Nucleation Of Precipitates Research Articles

Effect of Cu addition on the precipitation behavior of M2C carbides in maraging steel

In this study, two Ferrium S53 steels with and without 1.5% Cu were prepared. The microscopic morphology and structure of these steels during aging treatment were analyzed by transmission electron microscopy (TEM). The average equivalent spherical radius (Rv), volume fraction (ϕv), and number density (Nv) of Cu-rich precipitates and M2C carbides during aging treatment were analyzed by atom probe tomography (APT), and the spatial distribution, interfacial elemental segregation, and one-dimensional concentration profiles of the two types precipitates were also characterized. Based on the distribution of elements, 5%Cu and 9% (C + Mo) isoconcentration surfaces were selected to obtain Cu-rich precipitates and M2C carbides, respectively. The results demonstrate that the Cu-rich precipitates formed before the M2C carbides. The segregation of Cr and Mo elements at the phase interface due to the nucleation and growth of Cu-rich precipitates can promote the formation of the M2C carbides. The two types of precipitates display adjacent distribution relationship in steel. The number density of M2C carbides in steel contain 1.5%Cu can be greatly increased when the Cu-rich precipitates serving as nucleation particles. The formation of needle-like M2C carbides inhibited the coarsening of Cu-rich precipitates. More dispersive and fine M2C carbides were obtained in the steel.

Preparation of CaWO4-based glass-ceramics with uniform crystal distribution and enhanced luminous property by thermoelectric coupling treatment

To address the issue of crystal aggregation in CaWO4-based glass-ceramics under high temperature or prolonged durations, while also aiming to enhance the luminescent properties, a thermoelectric coupling treatment is introduced during the preparation process of glass-ceramics. In this study, the influence of applying an external DC electric field on the nucleation and crystallization was examined. Techniques such as DSC, XRD, and SEM are utilized to investigate the crystal types and microstructure of the glass-ceramics. Furthermore, the luminescent performance under ultraviolet and X-ray is characterized, with PL, PLE, and XEL spectra being measured. The findings reveal that thermoelectric coupling treatment augments the crystallization content, refines crystal distribution, and enhances luminescent properties. The applied DC electric field provides energy for thermodynamic reactions, reduces activation energy for bond breaking, accelerates ion migration, and facilitates crystal nucleation precipitation. Crucially, it induces polarization of Ti and Zr ions within crystal nuclei, generating electrostatic repulsion between nuclei. These effects mitigate crystal aggregation, refine distribution, and amplify crystal count. This underscores the potential of thermoelectric coupling treatment in synthesizing optical materials utilizing glass-ceramics as luminescent base materials, offering insights for similar researches endeavors.

Revisiting the precipitation mechanisms of Guinier-Preston zones, η′, and η precipitates in Al-Zn-Mg alloys

Al-Zn-Mg alloys stand among the strongest Al alloys due to the interaction of Guinier-Preston (GP) zones and η′ precipitates with dislocations. Understanding the process of precipitate nucleation is critical to design age-hardenable Al-Zn-Mg alloys with improved mechanical properties. In this investigation, the structure and formation path of GP zones, η′ and η precipitates was studied by means of first principles calculations and statistical mechanics. The structures of potential GP configurations on the FCC convex hull were revealed as well as the presence in the convex hull of FCC MgZn2 clusters. They were found to be precursors of η′ precipitates because of the formation of stacking faults. The η′-to-η transformation was governed by the advance of the η′ interface towards the FCC MgZn2 cluster, so the equilibrium η precipitate is the result of the growth and coarsening of η′. Finally, the critical nucleation temperature for η′ precipitates was determined from the thermodynamic driving force of the transformation. These results rationalize the experimental data available in the literature and offer insights to enhance the strength of Al-Zn-Mg alloys.

Effect of Coiling Temperature on Microstructures and Precipitates in High-Strength Low-Alloy Pipeline Steel after Heavy Reduction during a Six-Pass Rolling Thermo-Mechanical Controlled Process

Nb-Ti high-strength low-alloy pipeline steel was subjected to a six-pass rolling process followed by the coiling process at different temperatures between 600 and 650 °C using the thermo-mechanical testing system Gleeble 3500 (Gleeble, New York, NY, USA). This experimental steel was subjected to 72% heavy reduction through a thermos-mechanical controlled process. Thereafter, the microstructures were observed using optical microscopy, scanning electron microscopy, electron backscatter scanning diffraction, and transmission electron microscopy coupled with energy dispersive spectrometry and selected area electron diffraction. For the selected three coiling temperatures of 600, 625, and 650 °C, acicular ferrite, polygonal ferrite, and pearlite were observed, and morphology and statistical analysis were adopted for the study of precipitates. Based on the estimation by the Ashby–Orowan formula, the incremental strength through precipitation strengthening decreases with coiling temperatures and reaches 26.67 Mpa at a coiling temperature of 600 °C. Precipitation-time-temperature curves were obtained to explain the transformation of precipitates. The (Nb, Ti)(C, N) particles tended to precipitate in the acicular ferrite with [011](Nb, Ti)(C, N)//[011]α-Fe orientation. The lower coiling temperature provided enough driving force for the nucleation of precipitates while inhibiting their growth.

Effect of Zr on static recrystallization of deformed austenite and strain-induced precipitation in Ti microalloyed steel

The static recrystallization and strain-induced precipitation behavior of Ti and Ti-Zr low carbon microalloyed steels were studied through isothermal stress relaxation experiments at different deformation temperatures (875–1025 °C). The precipitation kinetic curves of microalloyed carbides were obtained, and the effect of Zr on the precipitation kinetics of microalloyed carbides was analyzed. Two static recrystallization kinetic models were established, and the activation energies for static recrystallization of Ti steel and Ti-Zr steel were calculated to be 312.44 and 246.59 kJ/mol, respectively. The results indicate that the addition of Zr lowers the nose point temperature of the precipitation–time–temperature curve, shortens the incubation period for the nucleation of precipitates, promotes the nucleation of strain-induced precipitates in Ti microalloyed steel, and reduces the coarsening rate of precipitates particles by an order of magnitude. In addition, the addition of Zr also advances the end time of static recrystallization of deformed austenite, inhibits the growth of static recrystallization austenite grains in Ti steel during the isothermal process, and makes the austenite grains finer and more uniform.

Effect of grain boundary misorientation and carbide precipitation on damage initiation: A coupled crystal plasticity and phase field damage study

A coupled crystal plasticity phase field damage framework has been developed and applied to modelling damage initiation. A novel implementation of a grain misorientation angle dependent critical energy release rate has been used to determine a reduction in the local critical energy release rate resulting from the effects of intergranular carbide precipitates and grain boundary misorientation. When applied to a notched high temperature 316H austenitic stainless steel specimen, a good correlation between experimental results and void nucleation statistics for a misorientation dependent critical energy release rate was obtained. This has been evaluated through comparison with correlative electron microscopy experimental results, showing the potential of phase field models in the area of early damage formation. Additions to include plastic strain and creep deformation effects were made, and comparisons were drawn with experimental data to investigate the contributions of microstructural geometry properties such as the difference in and average values of Schmid factors across grain boundaries, as well as the loading direction stress and dislocation densities. The limitations to this approach and opportunities for further work in this area are discussed, with specific interest in the need for additional literature data characterising grain boundary carbide precipitation and cavity nucleation analysis.

Unusual Precipitation at Grain Boundaries in Non-Equiatomic CoCrFeMnNi High Entropy Alloys

In Cr-rich CoCrFeMnNi alloys, the precipitation of the σ phase at grain boundaries during recrystallization is so fast that ultrafine-grained structure is formed due to the pinning effect of the precipitates. The average grain size of the fcc parent phase is found to be consistent with modified Zener-Smith model. If conventional alloys come to equilibrium, volume fraction of precipitates should approach a saturation value. However, it is interesting to note that the volume fraction of the σ phase in Cr-rich CoCrFeMnNi alloys is inversely proportional to the average grain size of the fcc parent phase. For instance, in Co20Cr25Fe20Ni15Mn20 alloys, the volume fraction changes from 6.5% to 1.2% with increasing average gran size from 14 μm to 210 μm even after annealing at 1273 K for 100 h. It is well known that heterogeneous nucleation of precipitates at grain boundary is energetically favorable and fast diffusion through grain boundary can assist the precipitation. However, they cannot account for the grain size dependence of the volume fraction after reaching equilibrium. Based on stereology, the reciprocal of grain size is proportional to grain boundary area. Thus, chemical fluctuation at grain boundaries (e.g. segregation) is considered to be related to the unusual precipitation at the grain boundaries.

Precipitate composition regulation and corrosion resistance improvement of boron-containing S31254 steels through a pre-aging treatment

Boron (B) at the grain boundary (GB) influences the segregation of Cr and Mo in S31254 super austenitic stainless steels, which inhibits the formation of precipitates. In this work, to make full use of B at the GB, a pre-aging treatment schedule at 300 ℃ for 50 and 150 min was proposed. After pre-aging treatment and high-temperature aging at 950 ℃, B-containing S31254 was used to investigate the effect of heat treatment on the precipitates. Results verified that pre-aging treatment for 50 and 150 min could promote the segregation of B at GB, and the nucleation of precipitates was inhibited during high-temperature aging. Consequently, the B-containing and Mo-rich precipitates were formed at 950 ℃. Especially, pre-aging treatment for 50 min showed the optimum effect on regulating precipitate compositions. B-containing and Mo-rich phase reduced the Cr-depleted region near the precipitates, which could remarkably improve corrosion resistance. Pre-aging treatment and high-temperature aging can regulate precipitate compositions, which can in turn improve corrosion resistance.

Transformation sequence of Mg-Si precipitates towards a precipitation of Mn and Cr containing phase governed by the high strain-rate collision during magnetic pulse welding of Al-Mg-Si alloy

This paper investigates the transformation of Mg-Si precipitates formed at an interface of an Al-Mg-Si alloy joint, welded by the high strain-rate collision during magnetic pulse impact welding. The Transmission Electron Microscopy (TEM) and Energy dispersive X-Ray (EDX) analysis of the precipitates reveal a nucleation of circular shape AlFeMnSiCr precipitate (size of ∼100 nm) from the dissolution of the native β″ (needle-shaped) precipitate. The AlFeMnSiCr phase can also reach a size up to several microns and grows into diverse shapes (rod-shaped, polygonal-shaped, ovoid-shaped). Based on the microstructural characterizations and on observations available in the literature including relevant data of Lodgaard and Ryum, the transformation sequence due to the high-speed collision can be described as follows: β″-Mg-Si (needle-shaped) → β’-Mg2Si (rod-shaped) ‘u-phase’ → Al(MnCrFe)Si (round-shaped) → Al(MnCrFe)Si +(+: Cr; Cu and/or Ni, rod-shaped, polygonal-shaped, ovoid-shaped). The comparison of samples welded at three different impact conditions (low impact intensity, intermediate impact intensity, high impact intensity) shows that the area fraction of the precipitates decreases with the increase of the impact intensity. The identification of the crystal structure of these different precipitates, from the nucleation to the final complete formation of the precipitation, can further enrich the transformation sequence in this paper. The role of the high strain kinematics on the deformation of the Al lattice and on the diffusion rate of alloying elements is also a perspective that can further explain this new observation of precipitation during magnetic pulse welding.

Understanding the effect of cobalt on the precipitation hardening behavior of the maraging stainless steel

The effect of cobalt on the precipitation hardening behavior of the maraging stainless steels was studied. The hardness test indicated that cobalt addition could advance peak aging time and generate a higher peak hardness. The statistical results of nano-size precipitates distribution demonstrated that cobalt increased the density of precipitates, thus resulting in a stronger precipitation hardening contribution. Based on the first-principles calculations and atom probe tomography (APT) results, a possible mechanism has been proposed to reveal the underlying mechanism: cobalt promotes the nucleation of precipitates by decreasing the diffusion activation energy of Ni and Ti.

Nucleation and evolution mechanism of precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy

The evolution process and nucleation mechanism of the precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy were studied using the combination of the Cs-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and first-principles calculations. A new nucleation mode of η' phase with the thickness of 7 at. layers was found, which is parallel to {111}Al planes and consists of 5-layer transition structures and 2 transition layers at the interface. The precipitate is the initial aged η' phase with the simplest structure. Its interfaces are coherent with the Al matrix and meet the stacking periodicity, which is more stable than ordinary η' phases. Besides, the interfaces have a tendency to segregate heavy elements. The presence of segregating atoms enhances the interface interaction and makes it more stable. These two aspects improve the stability of the nano-scale 7-layer particle and maintain the aging strengthening effect of Al alloy.

Change in the structure and mechanical properties of Al–Mg–Si alloys caused by the addition of other elements: A comprehensive review

Understanding 6xxx AA (Al–Mg–Si alloys) relies heavily on structural analysis at the atomic level. Because the nanoscale precipitates that form during artificial ageing essentially strengthen the alloys, it is crucial to lessen the negative effects of Natural Ageing (NA) and enhance the age hardening of Al–Mg–Si alloys. Numerous studies have shown that when different elements are added to 6xxx AA, NA is suppressed. This work includes a thorough and numerical description of the various elements that are added to 6xxx AA. This article gives information on the additional elements Li, Cu, Zn, and Ag that will form new phases and occupy a preferred site in 6xxx AA at different weight percentages. However, between metastable phases, 6xxx AA exhibit different element segregation, which improved the mechanical properties like hardness reported in this review work. Additionally, research on the addition of various other elements, such as Ni, Co, Au, and Cd to 6xxx AA is reported in this review paper to summarise the influence of these elements on the evolution of microstructural features and their correlation with mechanical properties. This area has received very little research and may be used in the future to enhance the mechanical properties of 6xxx AA. By encouraging precipitate nucleation, morphology, and mechanical properties through the addition of foreign elements, the results of this review paper reveal a novel pathway for precipitation hardening.

Mechanical behaviors of metallic alloys dominated by thermo-kinetic synergistic effects upon materials processing

For most metallic alloys, improving strength is usually accompanied by losing ductility (and via versa) even if the work hardening rate keeps unchanged, however, the behind reasons still remain elusive to date. In the present work, we explore the underlying physics for such mechanical performance paradox from the perspective of synergistic effects between thermodynamics and kinetics of phase transition and deformation involved in materials processing. With age-hardenable aluminum alloys as typical examples, the hardening and softening effects are analyzed in terms of system energy accumulation and dissipation during precipitation nucleation and growth along with dislocation multiplication and annihilation. The key thermo-kinetic parameters of driving force and energy barrier are determined by performing relevant atomistic calculations. Analytical models for bridging mechanical performances and thermo-kinetics in terms of microstructural characteristics, which derive from classical phase transition and deformation theories, endow connotation for directing precipitation design, as confirmed by a 6xxx series alloy at the pre-determined aging temperature with ideal strength and ductility. Our work offers an insightful perspective toward understanding the strength and ductility paradox, which can benefit to further establish quantitative connections among processing routes, microstructures and mechanical performances of metallic alloys.

Modeling of heterogeneous site energy distributions in precipitate nucleation

The simulation of heat changes resulting from phase transitions can help to interpret differential scanning calorimetry (DSC) measurements, e.g. of metallic alloy systems in which multiple reactions overlap during non-isothermal heat treatments. So far, simulated DSC curves mostly exhibit sharp reaction peaks as commonly just one mean energy value for a certain type of nucleation site is assumed. This work proposes an efficient model for treating heterogeneous nucleation site energy variations within the framework of classical nucleation theory (CNT). The site energies are assumed to vary according to a Rayleigh distribution and a scaling function. The effect on the nucleation behavior of precipitates is studied. A consideration of the distribution of heterogeneous site energies has the potential to significantly smoothen the numerical treatment of precipitation processes compared to the non-distributed case. The comparison to previously published simulations of DSC curves during the cooling of an AA6005 aluminum alloy demonstrates the advantages of this extension, especially for slow cooling rates.

Lattice Boltzmann modelling of salt precipitation during brine evaporation

Salt precipitation during brine evaporation in porous media is an important phenomenon in a variety of natural and engineering scenarios. This work establishes a multiphase multicomponent lattice Boltzmann (LB) method with phase change for simulating salt precipitation during brine evaporation. In the proposed LB models, the gas–brine multiphase flow, brine evaporation, salt concentration evolution, salt precipitate nucleation and growth are simultaneously considered. Simulations of the Stefan problem are first conducted to verify the proposed numerical models and determine the diffusion coefficient of brine vapour. Once the lattice Boltzmann models have been validated, salt precipitation during brine evaporation is simulated to investigate the competition mechanisms between salt precipitate nucleation and growth reaction. The results show that the typical salt precipitation patterns in existing experimental observation can be successfully reproduced, including the ring-like and pancake-like patterns. The difference in the salt precipitation patterns is explained by the competition mechanism between precipitate growth and nucleation according to the present study. Furthermore, the salt precipitation during gas injection into a microfluidic chip is investigated. The evolution of salt and brine saturation shows similar patterns to existing experimental results, and the effects of the gas injection rate on salt precipitation performance are clarified. The LB models in the present work can simulate salt precipitation with comprehensive consideration of multiphase brine evaporation, salt species mass transport, precipitate nucleation and growth, which have not been realized in previous studies. The numerical showcases demonstrate the excellent performance of the proposed models for the simulation of salt precipitation in porous media, which promise to guide practical engineering applications like CO2 sequestration.

Studying the STA effects on aluminium alloy joints by friction welding

This article examines the properties of joints fabricated by friction welding (FW) using AA6061-T6 and AA7075 as examples of both similar and dissimilar metals. It also looks at how heat treatment after welding affects the metallurgical characteristics of the joint. The heat treatment method under investigation combines solutionizing heat treatment with synthetic ageing (STA). The tensile characteristics, microhardness, and microstructure of the as-welded and STA-treated welded specimens are assessed in the paper using mechanical testing equipment and metallurgical characterization methods. The findings demonstrate that the joint characteristics are enhanced by precipitate nucleation occurring in the joint interface of the FW similar and dissimilar joint under STA treatment. On the other hand, the STA influence on the tensile strength (TS) of joints made with AA7075 was greater than that of similar joints made with AA6061-T6, as well as dissimilar joints made with both AA6061-T6 and AA7075. Additionally, after STA treatment, the similar joints showed a uniform distribution of hardness. However, in the dissimilar joint after STA treatment, the hardness dropped suddenly on the AA6061-T6 side, specifically from the partially deformed zone (PDZ) towards the heat affected zone (HAZ).

Inhibiting segregation enabled outstanding combination of mechanical and corrosion properties in precipitation-strengthened aluminum alloys

Segregation of solutes to grain boundary (GB) is the dominant restriction on enhancement of mechanical properties and corrosion resistance during aging in precipitation-strengthened aluminum alloys. Here, we innovatively introduce the cyclic deformation by the vibration during aging to eliminate the GB segregation, resulting in the formation of the narrow precipitate-free zone (PFZ) widths near the GB, as well as the fine and discontinuous grain boundary precipitations (GBPs). Compared with the traditional peak-aging, the 2014 aluminum alloy treated by thermal cooperative vibration aging (TCVA) exhibits a superb combination of impact toughness and corrosion resistance, and retains the strength and ductility. In addition, the atomic simulations show that TCVA generates numerous vacancies near the GB, but does not change the dislocation density with the increase of cycle time. These results indicate that the vacancy significantly promotes the nucleation and growth of precipitates in the vicinity of GB, resulting in the narrow PFZs and fine discontinuous GBPs. The present work provides the fundamental knowledge and method to inhibit the equilibrium segregation of solutes to GB during aging, and further realizes the precipitation-strengthened aluminum alloys with excellent mechanical and corrosion properties.

Hydride precipitation dependence on α-parent orientation and plastic deformation in zirconium alloys

Hydrides are at the center source of mechanical degradation affecting Zr-based nuclear fuel cladding integrity during in-reactor service and even spent fuel storage. This study presents investigations on hydride precipitation in response to thermo-mechanical treatment in a hydrogenated Zircaloy-4 plate. Correlated EBSD and ECCI characterizations demonstrate stress-induced variations in α-parent orientation and stacking structure between the in-plane and reoriented hydrides. Based on the hydride strains quantified by HR-EBSD, external tensile stress is found to favor the α-grains submitted to significant {0001}<10-10> shear stress as preferential nucleation sites for reorientated hydrides. Regardless of elastic or plastic stress, the interaction between external stress and hydride strains, particularly the shear component, is expected to govern hydride nucleation probability as a function of α-grain orientation and eventually induce hydride reorientation. Influence of plastic deformation is mainly manifested in hydride distribution, where the {10-10}<11-20> slipping promotes reoriented hydride interconnection up to hundreds of microns while the {10-11}<10-1-2> twining results in a fragmented distribution, as visualized by X-ray tomography. The presented results provide solid experimental evidences for the auto-catalytic nucleation and martensitic transformation of hydride precipitation, as well as new insights into stress-induced hydride reorientation in the light of stress interaction with hydride strain.

Designing ultrastrong and thermally stable FeCrAl alloys with the fine-grained structure

Designing microstructurally stable FeCrAl alloys with excellent strength-ductility synergy is highly desirable for their engineering applications. However, due to the preference nucleation of precipitates at grain boundaries (GBs), the improved precipitation strengthening of these alloys is usually accompanied by intergranular embrittlement. Here, we propose a novel thermomechanical processing route coupled with the Si alloying strategy via precipitation of coherent deformable Laves precipitates inside equiaxed fine-grains to achieve FeCrAl alloys with ultrahigh yield strength over ∼992 MPa, excellent uniform elongation of ∼7.6% at room temperature, and superior thermal stability at temperature ∼1200 °C. The Si alloying not only decreases the stacking fault energy of Laves precipitates favorable for their stacking-fault-mediated deformation but also hinders grain coarsening at 1200 °C due to the Si-enrichment favorable for GB pinning effects. Our results prove the possibility of achieving the collaborative enhancement of mutually exclusive properties in alloys, such as strength-ductility-thermal stability via nanoprecipitation engineering, and offer a promising route to prepare dispersion-strengthened materials.

Effect of gradient temperature rolling process on precipitation behavior in Q690D steel

In this paper, the second phase precipitation model is incorporated into the finite element simulation system of hot rolling process through the secondary development subprogram VUSDFLD of ABAQUS, and the simulation analysis and calculation of nucleation rate, volume fraction and size of precipitates under different temperature fields and strain fields are realized. The size, morphology and distribution of the microalloyed carbonitride precipitates in Q690D steel under gradient temperature rolling (GTR) and uniform temperature rolling (UTR) were studied by laboratory rolling experiments. The results show that the nucleation sites of the microalloyed carbonitride precipitates in the matrix increase under GTR, which is conducive to the nucleation of new precipitates and promotes the re dissolution of large precipitates. Finally, after tempering, the precipitate particles are distributed randomly on the ferrite matrix, and the size is refined by 20 ∼ 35 nm, resulting in obvious precipitation strengthening and microstructure refining effects.