Time Temperature Precipitation Diagram Research Articles

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.

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

The age-hardening behavior of the wrought Ni-based superalloy Alloy 625 was investigated in the temperature range between 923 and 1173 K for the application to advanced ultra-supercritical (A-USC) power plants. The carbon content of the alloy was controlled as low as possible to minimize the precipitation of carbides during aging. A two-step increase of hardness was detected for the alloy at temperatures between 1000 and 1100 K; the first increase of hardness results from the precipitation of the metastable γ′′ phase, and the second increase corresponds to the precipitation of the orthorhombic δ phase. In contrast, a single-step increase of hardness was detected below 1000 K derived from the precipitation of γ′′ phase and above 1100 K derived from the precipitation of δ phase. The TTP (time–temperature–precipitation) diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the nose temperatures of γ′′ and δ phases are determined as 1050 and 1123 K, respectively. The γ′′ particle coarsened along the Ostwald ripening. The activation energy for the γ′′ coarsening was evaluated as 202 kJ/mol, which is very close to that for the inter-diffusion of Nb in Ni.

A new numerical model to predict welding-induced sensitization in SUS304 austenitic stainless steel joint

In this paper, a SUS304/P92 dissimilar metal butt-welded joint was fabricated, and the welding-induced sensitization of SUS304 austenitic stainless steel in this joint was investigated via the numerical simulation and experiment. In the proposed new numerical model, the time–temperature-precipitation diagram and the time–temperature-sensitization diagram were used to simulate the region and the degree of sensitization, respectively. The predicted degree of sensitization (DOS) in the weld decay zone matched the electrochemical measurements well. Based on the simulation results, the evolution of DOS at the austenitic stainless steel side during the multi-pass welding process was examined in detail. The obtained results can help evaluate the material susceptibility to stress corrosion cracking in austenitic stainless steel joints.

Continuous Heating Dissolution and Continuous Cooling Precipitation Diagrams of a Nickel-Titanium Shape Memory Alloy

Binary NiTi alloys are the most common shape memory alloys in medical applications, combining good mechanical properties and high biocompatibility. In NiTi alloys, the shape memory effect is caused by the transformation of an austenite phase to a martensite phase and the reverse process. Transformation temperatures are strongly influenced by the exact chemical composition of the NiTi phase and the presence of precipitates in the microstructure induced by thermo-mechanical treatment, especially solution annealing and ageing. Isothermal time–temperature precipitation diagrams can be found in the literature. Cooling is frequently not considered, as water quenching is typically assumed to be sufficient. To the best of our knowledge, continuous heating dissolution (CHD) and continuous cooling precipitation (CCP) diagrams do not exist. Differential scanning calorimetry (DSC) is a common method to analyse the austenite/martensite transformation in shape memory alloys, but it has not yet been used to analyse precipitation processes during continuous temperature changes. We have enabled DSC to analyse dissolution and precipitation processes in situ during heating as well as during cooling from the solution annealing temperature. Results are presented as CHD and CCP diagrams, including information from microstructure analysis and the associated changes in the austenite/martensite transformation temperatures.

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

The age-hardening behavior of the wrought Ni-based superalloy Alloy 625 was investigated in the temperature range between 923 and 1173 K for the application to advanced ultra-supercritical (A-USC) power plants. The carbon content of the alloy was controlled as low as possible to minimize the precipitation of carbides during aging. A two-step increase of hardness was detected for the alloy at temperatures between 1000 and 1100 K; the first increase of hardness results from the precipitation of the metastable γ′′ phase, and the second increase corresponds to the precipitation of the orthorhombic δ phase. In contrast, a single-step increase of hardness was detected below 1000 K derived from the precipitation of γ′′ phase and above 1100 K derived from the precipitation of δ phase. The TTP (time–temperature–precipitation) diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the nose temperatures of γ′′ and δ phases are determined as 1050 and 1123 K, respectively. The γ′′ particle coarsened along the Ostward ripening. The activation energy for the γ′′ coarsening was evaluated as 202 kJ/mol, which is very close to that for the inter-diffusion of Nb in Ni.

Time–Temperature–Precipitation Relations for Nitrides and Evaluation of Internal Oxidation Theory for Nitridation of Austenitic Stainless Steel

Internal nitridation kinetics were determined for a UNS N08810/800H alloy using a general model of the form $$x^{n}=kt$$ . Nitridation behavior was studied at service-relevant temperatures 800 °C to 1000 °C in a 95 pct $$\hbox {N}_2/5\,{\text{pct}}$$ $$\hbox {H}_2$$ atmosphere for times 50 to 750 hours. Optical and scanning electron microscopy were used for microstructural characterization and measurement of nitride penetration. AlN, $$\hbox {Cr}_2\hbox {N}$$ , and CrN were formed, and the experimentally observed precipitation sequence was consistent with equilibrium calculations for this alloy using Thermo-Calc. A combination of diffusivity data determined using DICTRA and experimentally verified equilibrium calculations showed that Wagner’s analysis for internal oxidation kinetics was valid for AlN penetration. Parabolic kinetics closely approximated measured AlN penetration. This suggests that extension of AlN penetration models to other temperatures and Fe-Ni-Cr-Al alloy systems is reasonable. $$\hbox {Cr}_2\hbox {N}$$ penetration did not conform to Wagner’s analysis. Deviation from parabolic behavior was evident, and general model penetration predictions for $$\hbox {Cr}_2\hbox {N}$$ were experimentally validated. Using the experimentally determined models, time–temperature–precipitation diagrams for AlN and $$\hbox {Cr}_2\hbox {N}$$ penetration were constructed.

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.

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 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.

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.

Experimental Investigation and Analytical Prediction of σ-Phase Precipitation in AISI 316L Austenitic Stainless Steel

The phase precipitation in industrial AISI 316L stainless steel during aging for up to 80,000 hours between 823 K and 1073 K (550 °C and 800 °C) has been studied using transmission electron microscopy, scanning transmission electron microscopy, and carbon replica energy-dispersive X-ray microanalysis. Three phases were identified: Chromium carbides (M23C6), Laves phase (η), and σ-phase (Fe-Cr). M23C6 carbide precipitation occurred firstly and was followed by the η and σ-phases at grain boundaries when the aging temperature is higher than 873 K (600 °C). Precipitation and growth of M23C6 create chromium depletion zones at the grain boundaries and also retard the σ-phase formation. Thus, the σ-phase is controlled by the kinetic of chromium bulk diffusion and can appear only when the chromium reaches, at grain boundaries and at the M23C6/γ and M23C6/η/γ interfaces, content higher than a critical value obtained by self-healing. An analytical model, based on equivalent chromium content, has been established in this study and successfully validated to predict the time–temperature–precipitation diagram of the σ-phase. The obtained diagram is in good agreement with the experimental results.

Modelling Deformation-Induced Precipitation Kinetics in Microalloyed Steels during Hot Rolling

This paper reports the framework of a computer model that calculates the precipitation kinetics of MX type carbides or carbonitrides from austenite matrix in microalloyed steels during hot rolling. The kinetic model is based on the classical Johnson-Mehl-Avrami theory adapted to include the saturation of nucleation sites. The effect of deformation on precipitation kinetics is quantitatively described through its effect on flow stress, from which the number of potential nucleation sites can be estimated. The time-temperature precipitation diagram can then be calculated for a given alloy chemistry and deformation conditions. A preliminary study has been carried out to test its performance in both undeformed and deformed conditions.

Microstructure and intergranular corrosion of the austenitic stainless steel 1.4970

The precipitation behaviour of the DIN 1.4970 steel and its effect on the intergranular corrosion resistance were studied. Time–temperature–precipitation diagrams for the secondary phases (Ti, Mo)C, (Cr, Fe, Mo, Ni) 23C 6 and (Cr, Fe) 2B are presented and representative samples have been selected for corrosion studies. The susceptibility to intergranular corrosion of the samples was evaluated using the double loop electrochemical potentiokinetic reactivation technique. The results showed that the solution-annealed samples and those aged at 1173 K did not present susceptibility to intergranular corrosion, whereas aging treatment from 873 to 1073 K resulted in small susceptibility to intergranular attack that decreased with aging temperature. The preferential formation of (Ti, Mo)C at higher aging temperatures comparatively to M 23C 6, retained the chromium in solid solution preventing steel sensitization and, consequently, intergranular corrosion.

Microstructural thermostability of high nitrogen austenitic stainless steel

A high nitrogen austenitic stainless steel, Fe–24Mn–18Cr–3Ni–0.62N, was isothermally annealed from 750 to 900 °C for different times to investigate the thermostability of its microstructures. Results show that the precipitates were Cr 2N and initially formed along the grain boundary. The time–temperature–precipitation diagram was established, according to which the critical cooling rate of this material with less than 0.1–0.5 vol.% of precipitated Cr 2N was specified as 30 °C/min. The microhardness of the matrix in samples annealed at different temperatures decreased with the diffusion of nitrogen from the matrix to grain boundaries.

Time–temperature–precipitation diagrams of carbide evolution in low alloy steels

Previously published time–temperature–precipitation diagrams showing carbide evolution were modified with the use of additional experimental data. The same low alloy steels as investigated originally were annealed at 793 and 873 K for 3000 and 10,000 h which, compared to the longest annealing time in the previous investigation of 1000 h, brings the system closer to equilibrium. Carbides extracted into carbon replicas were characterised by transmission electron microscopy, including electron diffraction and energy dispersive X-ray spectroscopy. The results of the additional experiment presented in the modified diagrams provide more information about the evolution and stability of M 3C, M 2C, M 7C 3, M 23C 6, M 6C and MC in the investigated steels.

Effect of nitrogen on σ transformation in duplex stainless steel weld metal

Duplex stainless steels were gas tungsten arc welded without filler material in various Ar–N2 mixed gas atmospheres. Weld metals having different nitrogen contents were annealed at 900–1300 K to examine the single effect of nitrogen on the σ transformation. Nitrogen content of the weld metal did not influence the nose temperature and the time for σ initiation in time–temperature–precipitation diagrams for σ, but did decrease the σ content after annealing for long times. These results were correlated with the nitrogen content and microstructure of the weld metals. The effect of σ on the fracture toughness of the weld metal was evaluated by the small punch (SP) test. A small amount of σ in the weld metal led to a marked decrease in the SP energy.

Microstructural changes in austenitic stainless steels during long-term aging

AbstractThe microstructural changes, precipitation behaviour, and mechanical properties of typical austenitic stainless steels (304 H, 316 H, 321 H, 347 H, and Tempaloy A–1) have been examined after long-term aging. The steels were aged statically in the temperature range 600–800°C for up to 50000 h. The microstructural changes were observed by optical and transmission electron microscopy, and the extracted residue was identified using X-ray analysis. Time–temperature precipitation diagrams were made for each steel. The amount of σ-phase was measured in samples aged at 700°C. The hardness and impact-value changes, and the tensile properties of aged samples were measured.MST/358