Matrix Dislocations Research Articles

Hydrogen diffusion and trapping in a cryogenic processed high-Cr ferrous alloy

The effect of hydrogen diffusion and trapping was studied in a high-Cr ferrous alloy using an inverted scanning Kelvin probe and thermal desorption spectroscopy in correlation with microstructure and residual stress study. In addition, different processing of ferritic/martensitic 9Cr1WTaV alloy (EUROFER97) was tested in correlation with observed selected properties to observe induced changes in material degradation and surface. The activation energies for hydrogen traps were shown to have distinct peaks corresponding to different trapping mechanisms, including matrix dislocations and grain boundaries. For the cryogenically treated sample, an additional peak was also identified and correlated with increased carbide precipitation.

Effect of sintering temperature on microstructure and mechanical properties of MoNbVTa0.5Cr high-entropy alloys fabricated by powder metallurgy method

To address the challenges posed by the high density and low plasticity at room temperature of traditional refractory high entropy alloys (RHEAs), the MoNbVTa0.5Cr RHEA was synthesized through a combination of mechanical alloying (MA) and spark plasma sintering (SPS) techniques. The effects of different sintering temperatures on the microstructure and mechanical properties of the alloy were systematically studied. The MoNbVTa0.5Cr RHEA milled up to 60 h was found to have a metastable dual-phase containing a major BCC (a = 3.1474 Å) and Nb-Ta (a = 3.3058 Å) phase. The sintered MoNbVTa0.5Cr RHEA was composed of BCC phase and a small amount of Ta2VO6 at different sintering temperatures. There were a lot of edge dislocations in the BCC matrix near the BCC/Ta2VO6 interface. With the increase of sintering temperature, the grain size of sintered MoNbVTa0.5Cr RHEA gradually increased, while the mechanical properties gradually decreased with the increase of sintering temperature. When the sintering temperature was 1500 °C, the grain size of the alloy was 2.33 μm, the yield strength was 2783 MPa, the compressive strength was 3346 MPa, and the fracture strain was 20.45 %, showing the best comprehensive properties. And the density of MoNbVTa0.5Cr RHEA was 9.11 g·cm−3 and 26 % lower than that of WNbMoTaV RHEA. The specific yield strength of MoNbVTa0.5Cr RHEA prepared in this research was 295.28 kPa·m3·kg−1, which was superior to most RHEAs reported so far. This sintered MoNbVTa0.5Cr RHEA showed an excellent room temperature compression performance, which can be attributed to the co-existence of BCC phase along with the minor Ta2VO6 phases. The high yield strength of MoNbVTa0.5Cr RHEA was mainly attributed to the substitutional solid solution strengthening of metal atoms, the interstitial solid solution strengthening of O atoms and the grain boundary strengthening of ultrafine grains, with contributions to yield strength of 41 %, 28 %, and 24 %, respectively.

Contrasting irradiation behavior of dual phases in Ti-6Al-4V alloy at low-temperature due to ω-phase precursors in β-phase matrix

Aiming to simulate the radiation damage effect on a dual α+β phase Ti-6Al-4V alloy utilized as high-intensity accelerator beam window material, a series of irradiation experiments were conducted with a 2.8 MeV-Fe2+ ion beam in several dpa regions at room temperature. The nano-indentation hardness increased steeply at 1 dpa and unchanged up to 10 dpa, due to the saturation of defect clusters and tangled dislocations in the dominant α-phase matrix with a size of 2–3 nm and a density of about 1×1023 m−3. In contrast in the intergranular β-phase, larger loops of 20–30 nm diameter were observed with much less density of about 5×1020 m−3. The diffraction pattern showed rectilinear diffuse streaks between the β-phase reflections, corresponding to the ω-phase precursor, without dose dependency in its intensity. FFT/I-FFT analysis of the HREM revealed a sub-nanometer-sized lattice disorder with local fluctuations, not discrete but continuous, and homogeneously distributed within the matrix β-phase stably against the irradiation. The significantly low dislocation density and the absence of phase transformation in the β-phase matrix could be attributed either to the strong sink effect expected for this distinctive sub-nanometer-sized homogeneous lattice disorder or to the anomalous point defect recombination induced by the high mobility of vacancies, both of which are originated from the metastable ω-phase precursors specifically formed in the β(BCC) phase of group-4 transition metals.

Experimental and MD simulation study of surface residual stress and surface quality of SiCp/Al in pulsed laser-ultrasonic assisted grinding

The utilisation of aluminium-based silicon carbide composites (SiCp/Al) is on the rise. However, during the grinding process, where abrasive particles compel SiC particles into the aluminium matrix, stress concentration occurs near the interface between the two phases. This phenomenon results in adverse surface deformation accompanied by residual compressive stress (RCS). Consequently, a pulsed laser-ultrasonic-assisted grinding method (PL-UAG) has been proposed. The effectiveness of the method is verified through grinding experiments. Moreover, the influence process of the thermal effect of pulsed laser and the evolution mechanism of microscopic RS near the SiCp/Al two-phase interface are revealed through molecular dynamics simulation. The experimental results show that the surface RCS of SiCp/Al gradually decreases and transforms into residual tensile stress (RTS) with the increase of laser power. The RCS increases with the grinding depth. The RCS is reduced to 17 Mpa and the surface roughness Sa value is reduced to 0.110 μm with the laser power is 40 W and the grinding depth is 1 μm. The simulation results show that the energy dissipation process of pulsed laser helps to reduce thermal deformation and damage. The laser-induced thermal effect reduces the deformation of the aluminium matrix caused by particle extrusion and inhibits the formation and slip of dislocations in the aluminium matrix. In addition, the ultrasonic shock effect effectively weakens the RTS generated by the laser.

A novel under-aging design to improve the creep rupture life of a precipitation-strengthened Fe–Ni-based superalloy

The under-aging, peak-aging, and over-aging microstructures of a precipitation-strengthened Fe–Ni-based alloy were designed by applying different heat treatment procedures. Tensile creep tests were conducted at 700 °C and 250 MPa on samples exhibiting different strengthening γ′ precipitate volume fractions and diameters. The creep life of the alloy in the under-aged state was increased by more than 10% compared to the peak-aged condition, which provided a new approach to designing the heat treatment regime that improve the creep rupture life and plasticity of precipitation-strengthened alloys. The improvement in creep properties of the under-aged state was attributed to the transition from shearing to Orowan looping of γ′ particles by matrix dislocations during the dynamic precipitation and coarsening of γ′ phase, increasing intragranular deformation resistance. The under-aged microstructure design allowed reducing the grain boundary sliding contribution to overall strain during creep. As a result, the steady-state creep stage of the alloy in the under-aged state was extended. Additionally, relationships between applied stress and nucleation, growth, and coarsening of the γ′ phase have been evidenced.

Creep performance in a CoNi-based single crystal superalloy with super-high γ′ volume fraction at 760 °C and equivalent high stress

The creep performance at low temperature and high stress is one crucial property for the single crystal superalloys widely used for turbine blades. In this study, the creep behavior of a CoNi-based single crystal superalloy with super-high γ′ volume fraction (Vγ′ = ∼90%) was investigated at 760 °C/550 MPa based on microstructural (γ and γ′ phases) and deformation substructural (dislocations, anti-phase boundaries (APBs) and stacking faults (SFs)) analyses to reveal its creep mechanism under low temperature/high stress conditions. The super-high Vγ′ contributes to the rapid transformation of creep mechanisms from the matrix dislocation movement in the initial decelerating stage to the APB-coupled dislocation pairs and their evolving non-planar SF ribbons inside γʹ precipitates during the accelerating stage. In the following second decelerating stage, the interactions between SFs and APBs improve the deformation resistance of γ′ precipitates and decrease the creep rate. Subsequently, the high density of APBs is believed to play a significant role in microstructural stability and promotes the accumulation of matrix dislocations, which is responsible for the steady and final accelerating stages. Appropriate Vγ′ and increasing the APB energy are assumed to improve the creep resistance of CoNi-based single crystal superalloys by enhancing the strengthening effect of dislocation accumulation in γ matrix and critical shear stress of γ′ phase.

Effect of Post-heat Treatment on Microstructure and Mechanical Properties of Laser Cladded High Co-Ni Secondary Hardening Steel Coating

The effect of post-heat treatment on the microstructure and microhardness of laser cladded high Co-Ni secondary hardening steel coating was investigated. The microstructures were analyzed using a SEM equipped with an EDS, and the microhardness was measured with a Vickers indenter. Decomposition of the retained austenite in the coating occurred during the post-heat treatment. As the temperature increased from 200 °C to 600 °C, the quantity of the retained austenite at the boundaries decreased significantly, while that of the needle-shaped M3C cementite and M2C carbides increased. The M2C carbides evidently coarsened when the temperature was higher than 500 °C. The microhardness of high Co-Ni steel coating increased as the temperature of post-heat treatment increased from 200 °C to 400 °C because the fine-scale M2C carbides were coherent with the matrix and increased distinctly in this temperature range. It decreased sharply when the temperature further increased from 500 °C to 600 °C due to both the incoherency of the coarsened M2C carbides and the recovery of dislocations in the carbon-supersaturated matrix.

Diffusion and reaction behavior of palladium coating and copper matrix prepared using a halogen-free direct coating process

Herein, a palladium-coated copper wire was prepared using a halogen-free direct coating process, and the effects of the coating temperature on the morphology of the palladium coating and the mechanical properties of the wire were studied. The structural characteristics of the coating and copper matrix as well as the diffusion, reaction, and bonding characteristics of the elements at the copper–palladium interface were investigated. Results show that the deposition rate of palladium and the aggregation of palladium grains were accelerated with increasing coating temperature, promoting the formation of large palladium particles on the surface of the palladium-coated copper wire. The strength of the coated wire decreases and the elongation increases with increasing coating temperature. At 400 °C, the thickness of the coating was approximately 51 nm coating, consisting of approximately 4 nm nanocrystals. The improvement in plasticity was attributed to the distortionless recrystallized subgrain nucleus formed by the cell structure comprising dislocation tangles in the copper matrix, which reduces the matrix dislocation density. Positive and negative edge dislocations were present in the coating, and the two dislocation lines of the edge dislocation dipole offset each other. Numerous crystal defects were found in the copper–palladium bonding interface zone. Moreover, copper unidirectionally diffuses into the palladium coating, forming the intermetallic-compound CuPd in the interface bonding zone. Furthermore, the formation of twins was caused by the change in the stacking order of atoms due to the stacking fault occurring on the crystal surface, and the thickness of the diffusion zone was approximately 9 nm.

Influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys

The excellent mechanical properties of Ni-based superalloys rely on their unique two-phase microstructure. Here we probe the influence of interfacial dislocation network on strain-rate sensitivity in Ni-based single crystal superalloys. The interfacial dislocation could generate a weak long-range interaction to the matrix dislocations at low stress, and react with the incoming dislocation to form junctions at high stress. The weak interaction results in the dislocation pile-up around the interphase boundary, while the strong dislocation reaction makes the gradual loss of interface coherency. With the activation energies, we predict the dependence of strain-rate sensitivity on temperature in a regime with two bounds correlated with these two kinds of interactions. The predicted values match well with that measured in experiments at elevated temperatures.

Impacts of different designed microstructures on creep performance of a Ni-based wrought superalloy

The creep behavior of a novel Ni-based wrought superalloy with varying aging states, classified as large under-aged (UA), little UA, and little over-aged (OA) states, was investigated at a temperature higher than the intra-grain/grain-boundary iso-strength temperature (750 ​°C/250 ​MPa). In the samples before creep test, there are spherical γ′ precipitates homogeneously dispersed within the γ matrix and carbides precipitated at grain boundaries (GBs). After creep tests, precipitate-free zones (PFZs) appear on grain boundaries, with a large block-like γ′ phase present between carbides and PFZs, potentially serving as initiation sites for cracks. Little UA state samples exhibit the strongest creep resistance and intergranular fracture. The dominant creep deformation mechanism in the intra-grain is a combination of Orowan process involving slip and climb of matrix dislocations and shearing of γ′ precipitates. The results indicate that the optimizing the grain boundary microstructure may be more important than the change of intragranular microstructure when the creep temperature exceeds the iso-strength temperature.

Precipitation size and strain rate dependence of dislocation-amorphous nanopillar interaction mechanism in magnesium

Introducing amorphous phases into the crystalline matrix is a new design strategy to improve the mechanical properties of Mg alloys. In this work, the effects of strain rate and amorphous nanopillar size on the interaction mechanism between the dislocation and precipitation in the Mg matrix were investigated by molecular dynamics simulation. The results show that the interaction mechanism between dislocation and amorphous precipitation depends on strain rate and amorphous nanopillar size. At strain rates of 5 × 107/s and 5 × 108/s, the amorphous precipitation strengthens the Mg matrix due to the pinning effect of the amorphous precipitation relative to the dislocation. The interaction mechanism changes from merging and canceling of dislocation to the cross-slip mechanism with the increase of precipitation size. Moreover, the strengthening effect of the large-sized amorphous phase on the dislocation is noticeable. However, the amorphous precipitation softens the Mg matrix at strain rates of 5 × 109/s and 1 × 1010/s. The underlying mechanism varies from the dislocation shearing mechanism to the generation of dislocation loops with the increase of the amorphous precipitation size. This research can provide theoretical guidance for developing and designing new high-performance Mg alloys.

Chemical short-range order dependence of micromechanical behavior in CoCrNi medium-entropy alloy studied by atomic simulations

Chemical short-range order (CSRO) is a typical microstructural characteristic commonly found in multi-principal element alloys (MPEAs). For MPEAs with the face-centered cubic (FCC) structure, the presence of CSRO has been verified and characterized meticulously by using electron microscope. However, it is still challenging to reveal the effect of CSRO structure on the deformation microstructure and micromechanical behavior of those alloys. By applying atomic scale simulations, this study explores the micromechanical behavior of an equiatomic CoCrNi medium entropy alloy with random solid solution and three different degrees of CSRO under plane strain compression, respectively. The results show that CSRO can effectively promote planar slip of dislocations in the FCC matrix and elevate flow stress of the alloy. The planar slip and stacking of partial dislocations then lead to the formation of hexagonal-close-packed (HCP) structure lamellae. CSRO also modulates the morphology of dislocations. When dislocations cut through CSRO, it hinders the movement of dislocations while being destroyed by atomic plastic flow, causing CSRO and atomic clusters at the interface of the matrix to embed each other and generate antiphase boundaries which can significantly enhance the bulk yield strength. These findings provide fundamental insights into the interaction between CSRO and dislocations in CoCrNi MEA, guiding the design of MPEAs with superior combinations of strength and ductility.

Heat-Treatment Characteristics and Structure-Property Relationships of Thixomolded AZ91 and Ultralight LAZ771 Magnesium Alloys

Most magnesium alloys are produced by die casting due to their formability issues, but the thixomolding process has recently drawn considerable attention in manufacturing magnesium alloy casings for 3C electronic devices with lower porosity levels. This study compared the mechanical properties and microstructure of thixomolded AZ91 and ultra-light LAZ771 magnesium alloy for thin laptop cases. The experimental results firstly show that the microstructure of as-thixomolded samples contains fine αMg grains and a divorced eutectic network. After solution heat treatment, the ductility is improved due to the dissolution of the non-equilibrium secondary phases. Secondly, microstructural characterization for samples after aging treatment shows that the Mg17Al12 precipitates in AZ91 can be classified into continuous (CPs) and discontinuous precipitates (DPs) according to their morphologies. On the other hand, the AlLi precipitates in LAZ771 after aging treatment are spherical and incoherent with the αMg matrix. Those AlLi precipitates obstruct basal slip of dislocations in αMg matrix more effectively than Mg17Al12, resulting in higher yield strength of peak-aged LAZ771 specimens than heat-treated AZ91 specimens.

Synergic effects of Si and V micro-additions on microstructural and mechanical properties of a dilute Al–Sc–Zr alloy containing L12 Al3(Sc, Zr, V) nanoprecipitates

This study investigates the impact of incorporating 0, 0.05, 0.10 at% V into a bare Al-0.06Sc-0.06Zr-0.18Si at% alloy on the precipitation behavior of Al3(Sc, Zr, V) (L12-structure) nanoprecipitates and the consequent creep resistance of the alloy. Various techniques were used to obtain results, including Vickers microhardness, Electrical conductivity, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and tensile creep rupture testing. The findings show that the amount of V added affected the nanoprecipitate structures, precipitation kinetics, coarsening resistance, and creep resistance of the alloys. After isochronal aging to 425 °C (25 °C/1 h step), alloy V0.10 had a peak microhardness value of 603 ± 14 MPa. TEM revealed a microstructure with precipitate diameters ranging from 3 to 20 nm. The nanoprecipitates had a core-shell structure with a Sc-enriched core, a Zr-enriched shell, and, in some cases, a V outer shell. The phenomenon of vanadium partitioning at the precipitate shells occurred due to the incremental influence of silicon, which enhanced the diffusion rate of Zr and V. While the alloy V0.10 had a lower steady-state strain rate than the Al-0.06Sc-0.06Zr at% alloy at stresses below 16 MPa, it showed better creep resistance at stresses above 16 MPa. The threshold stress of the alloy V0.10 (10 MPa) was almost equal to that of the previously studied Al-0.06Sc-0.06Zr at% alloy (12 MPa). These results can be explained by the fact that a variation in the lattice parameter of Al3(Sc, Zr, V) (L12) affects the ability of matrix dislocations to traverse the nanoprecipitates, even in the absence of measurable lanthanides.

The influence of ZrO2 on tribological and mechanical characteristics of Al composite

Metal matrix composites based on aluminium alloys are emerging for industrial applications where a high strength to weight ratio is required. In this work, a reinforced composite made of an aluminiumalloy matrix grade 7010 with zirconium dioxide is developed. Utilizing the stir casting technique, zirconium dioxide is used as reinforcement in composite materials at weight percentages of 2 percent, 4 percent and 6 percent. The composite's mechanical and metallurgical characteristics are examined. SEM and EDS image demonstrating the homogeneous dispersion of the particles inside the matrix alloy. Ceramic particle addition increases material hardness by limiting alloy matrix dislocation. Tensile tests show that adding zirconium dioxide particles increases the material's strength. The Al7010/6% zirconium dioxide composite's ultimate strength (UTS) was 255 MPa, 26% higher than the Al7010 alloy's UTS, and the Brinell hardness of Al7010/6% is 93 BHN, which is 46% higher than the base alloy. Using a pin-on-disc wear testing system, the dry sliding wear behaviour of the Al7010/2 to 6 wt% zirconium dioxide composite is studied at ambient temperature. The prepared samples wear behaviour was evaluated under various weights and speeds. Compared to the base matrix alloy Al7010, the composites showed greater wear resistance.

In-situ TEM characterization of basal dislocations between nano-spaced long-period stacking ordered phases in MgYZn alloy

Long-period stacking ordered (LPSO) phases play strong barriers to the motion of non-basal dislocations and deformation twins, effectively improving the strength of Mg alloys. Corresponding to the crystallographic character of LPSO phases, it is generally believed that LPSO phases would not affect the motion of basal dislocations. Here, we perform in-situ TEM nanoindentation testing in a single crystal specimen of Mg97Y2Zn alloy that contains LPSO phases with various spacing within 100 nm, and report on a significantly low activity of basal dislocations in Mg matrix between LPSO phases with spacing less than 20 nm. Atomic HADDF-STEM results reveal a high concentration of solute atoms near LPSO phases, which could provide strong solute strengthening effect. These findings may provide possible guidance to design ultra-high strength of Mg alloys containing nano-spaced LPSO phases.

Dynamical analysis of a novel chaotic system and its application to image encryption

This study presents a novel four-dimensional chaotic system derived from the Lorenz-Haken equation. The paper investigates the system’s dissipative properties, equilibrium point, Poincaré map, 0–1 test, Lyapunov exponent, complexity, and bifurcation diagram to introduce the system’s dynamics. The research also examines the chaotic behavior of the system under various parameters, demonstrating the existence of symmetric attractors and offset boosting. To demonstrate the system’s feasibility, circuit simulation is performed using Multisim software, and the hardware implementation is carried out using a Field Programmable Gate Array (FPGA). An image encryption algorithm is designed by integrating matrix dislocation, backward diffusion, and deoxyribonucleic acid (DNA) encryption with the discretized chaotic system. The simulation results show that the algorithm has a good encryption effect. NIST tests show that the sequence has good randomness, correlation coefficient, and information entropy tests show that the encrypted image has strong randomness, and key space and key sensitivity prove that the system has a good ability to resist exhaustive attacks. The use of simulated differential attacks, cropping attacks, and noise attacks has demonstrated the high robustness of the system. The use of peak signal-to-noise ratio (PSNR) and structural similarity (SSIM) index proves that the encryption algorithm does not affect image quality. Further proving the excellent performance of the proposed encryption algorithm.

The development of a W-shaped channel extrusion for fabricating magnesium alloy shells by combining high amplitude shear strain with a shorter process

This paper introduced a novel single-pass severe plastic deformation method, W-shaped channel extrusion (WCE), for the direct manufacturing of magnesium alloy shells. Based on a strategically installed mandrel and die with a sloping asymmetric step structure, the WCE constructed a narrow severe plastic deformation gap specially reinforced by a backpressure-shear structure and an asymmetric shear-extrusion section. Therefore, slender bar with large height-to-diameter ratio (i.e. small diameter, large height) could easily be extruded into larger diameter shell in narrow gap through only one pass, and an impressive average plastic strain of over 4.1 was concurrently achieved, evidenced by upper bound theory calculation. The WCE process was further experimentally conducted on AZ80 alloy at 340 °C, demonstrating remarkable improvements in yield strength, ultimate tensile strength, and elongation of the extruded part by factors of 2.4, 2.2, and 4.3 over the as-cast condition. The consecutive multi-stage shear and high hydrostatic pressure catalyzed extreme grain refinement, favored high matrix dislocations and concurrently facilitated the early dynamic conception of granular β-Mg17Al12 particles in subgrains stage, all of which served the superior performance. Moreover, no prominent anisotropy detected in the extruded part due to the development of a special bimodal basal texture under shear stress, where its synergistic effect with grain refinement evidently promoted the balance of strength and plasticity. Finally, the mechanism of bimodal texture formation associated with specific slip activations during WCE and anisotropic weakening was revealed in detail.

Effects of different cooling conditions on microstructure and precipitation behavior of fine-grained Al–Zn–Mg–Cu alloy friction stir welding joint

The microstructure evolution and precipitation phase distribution of 7075 aluminum alloy friction stir welding (FSW) joint were investigated under liquid CO2 cooling conditions. The fine grained 7075 aluminum alloy was developed by four-pass equal channel angular pressing (ECAP) and FSW joints with CO2 cooling at room temperature. It was found that CO2 cooling could effectively reduce the peak temperature of the weld, leading to a refinement of the welded grain and an improvement of the welded joint. Various recrystallization mechanisms were found in different areas of the weld. The grain boundary and dislocation density of large angles gradually decreased from the weld stir-zone (SZ) to the heat-affected-zone (HAZ) regions, and the anisotropy increased. The type of precipitated phase and the relationship with the matrix interface were determined by detecting the spacing between the precipitated phase and the crystal plane of the matrix. CO2 forced cooling not only enhanced the matrix dislocation strengthening and grain boundary strengthening, but also effectively improved the size and type of precipitated phases and the colattice strengthening effect. In addition, liquid CO2 cooling can greatly reduce the PFZ width and enhance the Zener pinning effect.

Atomistic simulation of the dislocation interactions with the Al2Ca Laves phase in Mg–Al–Ca alloy

The mechanical properties of Mg–Al–Ca alloys are significantly affected by their Laves phases, including the Al2Ca phase. Laves phases are generally considered to be brittle and have a detrimental effect on the ductility of Mg. Recently, the Al2Ca phase was shown to undergo plastic deformation in a dilute Mg-Al-Ca alloy to increase the ductility and work hardening of the alloy. In the present study, we investigated the extent to which the deformation of Al2Ca is driven by dislocations in the Mg matrix by simulating the interactions between the basal edge dislocations and Al2Ca particles. In particular, the effects of the interparticle spacing, particle orientation, and particle size were considered. Shearing of small particles and dislocation cross-slips near large particles were observed. Both events contribute to strengthening, and accommodate to plasticity. The shear resistance of the dislocation to bypass the particles increased as the particle size increased. The critical resolved shear stress (CRSS) for activating dislocations and stacking faults was easier to reach for small Al2Ca particles owing to the higher local shear stress, which is consistent with the experimental observations. Overall, this work elucidates the driving force for Al2Ca particles in Mg–Al–Ca alloys to undergo plastic deformation.