identifying the strengthening mechanisms. Our results demonstrate that the incorporation of 0.5 wt% V significantly enhances the mechanical properties of alloys at both ambient and elevated temperatures. This improvement is primarily due to the formation of Al10V intermetallic particles, which hinder dislocation movement and increase dislocation density during deformation. Notably, the presence of twinning within Al10V contributes to a more uniform stress distribution, and it alleviates the typically detrimental effects on ductility observed with conventional brittle intermetallics. The joint addition of 0.2 wt% Ti and 0.5 wt% V further intensifies these benefits by increasing the number density of Al10V intermetallic particles. Our study highlights the considerable promise of V in improving the performance of aluminum alloys for high-temperature applications.

On the heterogeneous microstructure and strengthening mechanisms in a laser welding AZ80 magnesium alloy

This study investigates the microstructural characteristics and mechanical properties of a laser welded AZ80 magnesium alloy. The welding process led to the formation of coarse-grained fusion zone (FZ), where a secondary phase formed continuous network. Mg17Al12 precipitation and coarsening of grain boundaries occurred in the heat affected zone. The welded joint exhibited excellent mechanical properties with a yield strength of 202 MPa and a joint efficiency of 92%. The microstructure analysis via EPMA and EBSD in conjunction with synchrotron X-ray diffraction analysis reveals that precipitates and increased dislocation density in the fusion zone are primary strengthening mechanisms for the laser welded AZ80 Mg alloy.

Effect of pulse current on the creep ageing behavior of pre-strained 2195 Al-Li alloy

Pre-strained aluminum lithium (Al-Li) alloy plates have ultra-high strength and comprehensive performance after aging treatments, and are widely used in the aerospace industry. The initial increase in dislocation density caused by these pre-strains will significantly affect the evolution of deformation, microstructure, and mechanical properties during the pulse current assisted creep ageing forming process. Therefore, the effect of pulse current on the creep ageing behavior of 2195 Al-Li alloy with different pre-strains (0%, 4%, and 8%) was investigated. As the pulse current increases within the first 1h creep aging process of the various pre-strain samples, the 1h creep strain significantly increases and the subsequent steady-state creep rate slightly reduce. Moreover, the larger the pre-strain, the higher the total creep strain. Pulse current promotes the movement and recovery of initial dislocations, and promotes the fine formation of T1 phase and suppresses the nucleation of θ' and phase, thereby improving mechanical properties. Compared with the conventional creep ageing process, the pulse current acting on pre-strained 2195 Al-Li alloy significantly increased the peak aged strengthen and shorten the peak aging time. Meanwhile, the peak time platform period has also been significantly extended. It is evident that the application of pulse current assisted creep ageing forming process to pre-strained 2195 Al-Li alloy plates can significantly improve formability and mechanical properties, enlarges the forming process window and realize collaborative manufacturing of Al-Li alloy component shape and performance.

Ratcheting-fatigue behavior and fracture mechanism of 316H ASS under cyclic random loading block

In this study, a set of programmed random factors with non-zero mean were designed. Then various stress levels (15, 18 and 22 KN) were multiple superimposed to factors to form one random loading block (RLB), the blocks were repeated to failure to investigate the synergistic damage of 316H ASS under low-cycle fatigue (LCF), high-cycle fatigue (HCF) and ratcheting effect. The lifetime of cyclic RLB tests decreased with the increase of block-mean stresses σmBlock (208、255 and 311.5 MPa). The normalized strain amplitudes indicate that when the σmBlock amplitude below the yield strength (208 and 255 MPa), a stable ratchet accumulation phase allows the specimens to exhibit cyclic hardening behavior. When σmBlock exceeds the yield strength (311.5 MPa), the ratcheting strain increases significantly and the specimens exhibit cyclic softening behavior. Especially, the transgranular cleavage fracture, quasi-cleavage fracture and intergranular secondary cracks were identified when the failure of cyclic RLB tests were induced by LCF, HCF and ratcheting. With the increase of σmBlock amplitude, the decrease of LAGB proportion and the increase of dislocation density further reduce the fatigue resistance. In addition to dislocation motion, the α’-martensite phase transformation induced by ratcheting-fatigue has been further demonstrated as a mechanism for coordinated deformation. The percentage of stresses (within one block) that exceeds the diverge critical stress (375.6 MPa) of stacking faults (SFs) determines the α’-martensite nucleation mechanism.

Influence of increase in dislocation density due to Ni doping on the blue-photoluminescence of Mn-ferrite nanocrystals

Influence of increase in dislocation density due to Ni doping on the blue-photoluminescence of Mn-ferrite nanocrystals

Enhancing strength, ductility, and thermal fatigue resistance in Stellite 21 coatings via Mn alloying and Transformation-Induced Plasticity

This study investigates the effect of Mn on the microstructure and mechanical properties of Stellite 21 cobalt-based coatings prepared by laser cladding on 24CrNiMo steel substrates, focusing on utilizing the Transformation-Induced Plasticity (TRIP) mechanism to enhance toughness and thermal fatigue performance. The results indicate that the microstructure of the Stellite 21 alloy coating is primarily composed of γ-Co (FCC) with a minor presence of ε-Co (HCP), M23C6, and M7C3 carbides, which predominantly precipitate at or near the grain boundaries. Upon the addition of Mn, it dissolves into the γ-Co matrix, causing lattice distortion and increasing dislocation density, thereby contributing to solid solution strengthening. Compared to the Mn-free Stellite 21 alloy coating, the coating with 5% Mn exhibits an approximately 21% increase in tensile strength (∼1135 MPa) and a 74% increase in elongation (∼14.5%). This improvement is mainly attributed to the solid solution strengthening caused by lattice distortion due to Mn addition and the synergistic enhancement from the TRIP mechanism facilitated by the lower stacking fault energy. Moreover, the thermal fatigue crack propagation rate in the coating with 5% Mn is reduced by approximately 42.5% compared to the Mn-free Stellite 21 alloy coating. The enhancement in thermal fatigue performance is primarily due to the Mn addition, which stabilizes the ε-Co phase during thermal fatigue processes, enhancing plastic deformation capacity. The phase transformation process alleviates stress concentration, effectively reducing crack propagation rates and ultimately improving the material's thermal fatigue performance.

Microstructural Deformation and Fracture of Reduced Activation Ferritic-Martensitic Steel EK-181 under Different Heat Treatment Conditions

Abstract TEM studies were performed to examine the effect of holding of dispersion-strengthened heat-resistant reduced activation 12% chromium ferritic-martensitic steel EK-181 in static liquid lead for 3000 h at 600°C on the steel microstructure in comparison with the steel after conventional heat treatment by quenching and tempering at 720°C. It was found that the steel microstructure has good thermal stability under the specified experimental conditions. Microstructural deformation of EK-181 steel was studied in the neck region of tensile specimens tested at the temperatures 20, 680, 700, and 720°C with and without holding in liquid lead, and their fracture mechanisms were investigated. As a result of plastic deformation during tensile testing at room temperature, martensite plates and laths near the fracture surface are distorted and fragmented with the formation of new low-angle boundaries, and the dislocation density increases. At the deformation temperatures 680–720°C, nearly equiaxed ferrite grains are formed, the density and size of second-phase particles (M23C6 and MX) increases due to dynamic strain aging, and the dislocation density decreases locally. As the test temperature rises, the degree of martensite tempering increases. At T ≥ 700°C, some dynamic polygonization and dynamic recrystallization are observed. At elevated tension temperatures, ferrite coarsening is more significant in the specimens held in lead as compared to the conventionally treated material. The plastic deformation and fracture behavior of the steel are largely determined by the test temperature, rather than by the treatment mode.

Microstructure and strength-conductivity synergy of Al-Mg-Si-Cu alloy sheets prepared via vacuum melting and thermo-mechanical treatment

This work has investigated the response of microstructural evolution on the strength and electrical conductivity (EC) of vacuum melted Al-Mg-Si-Cu alloys during T6 heat treatment, cold rolling (CR) and re-aging (RA). The precipitation behavior of nano-scaled precipitates during thermo-mechanical treatment was revealed. The strength- conductivity improvement mechanisms of the alloy were discussed. The results showed that compared with the non-vacuum melting alloy, there were superior enhancements in the strength and EC of the alloy prepared by vacuum melting due to the high compactness. After T6 heat treatment, nanoscale β", Q' and L phases were precipitated in the alloy sheet. Owing to the increased dislocation density and precipitate dissolution during CR, the strength of the alloy sheet was improved, followed by a decrease in EC. After RA, a good combination of strength and EC of the alloy sheet was obtained, i.e., ultimate tensile strength and EC were 350MPa and 57.48% IACS, respectively, which was associated with the formation of nano-sized precipitates and the decrease in solute atom concentration. The precipitates volume, dislocation density, grain refinement and solid solubility during thermo-mechanical treatment were the main factors determining the strength and EC of the alloy. This work provides a strategy for the preparation of high strength-conductivity Al alloys.

The impact of cooling rate on the structure and properties of VGF-InP single crystals

InP is a III-V compound semiconductor with a zinc-blende crystal structure, widely used in optical communications, high-frequency millimeter-wave devices, optoelectronic integrated circuits, and solar cells. During the growth of VGF-InP single crystals, defects such as twinning, dislocations, and polycrystals are prone to occur. Experimental research on the cooling rate, an important control condition during the growth process, was conducted. By analyzing a large amount of discrete cooling data from the premium InP production and using spherical fitting algorithms for numerical analysis, the optimal cooling curve was obtained. Forward and reverse experimental verification results show that by adjusting the cooling rate at each growth stage, the recurrence of twinning and dislocations was successfully improved. Increasing the cooling rate during the shouldering process helps suppress intrinsic twinning but tends to increase dislocation density during the equal diameter stage process, leading to dislocation proliferation. Therefore, while increasing the cooling rate during shouldering, it is necessary to appropriately reduce the cooling rate during the equal diameter stage process to effectively suppress dislocation proliferation. By precisely controlling the temperature gradient and cooling rate inside the furnace, the thermal field conditions during the crystal growth process can be optimized, significantly improving the quality of InP single crystals.

Coupling CPFE-CA simulation for grain refinement in ultrasonic elliptical vibration diamond cutting of polycrystalline Cu

While microstructure evolution is commonly observed in severe plastic deformation of polycrystalline metals, modulating the cutting-induced grain refinement in subsurface of polycrystalline metals is promising for promoting the performance of machined surface. In this study, we demonstrate the effectiveness of applying ultrasonic vibration assistance (UVA) in effectively prompting grain refinement of polycrystalline Cu in ultra-precision diamond cutting by experiments and multiscale coupling simulations. Specifically, ordinary cutting (OC) and ultrasonic elliptical vibration-assisted cutting (UEVC) experiments of polycrystalline Cu are carried out, and subsequent cross-sectional characterizations of microstructure evolution in subsurface by metallurgical microscope and electron backscatter diffraction, as well as instrumented nanoindentation tests, are performed, which demonstrates significantly promoted grain refinement in subsurface and increased machined surface hardness by UVA, due to increased dislocation density that is beneficial for the nucleation and growth of dynamic recrystallization. In particular, the multi-scale coupling of crystal plasticity finite element (CPFE) simulation and Cellular Automata (CA) method is firstly established for exploring the microstructural evolution during UEVC and OC of polycrystalline Cu, which is capable of elucidating the underlying correlation of grain refinement behavior in subsurface with characteristics of stress and strain fields in cutting area. Furthermore, the influence of amplitude on the propensity of grain refinement is experimentally and theoretically evaluated, which suggests a critical amplitude of 4 μm that leads to a maximum reduction in grain size by 80.9% and a maximum increase in machined surface hardness by 55.8% in UEVC from that in OC, because of the most pronounced strain accumulation and dislocation activity. The findings reported in this study demonstrate the effectiveness of applying ultrasonic vibration assistance for modulating the grain refinement accompanying strengthening of machined surface in ultra-precision diamond cutting of polycrystalline metals.

Change in the qualitative composition of non-metallic mineral raw materials as a result of blasting operations

Purpose. The research purpose is to study the change in the qualitative composition of granite before and after blasting operations to determine its compliance with the criteria of marketable products. Methods. X-ray phase and X-ray structural research methods are used to study changes in the mineral composition of granites before and after blasting operations. To separate magnetic and non-magnetic fractions of the selected granite samples, a three-roller RST magnetic separator is used. X-ray phase research is conducted using a DRON-3 diffractometer. Additionally, an analysis of the unit cell dimensions of the quartz crystal lattice was conducted, and the dislocation density along the corresponding crystallographic planes was studied. Findings. It has been determined that after blasting operations, granite mass is redistributed from coarse fractions of 1-20 mm to small fractions of 0-1 mm with an increase in the latter by 4.2%. It has been found that the biotite content decrea-ses naturally and consistently, and the quartz content increases correspondingly in products in the following series: magnetic separator drum (90%, 2%) → lower roller (72%, 14%) → upper roller (55%, 31%) → non-magnetic product (48%, 34%) before blasting operations. Therefore, despite significant differences in the magnetic favorability of these two mineral phases, they are present in all magnetic separation products (with the exception of quartz in the non-magnetic product): magnetic separator drum → lower roller → upper roller. Originality. It has been established that along the crystallographic directions 101 and 211, the maximum gradient of dislocation density increase in the quartz crystal lattice in granite samples before blasting operations is observed during the transition from the lower roller product to the upper roller product, amounting to 1.55·1010 and 6.63·1010 cm-2, respectively. After blasting operations, in granite samples along the same directions, the maximum gradient of dislocation density increase is observed between the upper roller product and the non-magnetic product, amounting to 3.01·1010 and 4.67·1010cm-2. As a result of the thermodynamic impact of blasting operations, the weighted average dislocation density value along crystallographic planes 101 and 211 in the quartz crystal lattice increases by 47.21 and 25.72%, respectively. Practical implications. Understanding the quality characteristics of marketable products after blasting operations will contribute to optimizing the stages of further processing of non-metallic mineral raw materials (two-, three- or four-stage crushing) and expanding the scope of granite applications. This increases its competitiveness in the building materials market by reducing the costs for additional processing with a reduction in the labor intensity of the process.

Microstructural evolution and dynamic mechanical behavior of PM 2195 Al-Li alloy with resistance to shear instability in high strain rate deformation

The risk of adiabatic shear instability is common in traditional alloys covering high-strength steels, copper alloys and aluminum alloys during high strain rate deformation. However, the research on shear instability of 2195 Al-Li alloy prepared by powder metallurgy (PM) at high strain rates is still blank. The purpose is to explore its microstructure evolution and dynamic compression behavior. In this work, the 2195 Al-Li alloy prepared by PM amazingly demonstrates excellent resistance to shear instability at high strain rates during Split-Hopkinson pressure bar (SHPB) experiment. The microstructural evolution of grains and T1 (Al2CuLi) precipitate phase, and dynamic compression behavior of PM 2195 Al-Li alloy at various strain rates from 3000 s−1 to 9000 s−1 are investigated in detail employing correlative electron-backscatter diffraction (EBSD) and transmission electron microscopy (TEM). During high strain rate deformation, the alloy undergoes grain fragmentation and rotation, increasing dislocation density around the grains. When the strain rate is from 3000 s−1 to 7500 s−1, the Kernel Average Misorientation (KAM) value increases, indicating intensified localized strain. At a strain rate of 9000 s−1, a small number of grains are surrounded by nanocrystals (around 200 nm) that undergo stress release, while the average size of micrometer-sized grains is 0.81 μm. As the strain rate increases, the T1 phases in the alloy extends, vibrates, and fractures before dissolving in the matrix, while high-rate deformation promotes dislocation entanglement and defects, leading to the decomposition or ordering of precipitated phases through rapid plastic deformation. By virtue of energy dissipation strategy via the formation of nanocrystallines and dislocation behavior induced by the dissolution of T1 phases, the "positive-positive" effect is observed. The PM 2195 Al-Li alloy undergoes no shear failure even at the strain rate of 9000 s−1, showing great prospects in future applications in dynamic-loading conditions.

Mechanical property and corrosion performance in the as-rolled Mg-8Li-4Gd-1.5Ni alloy

As for the magnesium-lithium (Mg-Li) alloys for the applications of rapid corrosion material, enhancing their mechanical properties while maintaining the degradation rate poses a significant challenge. Simultaneously, it is crucial to elucidate the behavior of the long period stacking ordered (LPSO) phase in the corrosion process of as-rolled Mg-Li alloys. This study prepared the as-rolled Mg-8Li-4Gd-1.5Ni alloy with rolling reductions of 30 %, 50 %, 70 %, and 90 % at room temperature. Results indicate that, during the rolling process, the reticular LPSO gradually transitions to a fibrous LPSO and the GdNi3 particles are refined. Additionally, the amounts of twin increases. The alloy with 30 % reduction exhibits the highest corrosion rate, with a weight loss rate of 0.50 mg·cm−2·min−1 in a solute of 3 % KCl. The tensile strength reaches a maximum of 247 MPa. The increase in tensile strength is achieved at the expense of plasticity. The bending of the LPSO phase, the fracture of the secondary phases, and the increase in dislocations and twins increase the chemical reactivity of the alloy, leading to a higher corrosion rate. Rolling brings about the increase in dislocation density, dislocation entanglement, and the reduction in grain size, which indicate that the strengthening mechanisms of the alloy after rolling include work hardening and grain refinement strengthening.

Microstructure and mechanical properties of 2195 Al-Li alloy via friction stir additive manufacturing with different stirring paths

Friction stir additive manufacturing (FSAM) is increasingly becoming a feasible method for aluminum alloy structure due to the unique forming characteristics. In this study, two different stirring paths were explored for 2195 Al-Li alloy and the microstructure evolution and mechanical properties were ‌investigated. An ultrafine grained microstructure was formed in the FSAMed zone with an average grain size ranging from 1 to 5μm. The microstructure in the FSAMed zone at the bottom exhibited a consistent direction trend and a significant increase in dislocation density. Different stirring paths had minimal impact on the phase distribution. Frictional heating led to the re-dissolution and re-precipitation of precipitates, resulting in T1, θ', and δ'/β' were distributed within the grains. The samples of intersecting stirring path showed similar mechanical properties in different directions, while the UTS of LD was 52.56 MPa higher than TD of the reciprocating path samples. The microhardness range for the intersecting stirring path ranged from 87.0 HV0.2 to 126.6 HV0.2, while the reciprocating stirring path exhibited a microhardness range of 83.8 HV0.2 to 132.4 HV0.2. In summary, this study provides valuable insights into designing processing paths for FSAM of 2195 Al-Li alloy.

Microstructural evolution and interfaces in cold sprayed hybrid aluminum alloy powders of different hardness

The development of thick cold sprayed deposits of hard amorphous/nanocrystalline Aluminum High Entropy Alloys (Al HEA) is limited by their poor plastic deformability. To address this, Al HEA powder (Al90.05Ni4.3Y4.4Co0.9Sc0.35 (at. %)) is blended with soft Al 6061 in a 1:1 wt% ratio and cold sprayed. The microstructure of the composite deposits reveals that while Al HEA splats undergo limited deformation, Al 6061 splats deform extensively. Extreme deformation of Al 6061 captures the hard Al HEA particles, resulting in a composite deposit of 5 mm thickness, compared to 0.2 mm for pure Al HEA. Within the composite, Al HEA particles deform through shear band propagation and viscous flow, whereas Al 6061 splats deform via dislocation slip motion. These deformation mechanisms induce interface nanocrystallization, leading to a 123 % increase in lattice strain, a 67 % reduction in crystallite size, and an 800 % increase in dislocation density compared to the initial feedstock. This microstructural evolution, combined with the presence of hard Al HEA regions, results in a mean hardness of 2.56 GPa and an elastic modulus of 76 GPa. These values represent a 47 % and 6 % improvement over conventional polycrystalline cold-sprayed aluminum deposits. Thus, this study provides insights into the microstructural and interface evolution and its effect on indentation characteristics for making high-strength Al HEA deposits.

Revisiting the strain rate sensitivity of the flow stress of copper: Theory and experiment

One of the most important issues related to the strength of metals is the strain rate sensitivity of the flow stress. In this study, an analytical model of the flow stress as a function of strain rate is derived theoretically. The model can reproduce the strain rate sensitivity of the flow stress of copper over a wide range of strain rates (up to 109 s−1) quantitatively. Our theoretical derivations indicate that the strain rate sensitivity of the flow stress, especially that above 103 s−1, is a result of both the variation of the dislocation mobility mechanism with stress and the particular stress dependence of dislocation density but is not a result of each single mechanism. In particular, the stress dependence of the dislocation density and the initial dislocation density are critical to the quantitative relation of the flow stress–strain rate at high strain rate and the strain rate threshold, under which the upturn of the flow stress occurs, respectively. Moreover, experiments with copper of different initial dislocation densities at moderate and high strain rate are performed. The strain rate threshold of the flow stress upturn observed in the experiments grows considerably as initial dislocation density increases, which is in accordance with theoretical prediction by our model.