Initial Microstructure Research Articles

Influence of initial microstructure on surface integrity of ultrasonic impacting strengthened 34CrNi3MoA steel crankshaft

Influence of initial microstructure on surface integrity of ultrasonic impacting strengthened 34CrNi3MoA steel crankshaft

Optimizing the Strength and Toughness of V/Mo-Modified 0.22C-5.24Mn Steel by Short-Time Partial Austenitization Process.

In order to obtain the good match between yield strength and low-temperature toughness, the short-time partial austenitization (SPA) process was employed for V/Mo-bearing 0.22C-5.24Mn steel. The initial microstructure after intercritical tempering was dual-phase ferrite and reversed austenite (RA), while the final microstructure consisted of ferrite, RA, and secondary martensite (SM) after being subjected to the SPA process. (V, Mo)C with disclike morphology mainly precipitated during intercritical tempering, and the aspect ratio of particles decreased, leading to the appearance of near-spherical morphology. After being subjected to SPA process, the resultant multiphase hierarchical microstructure (three layers: outer layer of ferrite, interlayer of SM, and inner layer of RA) enabled a high yield strength of 1097 MPa, a total elongation of 14%, and an impressive impact energy of 33.3 J at -20 °C. The strengthening contribution of (V, Mo)C precipitation was estimated to be about 108 MPa.

Initial microstructure and temperature dependence of irradiation defects evolution in tungsten

Using the object kinetic Monte Carlo (OKMC) method and the sink strength model, we systematically investigated the influence of the initial microstructure and temperature on the aggregation, migration, removal, and resulting size distribution of defects emerging in tungsten (W) under neutron irradiation. It is found that the probability of defect absorption at grain boundaries is generally higher than that at dislocations. This suggests that the grain size (2 ∼ 20 μm) has a stronger effect on the evolution of irradiation defects compared to dislocation density (1010 m−2 ∼ 1013 m−2). As the grain size increases, the number of residual defects decreases but the size distribution of vacancies is basically unchanged. Besides initial microstructure, the evolution of radiation damage also depends on the irradiation temperature. The increase in temperature induces an initial increase in the number density of voids, and then followed by subsequent decreases, corresponding to the peak density at 800–1000 K. Furthermore, the defect evolution in irradiated W with different initial microstructures and temperatures is simulated, and the calculated results are compared with recent experiments. These results help us to understand the role of the initial microstructure of the material in the evolution of the irradiation defects.

Dynamic recrystallisation in Inconel®718 at creep conditions

Inconel®718 is a nickel-based superalloy primarily used in applications at high temperatures and loads. Its main characteristic is the high creep resistance due to a combination of microstructural features, mainly the presence of coherent intermetallic phases. Although the design of this type of alloy relies on controlling the distribution, nature and stability of the intermetallic phases, dislocations may play a determinant role on the creep resistance of nickel-based superalloys. This work investigates the creep resistance of Inconel®718 after two different heat treatments between 590 and 650 °C at 550 and 830 MPa stresses. After analysing the microstructure, we observe that dynamic recrystallisation happens at high temperatures and stresses, softening the material considerably. Therefore, we further develop mean-field models that predict the strain and microstructure evolutions during creep considering the size and fraction of precipitates, the dislocation densities, grain sizes and recrystallisation grade. Finally, we simulate the creep behaviour at different testing conditions and the initial microstructures and explore the robustness of the model beyond the measurement capabilities. The main conclusion is that an initial large amount of dislocations accelerates the nucleation and recrystallisation rates, decreasing the creep resistance of Inconel®718. In consequence, the material aged after forging presents lower creep resistance than the standard aged because of the larger initial dislocation density. On the contrary, the standard aged material undergo a strong reduction of the remaining dislocation density from forging during the solution treatment.

A Zr-45Ti-54Al-3V alloy with excellent elevated temperature mechanical properties: Millimeter-scale coarse grains with initial subgrains

For improved elevated temperature mechanical properties, a Zr-45Ti-54Al-3V alloy with initial subgrains microstructure and millimeter-scale coarse grains is prepared herein via an electric field-assisted high-temperature compression process at temperatures of 1200 °C and 1400 °C. After the treatment at 1400 °C, the tensile strength and elongation of the alloy at 500 °C are increased by 25.2% and 21.7%, respectively, compared with those of the as-forged 47Zr. Both treatment temperatures lead to a compressive strength of 1400 MPa, along with good elevated temperature compression deformation ability. The rapid grain growth is attributed to the promotion of atomic diffusion by the pulsed current, which also results in a uniform distribution of each solute element. The dynamic recovery and continuous dynamic recrystallization during hot deformation are the main reason of elevated temperature plasticity improvement of EFHC sample. The elevated temperature strength of EFHC sample is higher than that of as-forged sample because it has more α phase and LAGB than as-forged sample

Unlocking the Mechanisms behind Austenite Formation of Cold‐Rolled Ferrite–Martensite Dual‐Phase Steels: The Role of Ferrite Recrystallization

In this study, the austenite formation behavior is investigated for the ferrite–pearlite initial microstructure, considering the influence of ferrite recrystallization and cementite distribution. Special attention is also given to the effect of recrystallization state on cementite distribution for varied heating routes. Austenite nucleation for varied recrystallization state is elucidated in terms of activation energy evaluation and post microstructural characterization, and the austenite transformation kinetics are modeled and experimentally validated. In the uncrystallized state, austenite forms on the original pearlite colonies quickly due to the short carbon diffusion path. The high activation energy leads to the exceptionally difficult nucleation at the ferrite–cementite interface inside ferrite matrix. Once recrystallization occurs partially, priority of austenite nucleation is on those ferrite boundaries (both recrystallized and unrecrystallized boundaries) decorated by cementite. With complete recrystallization, preferential nucleation sites are found to be junctions of ferrite–cementite interface on recrystallized ferrite boundaries. Austenite transformation kinetics is well predicted by considering the varied nucleation density due to different recrystallization state, showing a good agreement with experimental data. It is demonstrated directly by experimental evidence that proceeding of recrystallization postpones the austenite transformation kinetics, while getting rid of the interference of heating rate and nucleation location.

Multiscale investigation of sintering kinetics of Astaloy 85Mo

Shrinkage during the sintering of powder compacts depends on numerous parameters, including green body characteristics such as particle size and green density. These parameters are also decisive for the initial microstructure and its evolution during sintering. In this study, a novel experimental setup is used to quantify the time-dependent microstructural evolution in water-atomised Astaloy 85Mo powder. Green bodies with different particle sizes and density levels were polished on the top surface and then subjected to an interrupted sintering procedure in a quenching dilatometer. Intermediate examinations of the microstructure by scanning electron microscopy revealed the pore morphology and the thermally etched austenite grain size. It was found that pore rounding relies solely on the local curvature only, whereas neck growth is in good agreement with analytical models. An increase in diffusivity was found on the macroscale and on the microscale due to the pre-deformation of the particles.

Influence of dynamic strain aging on low-cycle fatigue behavior in nano-sized κ-carbide strengthened FeMnAlC lightweight steel

The effects of dynamic strain aging (DSA) on the cyclic response and deformation behavior of a hot rolled Fe–22Mn–8Al–0.9C–0.02Nb (wt.%) lightweight steel have been investigated during strain-controlled low-cycle fatigue (LCF) tests at the temperature range of 25–400 °C. The initial microstructure consisted of a complete γ-matrix with nano-sized κ-carbides. During LCF testing, the cyclic stress amplitude displayed a monotonous softening until fracture at 25 °C, 100 °C, and 400 °C in all the investigated strain ranges (Δεt) of 0.8–2.2 %. However, at 200 °C and 300 °C, cyclic softening was followed by pronounced cyclic hardening. Notably, hysteresis loops at the cycle of half-life exhibited serrations in the flow stress at 200 °C and 300 °C. This indicated that cyclic hardening was attributed to the DSA phenomenon, which was confirmed by a negative strain rate sensitivity at 200 °C and 300 °C. A significant reduction in LCF life was observed under the DSA regime. During LCF testing, sequential transmission electron microscopic (TEM) observation indicated that deformation led to the formation of shear bands in multiple directions. Gliding of wavy dislocations was frequently observed at 200 °C compared to that at 25 °C, which resulted from the dragging effect by carbon solute atoms.

Application of a Closed-Form Model in Analyzing the Fracture of Quasi-Brittle Materials.

Fracture failure in quasi-brittle materials poses a persistent challenge in materials science and engineering. This study presents a thorough investigation of the Boundary Effect Model (BEM), offering a nuanced understanding of the size effect on fracture properties. The conceptual framework, evolutionary process, and applicability scope of BEM are elucidated, highlighting its accuracy and reliability in calculating fracture properties across various quasi-brittle materials. Through the integration of BEM with diverse fracture tests-such as three-point bending, four-point bending, and wedge-splitting-a linear correlation between maximum failure loads and material fracture properties is established. Notably, the study demonstrates that fracture properties, determined by BEM, can be regarded as consistent material constants across specimens of varying sizes, initial notch lengths, geometries, and microstructures. Validation of the BEM's reliability encompasses the analysis of 140 fracture test results involving concrete, hard rocks, and bamboo scrimber. The synergy of non-linear and linear BEM analyses emerges as a robust approach for accurately predicting the fracture behavior of quasi-brittle materials. This comprehensive exploration sheds light on the potential of the Boundary Effect Model as a valuable tool for predicting and understanding fracture mechanics in diverse materials and scenarios. This research serves as an effective approach to accurately evaluating the fracture properties of quasi-brittle materials, which is of great practical significance for material design, engineering construction, and various industrial applications.

The Corrosion Performance of Ti6Al4V Alloy after Dry Heat Sterilization with Respect to the Initial Microstructure

The Corrosion Performance of Ti6Al4V Alloy after Dry Heat Sterilization with Respect to the Initial Microstructure

Низкотемпературная сверхпластическая деформация никелевого сплава ЭК79 с ультрамелкозернистой структурой смешанного типа

One of the most effective ways to increase the processing plasticity of advanced superalloys (heat-resistant nickel-based alloys) is the formation of an ultrafine-grained (UFG) microstructure in bulk semi-finished products. Such a microstructure is a necessary condition for the manifestation of the structural superplasticity effect in the technological processes of manufacturing products from such superalloys. One of the most promising methods for producing UFG microstructures is thermomechanical treatment (TMT) according to the multiple isothermal forging scheme. It has been shown that the EK79 superalloy after TMT, with a gradual decrease in the processing temperature from 0.88 to 0.62 Ts (where Ts is the strengthening phase dissolution temperature) leads to the transformation of the initial microduplex fine-grained microstructure into a mixed UFG microstructure. Such a mixed UFG microstructure consists of: 1) relatively coarse (inherited from the fine-grain microstructure) particles – γ'-phase with a size of 3.0±0.8 μm; 2) γ-grains, and incoherent γ'-phase particles with a size of 0.3–0.5 μm; 3) strengthening coherent intragranular γ'-phase particles with a size of 0.05–0.1 μm, released upon cooling from the TMT temperature to room temperature. During uniaxial compression tests, the EK79 superalloy with such microstructure, demonstrates low-temperature superplasticity in the temperature range of 800–1000 °C. It has been found that an increase in the deformation temperature up to 1000 °C, leads to the increase of γ-phase grains to micron size. The maintenance of superplastic properties in the presence of relatively coarse incoherent particles in the microstructure of the second phase (γ'-phase) is apparently related to the fact that the deformation is localised in the UFG component.

Tailoring microstructure and mechanical properties of 17-4PH steel fabricated by wire-arc directed energy deposition via a combination of intrinsic and post-processing heat treatments

This paper examines the effect of intrinsic heat treatment, i.e., multiple transient heating from subsequent passes, and post-processing heat treatment on the microstructure and mechanical properties of 17–4PH stainless steel fabricated by wire-arc directed energy deposition. By carefully controlling the interpass temperature to remain below the martensite start (Ms) temperature, intrinsic heat treatment effectively triggers the formation of Cu-rich clusters and the reversion of austenite. This approach results in a concurrent enhancement in the ultimate tensile strength, increasing from 1036 MPa to 1128 MPa, and elongation, increasing from 13.2% to 18.5%, for the as-built part. During post-processing heat treatment, the microstructural response to direct aging treatment hinges on the initial microstructure induced by intrinsic heat treatment. Notably, direct aging treatment significantly enhances the hardness and ultimate tensile strength of the as-built part created at an interpass temperature surpassing the Ms temperature. Conversely, its influence on the mechanical properties of the as-built component, fabricated at an interpass temperature lower than the Ms temperature, remains minimal. Additionally, the application of a solution treatment effectively eradicates the inherent microstructural characteristics, regardless of the initial fabrication conditions. Nonetheless, the grain size of the solution-treated specimen is markedly influenced by the as-built microstructure. Ultimately, a judicious amalgamation of intrinsic and post-processing heat treatments yields an exceptional combination of properties, including high hardness (465 HV), ultimate tensile strength (1437 MPa), and elongation (14.5%). These results underscore the significance of a comprehensive consideration of intrinsic and post-processing heat treatments to tailor the microstructure and mechanical properties of 17–4PH steel fabricated by wire-arc directed energy deposition.

Effect of Initial Microstructures on Grain Refinement by Burnishing

Effect of Initial Microstructures on Grain Refinement by Burnishing

Strong yet ductile eutectic high-entropy FCC/Laves composite fabricated by powder plasma arc additive manufacturing: Mechanical property, microstructure evolution, and constitutive description over a wide range of temperatures and strain rates

To enhance the strength and toughness of metallic materials simultaneously over a wide range of temperatures and strain rates, a eutectic high-entropy FCC/Laves composite with a heterogeneous initial microstructure was fabricated by powder plasma arc additive manufacturing. The mechanical behavior of the composite over a wide range of temperatures and strain rates was tested with the aid of an electronic universal testing machine and an improved Split Hopkinson pressure bar. The high-entropy FCC/Laves composite possesses a unique combination of strength and ductility over the selected temperature and strain rate ranges due to the in situ composite nature with both soft high-entropy FCC phase and hard high-entropy Laves phase. Complicated thermal viscoplastic behavior is presented. To reveal the mechanisms of the complicated thermal viscoplastic behavior, microstructure evolution was characterized. The high-entropy Laves phase, as a kind of multi-component intermetallic, defies convention by displaying plastic deformation at room temperature and different strain rates. Superior damage tolerance of the high-entropy FCC/Laves composite can be achieved over the selected temperature and strain rate ranges with the contribution of deformation twin in FCC phase, as well as dynamic recrystallization over high temperature range. Finally, a constitutive description was developed, which is shown to be able to accurately describe the complicated plastic behavior over a wide range of temperatures and strain rates. These findings suggest promising prospects for advanced material design, opening a new avenue to achieve a fine balance between strength and ductility across different conditions.

Accelerating phase-field simulation of three-dimensional microstructure evolution in laser powder bed fusion with composable machine learning predictions

Phase-field (PF) modeling is a versatile physics-based computational method that has been used to simulate the evolution of microstructures. The PF method can produce accurate microstructures but suffers from a high computational cost, limiting its use in length scales relevant to additive manufacturing. Using small-scale PF simulations as training data, we trained a surrogate machine learning (ML) model as a computationally cheaper alternative. We use a three-dimensional (3D) U-Net convolutional neural network and learn microstructure evolution in a supervised fashion. With initial microstructure and thermal history as inputs, the ML model can predict the resulting grain orientations at a high accuracy compared to the PF model. Computationally, the ML model is orders of magnitudes faster than the direct PF simulation in a GPU implementation, and scales favorably with increasing number of cells. By spatio-temporally composing multiple ML model predictions at the small-scale, we demonstrate a large-scale 64-layer simulation of a 2 mm × 2 mm × 2 mm cube for a powder bed fusion additive manufacturing process. The ML results revealed a mixture of equiaxed, columnar, and curved grains, comparable to experimental observations. Microstructures resulting from different toolpath strategies, and lack of fusion defects are also demonstrated. This approach paves the way for microstructure-driven process design.

A dislocation-density-based crystal plasticity model for FCC nanocrystalline metals incorporating thermally-activated depinning from grain boundaries

A novel dislocation-density-based crystal plasticity model for nanocrystalline face-centered cubic metals is developed based on the thermally-activated mechanism of dislocations depinning from grain boundaries. Dislocations nucleated from grain boundary dislocation sources are assumed to be the primary carriers of plasticity in the nanocrystals. The evolution of the dislocation density thereby involves a competition between the nucleation of dislocations from grain boundary defect structures, such as ledges, and the absorption of dislocations into the grain boundary via diffusion processes. This model facilitates the simulation of plastic deformation in nanocrystalline metals, with consideration of the initial microstructure resulting from a particular processing method, to be computed as a direct result of dislocation-mediated plasticity only. The exclusion of grain boundary-mediated plasticity mechanisms in the formulation of the crystal plasticity model allows for the exploration of the fundamental role dislocations play in nanocrystalline plasticity. The combined effect of average grain size, grain size distribution shape, and initial dislocation density on the mechanical performance and strain-rate sensitivity are explored with the model. Further, the influence of the grain boundary diffusivity on post-yielding strain-hardening behavior is investigated to discern the impact that the choice of processing route has on the resulting deformation response of the material.

Thermally stable strong texture in additively manufactured cobalt-based superalloys

Cobalt-based γ'-strengthened superalloy exhibits tremendous potential in numerous fields, such as power generation equipment and the aerospace industry. The characteristics of laser powder bed fusion technology have aligned well with the demands for superalloy's strong texture or single crystal microstructure. However, the post-heat treatment processes often disrupt the initial microstructure of the as-printed parts due to unavoidable recrystallization. In this study, we obtained a strong <101> texture in an as-printed Co-5Al-14V (at.%) ternary cobalt-based superalloy, which is difficult to achieve in nickel-based superalloys. Intriguingly, this <101> texture remains thermally stable even after aging treatment at a near γ' solvus temperature. Hence, a <101> textured microstructure with an evenly distributed γ'(L12) precipitates was attained, which benefits the improvement of high-temperature creep resistance. The unique microstructure characteristics revealed in this work can be beneficial for the development of cobalt-based superalloys and shed light on the microstructural engineering of other LPBF-manufactured superalloys.

Initial microstructure and composition evolution of Al2O3 coatings fabricated by cathode plasma electrolytic deposition under constant voltage

At present, the research on cathode plasma electrolytic deposition (CPED) mainly focuses on improving the coating properties, while the initial structure and formation mechanism of CPED coating are less studied. This work aims to study the initial deposition behavior of Al2O3 coating by CPED at different time scales systematically. The results show that the coating consists of lamellar Al(OH)3 in the pre-plasma discharge period. After reaching the breakdown potential, the preparation process of Al2O3 can be considered as a cyclic process: plasma discharge, injection of molten oxide, sintering and solidification, and stacking. Such an iterative process results in a gradual thickening of the coating. Later in the deposition process, the raised structure of the coating surface flaps off and coating thickness decreases. In addition, the high temperature during plasma discharge results in local melting of the substrate surface, which is co-deposited with Al2O3.

A mesoscale crystal plasticity model to predict room-temperature deformation and martensitic transformation of high-strength Quenching and Partitioning (Q&P) Steels and validation with synchrotron X-ray diffraction

Renowned for the superior mechanical properties and adeptness at cold-forming, Quenching and Partitioning (QP) steels have gained prominence as a promising candidate material in fabricating safety-critical components in various industries. The pertinent research on QP steels focus on the martensitic transformation of the Retained Austenite (RA) phase during cold-forming, a crucial mechanism that substantially influences the overall strength and ductility of QP steels. The austenite stability and transformation rate heavily rely on the local strain path and the initial microstructure, which is challenging for analytical prediction. In this paper, a mesoscale model is developed to capture the deformation and transformation kinetics of QP steels inside the microstructure. The model integrates the detailed explicit microstructure, acquired from characterization experiments, into a high-resolution finite element (FE) mesh. It distinctly model the deformation and interaction between the various phases and the effect on the transformation of RA. The model is validated with high energy X-ray diffraction (HEXRD) data, and shows excellent capability in predicting the asymmetric stress-strain behavior under uniaxial tension and compression, as well as the martensitic transformation rate. The model is used to investigate the strain and load partitioning effect of surrounding matrix to the transformation of RA, offering insights into the complex behavior of QP980 and facilitates further material development.

Influence of preliminary compressive deformation on the spall strength of aluminum single crystal

Spall strength is an important property of material subjected to the shock wave loading. It substantially depends on the initial microstructure and its evolution during the preliminary shock compression. By an example of aluminum single crystal, a number of random axisymmetric compressed states are examined by means of molecular dynamics (MD) simulations, different deformation modes are revealed and the spall strength is calculated during subsequent volumetric tensile tests. A domain of elastic states with the spall strength of about 9 GPa, plastically deformed (dislocation plasticity) states with the spall strength of 6–7 GPa and the states with BCC phase transformations and the spall strength in the range 6–9 GPa are revealed as the main scenarios depending on the combination of strains at preliminary compression. MD results are generalized by means of artificial neural networks (ANNs), which allow interpolation between MD-studied points and plotting of continuous dependencies.