Precipitation Kinetics Research Articles

Achieving high strength and uniform ductility in high-entropy alloys via dynamic-precipitation accelerated transformation-induced plasticity

This study presents a novel approach to fabricating advanced transformation-induced plasticity (TRIP) high-entropy alloys (HEAs) that exhibit an exceptional combination of high strength and uniform ductility, far surpassing the mechanical performance of previously developed TRIP HEAs and non-TRIP multiphase HEAs. The ultrafine-grained Al5Cr20Fe35Co35Ni5 TRIP HEA sample exhibits a yield strength of gigapascal-level, ranging from 1100 to 1260 MPa, with large uniform elongations that range from 29 % to 39 %. Simultaneously, the alloy displays outstanding toughness, reaching 46,000±5390 MPa·%. The achievement of this significant breakthrough rests upon two key factors: (1) achieving an ultrafine-grained FCC matrix in a fully recrystallized microstructure with the aid of thermally induced B2 phase, and (2) dynamic precipitation of nanometer-sized B2 phase particles on high-density stacking faults and slip bands created within the interiors of ultrafine grains. The dynamic precipitation of the B2 phase, facilitated by the formation of an amorphous phase that serves as a precursor to the B2 crystalline phase, is a rare occurrence in HEAs at room temperature. This dynamic precipitation process greatly enhances TRIP-assisted strain hardening (from FCC to HCP phase) of the current alloy at ultrafine grains, enabling the overcoming of the inherent tensile ductility constraints typically associated with ultrafine-grained metals. The current development of TRIP HEAs with an outstanding combination of high strength and high toughness is believed to mark a remarkable milestone in the application field of TRIP HEAs.

Investigation of Microstructure and Mechanical Properties of Extruded Mg-6Bi and Mg-6Bi-1Ag Alloys.

The extruded Mg-6Bi alloy and Mg-6Bi-1Ag alloy subjected to extrusion at 300 °C with the extrusion ratio of 25:1 and die-exit speed of 2 m/min were used to investigate microstructure characteristics and mechanical behavior. The experimental results demonstrate that the bimodal microstructure, composed of coarse dynamic unrecrystallized (unDRXed) grains and fine dynamic recrystallized (DRXed) grains, was obtained after extrusion. The Ag addition can obviously promote dynamic recrystallization and average grain size. It also indicates that the dynamic precipitation is significantly promoted by Ag addition during extrusion, obtaining more fraction of the Mg3Bi2 precipitates. Moreover, the extruded Mg-6Bi-1Ag alloy has a high tensile yield strength of 304 ± 2.0 MPa, which is increased by 19% compared to the extruded Mg-6Bi alloy, and elongation of 11.0 ± 1.7%, almost the same as 11.9 ± 0.9% of the extruded Mg-6Bi alloy. This result also shows that the extruded Mg-6Bi-1Ag alloy exhibits better strain hardening capacity. Therefore, Ag exhibits an effective role in promoting dynamic recrystallization and dynamic precipitation, resulting in the enhancement of strength and strain hardening capacity of the extruded Mg-6Bi-1Ag alloy, as well as keeping high ductility.

Effect of equal-channel angular pressing on microstructure, aging kinetics and impact behavior in a 7075 aluminum alloy

Among the precipitation hardenable 7xxx series aluminum alloys, AA7075 alloys (one of the most important engineering alloys) due to their low density (lightweight), high toughness and strength, and enhanced fatigue behavior have been used over the years in aerospace, aircraft, automotive, construction and marine applications. In the present study, the influence of the artificial aging through conventional heat treatment (i.e., precipitation hardening) and the thermomechanical treatment (equal-channel angular pressing (ECAP) + post-ECAP aging) on the quasi-static tensile, impact toughness and precipitation behavior as well as on the microstructure of an AA7075 alloy is investigated systematically. The results indicate that the ECAP process as well as post-ECAP aging result in considerably increased strength and hardness of the AA7075 alloys because of the superimposed effects of grain boundary strengthening (Hall-Petch relation), strain hardening (by means of the increased dislocation density) and precipitation hardening (due to the fine precipitates formed in Al matrix). Multi-pass ECAP processing has been found to result in a considerable decrease of the impact toughness (from 15.3 J·cm−2 to 7.6 J·cm−2) associated with the failure mode (i.e., ductile-to-brittle transition) of the AA7075 alloys, whereas the artificial peak aging leads to a marked improvement (∼67 %) in the impact toughness in ECAPed conditions. Moreover, the ECAP process noticeably accelerates the precipitation kinetics of AA7075 alloys: The peak aging time in the ECAPed alloy is reduced significantly – from 30 h to 4 h. The results of the present study provide a deeper understanding of the combination of ECAP and heat treatment, and their effect on the fracture mode and toughness under impact loading, the aging kinetics and the microstructural evolution of an AA7075 alloy.

Impact of carbon segregation on transition carbides and cementite precipitation during tempering of low carbon steels: Experiments and modeling

Tempering of low-carbon steels (less than about 0.3 wt.% C) may differ from the traditional precipitation sequence, which includes transition carbide precipitation, retained austenite decomposition and cementite precipitation. The main difference is the partial or total absence of transition carbides. In the present work, the effect of the carbon segregation on the mitigation of the precipitation of the latter is analyzed by combining multi-scale experimental investigations with a new precipitation model accounting for the carbon heterogeneities induced by the segregation. The full precipitation sequence is considered in order to study the impact of the segregation not only on the transition carbides, but also on the cementite which forms afterwards. The experimental work includes APT characterization of carbon segregation, in-situ HEXRD experiments to reveal precipitation kinetics, and TEM observations for carbide size measurements.The here-in developed precipitation mean-field and physics-based model combines two previous ones dedicated to the nucleation and growth of transition carbides and cementite and to the segregation of carbon at dislocations. It is shown that, even after water-quench, carbon atoms are already segregated on dislocations. The mitigation of transition carbide precipitation is caused by the presence of such segregations, which decrease the driving force for the transition carbide nucleation and enhance cementite precipitation. In agreement with previous experiments, the model also demonstrates that inside a martensitic microstructure, the precipitation sequence is different between the first (formed close to Ms temperature) and the last martensite (formed at room temperature) formed upon cooling, because of the difference in dislocation density, which influences the intensity of the segregation phenomenon.

Dynamic Recrystallization Nucleation Mechanism and Precipitation Behavior of Homogeneous Al-Zn-Mg-Cu Alloy during Hot Deformation

The hot deformation behavior of Al-Zn-Mg-Cu alloy was investigated by flow stress curves in isothermal hot compression experiments with deformation temperatures of 350–450 °C and strain rates of 0.01 s−1 to 1 s−1, and the constitutive equation of homogeneous alloy was obtained. At the same time, the dynamic recrystallization and precipitation behavior during hot deformation and the relationship between them and the Z parameters were studied by using EBSD and TEM. DRV is the main mechanism of dynamic softening. With the decrease in Z parameter, the softening mechanism changes from dynamic recovery to discontinuous dynamic recrystallization or continuous dynamic recrystallization. At a higher Z parameter, the dislocation density and precipitated phase density are also higher because the high dislocation density provides heterogeneous nucleation sites of the precipitated phase. A large number of precipitates in the alloy also inhibit the nucleation and growth of dynamic recrystallization by hindering dislocation movement and grain boundary migration.

MoC‐MoSe2 Heterostructures as Multifunctional Catalyst Toward Promoting the Stepwise Polysulfide Conversion for Lithium‐Sulfur Batteries

AbstractCatalyzing polysulfides conversion for lithium‐sulfur batteries is an efficient strategy to overcome the sluggish kinetics of polysulfides conversion as well as its serious shuttling effect. Due to the multistep and complicated phase transformation of sulfur species, the monofunctional catalyst can hardly promote the overall polysulfides redox process. Herein, a molybdenum‐based heterostructure is proposed, that facilitates the entire reduction process by tandemly catalyzing liquid‐liquid conversion and liquid‐solid conversion. It is uncovered that the MoC physiochemically immobilizes the soluble long‐chain polysulfide and accelerates the conversion between S8 to Li2S4 through adsorbing Li2S8 and extending its S─S bond distance. Then, the kinetics of Li2S precipitation is enhanced by facilitating the migration of Li2S4 from MoC to MoSe2. This is driven by the internal electric field at the heterogeneous interface and the low diffusion energy barrier on MoSe2 for Li2S4. Moreover, MoC‐MoSe2 exhibits the smallest degree of Li2S2 disproportionation throughout the reduction process. Consequently, the cell with MoC‐MoSe2/C/S cathode delivers an initial discharge‐specific capacity of 841.1 mAh g−1 and long‐term cycling stability with a capacity attenuation rate of 0.08% per cycle at 1.0 C. This work presents a novelty view to design a rational multifunction catalyst for sequentially accelerating the stepwise conversion of polysulfides.

Deep learning for kinetics parameters identification: A novel approach for multi-variate optimization

This work presents an innovative deep-learning approach for multi-variate optimization, focusing on the identification of Mg(OH)2 precipitation kinetics parameters. The study employs three distinct experimental datasets, one for the Population Balance Model (PBM) fitting and two for validation. These datasets explore the impact on Particle Size Distributions (PSDs) of (i) increasing the initial reactant concentrations from 0.125 to 1 M and (ii) decreasing the flow rate from 12 to 4 m/s, both in a T-mixer, (iii) increasing the initial reactant concentration over a wider concentration range from 0.01 to 1 M in a more complex Y-mixer system. Leveraging PBM, we create a dataset to train a Neural Network (NN), referred to as the ‘mirror model,’ which predicts kinetics parameters based on experimental sizes. Notably, the PBM, fitted with dataset (i), excels at describing changes in flow rate (dataset (ii)) and substantial reductions in reactant concentrations in the Y-mixer (dataset (iii)), even though these conditions were not encountered during the fitting step. Key Performance Indicators (KPIs) reveal that the mirror model consistently outperforms two widely used algorithms, Conjugate Gradient (CG) and Particle Swarm Optimization (PSO), highlighting its remarkable potential for practical applications.

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

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

Precipitation kinetics of Cu in FeCu and FeCuX (X=B, N, and BN) alloys using kinetic Monte Carlo simulations

We employed kinetic Monte Carlo simulations to investigate the precipitation of Cu in FeCu and FeCuX (X=B, N, and BN) alloys. The parameters of kinetic Monte Carlo simulations were obtained through first-principles calculations. In this study, we utilized a vacancy-mediated model that considers the migration barriers dependent on local impurity atomic environments. The dynamical stability of both FeCu and FeCuX (X=B, N, and BN) alloys was examined by the virtual crystal approximation method. Through the assessment of advancement factor, cluster number density, and mean cluster radius, the simulation results indicated that both B and N promote precipitation of Cu. While B doping has a minor effect, N doping significantly promotes Cu precipitation.

Impact of Vanadium Presence on the Mechanical and Microstructural Characteristics of a Dilute Al–Sc–Zr–Si Alloy During Isothermal Aging

In this work, we studied precipitation tendency and resulting ambient and elevated temperature properties in low-V (Al-0.06Sc-0.06Zr-0.18Si-0.05V at.%), high-V (Al-0.06Sc-0.06Zr-0.18Si-0.10V at.%), and free-V (Al-0.06Sc-0.06Zr-0.18Si at.%) alloys subjected to isothermal aging at 300 °C and 400 °C. Scanning transmission electron microscopy revealed that V incorporated into Al3(Sc, Zr) nano-precipitates during aging, forming Al3(Sc, Zr, V) while retaining the spherical morphology and L12 order. Analysis showed that V delays precipitation kinetics by interacting with vacancies, requiring longer aging for peak strength. Both V alloys demonstrated increased peak microhardness around 50–100 MPa relative to the free-V alloy after extended aging. V also reduced precipitate coarsening rates during overaging. However, creep testing exhibited no improvements to threshold stress, indicating V did not expand the precipitate/matrix lattice mismatch needed for enhanced creep resistance. The threshold stress changed from 10.2 MPa in the free-V alloy to 10.4 MPa and 10.1 MPa in the low-V and high-V alloys, respectively. Instead, V localization in the interface of nano-precipitates with matrix contributed more stability. The precipitation strengthening mechanisms in all alloys were studied using established models for L12-reinforced systems. The results showed that coherency and ordering strengthening are the primary active mechanisms.

Improved Thermodynamic Descriptions of Carbides in Ni-Based Superalloys

The Ni-based superalloy René 41 has sparked recent interest for applications in next-generation aircraft engines due to its high-temperature strength that is superior to all similar grades. These desirable properties are achieved by careful control of the microstructure evolution during thermomechanical processing, and this is commonly informed by simulations. In particular, the grain boundary carbides M6C and M23C6 play an essential role in controlling the grain size and strength of the final product. Therefore, a solid understanding of the thermodynamic stability and thermokinetic evolution of these carbides is essential. However, thermokinetic simulations using existing thermodynamic databases have been demonstrated to have discrepancies between thermodynamic stabilities and experimental observations. Here, we collected a new experimental time–temperature–precipitation diagram. In conjunction with improved crystallographic descriptions, these experimental results are used to modify a CALPHAD database for M6C and M23C6. The modified database correctly identifies temperature regions with rapid carbide precipitation kinetics. Further, kinetic simulations and strengthening models successfully predict the hardness increase due to γ′ precipitation. The modified database has been applied to Udimet 700, Waspaloy, and Haynes 282, demonstrating improved results. These updates will facilitate more accurate simulations of the microstructure evolution during thermomechanical processing of advanced Ni-based superalloys for aerospace and other applications.

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.

Quantifying the energy of vacancy mediated diffusion of solute Cu, Mg, Ag and Zn atoms in an aluminum Matrix: An insight from ab initio calculations

In this work, a consistent and accurate set of energy barrier values for the migration process, assisted by a vacancy, of atoms of different chemical elements belonging to the two most relevant families of age-hardenable Al-Cu and Al-Zn-based alloys is given. These values were calculated using the Nudged Elastic Band method within the Density Functional Theory framework for a vacancy diffusing in simple configurations of solute atoms, specifically the main alloying elements Cu and Zn involving a single solute atom or more complex configurations, such as pairs of elements of the same or different chemical species. In both cases, in our calculations, the presence of Mg and/or Ag solute atoms is also considered. The results obtained are analyzed in terms of the energetic interaction among vacancies and the different solute atoms. The advantage of having a set of vacancy diffusion energy barrier values calculated under the same criteria is that it can be used as reliable input parameters for future kinetic Monte Carlo simulations to study the precipitation kinetics in a wide type of age-hardenable alloys.

Dynamic precipitation and twinning-induced dynamic recrystallization in Mg–6Al–3Sn–1Zn alloy during multi-pass high-speed rolling

Ultrafine-grained microstructure was successfully obtained in Mg–6Al–3Sn–1Zn (ATZ631) alloy by multi-pass high-speed rolling (HSR) at a rolling speed of 1100 m/min and a rolling temperature of 400 °C. The grain refinement mechanism of ATZ631 alloy during HSR was investigated, which is attributed to mechanically fragmented grains and twinning-induced dynamic recrystallization (TDRX). During the initial stages of rolling, the initial coarse grains are refined by the intersection of twins. With increasing strain, the grain refinement mechanism is the typical dynamic recovery and TDRX. In general, twins are filled with a high density of dislocations and nano-sized precipitates at the dislocations, which can accelerate DRX by increasing the driving force or particle-stimulated nucleation (PSN) within the twins. Subsequently, the coarse grains are refined by forming fine DRXed grains via dislocation entanglement, dislocation rearrangement, and dislocation absorption. As the process proceeds, grain boundaries (GBs) migration is suppressed by the precipitates at the GBs of new DRXed grains. Thus, the growth of the DRXed grains can be inhibited. Finally, ultrafine-grained microstructure with an average size of ∼116 nm is obtained. Moreover, the new grains formed by TDRX exhibit random orientation during this process, indicating that the basal texture gets effectively weakened by increasing the fraction of DRXed grains. These findings bring forward the microstructural evolution during dynamic precipitation and DRX, thus providing a novel notion for improving microstructures and grain refinement in Mg alloys.

Atomic Diffusivities of Yttrium, Titanium and Oxygen Calculated by Ab Initio Molecular Dynamics in Molten 316L Oxide-Dispersion-Strengthened Steel Fabricated via Additive Manufacturing.

Oxide-dispersion-strengthened (ODS) steels have long been viewed as a prime solution for harsh environments. However, conventional manufacturing of ODS steels limits the final product geometry, is difficult to scale up to large components, and is expensive due to multiple highly involved, solid-state processing steps required. Additive manufacturing (AM) can directly incorporate dispersion elements (e.g., Y, Ti and O) during component fabrication, thus bypassing the need for an ODS steel supply chain, the scale-up challenges of powder processing routes, the buoyancy challenges associated with casting ODS steels, and the joining issues for net-shape component fabrication. In the AM process, the diffusion of the dispersion elements in the molten steel plays a key role in the precipitation of the oxide particles, thereby influencing the microstructure, thermal stability and high-temperature mechanical properties of the resulting ODS steels. In this work, the atomic diffusivities of Y, Ti, and O in molten 316L stainless steel (SS) as functions of temperature are determined by ab initio molecular dynamics simulations. The latest Vienna Ab initio Simulation Package (VASP) package that incorporates an on-the-fly machine learning force field for accelerated computation is used. At a constant temperature, the time-dependent coordinates of the target atoms in the molten 316L SS were analyzed in the form of mean square displacement in order to obtain diffusivity. The values of the diffusivity at multiple temperatures are then fitted to the Arrhenius form to determine the activation energy and the pre-exponential factor. Given the challenges in experimental measurement of atomic diffusivity at such high temperatures and correspondingly the lack of experimental data, this study provides important physical parameters for future modeling of the oxide precipitation kinetics during AM process.

The Use of Global Sensitivity Analysis to Assess the Oral Absorption of Weakly Basic Compounds: A Case Example of Dipyridamole.

To utilize the global system analysis (GSA) in oral absorption modeling to gain a deeper understanding of system behavior, improve model accuracy, and make informed decisions during drug development. GSA was utilized to give insight into which drug substance (DS), drug product (DP), and/or physiological parameter would have an impact on peak plasma concentration (Cmax) and area under the curve (AUC) of dipyridamole as a model weakly basic compound. GSA guided the design of in vitro experiments and oral absorption risk assessment using FormulatedProducts v2202.1.0. The solubility and precipitation profiles of dipyridamole in different bile salt concentrations were measured. The results were then used to build a mechanistic oral absorption model. GSA warranted further investigation into the precipitation kinetics and its link to the levels of bile salt concentrations. Mechanistic modeling studies demonstrated that a precipitation-integrated modeling approach appropriately predicted the mean plasma profiles, Cmax, and AUC from the clinical studies. This work shows the value of GSA utilization in early development to guide in vitro experimentation and build more confidence in identifying the critical parameters for the mathematical models.

Hot compression behavior and microstructure evolution of Mg-Gd-Y-Zn-Zr alloy

In this work, the hot compression behavior and microstructure evolution of Mg-8Gd-3Y-xZn-0.6Zr (x = 0, 1.5) alloys were investigated by Gleeble-1500D thermal simulation test machine, SEM, EBSD and other equipment. The results showed that, prior to hot deformation, the grain size of Mg-8Gd-3Y-1.5Zn-0.6Zr (1.5Zn) alloy was smaller than that of Mg-8Gd-3Y-0.6Zr (0Zn) alloy. Both alloys exhibited weak texture strength with predominantly large angle grain boundaries. The true stress-strain curve of the 1.5Zn alloy distinctly revealed dynamic recrystallization features. Notably, the 1.5Zn alloy exhibited higher peak stress and thermal activation energy compared to the 0Zn alloy. Furthermore, the 1.5Zn alloy demonstrated a lower critical strain for dynamic recrystallization, suggesting a heightened likelihood and increased degree of recrystallization. After hot deformation, the Mg-8Gd-3Y-xZn-0.6Zr alloy experienced a notable reduction in grain size, particularly evident in the 1.5Zn alloy, where the grains were smaller compared to the 0Zn alloy. Moreover, the texture strength of 1.5Zn alloy was obviously weaker than that of 0Zn alloy. Dynamic precipitation and dynamic recrystallization were more likely to occur at grain boundaries. Simultaneously, the constitutive model, hot processing map and recrystallization kinetics model were constructed for Mg-8Gd-3Y-xZn-0.6Zr alloy.

Unexpected effects on creep resistance of an extruded Mg-Bi alloy by Zn and Ca co-addition: Experimental studies and first-principles calculations

In the present work, a new Mg-Bi based alloy is developed by the addition of Zn and Ca in equivalent atom fraction with Bi. Mg-Bi and Mg-Bi-Zn-Ca alloys were prepared by extrusion at a ram speed of 20 mm/s. Room temperature mechanical properties and creep behaviors at 423 K were investigated. The results show that Zn and Ca co-addition shows little influence on average grain size and texture intensity but changes the dispersive Mg3Bi2 into Mg2Bi2Ca particles in different sizes and a lower density. Twinning is largely activated during room-temperature deformation. Consequently, a slightly decreased proof strength but tripled elongation is shown at room temperature. Unexpectedly, large enhancement in creep resistance is detected after the co-alloying of Zn and Ca and the minimum creep rate is reduced by 10 to 20 times in the BZX621 alloy. Stress exponent n = 4–5 indicates that the creep is a dislocation-climb controlled type. Post-mortem characterization on microstructure shows slip of dislocation 〈c + a〉 are also largely found in B6 as well as BZX621 alloy and cross-slip is detected more severe in B6 alloy. Dynamic segregation and precipitation are also seen in both alloys. Bi-clusters are seen dispersive across the grains in B6 and so did the PFZs that could undermine creep resistance at the grain boundaries. By contrast, Zn-rich needle-like precipitates are developed at most “ends” of 〈c + a〉 dislocations, which would hinder the further dislocation motions and thus improve the creep resistance. First-principles calculations were adopted and the results show that the thermal stability and thermomechanical properties of Mg2Bi2Ca are much better than that of Mg3Bi2. Stacking faults energy is lowered down with the co-addition of Ca and Zn, which could inhibit the rate of dislocation climb and cross-slip. As a result, the improved creep resistance is obtained in the Mg-Bi-Zn-Ca alloys. Microstructural and controlling mechanism changes by thermal activation result in the unexpected enhancement in creep resistance with decreased room-temperature proof strength after co-addition. These findings could contribute to the development and optimization of creep-resistant Mg alloys in the future.

Effect of Si addition on the microstructure and creep properties of the forged titanium alloy

Creep experiments were carried out on pure Ti after annealed (720 °C/2 h, AC), Ti-0.4Si alloy after annealed (720 °C/2 h, AC) and thermal exposed (720 °C/2 h, AC+600 °C/500 h, AC) respectively. The presence mode of Si in Ti-0.4Si alloy after two heat treatments is partial precipitation and complete precipitation respectively. Combined with SEM and TEM characterization, the effects of Si addition on the creep properties and creep mechanism and the dominant roles of solid solution Si and silicide in improving the creep resistance of titanium alloys were investigated. The results show that the annealed Ti-0.4Si alloy has the best creep resistance, indicating that the addition of Si significantly improves the creep resistance of titanium alloys, and the Si has a creep strengthening effect whether it exists in solid solution state or in precipitated state. The high creep activation energy (Q = 253 kJ/mol) of the annealed Ti-0.4Si alloy is due to the strong inhibition of solid solution Si and dynamically precipitated silicide on matrix diffusion, indicating that the precipitation strengthening effect of dynamically precipitated silicide during creep is significantly stronger than that of precipitated silicide before creep, and obviously compensates the diffusion effect of dynamic precipitates during the precipitation process.

Dynamic evolution of the T1 phase and its effect on continuous dynamic recrystallization in Al–Cu–Li alloys

The T1 phase is the highest density and most prominent strengthening effect precipitate in Al–Cu–Li alloys, and it undergoes a complex dynamic evolution during hot deformation, which has significant effects on microstructure development and hot working. This study comprehensively characterizes and analyzes the dynamic evolution of the T1 phase at 400 °C/0.01 s−1 and its influence on continuous dynamic recrystallization (CDRX). The results indicate that the T1 phase successively undergoes coarsening, fracture, dissolution, dynamic precipitation, recoarsening, and spheroidization during deformation. A shear-coupled diffusion mechanism is proposed to explain the ultrafast coarsening rate of the T1 phase in early deformation. During the dynamic precipitation of the T1 phase, an anomalous inhomogeneous size and spatial distribution of the T1 phase are observed, and a high number density of fine T1 phases form in the matrix (with a denser, thinner and shorter size at the dislocation wall). The dynamically precipitated T1 phase is affected by the octahedral slip system during the coarsening process and exhibits a selective ripening phenomenon. The T1 phase formed by aging increases the inhomogeneity of the deformation and induces many substructures intragranularly, decreasing the percentage of CDRX grains but increasing the CDRX potential. Conversely, the dynamically precipitated T1 phase and its evolution accelerate CDRX development by promoting the transformation of LAGBs to HAGBs. In addition, the effects of dynamically evolving precipitates on the dislocations and formation modes of LAGBs at various deformation stages are elucidated. The results can provide valuable insights into the regulation of microstructure, and the development of high-performance Al–Cu–Li alloys, and also offer a theoretical and experimental basis for microstructure modeling.