Coarsening Mechanism Research Articles

Pore coarsening mechanism during hot rolling induced by hydrogen gathering in Al-Mg alloy

A novel mechanism for pore coarsening during hot rolling was proposed that the hydrogen in the ingot, driven by non-uniform hydrostatic pressure, accumulates into the tortuous shrinkage cavities at the thickness center of the slab. High pressure hydrogen gas gradually changes the stomatal curvature and finally leads to pore coarsening.

Powder coarsening mechanisms of Ti-6Al-4 V with multiple build cycles in laser powder bed fusion

Laser powder bed fusion (LPBF) is an additive manufacturing process which can produce complex 3D parts from digital models. The performance of parts fabricated by LPBF is largely dependent on the characteristics of the powder feedstock, in particular, the particle size distribution. The coarsening of powder particles may limit the potential for reusing powder in further builds, as consistency in powder quality is crucial for ensuring consistent parts performance when using reused powder, especially in aerospace applications. However, there is a lack of systematic understanding regarding the causes and nature of powder coarsening in LPBF. In this work, the characteristics of powder samples from different locations in the build chamber during LPBF were studied to understand the particle size evolution and determine the origin of coarsening, which has not been previously reported. Meanwhile, powder coarsening was found to have a detrimental effect on the relative density and surface quality of as-built parts, highlighting the importance of exploring the mechanisms of powder coarsening and finding ways to control it in LPBF. The relationship between powder in key locations in the build chamber and its effect on powder coarsening has been established. Layer thickness is identified as a critical factor in causing powder coarsening due to the fine powder size characteristic in the powder bed. Spatter, in its various forms, plays a direct or indirect role in powder coarsening. Sintered powders resulting from spatter and the laser scanning borders of as-built parts were observed to contribute to the powder coarsening. Therefore, three main mechanisms (layer thickness, spatter, sintered powder) associated with the powder coarsening are therefore proposed. This work provides insight and potential solutions to control powder coarsening and maintain consistent parts performance.

Revealing the coarsening behavior of precipitates and its effect on the thermal stability in Tʹ and ηʹ dual-phase strengthened Al-Zn-Mg-Cu alloys

High-strength Al-Zn-Mg-Cu alloys are widely utilized, but their strength deteriorates as strengthening precipitates coarsen rapidly at elevated temperatures, limiting their applications above 150°C. This study systematically investigates the microstructure evolution and its impact on the properties of peak-aged Al-Zn-Mg-Cu alloys with varying Zn/Mg ratios during thermal exposure at a series of temperatures from 150 to 300°C for 500 h. The results reveal that alloys A1 and A2 with an optimal Zn/Mg ratio (1.50−2.14) and relatively lower (Zn + Mg) content (7.0−8.8 wt.%), exhibit superior heat resistance properties compared to the other three alloys. Despite having lower strength relative to alloys with higher solute content, peak-aged alloys A1 and A2 retain the highest strength after thermal exposure. This performance is attributed to the high proportion (over 80%) of T′/T phases in the precipitates for alloys A1 and A2, which demonstrate better thermal stability in comparison to η′/η phases. Additionally, the lower solute content reduces the driving force for diffusion of Zn and Mg atoms, thus inhibiting the coarsening of precipitates. Moreover, the study elucidates that the coarsening mechanism of precipitates transitions from interfacial diffusion control at 150°C to matrix diffusion control at 200−300°C. These insights into the composition-dependent coarsening behavior of precipitates in dual-phase strengthened Al-Zn-Mg-Cu alloys offer valuable guidance for designing heat-resistant aluminum alloys with enhanced performance at elevated temperatures.

Binary phase separation in strongly coupled plasma

We investigated the two-dimensional binary phase separation process of plasma species using classical molecular dynamics in the strongly coupled regime. Both the plasma species interact via a pairwise screened Coulomb (Debye–Hückel) potential; however, the screening parameter κ is different for like- and unlike-species and is the cause for phase separation. We characterize the separation process by measuring the domain growth of equilibrium phases as a function of time—generally, the more significant the inhomogeneity in pairwise interaction, the faster the domain growth. Typically, the domain growth follows a power law in time with an exponent β characterizing the underlying coarsening mechanism. We demonstrate that the growth law exponent is β=1/2 for equal-number-density mixtures and 1/3 otherwise. Further, by comparing these with the corresponding growth laws in binary mixtures of viscous fluids, we show that the viscoelastic nature of plasma fluid modifies the coarsening dynamics, which in turn leads to the observed growth law exponents, notably in the unequal-number-density case.

Phase Transformations after Heat Treating an As-Cast Fe-30Mn-8.8Al-0.3Si-0.15C Steel

The phase transformations in an as-cast Fe-30Mn-8.8Al-0.3Si-0.15C steel were analyzed experimentally and numerically with a Calphad-based method during heat treatment. The nonequilibrium phases were determined using the Thermo-Calc Scheil module and the equilibrium phases with Themo-Calc based on the steel chemical composition. The precipitated phases were analyzed with TC-PRISMA using the chemical composition, nucleation site, and temperature among other factors. An ingot of this chemical composition was vacuum-melted using pure elements under an Ar gas atmosphere. As-cast steel specimens were annealed and solution-treated, quenched, and then aged at different temperatures. Heat-treated specimens were analyzed by different techniques. The results indicated that the microconstituents are the α and γ phases for the as-cast, homogenized, and quenched conditions. The main difference among these conditions is the distribution and size of the γ phase, which produced a change in hardness from 209 to 259 VHN. In contrast, the aging treatment at 750 °C caused a decrease in hardness from 492 to 306 VHN, which is attributable to the increase in volume fraction of the γ phase. On the other hand, the aging treatment at 550 °C promoted precipitation hardening from 259 to 649 VHN because of the κ precipitate formation. The calculated results for the different heat treatments with the Calphad-based method agreed well with the experimental ones. In addition, the intragranular precipitation of the κ phase could be simulated using the nucleation and growth and coarsening mechanisms based on a Calphad method.

Mechanical degradation behavior and γ′ coarsening mechanism of a Ni-based superalloy during long-term high-temperature thermal exposure

Aero engine design necessitates crucial components, such as turbine discs, to be highly stable in high-temperature environments. Therefore, the accurately determining of the mechanical properties and microstructure evolution of these components under long-term high-temperature exposure is ofs significant engineering and scientific value. Herein, we systematically analyzed the microstructure and performance evolution of a Ni-based superalloy during long-term high-temperature exposure, explored the relationship between service conditions and yield strength, and introduced novel strength decay factors, namely temperature (|α|) and time (|β|) decay factors. |α| was strongly positively correlated with the exposure temperature, whereas |β| exhibited a more complex behavior and was more affected by the microstructure before thermal exposure, especially at low temperatures (650–700 °C). Microstructural analysis revealed minimal changes in average grain size and Σ3 twin boundaries following long-term thermal exposure. However, γ′ particles coarsened after thermal exposure, particularly at higher temperatures and longer exposure times. The coarsening process under different conditions was examined using Lifshitz–Slyozov–Wagner and trans-interface diffusion-controlled models. The coarsening of γ′ particles led to mechanical degradation, as the critical failure stress decreased with increasing γ′-particle size.

Experimental and numerical study on the dynamic mode III fracture behaviors of rock using an axially notched flattened Brazilian disc in SHPB tests

The dynamic mode I and mode II fracture of rock have been widely investigated over the past decades. However, the lack of dynamic mode III fracture testing method and the rare understanding of mode III fracture mechanism of rock remain a significant knowledge gap in the realm of rock mechanics. In this work, an innovative axially notched flattened Brazilian disc (ANFBD) sample is proposed for the mode III fracture tests of rocks using the split Hopkinson pressure bar (SHPB). Firstly, the optimal geometrical dimension of the ANFBD sample for achieving true mode III fracture of rocks is comprehensively discussed using finite element method (FEM) analysis. Then, the reliability of ANFBD-SHPB method is experimentally verified through dynamic mode III fracture experiments on the Fangshan marble (FM). Laboratory experimental results show that the mode III fracture toughness (KIIId) values of FM are approximately 2.8 times of the mode I fracture toughness (KId) values at similar dynamic loading rates. Post-mortem examinations on the fracture surfaces show that the famous facet coarsening mechanism in mode III fracture is suppressed under high loading rates. Furthermore, insightful assessments regarding the inner progressive fracturing behaviors of the ANFBD sample are given by virtue of three-dimensional discrete element method (DEM) simulation. Moment tensor (MT) analysis indicates that the shear fracture mechanism mainly dominates the whole failure process of the ANFBD sample, while the tensile fracture mechanism slightly increases during the facet coarsening process. These results are of significant importance for revealing the failure mechanism of rock under dynamic mode III loading and give some beneficial implications for natural strike-slip fault structures.

Research on microstructure and mechanical properties of Ti–6Al–4V ELI fabricated by hybrid directed energy deposition with in-situ rolling and annealing

To achieve superior damage tolerance and fatigue properties, it is desirable for the microstructure of Ti-6Al-4V ELI (Extra Low Interstitial) titanium alloy to consist of equiaxed grains with a homogeneous lamellar structure. However, traditional forging and single additive manufacturing techniques require extreme processes such as quasi β heat treatment and hot isostatic pressing. In this paper, a hybrid directed energy deposition method that integrates in-situ rolling with simple annealing is proposed to simultaneously achieve these microstructures. The results indicate that the initial microstructures produced by hybrid directed energy deposition consist of fine equiaxed prior-β grains, inconspicuous interpass bright bands, randomly oriented lamellar structure and numerous small substructures at the TEM scale. Due to the plastic deformation caused by in-situ rolling, the primary β grains underwent significant refinement with a reduction in grain diameter from over 2000 μm to 113.8 μm. Besides, the unique microstructures and thickness of α laths are influenced by coarsening and dissolution mechanisms that are controlled by diffusion during phase transformation. Furthermore, a uniform microstructure without bright bands and excellent comprehensive properties can be achieved through heating at 880 °C for 2 h and natural cooling. These findings validate that hybrid directed energy deposition coupled with simple annealing present a novel and cost-effective approach for the direct manufacturing of uniform titanium alloy forgings.

Precipitation evolution of Al-Zn-Mg-Cu-(Ag) alloys with a low Zn/Mg ratio

The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu (wt.%) alloy from early stages to over-aged stages at 150 °C was investigated. Surprisingly, co-precipitation of strengthening phases T′ and η′ are found in both Ag-free and Ag-added alloys. With Ag addition, precipitation of both T′ and η′ is refined and increased, such that the age-hardening capabilities and peak-aged tensile strength are improved. In addition, the quantitative proportion of η′ precipitates increases with the increase of Ag content due to the increase in the (Zn+Cu)/Mg ratio of nucleating particles. The narrowed precipitate-free zones (PFZs) are considered responsible for the undiminished fracture elongation in Ag-added alloys. Essentially, these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions. In over-aged stages, the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy, which is related to precipitate coarsening mechanisms. The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion, while the Ag-added alloy follows two possible coarsening mechanisms, coalescence of aggregates and diffusion of atoms. The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing. Meanwhile, the transformation of metastable phases to stable phases is inhibited due to the addition of Ag, such that GP zones, T′, η′, η and T phases coexist even after 14 d of ageing. In terms of phase composition, the addition of Ag decreases the ratio of Mg/(Al+Zn) in T-type phase. For the Ag-added alloy, the sum concentration of Zn+Mg in η′ phase is about 10 at.% higher compared to T′ phase, and η phase continues to have a high sum concentration of Zn+Mg, besides, the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.

Structural degeneracy and formation of crystallographic domains in epitaxial LaFeO3 films revealed by machine-learning assisted 4D-STEM

Structural domains and domain walls, inherent in single crystalline perovskite oxides, can significantly influence the properties of the material and therefore must be considered as a vital part of the design of the epitaxial oxide thin films. We employ 4D-STEM combined with machine learning (ML) to comprehensively characterize domain structures at both high spatial resolution and over a significant spatial extent. Using orthorhombic LaFeO3 as a model system, we explore the application of unsupervised and supervised ML in domain mapping, which demonstrates robustness against experiment uncertainties. The results reveal the consequential formation of multiple domains due to the structural degeneracy when LaFeO3 film is grown on cubic SrTiO3. In situ annealing of the film shows the mechanism of domain coarsening that potentially links to phase transition of LaFeO3 at high temperatures. Moreover, synthesis of LaFeO3 on DyScO3 illustrates that a less symmetric orthorhombic substrate inhibits the formation of domain walls, thereby contributing to the mitigation of structural degeneracy. High fidelity of our approach also highlights the potential for the domain mapping of other complicated materials and thin films.

Novel Ni–Co-based superalloys with high thermal stability and specific yield stress discovered by directed energy deposition

We report on the rapid alloy screening of Ni–Co–Ti–Al–Mo superalloys with high thermal stability and specific yield stress by means of directed energy deposition. A laser directed energy deposition, a specific type of additive manufacturing, was employed using multiple powder feeders and elemental powders. Fifty superalloys of different compositions were deposited and the heat-treated microstructure and γ' solvus temperature were examined. The 43Ni–38Co–9Ti–6Al–4Mo superalloy (atomic percent composition) exhibited a uniform γ/γ' microstructure and a γ' solvus temperature of 1202°C. The beneficial properties of the superalloy were also found in the cast superalloy of identical composition. The cast superalloy exhibited a thermally stable γ/γ' microstructure with cuboidal γ' precipitates even after long-term aging heat treatments at 800°C, 900°C, and 1000°C up to 500 h. The γ'-coarsening mechanism was evaluated based on the Lifshitz–Slyozov–Wagner model and the trans-interface diffusion-controlled model. A transition between these two mechanisms was observed with an increase in aging temperature. Atom probe tomography analyses revealed that the sluggish interface diffusion of Co and corresponding reduction of the interfacial energy induced the transition of the γ'-coarsening mechanism. Moreover, the cast superalloy showed an enhanced specific yield stress attributed to its exceptionally low alloy density of 7.61 g/cm3.

Coarsening mechanism of over-aged δ-Ni2Si nanoscale precipitates in Cu-Ni-Si-Cr-Mg alloy

δ-Ni2Si precipitate is the crucial strengthening phase in Cu-Ni-Si alloy, and its coarsening behavior and mechanism impose dramatic effects on mechanical properties. In this paper, Cu-3Ni-0.7Si-0.1Cr-0.05 Mg (wt%) alloy was prepared to systematically investigate the coarsening behavior and elucidate the coarsening mechanism of δ-Ni2Si precipitate during over-aging treatment. Featured core-shell structure was observed via careful characterization during coarsening of δ-Ni2Si phase. Specifically, the core structure was stable δ-Ni2Si, while the shell structure was a metastable interface composed of Cu, Ni and Si elements. At later over-aging stage, the core-shell structure completely transformed into a stable and coarsened δ-Ni2Si phase being coherent with the Cu matrix. It is confirmed for the first time that the coarsening process of δ-Ni2Si precipitate can be interpreted by the trans-interface-diffusion-controlled (TIDC) coarsening theory. The core-shell structure was caused by the disparity between the diffusion activation energy and diffusion rate of Ni, Si, and Cu from the perspective of diffusion kinetics. Moreover, the presence of trace amounts of Cr and Mg components exhibited little effect on the coarsening process of δ-Ni2Si. During over-aging, tensile strength dramatically decreased owing to diminishing precipitation strengthening effect from coarsening of δ-Ni2Si precipitates.

Investigation on the effects of isothermal holding temperature and time on the coarsening mechanism and rheological properties of ADC12 Al semi-solid slurry

The current work's key emphasis is on investigating the influence of isothermal holding temperature and time on the microstructural development and coarsening rate during the directly generated semi-solid slurry of ADC12 Al alloy from the cooling slope process. Additionally, a thorough investigation is conducted to optimize process variables in the rheo-pressure die-casting process to determine the correlation between the non-Newtonian flow behaviour and its morphological characteristics of the quenched slurry of ADC12 Al alloy. The obtained results inferred that with increasing isothermal holding time, average grain size marginally increases initially, then significantly increases with increasing time, while shape factor increases first and then declines. Furthermore, when isothermal temperatures increase, the average grain size rises, while the shape factor rises first before decreasing. As a result of the increase in grain size due to elevated processing temperature and prolonged processing time, the aluminium alloy's hardness deteriorated. The ideal condition for more spheroidization of α-Al in the synthesized semi-solid slurry of ADC12 aluminium alloys was at an isothermal temperature of 565 °C and a holding time of 7 min. Under ideal processing conditions, the alloy's grain size, shape factor, and hardness were 31 μm, 0.89, and 62.32 HV, respectively. The semi-solids of ADC12 Al alloy have a low coarsening coefficient. The coarsening rate constant at 565 °C was calculated using the Lifshitz-Slyozov-Wagner relationship, and a value of 37 μm3/s was discovered. The findings of the isothermal rheology experiment demonstrate that viscosity decreases as the shear rate increases, confirming the existence of shear-thinning actions in semi-solid slurries.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of the Al0.6CoCrFeNi High-Entropy Alloy.

The effect of heat treatment on the microstructure and tensile properties of an as-cast Al0.6CoCrFeNi high-entropy alloy (HEA) was investigated in this paper. The results show that the as-cast Al0.6CoCrFeNi HEA presents a typical FCC dendrite morphology with the interdendritic region consisting of BCC/B2 structure and heat treatment can strongly affect the microstructure and mechanical properties of HEA. Microstructure analysis revealed the precipitation of a nano-sized L12 phase in the FCC dendrite and the formation of the FCC and σ phases in the interdendritic region after annealing at 700 °C. The coarse B2 phase was directly precipitated from the FCC dendrite in the 900 °C-annealed sample, with the coexistence of the B2, FCC, and σ phases in the interdendritic region. Then, the interdendritic region converted to a B2 and FCC dual-phase structure caused by the re-decomposition of the σ phase after annealing at 1100 °C. The tensile test results show that the 700 °C-annealed HEA presents the most significant strengthening effect, with increments of corresponding yield strength being about 107%, which can be attributed to the numerous nano-sized L12 precipitates in the FCC dendrite. The mechanical properties of 1100 °C-annealed alloy revert to a level close to that of the as-cast alloy, which can be attributed to the coarsening mechanism of B2 precipitates and the formation of a soft FCC phase in the interdendritic region. The observed variation in mechanical properties during heat treatment follows the traditional trade-off relationship between strength and plasticity.

Microstructure evolution and coarsening behavior of precipitated O phase in Ti–22Al–24Nb-0.5Mo (at. %) alloy during the heat treatment

Microstructure evolution and coarsening behavior of precipitated O phase were systematically investigated in this study, and the corresponding microstructures were characterized by scanning electron microscopy and transmission electron microscopy. The results showed that the original microstructure was composed of lath O phase, α2 phase and B2 matrix. The precipitated O phase presented different morphologies and sizes during heat treatment. With the increase of solution temperature and holding time, the O phase occurred coarsening, with a thickness ranging from 0.73 increased to 2.64 μm. But the volume fraction decreased from 57.65 to 18.40 %. During aging treatment, large amounts of fine secondary acicular O phase have precipitated from the matrix. As the aging temperature increases, the O phase underwent coarsening and the thickness increases from 0.70 to 1.40 μm. However, the aging temperature has little effect on the precipitation of acicular O phase, and the volume fraction decreases from 37.66 to 33.32 %. Research found that the small-sized O phase gradually dissolved in the matrix, while the large-sized O phase grew. This phenomenon can be explained by Ostwald ripening mechanism. The TEM analysis revealed that the O phase occurred separation, mainly due to the diffusion of solute atoms and the embedding of B2 matrix. Meanwhile, the O phase had been completed and segmented. The coarsening mechanism of the O phase was also explained by the boundary splitting mechanism and terminal migration mechanism.

Nitriding layer characteristics of modified AISI422 martensitic stainless steel in tempered state

In this study, the nitriding layer of the modified AISI422 martensitic stainless steel was characterized by several method, including optical microscopy (OM), micro-hardness tester, X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy (TEM), to study the phase transformation at the whole nitriding layer during the plasma nitriding. In order to investigate the evolution of the nitriding process the TEM samples of the nitriding layer has been prepared from the white layer to the un-nitriding zone by the focus ion beam method. With nearly 20 h nitriding process, the white layer, vein-like structure and the diffusion zone was clearly observed though the OM. Together, the TEM results show that the γ′-Fe4N and the micro holes existed in the white layer and the thick vein-like structure. And the morphology of the continuous precipitation of CrN was highly related with the depth of the nitriding layer. Excepted the normal coarsening mechanism, the EDS results indicate that the discontinuous precipitation of CrN may also originate from the transformation of the M23C6(M=Cr) during the nitriding process. Meanwhile, the residual stress value is not directly proportionate to the nitriding depth or the N contents. Furthermore, it could be affected by the N contents and the degree of the CrN spinodal decomposition, and these two factors lead the maximum value of the compress stress appears at the depth of 120μm.

Pore-level Ostwald ripening of CO2 foams at reservoir pressure

The success of foam to reduce CO2 mobility in CO2 enhanced oil recovery and CO2 storage operations depends on foam stability in the reservoir. Foams are thermodynamically unstable, and factors such as surfactant adsorption, the presence of oil, and harsh reservoir conditions can cause the foam to destabilize. Pore-level foam coarsening and anti-coarsening mechanisms are not, however, fully understood and characterized at reservoir pressure. Using lab-on-a-chip technology, we probe dense (liquid) phase CO2 foam stability and the impact of Ostwald ripening at 100 bars using dynamic pore-scale observations. Three types of pore-level coarsening were observed: (1) large bubbles growing at the expense of small bubbles, at high aqueous phase saturations, unrestricted by the grains; (2) large bubbles growing at the expense of small bubbles, at low aqueous phase saturation, restricted by the grains; and (3) equilibration of plateau borders. Type 3 coarsening led to stable CO2 foam states eight times faster than type 2 and ten times faster than type 1. Anti-coarsening where CO2 diffused from a large bubble to a small bubble was also observed. The experimental results also compared stabilities of CO2 foam generated with hybrid nanoparticle–surfactant solution to CO2 foam stabilized by only surfactant or nanoparticles. Doubling the surfactant concentration from 2500 to 5000 ppm and adding 1500 ppm of nanoparticles to the 2500 ppm surfactant-based solution resulted in stronger foam, which resisted Ostwald ripening. Dynamic pore-scale observations of dense phase CO2 foam revealed gas diffusion from small, high-curvature bubbles to large, low-curvature bubbles and that the overall curvature of the bubbles decreased with time. Overall, this study provides in situ quantification of CO2 foam strength and stability dynamics at high-pressure conditions.Article HighlightsA comprehensive laboratory investigation of CO2 foam stability and the impact of Ostwald ripening.Pore-level foam coarsening and anti-coarsening mechanisms insights.

Densification kinetics and microstructural evolution of binder jet printed and sintered porous Ni-Mn-Ga magnetic shape-memory alloys

Binder Jet Printing (BJP) of polycrystalline magnetic shape-memory alloys (MSMAs) allows to create complex geometries from pre-alloyed powders, while avoiding elemental evaporation and internal stresses. However, to increase functionality, polycrystalline MSMAs require high porosity and large grains to decrease internal constraints to twin boundary motion. This study focuses on the microstructural evolution of BJPed porous Ni-Mn-Ga during isothermal sintering to understand the densification mechanisms taking place at different temperatures (1070, 1080 and 1090 ℃) over time (0–8 h) in order to tailor the microstructure to regimes of large, interconnected porosity and large grains. Stereology was used to determine the sintering mechanisms as defined by Coble's model for intermediate stage sintering. Densification and grain size increased over time for all temperatures at different rates, while pore size decreased to a minimum near 1 h sintering. Sintering at 1070 ℃ resulted in an interplay of densifying and coarsening mechanisms, while at 1080 ℃ the dominating mechanism was volume diffusion and at 1090 ℃ grain boundary diffusion. No changes in electron concentration occurred during sintering, and thus, transformation temperatures and magnetization remained nearly constant. Transformation enthalpy and Gibbs free energy showed an inverse behavior to pore size, indicating larger pores reduce transformation energy.

Dynamic competition between coarsening mechanisms of Ti2Cu phase during thermal cycling

The dynamic competition between the coarsening mechanisms activates various coarsening behavior of the precipitate in multi-phase metallic materials, leading to different mechanical performances of alloys. In this work, the morphological evolution and coarsening kinetic of Ti2Cu phase in Ti–14Cu alloy were investigated comprehensively during thermal cycling at 200–600 °C. Our results suggest that the coarsening kinetic showed a gradual transition from the Lifshitz-Slyozov-Wagner (LSW) mechanism to the trans-interface diffusion-controlled (TIDC) mechanism. Ti2Cu phases precipitated at the triple junctions of α-Ti matrix by discontinuous precipitation under the control of LSW mechanism at lower temperatures (200–400 °C), and grew into lamellae along the high-energy grain boundaries. While the low-angle grain boundaries within Ti2Cu phase accelerated solute atom diffusion during the coarsening governed by TIDC mechanism at higher temperatures (400–600 °C), which leads to the boundary splitting of lamellae. In addition, it was also found that the blocky Ti2Cu phase improved the yield strength of the alloy as its high solid solution strengthening. This work provides a clear understanding for the coarsening of Ti2Cu phase, it could provide theoretical evidence for the hot working process design to improve the damage tolerance of Ti–Cu alloys.

Microstructural evolution and coarsening behavior of the precipitates in 2205 duplex stainless steel aged at 850 °C

The formation of the secondary phases in the 2205 duplex stainless steel (DSS) influences greatly its mechanical properties. Studies on the microstructural evolution and coarsening behavior of the precipitates in the 2205 DSS have both scientific and technological significance. In the present work, the evolution of the composition and morphology of the precipitates in the 2205 DSS aged at 850 °C for periods up to 200 h are systematically investigated by SEM/EDS (Scanning Electron Microscope/Energy Dispersive Spectrometer) and TEM (Transmission Electron Microscope). In addition, the size statistics for intermetallic precipitates, especially for the σ phase, are carried out based on the SEM images by using deep learning and digital image processing technique. Based on the above, the average interphase energy between the σ and γ phases is reasonably estimated according to the Ostwald coarsening mechanism. This work provides a comprehensive understanding of the microstructural evolution and coarsening behavior of the precipitates in 2205 DSS.