Σ-phase Precipitation Research Articles

Microtexture evolution of sigma phase in an aged fine-grained 2205 duplex stainless steel

Duplex stainless steels were developed using the synergic effect of the ferritic and austenitic phases on the mechanical and corrosion properties when they form a microstructure. Due to the large number of alloying elements added in these materials, precipitation of deleterious phases may occur when they are subjected to certain thermal cycles. The objective of this work was to evaluate the microtexture aspects of sigma (σ) phase precipitation in a cold rolled 2205 duplex stainless steel with fine grains, after isothermal ageing at 850 °C for different times. The microtextures of ferrite and austenite were also assessed and used to correlate with the crystallographic aspects of sigma phase precipitation and growth. In the initial moments of σ-phase precipitation, its orientation distribution function (ODF) was similar to the ones found for ferrite. For longer ageing periods, the σ-phase microtexture showed its main components to be (111)[1¯1¯2] and near to the known copper component (Euler angles) of austenite. The results indicate a severe dependence of σ-phase texture from the ones of ferrite, being decomposed, and on the secondary austenite formed in the eutectoid reaction. In addition, this work shows that an initial low crystallographically coherent interface between ferrite and austenite enhances σ-phase precipitation and deviates it from the Nenno orientation relationship.

Influence of ageing treatment temperature and duration on σ-phase precipitation and mechanical properties of UNS S32750 SDSS alloy

IntroductionSuper-Duplex Stainless Steeles (SDSS) proved an excellent potential for its use in many chemical and offshore applications due to their both high mechanical properties and a high corrosion resistance in chloride ion solutions. ObjectivesThis study evaluates the influence of ageing treatment temperature and duration on σ-phase precipitation and mechanical properties of UNS S32750 SDSS alloy. MethodsThe influence of ageing treatment on microstructural features was analysed by SEM-EBSD (Scanning Electron Microscopy - Electron Backscatter Diffraction) technique, while on mechanical properties by tensile and impact testing techniques. ResultsThe obtained results show that for the short duration ageing treatments the σ-phase nucleates, within the δ-phase matrix, at the δ/γ grains boundary by the δ → σ precipitation, while for long duration ageing treatments the σ-phase nucleates, within the δ-phase matrix, at the δ/δ grains boundary, or in other favourable nucleation sites, due to the eutectoid decomposition δ → σ + γ2. Experimental data showed that at low temperatures, i.e. 780 °C, in order to induce the precipitation of σ-phase, the minimum incubation time is situated close to 20 min, and that increasing treatment temperature decreases the minimum incubation time, with at 850 °C and 920 °C the σ-phase being firstly detected at 5 min. The σ-phase precipitation shows the highest precipitation kinetics at 850 °C, when the maximum weight-fraction is obtained for each treatment duration when compared with ageing treatments performed at 780 °C and 920 °C. ConclusionAll presented data brings valuable insight into the σ-phase precipitation phenomena and its influence on UNS S32750 SDSS alloy’s exhibited mechanical properties and, also, can provide researchers and industrial steel processors a guide regarding the selection of optimal ageing treatment parameters to avoid/minimise the embrittlement induced by the precipitation of deleterious σ-phase.

Investigation of Thermally Induced Deterioration Processes in Cold Worked SAF 2507 Type Duplex Stainless Steel by DTA

Thermally induced deterioration processes were studied in cold worked (up to 60% deformation) SAF 2507 type super-duplex stainless steel (SDSS) by differential thermal analysis (DTA). DTA results revealed two transformations. Parent and inherited phases of these transformations were examined by other methods too, such as micro-hardness tests, optical metallography and X-ray diffraction (XRD). Finally, these transformations were identified as the formation of α’- and σ-phases. Formation of strain-induced martensite (SIM) and recrystallization were not experienced until 1000 °C, despite high degree of cold working. Activation energies of the σ-phase precipitation and α’-phase formation were determined from the Kissinger plot, through DTA measurements—they are 275 and 220 kJ/mol, respectively—in good agreement with the values found in the literature.

Lattice distortion and atomic ordering of the sigma precipitates in CoCrFeNiMo high-entropy alloy

This study aims to understand the critical lattice distortion effect of σ-phase precipitation in CoCrFeNiMo high-entropy alloys (HEAs) using various quantitative methods. The high-angle annular dark-field (HAADF) STEM image illustrates the actual atomic arrangement of materials. From the σ-phase atom image, we can distinctly observe the atomic distortion and realize that the lattice constant coefficient of variation is approximately 2.9%, which is similar to the difference in the lattice constant calculated using XRD (ε∼2.74%). The values 2.9% and ∼2.74% represent the values of lattice distortion from the microscopic and macroscopic perspectives, respectively; however, they are significantly different from the lattice distortion value (∼9.9%) obtained from atomic size difference. Therefore, it is speculated that factors such as atomic bonding and electronic structure can significantly affect the lattice distortion effects. In addition, we use EELS elemental mappings at the atomic scale for understanding compositional distributions in the σ phase. We found that 8j sites have Mo, Fe, Co, and Cr signals; however, they do not contain Ni signal. Based on these findings, it can be explained that the ordering degree of the σ phase is neither entirely ordered nor disordered. This study not only provides the most direct evidence of the critical lattice distortion effect of high-entropy alloys but also proves that the σ phase is not entirely disordered, unlike the conventional HEAs.

Analysis of a Directionally Solidified (DS) GTD-111 Turbine Blade Failure

The purpose of this paper is to clarify the impact of pitting corrosion and erosion on the directionally solidified (DS) GTD-111 turbine blade behavior. Moreover, the pitting corrosion and oxidation phenomena engendered inside cooling channels of the turbine blade are utterly highlighted. Other features such as the η (Ni3, Ti) platelets nucleation, needle σ-phase precipitation at the interface NiPtAl/(DS) GTD-111 substrate are exhibited as well. Finally, the different microstructural changes in (NiPtAl) coating strata against hot corrosion, oxidation and interaction with (DS) GTD-111 substrate are revealed and argued.

On the enhanced wear resistance of CoCrFeMnNi high entropy alloy at intermediate temperature

Sigma (σ) phase precipitation was observed in the recrystallized wear subsurface of a face centred cubic CoCrFeMnNi high entropy alloy after sliding wear at intermediate temperatures (600–800°C). This had a marked effect on the wear resistance of the alloy. The effect of load and temperature on the formation of the wear subsurface microstructure and wear behaviour is studied. The alloy showed excellent wear resistance at an intermediate temperature of 600°C, compared to other testing conditions, due to the formation of a fine-grained wear subsurface along with the precipitation of σ-phase.

σ-Phase Formation in Super Austenitic Stainless Steel During Directional Solidification and Subsequent Phase Transformations

The solidification path and the σ-phase precipitation mechanism in the S31254 (UNS designation) steel are investigated thanks to Quenching during Directional Solidification (QDS) experiments accompanied by scanning electron microscopy observations and electron backscattered diffraction (EBSD) analysis. Considering experimental conditions, the γ-austenite is found to be the primary solidifying phase (1430 °C), followed by δ-ferrite (1400 °C, ≈ 87 pct solid fraction). The σ-phase appears in the solid-state through the eutectoid decomposition of the δ-ferrite: δ → σ + γ2 (1210 °C), whereas the σ-phase is predicted to form from the austenite at 1096 °C in equilibrium conditions. The resulting temperatures of solidification path and phase transformation are compared with Gulliver–Scheil model and equilibrium calculations predicted using Thermo-Calc© software. It is shown that the thermodynamics calculations agree with experimental results of solidification path. The EBSD analysis show that the δ-ferrite has δNW2 ORs with the σ-phase.

Oxidation behavior of Al/Y co-modified nanocrystalline coatings with different Al content on a nickel-based single-crystal superalloy

Isothermal oxidation behavior of Al/Y co-modified sputtering nanocrystalline coatings with various Al content on the René N5 single-crystal superalloy is investigated. Results indicate that precipitation and redissolution of the Y-containing σ-phase during oxidation are strongly dependent on the original Al content, which leads to different segregation behavior of Y-enriched oxides in alumina scales and oxidation kinetics. Higher Al content inhibits the intensive oxidation of Y, but causes elements interdiffusion with the alloy substrate. The nanocrystalline coating with moderate content of Al and equilibrium single γ′ phase, possesses the best comprehensive properties, i.e. low oxidation rate, meanwhile little elements interdiffusion.

Влияние старения на микроструктуру, фазовый состав и микротвердость высокоазотистой аустенитной стали

The authors studied the effect of duration of age hardening at the temperature of 700 °C on the microstructure, phase composition and microhardness of high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N (wt. %) steel. The study showed that age hardening at the temperature of 700 °C for half an hour causes the complex of phase transformations: the decomposition of δ-ferrite (with the formation of σ-phase and austenite) and the formation of cells of discontinuous decomposition on the austenitic grains boundaries (the formation of particles based on the chromium nitride Cr 2 N and the depletion of austenite by interstitials). After age hardening for more than 10 hours, besides the discontinuous decomposition of austenitic grains, a homogeneous (continuous) precipitation of chromium nitride occurs in those austenitic grains, which have not undergone discontinuous decomposition in the initial stages of aging. With an increase in the aging duration up to 50 hours, the authors observed the growth of decomposition cells in austenitic grains and the formation of mixed structure. Such structure consisted of austenite grains, which underwent discontinuous decomposition with the formation of lamellar precipitations of chromium nitride in austenite; austenitic grains with the dispersed particles formed by the mechanism of continuous decomposition; and the grains with σ-phase, chromium nitrides, and austenite formed as a result of the high-temperature ferrite decomposition during aging. The aging caused the increase in the microhardness, which value depends on the mechanism of precipitation hardening - continuous or discontinuous decomposition in austenite or the precipitation of intermetallic σ-phase and chromium nitrides plates in the grains of high-temperature ferrite.

Effect of Cold Deformation and Solid Solution Temperature on σ-phase Precipitation Behavior in HR3C Heat Resistant Steel

Effect of Cold Deformation and Solid Solution Temperature on σ-phase Precipitation Behavior in HR3C Heat Resistant Steel

Kinetics of phase separation, border of miscibility gap in Fe–Cr and limit of Cr solubility in iron at 832 K

Kinetics of phase decomposition accompanied by precipitation of σ-phase in a Fe–Cr alloy with 26.3 at% Cr isothermally annealed at 832 K was studied by means of Mössbauer spectroscopy. Two stage decomposition process has been revealed by three different quantities viz. the average hyperfine field, <H>, the short-range parameter, α1, and the probability of atomic configuration with no Cr atoms within the first two coordination shells around the probe Fe atoms, P(0,0). The first stage, that has terminated after ∼300 h of annealing, has been associated with the decomposition into Fe-rich phase in which the concentration of Cr, determined as 20.7 at.%, can be interpreted as the border of the metastable miscibility gap at 832 K. The second stage can be regarded as a continuation of the phase decomposition process combined with a precipitation of σ. Its termination can be associated with the limit of Cr solubility estimated as 19.7 at% Cr. The three relevant parameters for this stage have also saturation-like behavior vs. annealing time and the saturation can be interpreted as termination of the two processes. The concentration of Cr in the Fe-rich phase has been determined as 19.7 at.% and this value can be regarded as the limit of Cr solubility in iron at 832 K. Both stages of the kinetics were found to be in line with the Johnson-Mehl-Avrami-Kolgomorov equation yielding values of the rate constant and the Avrami exponent. The activation energy of the second-stage process was determined to be by ∼12 kJ/mol higher.

Microstructure Evolution and Some Properties of Hard Magnetic FeCr30Co8 Alloy Subjected to Torsion Combined with Tension.

The hard magnetic alloy FeCr30Co8 alloy was subjected to severe plastic deformation (SPD) by torsion combined with tension in the temperature range of 750 °C to 850 °C. This range of deformation temperatures corresponds to the α solid solution on the Fe–Cr–Co phase diagram. The study of the alloy after SPD by means of X-ray diffraction (XRD) and scanning and transmission electron microscopy techniques showed the formation of a gradient microstructure with fine grain size in the surface layer and precipitation of the hard intermetallic σ-phase. Next, the magnetic and mechanical properties of the deformed alloy after short annealing at 1000 °C and magnetic treatment were studied. A slight decrease in coercive force was found, along with a significant gain in plasticity and strength. The effective deformation temperature was determined to obtain the optimal magnetic and mechanical characteristics of the alloy. This method of deformation can be applied for the improvement of the mechanical properties of some magnets (high-speed rotors) which should have good magnetic properties within their volume while maintaining good mechanical properties on the surface.

Solidification path and phase transformation in super-austenitic stainless steel UNS S31254

The solidification path and the σ-phase precipitation mechanism of UNS S31254 alloy were studied on the basis of directional solidified experiments accompanied by scanning electron microscopy observations and energy dispersive X-ray a nalysis. The resulting temperatures of solidification paths and phase transformation were compared with Gulliver-Scheil and equilibrium calculations predicted using ThermoCalc© software. It was confirmed that the experimental solidification path was in agreement with the thermodynamic calculations. The complementarity of the results have made it possible to propose a solidification path and a σ-phase precipitation mechanism for the UNS31254 steel.

Zinc diffusion induced precipitation of σ-phase in austenitic stainless steel

Zinc diffusion-induced degradation of AISI 316LN austenitic stainless steel pot equipment used in 55%Al-Zn and Zn-Al-Mg coating metal baths is described. SEM/EDS analyses results showed that the diffused zinc reacts with nickel from the austenite matrix and results in the formation of Ni-Zn intermetallic compounds. The Ni-Zn intermetallic phase and the nickel depleted zones form a periodic and alternating layered structure and a mechanism for its formation is proposed. The role of cavities and interconnected porosity in zinc vapour diffusion-induced degradation and formation of Ni-Zn intermediate phases is also discussed. The formation of Ni-Zn intermediate phases and the depletion of nickel in the austenite matrix results in the precipitation of σ-phase and α-ferrite in the nickel depleted regions of the matrix. This reaction will lead to increased susceptibility to intergranular cracking and accelerated corrosion of immersed pot equipment in the coating bath. Zinc diffusion induced precipitation of σ-phase in austenitic stainless steels that we are reporting in this work is a new insight with important implications for the performance of austenitic stainless steels in zinc containing metal coating baths and other process industries. This new insight will further lead to improved understanding of the role of substitutional diffusion and the redistribution of alloying elements in the precipitation of σ-phase in austenitic stainless steels.

Phase stability and kinetics of σ-phase precipitation in CrMnFeCoNi high-entropy alloys

Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system.

Impact of minor alloying with C and Si on the precipitation behavior and mechanical properties of N-doped Co–Cr alloy dental castings

The addition of carbon and silicon as minor alloying elements was examined as a means to improve the mechanical properties of novel nitrogen-doped Co–Cr-based alloy dental castings. Samples of Co–32Cr–9W–Si–0.25N–C (mass%) alloys were prepared using a dental-casting machine. Microstructural analysis was performed on the alloys using scanning electron microscopy, electron-backscatter diffraction, electron-probe microanalysis, and X-ray diffraction, with a particular focus on the precipitation behavior. The findings were compared with thermodynamic predictions and examined in relation to the tensile properties and Vickers hardness at room temperature. All of the prepared alloys had a face-centered-cubic γ-phase matrix, with grains measuring a few millimeters in diameter and consisting of dendritic substructures. The precipitation of the intermetallic σ-phase, which occurred in the interdendritic regions with solidification segregation of Cr and W, was replaced with M23C6 through the addition of carbon. This significantly increased the ultimate tensile strength of the alloys without severe loss of ductility, although the 0.2% proof stress did not change. The addition of silicon, on the contrary, promoted the formation of the precipitates, which included M6C and the σ-phase, making the alloys brittle. The results of this study highlight the role of minor alloying elements, such as carbon and silicon, on the microstructural and mechanical properties; the findings also shed light on the significance of precipitation control in dental castings of Co–Cr alloys, which should aid the design of novel dental alloys.

Investigation on Aging σ-Phase Precipitation Kinetics and Pitting Corrosion of 22 Pct Cr Economical Duplex Stainless Steel with Mn Addition

The influence of Mn addition on σ-phase precipitation kinetics and pitting corrosion of Fe-22Cr-1.9Ni-2.3Mo-0.2N-xMn low nickel type duplex stainless steel was investigated by medium- and high-temperature aging treatments of 600 °C and 800 °C. The microstructure analysis showed that the fine rod-shaped and coarsening dendritelike σ-phase precipitates formed at 600 °C and 800 °C, respectively, and the precipitate growth with the higher temperature was accelerated due to the partition of Mn, but Mn is not a strong σ-phase forming element like Cr, Mo during aging treatment at these two temperatures. At an early aging time of 800 °C, more precipitated nuclei with more Mn addition promote refinement of σ precipitates in later aging time. The kinetic behavior at 600 °C and 800 °C is related to diffusion-controlled growth of σ phase, and the σ-phase nucleation and growth are enhanced with more Mn addition and higher aging temperature due to a faster Mn diffusion rate. The difference in precipitation morphology for two aging temperatures was attributed to the different nucleation modes caused by kinetics parameter n variation. Increasing the aging temperature from 600 °C to 800 °C increased the susceptibility to pitting with higher Mn addition due to faster σ-phase precipitation kinetics.

Effect of Cr and Zr on phase stability of refractory Al-Cr-Nb-Ti-V-Zr high-entropy alloys

The effect of annealing at 800 °C or 1000 °C for 100 h on the structure and mechanical properties of refractory AlCrxNbTiV and AlNbTiVZrx (x = 0–1.5) high-entropy alloys was studied. In the initial condition (annealing at 1200 °C for 24 h), the AlNbTiV and AlCr0.5NbTiV alloys were composed of the B2 phase while the AlCrNbTiV and AlCr1.5NbTiV alloys had a mixture of the B2 and Cr2Nb-type C14 Laves phases. The AlNbTiVZrx (x = 0.5–1.5) alloys in the initial condition consisted of the B2, Zr5Al3-type, and ZrAlV-type C14 Laves phases. Annealing of the AlCrxNbTiV (x = 0–1.5) alloys at 800 °C or 1000 °C resulted in (i) precipitation of the Nb2Al-type σ-phase or an increase in the fraction of the Laves phase; (ii) an increase in the microhardness and a pronounced drop in ductility. Annealings of the AlNbTiVZrx (x = 0.5–1.5) alloys at 800 °C or 1000 °C led to insignificant changes in the structure and microhardness but somewhat decreased ductility. Quasi-binary AlNbTiV-Cr and AlNbTiV-Zr phase diagrams constructed with a Thermo-Calc software and TCHEA2 database were used to analyze phase transformations in the experimental alloys. The differences in the behavior of the Cr/Zr-containing alloys and the relationships between the phase composition and mechanical properties of the alloys were discussed.

The dilatometric technique for studying sigma phase precipitation kinetics in F55 steel grade

Sigma phase precipitation occurring during the exposure of duplex stainless steels in the temperature range from 800 to 900 °C deeply affects the material toughness and corrosion resistance. σ-Phase precipitation process is strongly influenced by many physical parameters, such as the specific chemical composition, the ferrite amount and its average grain size, and the entity of plastic deformation due to the previous technological process. The strong dependencies of σ-phase precipitation on all these factors justify the continuous study of the process kinetics. This paper focuses on the σ-phase precipitation kinetics in F55 steel grade. The investigation has been performed by an innovative experimental method, such as the anisothermal dilatometric technique. The application of the Kissinger’s method has been used for deriving the process activation energy and kinetics. The results have been compared with the ones obtained by metallographic analysis and hardness tests performed on isothermally aged samples, heat-treated in a laboratory furnace at 850 °C.

Study of the effect of sigma phase precipitation on the sliding wear and corrosion behaviour of duplex stainless steel AISI 2205

The effects of σ-phase on the corrosion and unidirectional sliding wear behaviour of wrought duplex stainless steel (DSS) 2205 were investigated. The precipitation of σ-phase was induced by an isothermal aging at 850°C during 5, 15, 30 and 60min. Microstructural characterisation shows that the volume of δ-phase decreases as σ-phase increases. Dry sliding wear tests were carried out in a tribometer (model TRB, Anton Paar, Switzerland) with pin on disc technique at 7 and 15cm/s sliding rate with a load of 1N and an α-Al2O3 ball as counterpart. Results shown an important increase on wear resistance under dry sliding conditions after 5min of heat treatment due to the hardness provided by σ-phase on the DSS compared to base metal (BM). Nonetheless, cracks by Hertzian contact stress due to the difference of plastic deformation of matrix and σ-phase were observed at 15cm/s sliding rate in heat treated samples, induced by stress fatigue during the abrasion wear mechanism on the surface. Also there was observed that δ-phase is prone to dissolution in acidic media as σ-phase volume increases. Thus, results shown an improvement on wear resistance on samples aged for 5min in the starting critical intergranular corrosion susceptibility.