Time Temperature Precipitation Research Articles

Thermodynamic analysis of precipitation behavior of M23C6 carbide in Nimonic 105 superalloy

AbstractThe Time–Temperature–Precipitation diagram ofM23C6carbide formation was established for a Nimonic 105 Ni-based superalloy by means of microstructural observation and mathematical-thermodynamic analysis. The results showed that during heat treatment at 750 °C,M23C6carbide usually forms at grain boundaries due to the decomposition ofMC carbides, indicating that the grain boundaries promoteM23C6carbide precipitation. Also, by increasing temperature or time, diffusion rate increases and new diffusional paths (such as γ matrix and twinning) are provided leading to the nucleation of these phases inside the matrix by means of γ→M23C6+ γ′ reaction. According to thermodynamic analysis, the interactions betweenM23C6formation Gibbs free energy and diffusion activation energy of elements, especially carbon, definesM23C6precipitation behavior. The peak of the Time–Temperature–Precipitation diagram was calculated to be about 900 °C based on experimental results, and about 927 °C based on thermodynamic results. These two temperatures can be considered as approximately equal.

Micro-mechanisms and precipitation kinetics of delta (δ) phase in Inconel 718 superalloy during aging

The formation of δ phase in Inconel 718 superalloy during aging was quantitatively studied based on improved analyzing techniques to unambiguously determine the mechanism of δ phase precipitation and its kinetics. Based on the general transformation rate expressed in terms of the time corresponding to the transformed fraction of 0.5 or 50% and also the modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation, the average activation energy for the formation of δ phase was determined as 212.4 kJ/mol and 197.5 kJ/mol, respectively. These values are comparable to the activation energy for diffusion of Nb in Ni (202.59 ± 4.71 kJ/mol). Therefore, the micro-mechanism for the formation of δ phase was characterized as the diffusion of Nb in the matrix. Based on the kinetic studies, the temperature of 1055.7 K (782.7 °C) was found as the lower bound for the formation of δ phase, which was consistent with the precipitation-temperature-time (PTT) plot developed for this material. Finally, a formula was obtained to quantitatively show the effects of aging temperature and cold rolling reduction on the transformation rate for the precipitation of δ phase.

Analysis of β´(Cu4Ti) Precipitation During Isothermal Aging of a Cu-4wt.%Ti Alloy

This work analyzed experimentally and numerically the growth kinetics of β′ precipitation of a Cu–4 wt%Ti alloy after aging at 400, 500, and 600 °C for times from 0.0166 to 200 h. Results indicated that the precipitation process is almost controlled by nucleation and growth during aging at 400 °C, originating a slow growth kinetics of precipitation. In contrast, the coarsening of precipitates dominates the precipitation process during aging at 500 and 600 °C. The interfacial energy of interface between the α matrix phase and β′ precipitates was determined to be about 0.1135, 0.0980, and 0.0725 Jm−2 for aging at 400, 500, and 600 °C, respectively. These values suggest a coherent interface, which is in agreement with the flat faces of β′ cuboid precipitates. Calculated Time–Temperature–Precipitation diagram for the β′ precipitation indicated good agreement with experimental results. Precipitation hardening was higher for the slower growth kinetics of precipitation.

Precipitation Behavior of VN in High Nitrogen and Vanadium Micro-alloyed Low Carbon Weathering Steel

In this study, the high nitrogen and vanadium micro-alloyed low carbon weathering steel (0.0320%N–0.096V) with excellent mechanical properties was produced. TEM and EDS tests of the hot rolled specimens were conducted and it was found that a great number of VN precipitates were generated and the size of VN particles was in the range of 10–300 nm, which exceeded the critical size of 7.89 nm for pinning dislocation line. Thermodynamic calculation results showed that, at high temperature, the precipitation performance of VN remarkably increased due to high nitrogen content. For the experimental steel, the initial precipitation temperature of VN reached 1133 °C and the precipitation rate of V exceeded 90% at the finish rolling temperature of 850 °C. The precipitation temperature time (PTT) curves drawn from kinetic data showed that the fastest precipitation temperature was about 950 °C. The total precipitation time was shortened from 110 to 10 s at 850 °C due to the high dislocation density caused by deformation, and the VN particles could thus be completely precipitated with a brief relaxation process. High nitrogen and vanadium micro-alloying was beneficial for improving the grain refinement and precipitation strengthening effect of vanadium.

Strain-Induced Precipitation Kinetics of Vanadium Carbonitride Precipitates with the Cubic Structure in High-Strength Weathering Steels

The strain-induced precipitation kinetic model of vanadium carbonitride [V(C, N)] precipitates with the cubic structure in vanadium-nitrogen (V–N) microalloyed high-strength weathering steels was constructed and validated. The V(C, N) precipitates possess an fcc structure with the lattice sites and octahedral interstitial sites (O-sites) being occupied with V atoms, and C and N atoms, respectively. The model is based on the cubic structure for V(C, N) precipitates instead of the spherical one, since recent experimental results clearly demonstrate that the strain-induced precipitation of V(C, N) in austenite is of the cubic structure. The nitrogen content effect on precipitation–temperature–time (PTT) curve pattwerns was also studied. With the nitrogen content, the PTT curve nose temperature increased, and the curves shifted left. When the nitrogen content in steels was dropped, PTT curves of V(C, N) precipitates in austenite acquired an S-shape, since free energy conversion is nonlinear. Additionally, thermodynamic software was employed to calculate the phase equilibrium of V(C, N) precipitates in the new high-strength weathering steels. The N site fraction of N during the V(C, N) precipitation was much larger for steels with high N contents, compared to those with low ones.

Precipitation Kinetics of M23C6 Carbides in the Super304H Austenitic Heat-Resistant Steel

The precipitation kinetics of M23C6 carbides in Super304H and TP304H steels were investigated using the selective-etching method, SEM backscattered electron images and Image-Pro-Plus 6.0 software. Precipitation–temperature–time (PTT) diagrams of M23C6 carbides in the as-received Super304H (fine grains), coarsened Super304H (coarse grains) and TP304H (coarse grains) steels all show the typical C-shaped character with nose temperature range from 800 to 850 °C. Compared with the TP304H steel, the same trend is found of the PTT curve of M23C6 carbides for both kinds of Super304H steels, but their start lines move to the right and finish lines to the left. The preferential formation of Nb(C,N) phase at grain boundaries in the Super304H steels inhibited the nucleation of M23C6 carbides in the early stage of precipitation, causing the right shift of the start line of PTT curve. The main reason for the left shift of the finish line of the two Super304H steels was the quicker growing and coarsening rate of M23C6 in the later precipitation stage due to their higher C content than in TP304H. For the difference in PPT curves between the two grain sizes of the Super304H steel, the lower diffusion rate of atoms in the coarse-grained Super304H steel may explain its righter finish line than the fine-grained counterpart, while the reason for its lefter start line is due to the higher solute segregation along coarse-grained boundaries.

Precipitation analysis of as-cast HK40 steel after isothermal aging

As-cast HK40 steel was aged at 700, 800, or 900°C for times as long as 2000 h. Microstructural characterization showed that the primary M7C3 carbide network contained a substantial content of manganese, in agreement with the microsegregation of manganese calculated by Thermo-Calc using the Scheil–Gulliver module. The dissolution of primary carbides caused the solute supersaturation of austenite and subsequent precipitation of fine M23C6 carbides in the austenite matrix for aged specimens. During prolonged aging, the carbide size increased with increasing time because of the coarsening process. A time–temperature–precipitation diagram for M23C6 carbides was calculated using the Thermo-Calc PRISMA software; this diagram showed good agreement with the experimental growth kinetics of precipitation. The fine carbide precipitation caused an increase in hardness; however, the coarsening process of carbides promoted a decrease in hardness. Nanoindentation tests of the austenite matrix indicated an increase in ductility with increasing aging time.

Precipitation Kinetics in a Nb-stabilized Ferritic Stainless Steel

The precipitation occurring in a Nb-stabilized ferritic stainless steel, containing initially Nb(C, N) carbonitrides and Fe3Nb3X precipitates, was investigated during aging treatments performed between 923 K and 1163 K (650 °C and 890 °C) by combining different techniques, (thermoelectric power (TEP), scanning/transmission electron microscopy (SEM/TEM), and atom probe tomography (APT)), in order to determine the precipitation kinetics, the nature and morphology of the newly formed precipitates as well as the chemistry of the initial Fe3Nb3X precipitates, where X stands for C or N. The following composition was proposed for these precipitates: (Fe0.81 Cr0.19)3 (Nb0.85 Si0.08 Mo0.07)3 (N0.8 C0.2), highlighting the simultaneous presence of N and C in the precipitates. With regard to the precipitation in the investigated temperature range, two main phenomena, associated with a hardness decrease, were clearly identified: (i) the precipitation of Fe2Nb precipitates from the niobium initially present in solution or coming from the progressive dissolution of the Fe3Nb3X precipitates and (ii) the precipitation of the χ-phase at grain boundaries for longer aging times. From the TEP kinetics, a time–temperature–precipitation diagram has been proposed.

Mathematical-Thermodynamic Prediction and Kinetics of Eta Phase Formation in Fe–Ni-Based Superalloys

Time–temperature–precipitation diagram of eta phase formation was established for an Fe–Ni based superalloy by means of mathematical-thermodynamic analysis, kinetic investigation, and microstructural observation. The eta phase started to precipitate at the expense of the γ′ phase after prolonged aging. On the basis of thermodynamic prediction, the interaction between Gibbs free energy of eta phase formation and activation energy for diffusion of elements, especially titanium, was considered. The tip of TTP diagram was obtained at 817 °C, on the basis of mathematical-thermodynamic prediction, and 825–840 and 840 °C, on the basis of microstructural evolution and kinetic analysis, respectively. Kinetic investigations predicted that the eta transformation started with diffusion of Ti into Ni. This indicated that, with the progress of transformation, as n in Avrami’s equation predicts, other eta formation mechanisms such as γ′ to eta transformation, instant nucleation from the matrix, coincidence of eta lamellas, etc. increased n and activation energy.

Morphology Evolution of γ′ Precipitates during Isothermal Exposure in Wrought Ni-Based Superalloy Inconel X-750

The morphological evolution of γ′ precipitates and lattice misfit with isothermal aging were closely investigated in wrought Ni-based superalloy Inconel X-750. The γ′ morphology dramatically changes in terms of shape, distribution, coalescence and coherency at the γ/γ′ interface. These processes and their dependence on temperature are summarized as a γ′ morphology map together with a time–temperature–precipitation (TTP) diagram through quantifying relevant morphological parameters. The lattice misfit was measured by X-ray diffraction and is positive; it decreases from 0.6% at room temperature to 0.1% at the aging temperature. These results suggest that the morphological changes of the γ′ precipitates are attributable to very low lattice misfit, the interaction of the elastic field, the volume fraction of the precipitates and incoherence in γ/γ′ interface.

Effect of Nb on solution and precipitation of inhibitors in grain-oriented silicon steel

In this study, three grain-oriented silicon steels with different Nb contents were designed to investigate the effect of Nb on the behaviors of inhibitor solution and precipitation by Thermo-Calc software combined with TEM technology. The influence of Nb on precipitation rate of inhibitors in the silicon steels was also discussed. The results show that besides MnS and AlN, Nb(C,N) also exists in Nb-containing silicon steels, and with the increase of Nb content, the precipitation temperature and maximum mole fraction of Nb(C,N) increases significantly. Different Nb contents have no obvious effect on MnS. While the precipitation temperature of AlN decreases from 1190°C to 1140°C with Nb addition from 0.044wt% to 0.09wt%, but the maximum mole fraction of AlN does not change. According to the PTT (Precipitation-Time-Temperature) curve, Nb addition accelerates the precipitation process of second phase particles and shortens the incubation period. Combining the theoretical calculations with TEM analyses, it is known that proper Nb addition in the silicon steel can reduce the slab reheating temperature. Whereas, with excessive Nb addition, the inhibitors can not dissolve into the matrix totally at 1250°C, which will have a negative effect on the production of grain-oriented silicon steel.

High-Temperature Deformation of Inconel 718PlusTM

Since its introduction in 2003, alloy 718PlusTM spurred a lot of interest owing to its increased maximum service temperature over conventional Inconel 718 (704 °C versus 650 °C), good formability, and weldability together with its moderate cost. Understanding the high-temperature deformation characteristics and microstructural evolution is still of interest to many. It is known that the service performance and hot-flow behavior of this alloy are a strong function of the microstructure, particularly the grain size. To develop precise microstructure evolution models and foresee the final microstructure, it is important to understand how and under which forming conditions softening and precipitation processes occur concurrently. In this work, the softening behavior, its mechanisms, and the precipitation characteristics of 718PlusTM were investigated in two parallel studies. While cylindrical compression tests were employed to observe the hot-flow behavior, the precipitation behavior and other microstructural phenomena such as particle coarsening were tracked via hardness measurements. A precipitation–temperature–time (PTT) diagram was reported, and modeling of the flow curves via hyperbolic sine model was discussed in the light of the PTT behavior. Both “apparent” approach and “physically based” approach are implemented and two different sets of parameters were reported for the latter. Finally, recovery and recrystallization kinetics are described via Estrin–Mecking and Bergstrom, and Avrami kinetics, respectively.

Isothermal precipitation kinetics of carbides in undercooled austenite and ferrite of a titanium microalloyed steel

A new method for isothermal precipitation kinetics of carbides in undercooled austenite and ferrite has been established based on measuring the strength increments. Precipitation-time-temperature (PTT) diagrams of carbides were determined with this method performed on a Gleeble simulator, for a low-carbon titanium microalloyed steel in a temperature range from 530°C to 670°C. Fine precipitates were studied by using high resolution transmission electron microscope (HRTEM), and their volume fractions together with their attributions to strength increments were calculated and analyzed. Results show that the strength increments are mainly attributed to the γ/α interphase precipitation whose nano-scale TiC precipitates obey the Baker-Nutting orientation relationship with respect to the ferrite matrix: (100)TiC ǁ (100)α-Fe and [011]TiC ǁ [001]α-Fe. The PTT curves obtained have a classic C-shape with their noses at about 600°C, where the strength increment reaches 114MPa after isothermally holding for 1h. Additionally, the corresponding precipitation kinetics curves are presented in an S-shape, and a modified-form of Avrami formula is introduced to describe their growth periods. The presented mathematical expression fits the experimental data quite well. Consequently, the proposed new method is intuitive, effective and convenient, particularly suitable for detecting the precipitation kinetics in low temperature range.

Strain-induced Precipitation in Ti Micro-alloyed Interstitial-free Steel

Stress relaxation method was carried out on a Ti micro-alloyed interstitial-free (IF) steel at the temperature ranging from 800 to 1000 °C. The results show that the softening kinetics curves of deformed austenite can be divided into three stages. At the first stage, the stress has a sharp drop due to the onset of recrystallization. At the second stage, a plateau appears on the relaxation curves indicating the start and finish of strain-induced precipitation. At the third stage, the stress curves begin to descend again because of coarsening of precipitates. Precipitation-time-temperature (PTT) diagram exhibited a “C” shape, and the nose point of the PTT diagram is located at 900 °C and the start precipitation time of 10 s. The theoretical calculation shows that the strain-induced precipitates were confirmed as almost pure TiC particles. The TiC precipitates were heterogeneously distributed in either a chain-like or cell-like manner observed by transmission electron microscopy (TEM), which indicates the precipitates nucleated on dislocations or dislocation substructures. In addition, a thermodynamic analytical model was presented to describe the precipitation in Ti micro-alloyed IF steel, which shows a good agreement between the experimental observation and the predictions of the model.

Carbide precipitation kinetics of a 2000 MPa grade highstrength spring steel

The carbide strain-induced precipitation kinetics of a 2000 MPa grade Nb-V high-strength spring steel during isothermal deformation is quantitatively investigated using the stress relaxation method. The stress relaxation technique was employed to determine the precipitation start (P s ) and finish (P f ) times. The evolution of carbides during the isothermal holding is observed by Transmission Electron Microscope (TEM). The results show that the precipitation-time-temperature (PTT) diagrams of the carbides in the experimental steel exhibit a C-curve, with a nose temperature of 900 °C. The precipitation kinetics of carbides in the experimental steel is slower than that of plain carbon steels, which is attributed to the reduced activity of Nb. The result indicates that the precipitate is a Nb, V, Ti and C-containing complex carbide at deformation temperatures ranging from 850 °C to 1050 °C. Large parts of the precipitates are (Nb,V)C, and there is also a tiny amount of TiC. The carbides are dispersed below 900 °C and distributed in clusters near dislocations and grain boundaries above 950 °C.

Precipitation model in microalloyed steels both isothermal and continuous cooling conditions

Niobium and vanadium precipitates (nitrides and carbides) can inhibit the static recrystallization of austenite but this does not happen for Ti, which form nitrides at high temperatures. RPTT diagrams show the interaction between recrystallization and precipitation allowing study the strain induced precipitation kinetics and precipitate coarsening. Based on Dutta and Sellars’s expression for the start of strain-induced precipitation in microalloyed steels, a new model has been constructed which takes into account the influence of variables such as microalloying element percentages, strain, temperature, strain rate and grain size. Recrystallization- Precipitation-Time-Temperature (RPTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels with different Nb, V and Ti contents. Mathematical analysis of the results recommends the modification of some parameters such as the supersaturation ratio (ks) and constant B, which is no longer a constant but a function of ks. The expressions are now more consistent and predict the Precipitation-Time-Temperature (PTT) curves with remarkable accuracy. The model for strain-induced precipitation kinetics is completed by means of Avrami’s equation. Finally, the model constructed in isothermal testing conditions, it has been converted to continuous cooling conditions in order to apply it in hot rolling.

Thermo-kinetic modeling of Cu precipitation in α-Fe

Abstract Consistent and predictive simulation of precipitation in the Fe–Cu system is a complex task. So far, no comprehensive approach is reported in literature that is capable of reproducing simultaneously all fundamental precipitation parameters (phase fraction, mean radius and number density) within a single set of model parameters and over an extended range of temperatures. In the present paper, we address this problem and present a novel approach for simulation of Cu-precipitation based on CALPHAD-type Gibbs energies, a mean-field model for precipitate growth, advanced classical nucleation theory and a predictive model for interface energy calculation. For successful simulation, several physical mechanisms are accounted for in addition to conventional Kampmann–Wagner-based precipitation modeling to achieve a consistent evolution of precipitation parameters. These comprise of models for the transition from bcc to 9R to fcc structure, curvature and temperature effects on interfacial energy, particle coalescence as well as a thermodynamic reassessment of the Fe–Cu system. We compare our results with experimental data as well as first-principles calculations from literature and, finally, predict a time–temperature-precipitation (TTP) diagram that reproduces the experimental one well within the experimental scatter.

Carbide precipitation in austenite of Nb–Mo-bearing low-carbon steel during stress relaxation

The carbide precipitation in austenite of Nb–Mo bearing low-carbon steel was investigated by stress relaxation and transmission electron microscopy (TEM). The precipitation-time-temperature (PTT) diagram exhibits a typical “C” shape curve, the nose of which was located at 950°C. Mo was found to be incorporated into NbC lattice to form a complex (Nb, Mo)C particle during precipitation. Compared to NbC particles in Nb-bearing steel, (Nb, Mo)C particles in Nb–Mo-bearing steel exhibit superior coarsening resistance.

Promoting Ti4C2S2 strain induced precipitation during asymmetrical hot rolling to improve r value and advantaged texture in Ti stabilized IF steel

The kinetic of Ti4C2S2 strain induced precipitation in a Ti stabilized IF steel was investigated using two stage interrupted compression test with high true strain (0.5). The PTT (precipitation–time–temperature) diagram was obtained by plotting 1/Ar–time curves. TEM (transmission electron microscopy) observation confirmed that the evolution of Ti4C2S2 precipitate in the quenched samples of thermal simulation is in good agreement with the PTT diagram. Hot strips were produced at three different rolling temperatures with high strain and slight shear deformation. It was found that hot rolling at the nose temperature of the Ps line of the PTT diagram can effectively promote the precipitation of Ti4C2S2 and retard the precipitation of TiC. Cold rolled and annealed sheets from hot strip containing higher volume fraction of Ti4C2S2 exhibited higher r value and stronger γ fiber texture with equal {111}〈112〉 and {111}〈110〉 components. By contrast, cold rolled and annealed sheets from hot strips containing lower volume fraction of Ti4C2S2 represented lower r values and weaker γ fiber texture with significant drops from {111}〈112〉 to {111}〈110〉 component.

Understanding sigma-phase precipitation in a stabilized austenitic stainless steel (316Nb) through complementary CALPHAD-based and experimental investigations

Sigma-phase precipitation in a 316Nb “stabilized” austenitic stainless steel was studied through complementary CALPHAD-based and dedicated experimental investigations. Thermokinetic calculations performed using Thermo-Calc (with the DICTRA module) and MatCalc software showed that the sigma phase (σ) precipitated directly at γ-austenite grain boundaries (GB) via a common solid-state reaction when carbon and nitrogen contents fell below a critical threshold. Residual δ ferrite was found to be more susceptible to σ-phase precipitation; this type of precipitation occurred via two mechanisms that depended on the concentration profiles of δ-ferrite stabilizing elements induced by previous thermomechanical processing: direct σ precipitation (δ→σ) along the periphery of δ islands followed by a eutectoid decomposition (δ→σ+γ2) within these islands. Both simulations and experiments revealed that the σ phase at γ GB contained higher amounts of Mo and Ni, while σ within δ ferrite possessed higher contents of Fe and Cr. Finally, the simulated time–temperature–precipitation diagrams for the σ phase in residual δ ferrite were found to be in very good agreement with the experimental ones and comparable to those observed in duplex stainless steels.