Formation Mechanism Of Microstructure Research Articles

Microstructure characteristic, formation mechanism and mechanical properties of AZ91D/A356 bimetallic interface with a novel Ni-Cr composite coating

Microstructure characteristic, formation mechanism and mechanical properties of AZ91D/A356 bimetallic interface with a novel Ni-Cr composite coating

High-temperature dependence of yield strength and microstructure in Ni54Fe19Ga27 and (Ni54Fe19Ga27)99.7B0.3 alloys

The deformation behavior, yield strength temperature dependence, and plasticity of the high-temperature L21(B2)-phase during compression in the temperature range 773–1173 K for Ni54Fe19Ga27 and (Ni54Fe19Ga27)99.7B0.3 shape memory alloys was determined for the first time. The studied alloys were characterized by high plasticity (over 70 %) and minimum yield strength level (13–65 MPa) at temperatures from 973 to 1173 K. Possible mechanisms of yield strength decrease, plasticity increase, and microstructure formation during the deformation process depending on test temperature and chemical composition are discussed. The possibility of hot rolling of Ni54Fe19Ga27 alloys at elevated temperatures was demonstrated.

Phase and microstructure formation during reactive spark plasma sintering of AlxCoCrFeNi (x=0.3 and 1) high entropy alloys

This paper is one of the first studies focusing on phase and microstructure formation mechanisms during reactive sintering of AlxCoCrFeNi (x = 0.3, 1) high entropy alloys (HEAs). The alloys have been prepared by mechanical activation followed by spark plasma sintering (SPS). Both powders are subjected to gluing to the milling vials and the phenomenon intensifies with Al content, resulting in a more important mechanical alloying delay for the equiatomic composition. The temperature influence on phase and microstructure formation was investigated by SPS from 550 to 1100 °C. Al0.3CoCrFeNi reactive sintering led to a face-centered cubic (FCC) solid solution with a grain size around hundreds of nanometers with nanometric intergranular B2 precipitates which content and size reduced with the temperature increase. Grain growth was observed at prior particle boundaries due to dynamic recrystallization, a phenomenon that can be controlled through modification of the milling and sintering steps, leading thus to tailored grain sizes. For the AlCoCrFeNi composition, temperature increase favors secondary FCC formation and leads to sigma phase precipitation. A dual homogeneous-heterogeneous microstructure is obtained due to the important gluing leading to insufficient milling. The dynamic recrystallization at prior particle boundaries promotes also body-centered cubic (BCC) to FCC phase transition. Therefore, the proposed reactive sintering route allows to obtain, for the equiatomic composition, alloys exhibiting very fine microstructures, with higher FCC fraction content than by conventional metallurgy processes.

A novel ultrasonic rolling assisted direct energy deposition method with semi-solid thixo-forming characteristics for AA6061

In order to overcome the defects during the process of additive manufacturing for AA6061, a novel ultrasonic rolling assisted direct energy deposition method with semi-solid thixo-forming characteristics was proposed in this study, which used cold deformed AA6061 wire as raw material, and a corresponding process platform was established. The results showed that under the conditions of laser heating temperature of 750 ℃ and heating plate temperature of 200 ℃, the sample could achieve continuous multi-pass deposition while retaining some of the cold-drawn plastic deformation energy. When the vertical deformation rate was 0.153 and the ultrasonic amplitude was 20 μm, the microstructure of deposition sample after semi-solid heat-treat exhibited typical equiaxed grain characteristics, corresponding to average grain diameter of 95.6 μm and shape coefficient of 1.263. Thin-walled samples with different shapes were formed to verify the capability of process platform. The inference of the formation mechanism of semi-solid microstructure was revealed at last: the introduction of ultrasonic rolling supplemented the plastic deformation energy, and semi-solid heat-treat provided temperature and time for the incubation of equiaxed grains, both of which were crucial for the formation of the semi-solid microstructure.

Microstructure formation mechanism and mechanical properties of super-thickness TC11 titanium alloy joint by electron beam welding and laser additive manufacturing hybrid connection technology

Advanced connection technology for extra-large/super-thickness titanium wrought components is urgent demand in aerospace manufacturing industries. For achieving high-performance titanium alloy super-thickness joint with a high connecting efficiency, a novelty concept of hybrid connection technology with electron-beam welding and subsequent laser additive connection is proposed. A super-thickness (200 mm) TC11 titanium alloy joint has been successfully prepared, and microstructure, tensile properties and fracture behavior of different regions in which are investigated. The results suggest that in stress-relieve heat treatment (SRT) condition, deformation and fracture of the joint are prone to occur in its low-strength wrought base material zone; after double annealing heat treatment (DAT), lamellar α phases in different zones of the joint become similar, associating with a higher fraction of fine secondary α phases in wrought base material zone. Therefore, the tensile properties of the joint are comparable to wrought properties. Although strength in different zones of hybrid connection joint is similar, ductility is not uniform and apparently lower in middle fusion zone due to grain heterogeneity. Overall, these findings indicate that hybrid connection can be one potential technology for fabricating high performance super-thickness titanium alloy joint with a high connection efficiency in engineering application fields.

Role of {332} twinning on adiabatic shear behavior during dynamic loading in β-type Ti–15Mo alloy

The microstructural evolution and formation mechanism of the adiabatic shear band (ASB) in {332}<113> twinning-induced plasticity (TWIP) Ti–15Mo alloy were investigated by applying stopper rings in a split Hopkinson pressure bar apparatus. Accompanied by activation of numerous {332}<113> twins, ASB consisting of elongated fine-grains initiated in the dynamically compressed sample with 0.28 true strain. Multiple twins adjacent to the ASB region were severely distorted and refined along the shear direction. Further deformation led to refinement of elongated twins into elongated subgrains, then some of them were subdivided into equiaxed fine-grains. The morphology of the fine-grains within the ASB underwent a complete transition from elongated to equiaxed as the strain increased to 0.33. The equiaxed fine-grains exhibited a random distribution of orientation, which was regarded as the result of dynamic recrystallization. The formation of equiaxed fine-grains was found to obey rotational dynamic recrystallization mechanism through the thermodynamic and kinetic calculations.

Microstructure evolution during solidification in a low superheat casting process of the Al-Mg2Si composites having excess Si: A phase field study

A two-dimensional phase field (PF) model of solidification during low superheat casting has been developed to grab insight into the microstructure formation mechanism of the Al-xMg2Si-ySi composite, having extra Si as the grain refining agent. The developed PF model employs a seed undercooling based nucleation model to simulate the formation of primary Mg2Si and α-Al grains, wherein the interfacial energy of the Al-melt interface is taken from literature, and a Molecular Dynamics (MD) model is employed to calculate the interfacial energy of the Mg2Si-melt interface. The simulation study predicts the microstructural parameters such as grain size, and sphericity of the primary Al and primary Mg2Si phases, as well identifies the appropriate numerical parameters such as mobility, anisotropy parameters to study the microstructure evolution in the Al-xMg2Si-ySi composites. The kinetics of grain growth of both α-Al and primary Mg2Si have been studied. Novelty of the study lies in developing a near-accurate PF model capable of optimising the weight fraction of Mg2Si and excess Si in the low superheat cast Al-xMg2Si-ySi composite system, in view of obtaining the lower grain size and higher sphericity of both primary Al and primary Mg2Si grains, which is in line with the experimental observations. The proposed PF model is capable of predicting the faceted growth of Mg2Si particles in the Al-xMg2Si-ySi composite, in presence of extra Si and during low superheat casting, and the dendritic growth of Mg2Si in the binary Al-Mg2Si composite having high Mg2Si weight fraction. Following the Jackson’s model of interface growth, it has been demonstrated that the modification of kinetic coefficient leads to a transformation of morphology of Mg2Si particles from large dendritic to faceted ones.

Identifying the in-situ origin of supercluster-induced ductile precipitates in Zr-based bulk metallic glasses

A sandwich architecture including double crystal substrates and a single glass-forming droplet has been successfully designed based on the simplified liquid-bridge methods in multicomponent alloys. The multiscale precipitates mainly consisting of β-phase dendrites with a body-centered cubic (BCC) structure, cubic CuZr2 phase, and icosahedral quasicrystalline phase (I-phase) could be simultaneously identified in dissimilar regions of the glass-forming matrix during heat-induced dissolution or diffusion from crystal substrate to glass-forming droplet. The gradient distributions of crystal-like clusters not only accelerate diffusion velocity due to the collective atomic motion but also can simultaneously modulate the atomic-scale structure, mechanical response, and chemical composition via tracing a high-throughput strategy. The present work provides new insights to discover the formation mechanism of the representative microstructures strongly associated with the atomic-scale structure-property-composition relationships and is also helpful to shed light on the in-situ origin or structural mechanism of supercluster-induced multiscale precipitates in developing bulk metallic glasses (BMGs) and in-situ formed BMG composites (BMGCs).

Formation mechanism of surface gradient microstructure and mechanical properties evolution of Mg–Y-Nd-Gd-Zr alloy by ultrasonic rolling

The evolution mechanism of the microstructure and mechanical properties of a Mg–Y-Nd-Gd-Zr alloy subjected to different numbers of passes of the ultrasonic surface rolling process (USRP) was investigated. The results demonstrated an initial decrease followed by an increase in the surface roughness of the alloy. The optimal surface roughness was achieved after four passes, with a contour mean deviation of 0.06 μm and micro-roughness 10-point height of 0.55 μm. After the USRP, the alloy exhibited a composite gradient structure characterized by increased grain size and decreased dislocation, lamination, and twin densities. The depth of the gradient layer increased with each additional USRP pass, ranging from 650 μm (1 pass) to 910 μm (8 passes). Furthermore, the USRP treatment effectively enhanced the alloy strength; compared with the untreated samples; the yield strength increased by 34.2% in the gradient sample after one pass. No significant change was observed in the YS with an increasing number of rolling passes for Mg alloys, whereas both the tensile strength and elongation showed improvements. High-density twinning played a crucial role in enhancing the YS of the gradient samples without significant variation across different numbers of rolling passes, thus following a consistent pattern. The formation of more nanocrystals on the surface of Mg alloys after multiple rolling cycles is key to preventing premature fracture failure and improving the work-hardening rate.

Phase-field simulation framework for modeling martensite and bainite formation in steel

In this study, we present a combination of phase evolution, chemical diffusion, temperature evolution, and finite strain elasto-plasticity to simulate the martensitic and bainitic transformation using the phase-field software library OpenPhase (OpenPhase, 2023). It is demonstrated how the carbon concentration significantly influences the martensite start temperature and the resulting microstructure. Furthermore, the kinetics of the transformation is strongly influenced by plasticity. For bainitic transformation, it is demonstrated how the holding temperature significantly influences carbon partitioning and the resulting microstructure: higher holding temperatures allow increased carbon diffusion and partitioning, stabilizing retained austenite, which is in good agreement with experimental observations. The present study offers new insights into the microstructure formation mechanisms during martensitic and bainitic transformations in low-carbon steel and offers a consistent modeling approach to model complex phase transformation scenarios in steel and other construction materials.

An improved preparation method of core-shell Al@Sn-Bi microspheres and its microstructure formation mechanism

This paper presents an improved method for achieving controllable preparation of the composition and size of the monotectic alloy core-shell structure microspheres. This advancement is expected to enhance the commercial application of monotectic alloy core-shell structure microspheres. By installing a heating device at the top of the drop tube in the pulsated orifice ejection method setup, we achieved in-situ heat treatment of falling droplets, resulting in the successful preparation of Al45(Sn42Bi58)55 core-shell structure microspheres. Furthermore, we investigated the formation mechanism of the Al@Sn-Bi core-shell structure. Nine sets of Al-Sn-Bi microspheres with varying in-situ heat treatment temperatures were prepared. Analysis via X-ray diffraction and transmission electron microscopy revealed that the microspheres consisted of Al-rich, Sn-rich, and Bi-rich phases, without any intermetallic compounds. Scanning electron microscopy results indicated that as the in-situ heat treatment temperature increased, the structure of the Al-Sn-Bi microspheres transitioned from irregular to regular core-shell configurations. This suggests that higher heat treatment temperatures prolong liquid phase separation time, leading to increased repetition of Marangoni motion within the droplet, Ostwald ripening of the minor phase droplets, and Soret effect of the matrix phase. However, at temperatures exceeding 700 °C, the solidification rate of alloy droplets decreased, causing the Al-rich phase core to deviate from the center and form a crescent structure due to Stokes motion.

Synergy effect of multi-strengthening mechanisms in CoNiCr-based high-entropy superalloy at cryogenic temperature

The evolution and formation mechanism of microstructures in a high-entropy CoNiCr-based superalloy induced by cold-drawing deformation at cryogenic temperature were investigated. The improvement of strength while maintaining the ductility during cryogenic-drawing deformation in the CoNiCr-based high-entropy superalloy originated from the activation of multiple deformation mechanisms including deformation twins, dislocations, 9R phase and hexagonal close-packed (HCP) phase transformation. Two dominant contributions have been suggested for the increase in strength, namely, nanotwins strengthening and the existence of 9R and HCP phase. In this paper, cryogenic-deformation-induced phase transformation from face-centered cubic (FCC) to HCP in the CoNiCr-based high-entropy superalloy was found, and the atomic-scale mechanism of FCC-9R-Twin/HCP transformation was revealed. A new crystallographic model was established to explain the FCC→9R→Twin/HCP transformation. The phase transformation is significant for the comprehensive understanding of the deformation behavior of this CoNiCr-based high-entropy superalloy at cryogenic temperature, and for guiding the design of high-entropy superalloy with advanced mechanical properties.

Grasping the behavior of magnetorheological fluids in gradient pinch mode via microscopic imaging

Magnetorheological (MR) fluids are suspensions of micrometer-sized ferromagnetic particles in a carrier fluid, which react to magnetic fields. The fluids can be operated in several fundamental modes. Contrary to the other modes, the rheology and microstructure formation of the MR fluid in the gradient pinch mode have been studied to a far lesser extent. The magnetic field distribution in the flow channel is intentionally made non-uniform. It is hypothesized that the Venturi-like contraction is achieved via fluid property changes, leading to a unique behavior and the presence of a pseudo-orifice. The main goal is to investigate the presence of the Venturi-like contraction effect in the fluid by means of optical imaging and hydraulic measurements. To accomplish the goal, a unique test rig has been developed including a fluorescence microscope and MR valve prototype. The Venturi-like contraction hypothesis was confirmed. The results indicate that the effective flow channel size decreases by 92% at the maximum magnetic flux applied. This has a direct impact on the flow characteristics of the MR valve. The variation of the pressure–flow rate curve slope with magnetic field was demonstrated. The results provide valuable information for understanding the rheology and microstructure formation mechanism in MR fluids in the pinch mode.

Effect of Al3+ on the surface microstructure and film formation mechanism of Zr-based conversion coatings with improved corrosion resistance performance on cold rolled steel

Most Zr conversion coatings are prepared on Al alloys, with Al3+ from the substrate contributing significantly to the film growth process. Thus, the effect of Al3+ on the electrochemical characteristics, surface microstructure and film-formation mechanism of Zr-based conversion coatings on old- rolled steel (CRS) substrates was investigated by growing the coatings with and without Al3+ as an additive. Electrochemical tests indicated that the anticorrosion performance of the Zr-Al coatings prepared in the presence of Al3+ was greatly superior to that of Zr conversion coatings obtained in the absence of Al3+. Field-emission scanning electron microscopy and X-ray photoelectron spectroscopy analysis indicated that adding of Al3+ in the conversion bath changed the surface microstructure of the Zr coating, greatly promoted the deposition of Zr and F on the CRS substrate and increased the film thickness and density. Both coatings were embedded into the etched CRS substrates. The presence of Al3+ additive induced the formation of different coating components. The Zr coating mainly consisted of ZrO2 and some FeFx, whereas the Zr-Al coatings comprised of ZrO2, ZrOxFy, Al2O3, AlFx and some FeFx. The present study results confirm the key role of Al3+ as an additive in the film formation and growth process of Zr coatings and its effect on enhancing the anticorrosion performance of CRS substrates in NaCl solutions.

Melting and solidification dynamics during laser melting of reaction-based metal matrix composites uncovered by in-situ synchrotron X-ray diffraction

Laser additive manufacturing (AM) of reaction-based metal matrix composites (MMCs) involves highly complex and non-equilibrium material transformation behavior, including melting, dissolution, precipitation, and solidification. Yet, the dynamics and interplay of these phase transformation processes remain poorly understood, posing substantial challenges in identifying the microstructure formation mechanism, and predicting and controlling the microstructure in the printed parts. Here we performed the in-situ X-ray diffraction experiment to characterize the phase evolution dynamics of the 316L + 10 vol.%TiC system during laser melting, which provides direct and quantitative insights of the complex phase reaction and evolution dynamics under rapid heating and cooling conditions relevant to additive manufacturing of reaction-based MMCs. Further in-depth thermodynamic and kinetic calculations revealed that most of the phase evolution behavior observed in the in-situ X-ray diffraction experiment cannot be solely explained by widely used equilibrium thermodynamic models, and diffusion-controlled nonequilibrium dissolution and precipitation kinetics must be considered to elucidate the complex phase evolution behavior, including incomplete TiC dissolution, and three-step TiC precipitation. The three distinct types of precipitates generate unique hierarchical TiC micro- and nanostructures, which enhances the yield strength from 513 MPa to 877 MPa by 71 %, tensile strength from 628 MPa to 1054 MPa by 68 %, and Young's modulus from 193 GPa to 221 GPa by 14 %. The findings of our research provide the knowledge foundation for the design of unique microstructures and advanced MMC materials through laser AM.

Laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities

Ceramic matrix composite (CMC) is typical difficult-to-machine material, laser assisted machining (LAM) is an effective method to solve the problem of poor machinability, and a deep understanding of the interaction between laser and material can lay a foundation for LAM. Aiming at the unclear mechanism of microstructure formation, crack propagation and material hardness evolution of laser-irradiated CMC, laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities were studied in this paper. The results show that Cf/C–SiC presents two different states of modification and ablation under irradiating. In modified state, fiber interface in central area is oxidized, and crystal transformation occurs. Amorphous Si–O–C, Si and spherical SiO2 exist in edge deposition area, and continuous phase disappears in heat affected zone. In ablative state, carbon nanosheets and clusters are formed in ablation groove. Surface of convex and deposition area are composed of sedimentary basement and clusters, and there are recast layer and heat affected zone in cross section. When thermal stress exceeds interfacial bond strength and critical matrix stress, the fiber interface debonding and matrix cracking occur, and both transgranular and intergranular fracture exist in the matrix. When laser scanning direction is perpendicular to fiber extension, crack propagation can be inhibited. The propagation and intersection of cracks and coarsening of SiC crystal phase in heat affected zone lead to the decrease of matrix microhardness. The results of this study can be used to provide references for parameters selection and theoretical support for LAM of CMC.

Quantification of melt pool dynamics and microstructure during simulated additive manufacturing

The dynamics of laser spot melting and metallic alloy solidification are investigated through synchrotron x-ray radiography, 3D electron backscatter diffraction data, and computational fluid dynamics (CFD) simulations on a model NiMoAl alloy. Solidification velocities are measured from in-situ images to validate a CFD model of the spot melt. TriBeam tomography is used to reconstruct the melt pool in 3D, characterize the microstructure, and validate the CFD model melt pool dimensions. 3D geometrically necessary dislocation (GND) density calculations reveal extensive deformation at the sample surface. GND calculations integrated with the CFD model reveal that the halo of small, equiaxed grains adjacent to the fusion line is a result of recrystallization that occurred in the heat affected zone. By combining in-situ and ex-situ data modalities across a variety of temporal and spatial length scales, the microstructure formation mechanisms and their relationship to melt pool solidification are quantified.

Effects of calcined phosphogypsum replacement on hydration and properties of calcium sulfoaluminate cement at different curing temperatures

The present research aims to understand the cement hydration and microstructure formation mechanism of calcined phosphogypsum (CPG) supplemented calcium sulfoaluminate (CSA) pastes cured under hot-weather temperatures and to ensure the appropriate application and effective service of CSA cement-based materials, as well as to address the problem of overstocking of CPG. The effects of CPG dosage on the heat of hydration, electrical resistivity, hydration products, and bound water content of CSA cement pastes were investigated at curing temperatures of 20 °C, 35 °C, and 50 °C. Hydration kinetics was studied to elucidate the hydration mechanism, and thermodynamic modelling was performed to predict the phase assemblages. The results show that the incorporation of CPG facilitated the formation of ettringite and significantly reduced the transformation of ettringite to monosulfate, and these phenomena were enhanced with an increase in the CPG. The addition of CPG at elevated curing temperature (35 °C and 50 °C) significantly improved the compressive strength of CSA cement pastes. The growth rate and nucleation rate predicted by the BNG model for the CSA cement pastes cured at 50 °C were substantially higher compared to those at 20 °C.

Microstructure evolution and formation mechanism of interfaces in parallel gap resistance welding of stranded Ag-plated Cu conductor to Ag interconnector

In space flexible solar arrays, stranded Ag-plated Cu conductors and Ag interconnectors are preferred materials for energy transmission. Parallel gap resistance welding is a solderless micro-joining process that favors the reliability of joints between stranded Ag-plated Cu conductors and Ag interconnectors. It was found the interfacial microstructure presented diversity and complexity while mechanical and electrical properties showed unusual nonlinear variations with welding voltage. This study aims to clarify the interfacial microstructure evolution and its effects on the joint properties while elucidating the microstructure formation mechanisms. A shift in the bonding mechanism at interfaces from solid-state diffusion to brazing was found as welding voltage reached a critical value (1.5 V). It eliminated micro gaps and built nano-scale interlocking structures, resulting in substantial improvement in mechanical properties. Increasing the welding voltage from 1.2 V to 1.4 V improved electrical conductivity due to the enlarged bonding area at interfaces. However, high welding voltage (1.6 V) led to degradation in the electrical conductivity of joints due to excessive Ag-Cu solid solution formed at interfaces. The key to fabricating high-strength and high-conductivity joints lies in achieving appropriate interfacial melting while reducing alloying by controlling peak temperature and shortening the duration above the Ag-Cu eutectic point.

Multiscale microstructure containing nanometer-scale precipitations and stacking faults yields a high-strength Al-5Cu alloy by electron beam freeform fabrication

Electron beam freeform fabrication (EBF3) additive manufacturing technique has been successfully employed to print the non-weldable Al-5Cu thin-walled parts through an optimized set of process parameters. Post-heat treatment is conducted to engineer the desirable microstructure containing multiple nanostructures, i.e., the nanoscale θ" and θ' precipitates, stacking faulted zones and Al-Cu-Cd clusters, leading to a high-strength Al-5Cu alloy with good ductility. The grain formation mechanisms are revealed via a combinatorial simulation and experimentation. It is shown that EBF3 process can produce three different types of grain microstructures: fully equiaxed, fully columnar, and the mixture. In the layerwise melt pool solidification process, wide partially melted zone is observed to favor the formation of equiaxed grains due to its inheritance characteristic. After solution and aging heat treatment, Al-Cu-Cd nanoclusters (∼ 2 nm in Dia.) are generated, which facilitates the formation of nanoscale stacking faulted zones (∼11 nm). Meanwhile, as heterogeneous nucleation sites, Al-Cu-Cd nanoclusters effectively promote θ'-Al2Cu phase (∼ 19 - 114 nm) precipitation to enable the coexistence with fine θ"-Al3Cu precipitates (∼ 7 - 37 nm). The multiscale microstructure, especially incorporating the newly identified nanoscale stacking faulted zones, activates multiple strengthening mechanisms. Consequently, its tensile properties at room temperature reach an ultimate tensile stress of ∼ 496.5 MPa, yield strength of ∼ 435.7 MPa, and elongation of ∼ 9.6%, which are superior to other as-cast and additively manufactured Al-Cu alloys. This work offers a promising processing route to directly fabricate high-strength near-net-shape Al-Cu alloy parts for the aerospace and automotive industries.