Microstructural Evolution Research Articles

The influence of carbon content gradient and carbide precipitation on the microstructure evolution during carburizing-quenching-tempering of 20MnCr5 bevel gear

Due to the mechanism of microstructural transition, high-strength low-carbon alloy steel gears exhibit high strength and surface hardness after Carburizing-Quenching-Tempering (CQT) combined heat treatment, while maintaining good toughness in the core. The carbon potential gradient and the precipitation of carbides are the main factors affecting the final microstructure, yet there is a shortage of related research. To address this gap, this study developed a multi-physics coupled model considering the impact of carbon content to simulate microstructural transformation. Using finite element analysis, we simulated the microstructural evolution during the CQT heat treatment of 20MnCr5 bevel gear. By comparing the simulation results with the calculations from the commercial heat treatment software DANTE®, the accuracy of this model in predicting microstructural distribution and hardness has been validated. Furthermore, the study established a carbide precipitation kinetics model that takes into account the influence of grain size, to investigate the precipitation behavior of carbides within the matrix during the tempering stage. The cellular automaton method was applied to demonstrate the evolution process of the quenched microstructure during tempering. The study shows that during tempering, carbides primarily precipitate in areas with higher carbon content, preferentially at the boundaries of fine grains with a high density of lattice defects. As the carbon content increases, the number density of precipitates rises, while their average radius decreases. The reduction in matrix supersaturation due to carbide precipitation is one of the causes for the reduction of tooth surface hardness during tempering. The model constructed in this study provides a new perspective for understanding the laws governing material microstructural evolution and offers a solid theoretical foundation for the study of stress and deformation during the heat treatment process.

Ab initio informed solute drag assessment for ferritic steels

Linking atomistic information on solute interactions with microstructure evolution is a key challenge for predictive modelling of chemistry effects on material properties. The objective of the present work is to provide a link between grain boundary segregation energies with GB migration kinetics on the example of recrystallization in multi-component ferritic steels. For that purpose, the segregation of 64 elements from the periodic table to a representative grain boundary is computed with ab initio density functional theory. To connect this data to grain boundary migration kinetics, the solute trend parameter from a simplified solute drag treatment is employed. The solute trend parameter for individual solutes is presented, which highlights solutes with large impact on grain boundary migration. Furthermore, an extension of the solute trend parameter is introduced that allows to evaluate the solute drag potential of realistic steel compositions. The necessity to include solute co-segregation, site competition, and precipitation effects is shown in a comparison to experimental data on recrystallization kinetics. The comparison to experimental data demonstrates the qualitative predictability of recrystallization kinetics by the extended solute trend parameter.

Microstructural evolution and corrosion behavior of non-isothermally heat treated hypoeutectic Al-Si-Cu alloy

In the present work the effect of non-isothermal heat treatments on the microstructure and corrosion resistance of a hypoeutectic Al-Si-Cu die-cast alloy was studied. The microstructural aspects were examined by confocal laser scanning microscopy and scanning electron microscopy. The corrosion behavior was studied by potentiodynamic polarization tests. In the as-cast condition, the microstructure consisted of needle-like Si particles and Fe-rich intermetallics, besides the Al matrix. The non-isothermal treatments promoted spheroidization of Si particles, decreasing its volume fraction. The corrosion resistance was affected by concentration of the Si particles. The corrosion mechanism is discussed based on the microstructural evolution of the Al-Si-Cu alloy. The main influencing factor was the volume fraction of Si particles. The non-isothermally treated samples exhibited lower corrosion current density than the as-cast alloy, showing superior corrosion resistance.

Evolution of Microstructure and Mechanical Behavior of a Lean Alloyed Continuously Cooled Bainitic Steel

Evolution of Microstructure and Mechanical Behavior of a Lean Alloyed Continuously Cooled Bainitic Steel

The Cahn–Hilliard model of coherent lamellar microstructure: application to alkali feldspar

The temporal evolution of coherent lamellar microstructures is significantly influenced by their elastic properties, particularly when the exsolved phases are coherent. This research utilizes the Cahn–Hilliard model to examine the morphological and energetic evolution of binary alkali feldspar, integrating anisotropic elastic energy into the Gibbs energy equation. The Cahn–Hilliard model successfully simulated the orientation of lamellae observed in natural samples and the elastic strain was consistent with previous research. We also computed the coherent solvus from the annealing simulation of various precursor compositions and temperatures. The temperature difference (ΔT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta T$$\\end{document}) between the strain-free solvus and the coherent solvus was ΔT=85∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta T = 85\\,{}^\\circ \ ext {C}$$\\end{document}, which is slightly lower than previously reported values obtained from similar parameters. This discrepancy is likely due to the presence of non-planar lamellae at the onset of phase separation, which are more stable than planar ones. We also simulated the binodal curves of the coherent solvi for different precursor phase compositions. The computed solvi were not unique but varied depending on the precursor composition. Our model is flexible because it does not assume any specific shapes for the lamellar interfaces and is applicable to various coherent binary systems.

A general model of microstructure evolution including phase transformation in elasto‐plastic materials

A general model of microstructure evolution including phase transformation in elasto‐plastic materials

In situ study of microstructure evolution and α → ω phase transition in annealed and pre-deformed Zr under hydrostatic loading

The detailed study of the effect of the initial microstructure on its evolution under hydrostatic compression before, during, and after the irreversible α→ω phase transformation and during pressure release in Zr using in situ x-ray diffraction is presented. Two samples were studied: one is plastically pre-deformed Zr with saturated hardness and the other is annealed. Phase transformation α→ω initiates at lower pressure for a pre-deformed sample but for a volume fraction of ω Zr, c>0.7, a larger volume fraction is observed for the annealed sample. This implies that the proportionality between the athermal resistance to the transformation and the yield strength in the continuum phase transformation theory is invalid; an advanced version of the theory is outlined. Phenomenological plasticity theory under hydrostatic loading is outlined in terms of microstructural parameters, and plastic strain is estimated. During transformation, the first rule is suggested, i.e., the average domain size, microstrain, and dislocation density in ω Zr for c<0.8 are functions of the volume fraction, c of ω Zr only, which are independent of the plastic strain tensor prior to transformation and pressure. The microstructure is not inherited during phase transformation. Surprisingly, for the annealed sample, the final dislocation density and the average microstrain after pressure release in the ω phase are larger than for the severely pre-deformed sample. The results suggest that an extended experimental basis is required for the predictive models for the combined pressure-induced phase transformations and microstructure evolutions.

Effects of Ga Content on Microstructure Evolution and Mechanical Response of Heterostructured Dual-Phase Ag-49Cu Alloys

Effects of Ga Content on Microstructure Evolution and Mechanical Response of Heterostructured Dual-Phase Ag-49Cu Alloys

Microstructure evolution and catalytic activity of AlCo1−xFeNiTiMox high entropy alloys fabricated by powder metallurgy route

This investigation presents the synthesis of equiatomic and non-equiatomic AlCo1−xFeNiTiMox (x = 0, 0.1, 0.25 and 1.0) high entropy alloys fabricated by mechanical alloying. Mo partially replaced Co. Classic thermodynamic calculations, such as mixing enthalpy (ΔHmix), configurational entropy (ΔSmix), the atomic size difference (δ), entropy to enthalpy ratio (Ω), electronegativity difference (△χ), and valence electron concentration (VEC) were used. Considering δ, Ω and VEC parameters, a BCC solid solution and an intermetallic phase can be predicted due to the partial replacement of Co by Mo. X-ray and electron diffraction of equiatomic HEA without Mo content revealed that after 35 h of milling, a Fe-type BCC lattice phase was formed in the alloy and two L21 phases, in addition to a minimal amount of FCC phase. As the Mo content increased, the Fe-type BCC phase was steadily replaced by the Mo-type BCC phase and the Fe-type FCC phase, and two L21 phases were also developed. When the 5 at% Mo-containing (x = 0.25) alloy was further milled for 80 h, the amount of phases remained almost the same; only the grain size was strongly reduced. The influence of the Mo addition on the properties of studied alloys was also confirmed in the decolourisation of Rhodamine B using a modified photo-Fenton process. The decolourisation efficiency within 20 min was 72% for AlCoFeNiTi and 87% for AlCo0.75FeNiTiMo0.25 using UV light with 365 nm wavelength.

The Influence of Deep Cryogenic Treatment (DCT) on the Microstructure Evolution and Mechanical Properties of TC4 Titanium Alloy

The Influence of Deep Cryogenic Treatment (DCT) on the Microstructure Evolution and Mechanical Properties of TC4 Titanium Alloy

Microstructural Evolution of Pre-oxidized T91 Steel During LBE Dissolution Corrosion

Microstructural Evolution of Pre-oxidized T91 Steel During LBE Dissolution Corrosion

Effect of Superfilling and Leveling Inhibitors in Hybrid Additives on the Microstructure Evolution of Electroplated Copper Interconnect Films during Self-Annealing

Abstract As the feature size shrinks, the main challenge for copper interconnect electroplating is to minimize interconnect resistance while ensuring the void-free feature filling, which is realized by the interaction of additives during electroplating. Among additives, hybrids are considered suitable for damascene filling because they combine type-I (superfilling) and type-II (leveling) inhibitors. However, it is generally believed that the more inhibitory an additive is, the more impurities it introduces into the films, thereby hindering grain growth during self-annealing, which paradoxically limits the conductivity of copper interconnects. To balance the inhibition of hybrid additives with their ability to introduce impurities, the effect of hybrid additives on the microstructure evolution of the films during self-annealing is explored in terms of texture, grain size, and impurity density, which are considered to determine the conductivity of copper films. We find that the {111} texture increases as the percentage of type-I inhibitors increases, whose relative texture coefficient reaches 97% after self-annealing due to the adsorption of hybrid additives on non-<111>-oriented crystal planes. Additionally, the impurity content and grain size increase as type-I inhibition increases, and so does the film conductivity after self-annealing, due to the possible promotion from impurities to the grain growth.

Location-Dependent Phase Transformation Kinetics During Laser Wire Deposition Additive Manufacturing of Ti–6Al–4V

AbstractThis work models the microstructural evolution as a function of position in laser-wire deposited Ti–6Al–4V parts using a classical multi-component JMAK nucleation and growth model with additive isothermal time-steps. Model predictions are compared to experimental observations. The model can be used to interpret nucleation and growth kinetics of various characteristic features of the microstructure that are inaccessible by experiments. It explains the presence of “layer bands” at specific locations unrelated to the weld bead structure. The last re-heating (of multiple thermal cycles) of a solidified layer, and where it peaks, plays a key role in increasing the nucleation density at specific locations, resulting in full $$\alpha $$ α -colony microstructures constituting the “layer bands,” while the rest of the build is predominantly basketweave. Additionally, changes in the basketweave $$\alpha $$ α -lath thickness as a function of the distance from the substrate are studied and compared with an Arrhenius-type function, with results highlighting a more complex relationship than an Arrhenius-type function would suggest.

Influence of Oxygen and Nitrogen Flow Ratios on the Microstructure Evolution in AlCrTaTiZr High-Entropy Oxynitride Films

This study explores the influence of oxygen and nitrogen flow ratios on the microstructure and mechanical properties of AlCrTaTiZr high-entropy oxynitride films. Oxygen flow rates (0%–0.75%) were adjusted while maintaining a fixed nitrogen flow ratio (RN = 15%) to fabricate films with similar compositions. The results show that increasing oxygen flow enhanced hardness through solid solution strengthening and grain refinement, though excessive oxygen caused an amorphous structure and reduced hardness. After annealing at 900 °C, the hardness of all films was further increased. The film with a nitrogen flow ratio 40 times higher than oxygen exhibited the highest hardness of 21.8 GPa, along with superior mechanical performance. These findings highlight the potential of high-entropy oxynitride films for applications requiring high wear resistance and adhesion.

Microstructural evolution and formation of defect complexes in diamond films by nitrogen-containing plasma

Abstract The morphology and crystalline quality of polycrystalline diamond samples were studied by systematically varying the flowrate of nitrogen gas in the microwave plasma. A slight improvement in both crystallite size and crystalline quality is observed for a low concentration of 0.5 sccm nitrogen. With a further increase in nitrogen concentration, diamond switches from micro-crystalline (MCD) to nanocrystalline (NCD) with a nitrogen flow of 2.5 sccm (10% of methane concentration). The surface roughness of the sample is found to depend strongly on the crystallite size of the sample. Extensive spectroscopic studies have been done to understand the presence and formation of different defect complexes in diamond. The presence of nitrogen-containing defect complexes has been studied thoroughly and their concentration has been found to be limited by the solubility limit rather than the availability of reactants in the gas environment. In contrast, the effect these complexes have on the strain of the diamond film is found to be negligible. Optical emission spectroscopy (OES) of the plasma reveals the presence of C2 dimers as well as C-N radicals. However, they have little role in modifying diamond grain morphology or crystalline quality.

Effect of continuous and discontinuous non-proportional loadings on formability of DX54 sheet material

Both continuous and discontinuous non-proportional loadings occur in multi-stage automotive stamping processes. Discontinuous loading is widely studied, but due to requiring sophisticated experimental procedures, continuous loading has been studied less. This study explores the impact of continuous loading on DX54 steel utilising an innovative experimental setup that enabled cruciform samples to undergo uniaxial to biaxial strain path change continuously without unloading. A similar two-stage discontinuous loading from uniaxial to biaxial with unloading in between was generated in DX54 to understand the differences in macro-strain, micro-strain, microstructure, and micro-texture evolution between the continuous and the discontinuous loadings. The stress state of the material during continuous loading was different to that during discontinuous loading. In this study, the occurrence of ‘pseudo-localisation’ was observed during continuous loading, and the observed rotation of high-strain bands differed between continuous and discontinuous loading. The discontinuous loading induced a higher strain, hardening rate, and increased elongation compared to the continuous loading. These results suggest the potential for higher formability during the discontinuous loading compared to the continuous loading.

A hybrid mesoscale-continuum approach to understand and predict melting kinetics of Al powders during laser processing

Abstract Laser interaction with metallic powders during additive manufacturing (AM) leads to fast heating and cooling rates that can affect the quality of the final products due to the formation of defects. One of the first steps towards predicting microstructures generated during AM, therefore, requires an accurate understanding of the laser energy deposition mechanisms that determine the melting kinetics at the level of individual powders. The critical challenge, however, is the availability of computational methods that can model the laser energy absorption, heat transfer, and the related microstructure evolution in individual metal powders at the length and time scales of AM. This manuscript demonstrates the capability of a novel scale-bridging methodology that combines the mesoscale quasi-coarse-grained dynamics (QCGD) simulations with a continuum two-temperature model (TTM) to account for the atomistic mechanisms of laser energy deposition and microstructure evolution and predict the kinetics of melting of individual powders at the experimental time and length scales. The scale-bridging capability of the hybrid QCGD-TTM simulations is demonstrated here by investigating the laser-induced microstructure evolution in aluminum powders with various sizes ranging from 200 nm to 20 µm. The analysis of the evolution of temperature, pressure, phase fraction, and melt fronts suggests the melting mechanism is heterogeneous due to the interaction with a laser, and the melting time is observed to decrease exponentially as the laser intensity increases. The solid-liquid interface velocity can be quantified to identify correlations with interface temperatures, and the predicted values satisfy the theoretically reported limits of crystal stability of metals against homogeneous melting. In addition, the pre-melting is found at the grain boundaries of 20 µm polycrystalline aluminum powder, while a minute contribution to melting is observed. This manuscript demonstrates the capability of the QCGD-TTM method to capture laser-powder interaction and allow the investigation of the kinetics of laser melting.

Microstructure evolution and mechanical properties of NiAlCrFeMo high entropy superalloy after different annealing treatment

A novel NiAlCrFeMo high entropy superalloy was prepared using the vacuum arc melting method, followed by annealing at 800 °C, 1000 °C, and 1200 °C for 10 h. The microstructural characteristics and mechanical properties of the alloy after annealing were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Vickers hardness testing, and high-temperature tensile testing. The results indicate that the as-cast alloy consists of dendritic γ+γ′ phases and interdendritic B2-type β phase, with hemispherical α-Cr phases present within the β phase. Compared to the as-cast status, the volume fraction of the β phase in the annealed state increased from 18.52 % to 26.13 %. Notably, at 800 °C/10h, acicular γp’ phases precipitated within the β phase. The alloy exhibited varying degrees of improvement in both strength and ductility after annealing. The specimen annealed at 800 °C/10h showed the highest strength (σYS = 181.06 MPa) and good ductility (εEI = 12.45 %), with strength increasing by approximately 13.10 % compared to the as-cast status. This improvement is attributed to the coarsening of the α-Cr phase, the transformation in γ′ morphology, and the precipitation of acicular γp’ phase. At 1200 °C/10h, all precipitates dissolved into the matrix, resulting in the lowest strength (σYS = 134.08 MPa) and the highest ductility (εEI = 15.54 %).

Investigation on the microstructural evolution and corrosion behaviors of Zn-microalloyed Al-Cu-Li alloys

Zn microalloying made great contributions to microstructures evolution and performances enhancement. The effects of various contents of Zn on the precipitate microstructure, tensile property and corrosion behavior of Al-3.2Cu-1.5Li alloys with medium Cu/Li mass ratio were examined by tensile testing and corrosion measurement. Experimental results demonstrated that the tensile strength of the Zn microalloyed samples dropped slightly as the Zn content increased due to the number density of precipitates with minor size variation showed a downward trend within the grains. Corrosion testing revealed that the corrosion type of Zn microalloyed samples was primarily localized intragranular corrosion, slight intergranular corrosion and local pitting corrosion was also partially involved. The emergence of Zn element in grain boundary (GB) coarse (T1) phases and the suppressed precipitation of intragranular phases in the alloys with higher content Zn were found to reduce corrosion sensitivity with the increase of Zn content, which was ascribed to the decreased potential difference between the GB and adjacent matrix, and between the GB precipitates and precipitation free zone (PFZ).

High-temperature mechanical behavior and microstructural destabilization evolution of the lightweight refractory high-entropy alloy prepared by laser additive manufacturing

Lightweight refractory high-entropy alloy (LRHEA) has lower density and higher specific strength, which makes it very promising for future applications in rocket engine components. Their high-temperature resistance to softening and high microstructure stability in a wide temperature range are also essential to sustaining their performance due to being used in harsh, high-temperature situations. In this work, the high-temperature tensile behavior of Al0.3NbTi3VZr1.5 LRHEA fabricated by laser additive manufacturing has been systematically investigated from 400 °C to 800 °C, and the relationship between the deformed microstructure and the mechanical behavior was revealed, and analyzing the reasons for the thermodynamic instability of the solid solution in LRHEA are discussed in detail. It is found that the weakening of the high entropy effect at intermediate temperatures reduces the stability of the solid solution, and the accelerated diffusion between elements further promotes the solid solution destabilization, mainly manifested as the Laves phase and the local chemical fluctuation (LCFs). The high dislocation density introduced by the additive manufacturing technology leads to the irregular connection of the Laves phases within the grains, and it can be improved by grain growth. The high negative mixing enthalpy competes with the high-entropy effect, and temperature is the decisive factor. In this situation, the atomic clusters were promoted and induced spinodal decomposition for generating LCFs. The formation of LCFs exhibits solute competition with the emergence of the C14 Laves phase, and the phase proportions fluctuate at different temperatures. This work provides new thinking and attention for developing lightweight, heat-resistant materials.