Precipitation Of Secondary Phases Research Articles

Phase field modeling of precipitation in Zr-1%Nb-1%Sn alloy: a study of statistical properties and mechanical response

We develop the phase field model to simulate precipitation of secondary phase in ternary alloys with extra-small content of doping. This approach is applied to study β-phase precipitation in the model system of commercial alloy Zr-Nb-Sn at thermal treatment. An analysis of local rearrangement of doping and equilibrium vacancies during precipitation has shown that the dissolved Tin is mostly segregated around phase interface by trapping vacancies. Kinetics of precipitation, size and distribution of the precipitates, concentration of the species in precipitates and matrix are studied where it is revealed that Lifshits-Slyozov-Wagner distribution can be used to predicate statistical properties of precipitates. Mechanical response including the plastic deformations in precipitated solid is discussed. It is shown that yield strength change increases during precipitation. Yield and ultimate stresses are studied at different shear rates for the annealed alloy. A transition to plastic flow is described by means of dislocation structure evolution. Formation and growth of slip planes and dislocation loop-precipitate interaction governed by elastic moduli difference is analyzed. It is shown that emergence of dislocation loops around precipitates follows the Orowan mechanism.

Tailoring Microstructure of Austenitic Stainless Steel with Improved Performance for Generation-IV Fast Reactor Application: A Review

Austenitic stainless steels are selected as candidate materials for in-core and out-of-core components of Generation-IV fast reactors due to their excellent operating experience in light-water reactors over several decades. However, the performance of conventional austenitic stainless steels proves to be inadequate through operation feedback in fast reactors. To withstand the demands for material performance exposure to the extreme operating environment of fast reactors, modified austenitic stainless steels for in-core and out-of-core components have been developed from the first-generation 300-series steels. The design of an appropriate microstructure becomes a top priority for improving material performance, and key metallurgical features including δ-ferrite content, grain size and secondary phase precipitation pertinent to austenitic stainless steel are focused on in this paper. δ-ferrite content and grain size are closely correlated with the fabrication program and their effects on mechanical properties, especially creep and fatigue properties are critically assessed. Moreover, the impacts of some major elements including nitrogen, stabilization elements (Nb, Ti, V), phosphorus and boron on secondary phase precipitation behaviors during aging or creep are reviewed in detail. Based on the role of the aforementioned metallurgical features, the recommended specification of nitrogen content, stabilization ratio, phosphorus content, boron content, δ-ferrite content and grain size are put forward to guarantee the best-expected performance, which could provide reactors designers with attractive options to optimize fast reactor systems.

Investigation of Microstructures and Mechanical Properties of Ultra-High Strength Al-Zn-Mg-Cu Alloy Prepared by Rapid Solidification and Hot Extrusion

Al-Zn-Mg-Cu aluminum alloys have the advantages of high specific strength, easy processing, and high toughness, showing great potential application in the aerospace field. However, ultra-high strength aluminum alloys usually contain coarse microstructures, micro-segregation, and casting defects that seriously deteriorate mechanical properties. Here, we report a high-strength aluminum alloy (Al-10.5Zn-2.0Mg-1.2Cu-0.12Zr-0.1Er) prepared by rapid solidification and hot extrusion to explore the microstructure modification of the alloy based on this strategy. The results show that: rapid-solidification technology can significantly refine alloy grains, alloy ribbons were composed of α (Al) equiaxed fine grains, and the average grain size was less than 6 μm. After extrusion, the alloy had partially recrystallized, existing coarse second-phase (T-phase) and needle-shaped precipitates were MgZn2 (η-phase), and the tensile strength and elongation of the extruded bar were 466.4 MPa and 12.9%, respectively. After T6 heat treatment, the tensile strength of the alloy reached 635.8 MPa, while elongation decreased to 10.5%. According to microstructure analysis and considering the contributions of grain boundary, dislocation, and precipitation-strengthening to the improvement of the mechanical properties, it was found that precipitation-strengthening is the main strengthening mechanism. Our research shows that rapid-solidification and hot-extrusion technology have great potential for improving the microstructures and mechanical properties of aluminum alloys.

In situ synchrotron study of sintering of gas-atomized Ti-6Al-4 V powders using concomitant micro-tomography and X-ray diffraction: Effect of particle size and interstitials on densification and phase transformation kinetics

Compared to the well-established powder bed fusion techniques, sinter-based additive manufacturing of titanium alloys remains extremely challenging. This technique involves three steps: (i) shaping of a part composed of metallic powders bound with polymeric binder (ii) debinding (iii) sintering. One main issue is that densification during the solid sintering is promoted by small powder particles whereas the latter have a high propensity to carbon and oxygen uptakes from the binder, which are detrimental to ductility of titanium parts. In this article, we report a unique in-depth characterization of solid sintering of titanium powders using in situ coupled micro-computed tomography (µCT) and X-Ray diffraction under synchrotron radiation at high temperature, combined with in situ environmental scanning electron microscopy (HT-eSEM). Evolution of global porosity, pore size distribution and interconnectivity as well as allotropic titanium phase transformation and precipitation of second-phase precipitates (titanium carbides) were determined, allowing a discussion on the densification/phase transformation relationship. This multi-scale in situ study of solid sintering was used to identify the effect of powder particle size on the contamination / densification trade-off. Carbon/oxygen uptakes clearly increase the α-to-β transus but do not affect significantly the final porosity of the sintered parts, which argues for a secondary role of the β phase on the sintering kinetics of titanium alloys. Reducing the powder particle size has a tremendous effect on both the densification kinetics and the final pores structure.

High-performance reality of 55SiCrV spring steel processed by the rapid-induction heat treatment

Heat treatment methodology is vital to achieve high performance of advanced spring steel. The present study has investigated the microstructure and mechanical properties of the 55SiCrV spring steel processed by a novel online rapid-induction heat treatment. The results showed that rapid heating refines the prior austenite grain size of the 55SiCrV spring steel, and the vanadium mainly exists as the solid solution in the matrix instead of the dispersive precipitation of the V-containing secondary phase under the rapid induction heat treatment condition. An optimal combination of mechanical properties simultaneously with ultrahigh strength and high plasticity was obtained and the properties of the steel are mainly enhanced by the hardenability and tempering resistance of solute V in the material.

Enhanced thermal stability and mechanical properties of the nanocrystalline multicomponent Fe–(ZrNbMoTa)x alloys

Thermal stability and mechanical property of the multicomponent nanocrystalline Fe–ZrNbMoTa alloys were investigated. Single-phase nanocrystalline Fe–(ZrNbMoTa)x (x = 0.1, 0.2, 0.5 and 1 at. %) alloys were prepared by ball milling and followed by annealing in a set of predesigned condition. It was found that the Fe-based multicomponent alloys possess good thermal stability through 10 h annealing at 900 °C, which is attributed to the thermodynamics effect in the reduction of grain boundary energy caused by multicomponent co-segregation of Zr, Nb, Mo and Ta, and kinetics effect related to sluggish diffusion and Zener pinning of a small amount of second-phase precipitation. Additionally, nanocrystalline Fe–(ZrNbMoTa)x alloys show excellent mechanical properties. The hardness is 738.6 HV, and its yield strength and compressive strength reach up to 3100 MPa and 3600 MPa, respectively. These are attributed to the comprehensive effects of nanocrystalline, solid solution and second phase strengthening.

The Superstrength of Nanostructured Metallic Materials: Their Physical Nature and Hardening Mechanisms

Ultrafine grained (UFG) metallic materials obtained by severe plastic deformation (SPD) typically exhibit very high strength properties, whose values are much higher than those predicted by the well-known Hall–Petch relation. Our studies show that the basis for this to occur is that SPD not only forms the UFG structure, but also leads to the formation of other nanostructural features, such as dislocation substructures, nanotwins, and nanosized precipitates of second phases, which additionally contribute to strengthening of materials. At the same time, this analysis of hardening mechanisms indicates that the structure and condition of grain boundaries, namely, their nonequilibrium state and the presence of grain boundary segregations, also substantially contribute to hardening. Taking this into consideration, approaches are discussed to achieve very high strengths in metallic materials by SPD.

Characterization of Microsegregation and Secondary Phase Precipitation in Nickel-Based Superalloy after Wire-Feed Electron Beam Additive Manufacturing

Characterization of Microsegregation and Secondary Phase Precipitation in Nickel-Based Superalloy after Wire-Feed Electron Beam Additive Manufacturing

Characterization and Control of Secondary Phase Precipitation of Nb–V–Ti Microalloyed Steel during Continuous Casting Process

Characterization and Control of Secondary Phase Precipitation of Nb–V–Ti Microalloyed Steel during Continuous Casting Process

Corrosion-erosion behavior and mechanism of Cu[sbnd]Mo co-doped CoCrFeNi high-entropy alloy coating prepared by directed energy deposition

Developing cost-effective and high-performance high-entropy alloy (HEA) coatings on demand is imperative. This study designed a class of HEAs on-demand by synergistic doping of Cu and Mo elements based on the first-principles and multi-principal alloy design concept. A CoCrFeNiCu0.25Mo0.75 HEA coating with outstanding corrosion and slurry erosion wear performances was fabricated on the duplex stainless steel using the directed energy deposition technique. The corrosion mechanism of the HEA coating in 3.5 % NaCl and 0.5 M H2SO4 solutions and the slurry erosion wear mechanism under a slurry concentration of 5 wt% were analyzed in combination with the density functional theory. The results indicate that the SQS crystal model is a better predictor of phases and properties of HEA than the VCA model. The HEA coating is a cellular structure that consists mainly of a face-centered cubic phase structure and contains a small amount of homogeneous sigma nanocrystalline secondary phase precipitation. Meanwhile, the passive film of the HEA coating is mainly composed of metal oxides and hydroxides. Cu is not oxidized during the corrosion process and always exists in the metallic form, which serves as a barrier layer to prevent the transfer of point defects. Mo with a larger atomic size can induce lattice distortion, enhance the coating strength and hardness, and reduce the density of point defects. Further, the slurry erosion wear rate of the HEA coating is lower at a 30° impingement angle than 90° for brittle erosion mode. At the 30° impingement angle, micro-cutting and ploughing are the main wear mechanisms of the HEA coating, while at the 90° impingement angle, micro-indents and craters are the main wear mechanisms. In addition, the corrosion and slurry erosion wear performances of CoCrFeNiCu0.25Mo0.75 HEA coating are better than the most reported once-formed HEAs.

Revealing strengthening contribution of grain refinement and phase precipitation in CrMnFeCoNi high-entropy alloy prepared from different powders

Equiatomic CrMnFeCoNi high-entropy alloys (HEAs) were prepared by vacuum hot-pressing sintering with single phase precursor powders made from mechanical alloying (MA) or spray-drying (SD), respectively. MA process was found beneficial for the following precipitation and formation of secondary phases during sintering, after which multiphase bulk HEAs were formed consisting of FCC, M23C6, and precipitated nanoscale σ phases. The tensile and compressive yield strengths of bulk HEAs fabricated from MA powders reached 1150 and 1200 MPa, respectively, which were about three times than SD powders. The excellent mechanical properties of the former were found due to the precipitation of nanoscale σ phase and M23C6 intermetallic resulted from the MA treatment and sintering processing. Nanoscale σ and M23C6 phase can effectively strengthen the alloy not only by self-reinforcement but also through the grain boundaries of the refined grains induced by the precipitation.

Is there still something to eat for trees in the soils of the Strengbach catchment?

The global demand for wood is growing and the sustainability of forests has become an increasingly important issue. The first metre of soil represents the reserve of nutrients cations (K, Ca, Mg) for trees. The depletion of these elements in the soil gradually leads to a decrease in soil fertility, which can have a strong impact on the development and health of forests.The aim of this study was to better understand the source and dynamics of nutrients in forested soils from a base poor environment by combining mineralogic, chemical, isotopic and numerical tools through an experimental approach. We designed batch experiments to follow the leaching of soils from two experimental plots (under spruce and under beech from the Strengbach catchment, Vosges Mountain, NE France). The studied elements show highly contrasting behaviours. Mg release results from a mixing between cationic exchange, mineral dissolution (mainly smectite) and organic matter mineralization. The dynamics of K are underestimated by modelling, which may be attributed either to an unknown source of K or to an overestimation of the secondary phase precipitation. The Sr isotopic approach, used as a proxy to identify the sources of Ca, shows that the exchangeable complex supplies nutrients first and rapidly to the solution, followed by the clay compartment and at the very end, the bulk soils. Na is mainly controlled by albite dissolution.The release of Ca is significantly lower than that of other cations as Mg or K. Our results suggest that the exchangeable Ca, estimated through classical extractions, is only partly available for plants, due to strong chemical binding with organic matter and/or the occurrence of Ca-chemical bridges between clay and organic matter. Sustainable management should thus consider that exchangeable Ca, conventionally determined, does not necessarily provide an accurate picture of the real availability of Ca and therefore of the fertility of soils, with a risk of overestimation of nutrient reservoir for trees. Calcium tree nutrition at these sites has to rely on the long term on atmospheric deposition and an efficient recycling through biological cycling. This is also true for the other nutrient cations, but to a lesser extent since soil intrinsic fluxes (i.e., weathering, mineralisation of OM…) are not null and may thus supply the exchangeable pools and participate substantially to tree nutrition. Harvesting and biomass exportation could rapidly impact the biological cycle and threaten the sustainability of these ecosystems, similar to changing tree species that may influence the nature of the litter, its turnover and the input of nutrients to the soil.

Effect of hydrogen accumulation on θ' precipitates on the shear strength of Al-Cu alloys

We investigate the effect of accumulation of H atoms on θ' strengthening precipitates in Al-Cu alloy. A relationship is established between the H accumulation and decrease in shear strength of alloy depending on the H content in precipitates of second phase. We use the previously proposed multiscale approach combining molecular dynamics (MD), continuum modeling and discrete dislocation dynamics (DDD). H atoms are located inside precipitate and at interphase boundaries. Energetically preferable places of H accumulation near and inside θ' phase in Al-Cu alloy are revealed. It is shown that the H accumulation on θ' leads to a decrease in precipitate shear resistance and causes the change in the dislocation-precipitate interaction mechanism from the Orowan loop formation to the cutting of the inclusion. The H accumulation on precipitates causes an increase in the threshold stress for the dislocation nucleation on the inclusion. The mechanism of softening of inclusions associated with a change from looping to cutting is transferred to the DDD level. The DDD results demonstrate that H charging of Al-Cu alloy can substantially decrease its shear strength. For high strength alloys with small precipitate size and/or inter-precipitate distance and with the yield stress above about 460 MPa, the reduction of strength can reach 10–25%.

Structure, crystallographic texture and anisotropy of properties of VZh159 alloy and how they are influenced by regimes of selective laser melting and final heat treatment

This paper examines the effect of scanning strategy (unidirectional scan, island scan and a scan with a 60о turn of the laser beam between layers) during selective laser melting and heat treatment on the crystallographic texture and mechanical properties of VZh159 alloy specimens. Application of different scanning strategies leads to two types of crystallographic texture formed in specimens: the axial component <100> gets reinforced in the unidirectional strategy, while it is the texture component {100}<001> in the other cases. After printing, the specimens were subjected to different regimes of heat treatment. They included quenching from 1,100 oC; quenching from 1,100 oC and ageing at 800 and 700 oC; quenching from 1,100 oC and ageing at 900, 800, 700 and 650 oC. When heat treatment (ageing) tests are conducted at room temperature, it leads to increased yield point and strength as a result of secondary phase precipitation at grain boundary and in grain core, while plasticity drops. During high temperature tests, no strength gain was found after heat treatment. In all heat treatment regimes, the authors observed anisotropy of mechanical properties in the specimens caused by the crystallographic texture that remains after heat treatment. The maximum strength and yield point were detected at 45о load application to growth direction during printing. At the same time, the anisotropy of yield point does not change during high temperature mechanical testing (at 850 oC) and has no dependence on the amount of additional precipitates. It suggests that crystallographic texture plays a defining role for anisotropy of properties.The research was conducted under financial support of Russian Federation presented by the RF Ministry of Science and Higher Education (Agreement No. 075-15-2021-1352).

Effect of Secondary-Phase Precipitation on Mechanical Properties and Corrosion Resistance of 00Cr27Ni7Mo5N Hyper-Duplex Stainless Steel during Solution Treatment.

In this work, the effect of secondary-phase precipitation on the microstructure, mechanical properties, and corrosion resistance of 00Cr27Ni7Mo5N hyper-duplex stainless steel (HDSS) during solution treatment was investigated. The results reveal that σ-phase precipitates at the interface between the α and γ phase when the solution treatment temperature is lower than 1070 °C. It is not only brittle, but also prone to create a Cr-depleted zone, which significantly deteriorates the mechanical properties and corrosion resistance. With the increase in the solution treatment temperature, the volume fraction of ferrite gradually increases. The yield strength and tensile strength increase slightly, but the elongation decreases. At the same time, the impact toughness shows a trend of first increasing and then decreasing. When the solution treatment temperature is higher than 1130 °C, Cr2N precipitates in the ferrite. The precipitation of Cr2N causes a decrease in the plastic toughness, but it does not deteriorate the mechanical properties as significantly as the σ phase. However, it can also cause the formation of a Cr-depleted zone that significantly decreases the corrosion resistance. There is no secondary-phase precipitation in the sample after solution treatment at 1100 °C, which shows the best mechanical properties and corrosion resistance.

Microstructural Evolution of T91 in High Temperature Conditions and under Combined Effect of Stress and Temperature

ASTM A213 T91 steel is widely used in power plants and petrochemical industry for long-term service components. Due to its high resistance to creep, thermomechanical fatigue and corrosion, the use of grade 91 steel allows usual plant service parameters to be raised up to ultra-supercritical conditions (600 °C, 300 bar) so that performances are remarkably increased. The strongest factors that affect performances are the time of exposure, the temperature and the applied stress: such parameters can dramatically decrease the service life of a plant component. The improved mechanical properties of grade 91 are strictly related to its specific microstructure: a tempered martensite matrix with fine precipitates embedded in. Two typologies of secondary phases are present: M23C6 carbides (where M = Cr/Fe/Mo/Mn) and finely dispersed MX-type carbonitrides (where M = V/Nb and X = C/N). This study is focused on the microstructure evolution of grade 91 steel under creep conditions. First, three sets of laboratory-aged specimens heated in oven at 550 °C, 600 °C and 650 °C were examined; the exposure time was up to 50,000 h. Furthermore, the influence of stress on the microstructure in two sets of samples was evaluated: a first batch of specimens cut from an ex-service tube of a petrochemical plant (over 100,000 h of service at 580 °C and 19–25 bar) and a second set of samples coming from another ex-service tube under ultra-supercritical conditions (605 °C, 252 bar) in a power plant. The microstructures were characterized by optical, scanning electron and transmission electron microscopy and the results were compared to the literature. Some interesting trends were evidenced, in terms of secondary phases precipitation and coarsening, as well as martensite recovery. Furthermore, the applied stress seems to influence size and number of Laves phase particles.

Microstructural Evolution of Inconel 718 during Aging and Its Correlation with Electric Conductivity Measurements

Abstract The microstructural evolution of the nickel-base superalloy Inconel 718 after exposition at 750°C for different holding times (10 min–1,000 h) was characterized and correlated with the Eddy current (EC) technique. The results showed that electrical conductivity increases with aging time. This behavior was associated with the precipitation and coarsening of the metastable γ″ phase and its transformation into the detrimental δ phase after 100 h. EC nondestructive evaluation was correlated with X-ray diffraction patterns in terms of the precipitation and growth of secondary carbides that were observed in the scanning electron microscope. Vickers microhardness values were also in agreement with microstructural changes and EC measurements, as the maximum hardened condition can be associated with the precipitation of secondary phases before the transformation of the γ″ phase into the δ phase.

Influence of 40% Cold Working and Annealing on Precipitation in AISI 316L Austenitic Stainless Steel.

Intergranular corrosion is one of the most important processes affecting the behaviour of austenitic stainless steels. Factors such as steel chemical composition, the degree of prior deformation and the exposure temperature affect the degree of sensitisation. AISI 316L (0% CW) steel was annealed at 650 °C for 5, 10, 30, 100, 300 and 1000 h to analyse the influence of isothermal annealing on the precipitation of secondary phases. AISI 316L steel after 40% cold working and subsequent annealing at 650 °C for 1, 1.5, 2, 5 and 10 h was investigated. Time–temperature sensitisation (TTS) diagrams were created based on corrosion test (ASTM A 262, practice A) results. In the case of AISI 316L (0% CW), M23C6, chi and sigma phases precipitated at grain boundaries, and the Laves phase was mainly inside of the grains. In the case of AISI 316L (40% CW), sigma, chi, Laves and M23C6 were identified and precipitated mainly along the grain boundaries as well as on the shear bands within different annealing times. It was confirmed that the increase in the annealing time caused an increase in the amounts of secondary phases. Secondary phases in the equilibrium state were calculated using Thermo-Calc software.

Revealing a correlation between microstructure, stoichiometry, coherency strains and mechanical behavior of bulk polycrystalline oxide-based age hardened/toughened ceramic alloys

The processing challenges and microstructural inhomogeneity associated with ceramic (nano)composites render it important to develop hardened/toughened oxide-oxide ceramic alloys, possessing good mechanical properties, via a facile solid-state precipitation-based approach. However, detailed understanding of processing-microstructure-property correlations in age-hardened/toughened polycrystalline ceramics is still lacking. The present research, therefore, focuses on understanding the possible effects of variation of dimension/stoichiometry of coherent second-phase precipitate particles in such ceramic alloys on the coherency strains, associated residual stresses and mechanical properties. Polycrystalline MgO-based ceramic alloys have been developed via controlled aging treatments of partially supersaturated Fe(oxide)-containing MgO solid solution. Air-quenching post sintering-cum-solution treatment results in the formation of ultra-fine (∼5–20 nm) coherent intragranular Mg-ferrite precipitates; with subsequent aging treatments causing increases in their size and amount. The intragranular precipitate particles remain coherent with the MgO matrix even upon aging at 1000 °C for 20 h, despite coarsening to ∼130 nm. Detailed compositional analysis using EDS (with HAADF-STEM) and 3D APT reveal progressive increment in Fe-content of the precipitates with aging duration, such that the ultra-fine particles obtained right after solution treatment correspond to non-stoichiometric Fe-deficient Mg-ferrite (with ∼6 at.% Fe), whereas the relatively coarser particles obtained after 20 h of aging correspond to near-stoichiometric MgFe2O4 (with ∼27 at.% Fe). Such increment in Fe-content results in increase in lattice misfit/coherency strains, which dominates the hardening response and improves the hardness despite loss of solution hardening. Furthermore, increase in compressive residual stress in the matrix due to increment in misfit/coherency strains results in improved resistance to crack propagation.

Microstructure design via novel thermodynamic route to enhance the thermoelectric performance of GeTe

Defect engineering is the core of thermoelectric study: no thermoelectric material can achieve the best performance without implementing proper defects. Designing the appropriate feature size of defects can promote the trade-off between carrier mobility and lattice thermal conductivity. Taking GeTe as an example, the lattice thermal conductivity is mainly contributed by mid- and long-wavelength heat-carrying phonons, while carrier mean free path is close to point defects. Different from the mainstream strategy of oversaturated precipitation of secondary phases, we propose a novel thermodynamically route to rational design microstructures and enhance the thermoelectric performance of GeTe. Thermodynamically, Zn–Cd co-doping reduces the solubility limit of Zn or Cd in GeTe and promotes the generation of nano and micron scale Zn0.6Cd0.4Te secondary phases, thereby reducing lattice thermal conductivity meanwhile leaving a minimal carrier mobility reduction. The further Sb doping significantly reduces carrier concentration and enhances the density-of-state effective mass, thus improving the Seebeck coefficient. In addition, the formed multiscale microstructures via novel thermodynamic route further restrain the lattice thermal conductivity, leading to a maximum zT of 1.60 at 823 K in Ge0.80Sb0.06Zn0.07Cd0.07Te. This work attest to the efficacy of the rational design of microstructures by novel thermodynamic route toward the high zT in GeTe and other TE materials.