Sigma Phase Precipitation Research Articles

Effect of Deformation and Subsequent Heat Treatment on Sigma-Phase Precipitation and Mechanical Property of CoCrFeMnNi High Entropy Alloy

Effect of Deformation and Subsequent Heat Treatment on Sigma-Phase Precipitation and Mechanical Property of CoCrFeMnNi High Entropy Alloy

Phase and microstructure formation during reactive spark plasma sintering of AlxCoCrFeNi (x=0.3 and 1) high entropy alloys

This paper is one of the first studies focusing on phase and microstructure formation mechanisms during reactive sintering of AlxCoCrFeNi (x = 0.3, 1) high entropy alloys (HEAs). The alloys have been prepared by mechanical activation followed by spark plasma sintering (SPS). Both powders are subjected to gluing to the milling vials and the phenomenon intensifies with Al content, resulting in a more important mechanical alloying delay for the equiatomic composition. The temperature influence on phase and microstructure formation was investigated by SPS from 550 to 1100 °C. Al0.3CoCrFeNi reactive sintering led to a face-centered cubic (FCC) solid solution with a grain size around hundreds of nanometers with nanometric intergranular B2 precipitates which content and size reduced with the temperature increase. Grain growth was observed at prior particle boundaries due to dynamic recrystallization, a phenomenon that can be controlled through modification of the milling and sintering steps, leading thus to tailored grain sizes. For the AlCoCrFeNi composition, temperature increase favors secondary FCC formation and leads to sigma phase precipitation. A dual homogeneous-heterogeneous microstructure is obtained due to the important gluing leading to insufficient milling. The dynamic recrystallization at prior particle boundaries promotes also body-centered cubic (BCC) to FCC phase transition. Therefore, the proposed reactive sintering route allows to obtain, for the equiatomic composition, alloys exhibiting very fine microstructures, with higher FCC fraction content than by conventional metallurgy processes.

Flux-cored arc welding of 316L austenitic stainless steel: the effect of different shielding gas mixtures on microstructural features and weld performance

ABSTRACT Double-pass flux cored arc welding (FCAW) of 316 L austenitic stainless steel was performed using different shielding gas mixtures. Microstructural features and weld performance were evaluated. Based on the Creq/Nieq ratio of fusion zone and cooling rate, different morphologies of ferrite (skeletal, lathy, and dendritic) and austenite with various volume fractions were produced. An increase in Creq/Nieq ratio can increase the strength level because the presence of ferrite in the weld metal acts as a second-phase strengthening agent. On the other hand, ferrite is a suitable place for sigma phase precipitation. Highest joint efficiency resulted in the specimen welded via GTAW. The ductility of welds was ranked as CO2 +Ar shielded FCAW > GTAW > CO2 shielded FCAW > self-shielded FCAW. At room temperature, variation of delta-ferrite content (Creq/Nieq ratio) does not affect toughness. However, an increased amount of CO2 in shielding gas will result in decreased toughness values because of the enhanced formation of oxide inclusions. Increasing CO2 content of shielding gas from 0% to 18% and finally to 100% will result in decreasing toughness values from the highest value of 76J to 40 J and finally the lowest value of 38J.

Suppressed Plastic Anisotropy via Sigma-Phase Precipitation in CoCrFeMnNi High-Entropy Alloys.

The effect of sigma-phase precipitation on plastic anisotropy of the equiatomic CoCrFeMnNi high-entropy alloy was investigated. Annealing at 700 °C after cold-rolling leads to the formation of the Cr-rich σ phase with a fraction of 2.7%. It is experimentally found that the planar anisotropy (∆r = -0.16) of the CoCrFeMnNi alloy annealed at 700 °C is two times lower than that of the alloy annealed at 800 °C (∆r = -0.35). This observation was further supported by measuring the earing profile of cup specimens after the deep drawing process. The plastic strain ratio, normal anisotropy, and planar anisotropy were also predicted using the visco-plastic self-consistent model. The results indirectly indicated that the reduction of plastic anisotropy in alloy annealed at 700 °C can be attributed to the formation of the σ phase.

Interpass temperature effects on WAAM ER316L stainless steel corrosion using potentiostatic pulse tests

The effect of interpass temperature on the microstructure and pit nucleation behaviour in wire and arc additive manufacturing (WAAM) deposited ER316L stainless steel is investigated. Lowering the interpass temperature increases the ferrite content and decreases the sigma phase precipitation. Potentiostatic pulse measurements show that the WAAM ER316L pits develop predominantly near sigma phases and have higher pit density compared to a wrought 316L (UNS S31600). The pitting susceptibility of WAAM ER316L increases with increasing interpass temperature due to the precipitation of larger sigma phase enhancing the elemental segregation.

Unveiling the effects of Ti microalloying and thermomechanical processing on recrystallization behavior and mechanical properties of type 316 stainless steel

For the titanium-stabilized austenitic stainless steels, the interaction of precipitation due to the addition of Ti and the recrystallization process is an important aspect of grain refinement, which needs more systematic investigations. Accordingly, in the present work, the effects of Ti microalloying and thermomechanical processing on the recrystallization behavior and mechanical properties of type 316 stainless steel were investigated. It was revealed that the strain-induced α′-martensitic transformation in these metastable alloys is sensitive to the presence of Ti and C in solid solution or their presence as TiC carbides in the aged condition, as well as the temperature of rolling and reduction in thickness. During annealing, the reversion of deformation-induced α′-martensite to austenite and primary recrystallization resulted in the formation of fine-grained equiaxed microstructures, where the precipitation of TiC and sigma-phase was found to be important in the retardation of recrystallization and grain growth (by Zener pinning mechanism) as well as refining the grain size. Annealing the cold rolled 316Ti stainless steel at 800 °C led to a fine grain size of ∼2.5 μm with the precipitation of sigma-phase; while annealing at 900 °C resulted in a coarser grain size of 4 μm but without the presence of the unfavorable sigma-phase. The yield stress of 476 MPa, ultimate tensile strength of 805 MPa, and total elongation of 68 % were obtained for the latter, revealing the merits of Ti addition and thermomechanical processing for the improvement of mechanical properties.

Influence of Si and Mo on the Sigma-Phase Precipitation in S32750 Super Duplex Stainless Steel Slab

In this study, we investigated the effect of Si and Mo on the sigma-phase precipitation in S32750 super duplex stainless steel slab. The activity for Mo with increasing Si and Mo was calculated by the Wagner formula, and the equilibrium solidification phase diagrams of S32750 duplex stainless steels with different Si and Mo contents were calculated using the thermo-calc software. The sigma phase precipitated mainly at ferrite/austenite phase boundaries and grew up towards the interior of ferrite phase in S32750 SDSS slab. The area fraction and size of the sigma phases significantly increased with increasing Mo content and Si content. Also, the increment in Mo and Si content affected the Mo concentration in sigma phase. The sample（Mo:3.4%,Si：0.3%） had a lowest sigma-phase area fraction of 2.84% and had lowest Mo content in σ phase.The calculation results showed that the increase of Mo and Si content increased the initial precipitation temperature and maximum precipitation amount of σ phase in S32750 SDSS equilibrium phase diagram. The activity of Mo also increased with increasing Si content and Mo content. That is, Mo and Si elements promoted the precipitation of σ phase.

Sigma phase precipitation and growth in solution annealed 321 stainless steel during thermal aging: Effects on toughness & intergranular corrosion

Sigma (σ) phase is considered as one of the most deleterious phases in stainless steels (SS) due to its detrimental effects on mechanical and corrosion properties. In this work, σ phase formation and growth in annealed SS 321 (without δ-ferrite) and its effects on impact toughness and intergranular corrosion (IGC) were investigated, following thermal aging at 750 °C up to 5500 h. Microstructural characterization using X-ray diffraction and electron microscopy techniques revealed (a) precipitation of intergranular Cr-rich M23C6 and fine Ti-rich intra & intergranular carbides after short aging time of 27 h, (b) σ phase nucleation at the Cr-rich M23C6 - austenite interfaces after 100 h of aging and (c) σ phase at the austenite grain boundaries (GBs) beyond 700 h of aging. The experimental results, combined with thermodynamic calculations (ThermoCalc) showed that the meta stable Cr-rich carbide is the source of σ phase nucleation. σ phase started to grow at the expense of Cr-rich M23C6, and further, its size increased preferentially along the GB and into one of the austenite grains with increase of aging duration. Hardness remained in the range of 190–200 HV for all the specimens aged beyond 100 h and is shown to be due to stable Ti-rich intragranular carbides of 7–20 nm size. Impact toughness values decreased with the growth of σ phase, and up to 50% reduction in toughness was found by about 1% σ phase, with a ductile-brittle cleavage fracture. The normalized values of σ phase fraction, determined from its size and the toughness values, with aging duration followed Kolmogorov-Johnson-Mehl-Avrami (KJMA) model and both yielded the same empirical rate constant (k) of 2.7 × 10−7/s and the n value of ∼1. The lower k value and the n value of 1 suggest slower kinetics and one-dimensional growth of σ phase, which is in well agreement with microstructural observations. Susceptibility to IGC was seen in specimens with sub-microscopic σ phase (aged up to 100 h). However, aging beyond 700 h decreased resistance to uniform corrosion due to σ phase growth.

Effect of yttrium addition on precipitated phase, corrosion resistance, and mechanical behavior of 6Mo superaustenitic stainless steel

Optimizing alloy composition is an effective method to regulate the microstructure and improve the properties of superaustenitic stainless steel (SASS). In this research, 6Mo SASS containing 16 ppm and 83 ppm yttrium (Y) was prepared, and the effects of Y microalloying on secondary phase precipitation and materials performance were investigated. It was found that higher Y content could better inhibit the sigma(σ) and Cr2N phase precipitation along the grain boundary, leading to the improvement of the intergranular corrosion (IGC) resistance and mechanical properties of aged steels.

Cooling Optimization of a Large Duplex Forged Bar and Subsequent Improvement in Impact Toughness over Its Thickness

The machining of a central axial hole in a duplex stainless steel 2205 bar is sufficient to optimize cooling and thus to achieve the toughness requirements of the most demanding international standards. The cooling rates are obtained by finite element methodology simulation. The increased cooling rate in the central zone of the bar is directly related to the improvement of the Charpy impact toughness. Microstructure at different bar locations is characterized by optical microscopy, scanning electron microscopy, and energy‐dispersive X‐ray spectrometry. In the absence of sigma phase precipitation, an austenite spacing over 45 μ provokes a decrease in absorbed energy close to 40 J. Moreover, the pierced bar complies with the international standards requirements in tensile with yield strength higher than 450 MPa and Rockwell hardness lower than 31 (HRC) over the section and along the bar. Finally, the resistance to pitting corrosion required is also satisfactory due to the absence of chromium nitrides.

Enhanced thermoplasticity of 7Mo-0.4 N contained super austenitic stainless steel through pre-deformation

This study investigated the hot tensile deformation behavior of super austenitic stainless steel (SASS) with different pre-deformation degrees within 1150–1250 ˚C. The influence of pre-deformation on the number of annealing twin (AT), grain size, dislocation evolution, sigma phase precipitation, static recrystallization (SRX), and dynamic recrystallization (DRX) behaviors at hot tensile deformation stages is analyzed. Results show that the thermoplasticity of the SASS increases first and then decreases with the increase of pre-deformation degree at each deformation temperature. The area reduction of the samples pre-deformed for 0 %, 10 %, 20 %, and 25 % are 0.25, 0.32, 0.37, and 0.35, respectively at a tensile temperature of 1150 ˚C. When the pre-deformation degree is within the range of 0–20 %, the deformation storage energy (DSE) gradually increases with the pre-deformation degree. The SRX amount at the high-temperature holding stage and the DRX amount in the high-temperature deformation stage significantly increases, and the grain size gradually decreases. As a result, crack initiation and propagation are inhibited, and thermoplasticity improves. However, when the pre-deformation degree is increased to 25 %, the grains grow abnormally during the high-temperature tensile deformation, and a large number of sigma phases are induced, which deteriorates the thermoplasticity of the SASS.

Effects of Welding Parameters on Sigma Phase Precipitation in 25Cr-5Ni-1Mo-2.5Cu-1Mn-0.18N Duplex Stainless Steel

Recently, a new grade of duplex stainless steel, UNS S82551 (25Cr-5Ni-1Mo-2.5Cu-0.18N), has been developed to overcome the drawbacks in super martensitic stainless steel, conventional 22Cr and 25Cr super duplex stainless steels in terms of productivity and cost. The characteristic of the alloy design of UNS S82551 is to use Cu, instead of Mo, to ensure comparable corrosion resistance and strength. In addition, due to the significant decrease in Mo content, UNS S82551 is expected to be less sensitive to sigma phase precipitation during single or multi-pass welding compared with conventional and super duplex stainless steels. There is a trade-off between achieving better properties and avoiding sigma phase precipitation when increasing alloying elements such as Mo, Cr and Cu. In order to utilize the new UNS S82551 steel in industry for welding in a similar manner to conventional and super duplex stainless steels, the prevention of sigma phase precipitation is important. This work investigated the effect of weld thermal cycling on sigma phase precipitation behaviour in UNS S82551 welds. During thermal cycling in the tests, the amount of sigma phase for UNS S31803 and UNS S32750 increased with increasing the number of thermal cycles, and lower cooling rate, but it was not observed for UNS S82551. Based on the isothermal kinetics of sigma phase precipitation, the amount of sigma phase precipitated during thermal cycle can be predicted by applying the additivity rule to the physical model. The area fractions of sigma phase calculated have a good fit to the experimental ones.

Effect of Temperature on Tribological Behavior of FeCrNi Medium Entropy Alloy

FeCrNi medium entropy alloy (MEA) is a promising structural material due to its outstanding mechanical properties and corrosion resistance. However, study on the high-temperature tribological behaviors of FeCrNi MEA is still scarce. This work investigates the tribological performances of the FeCrNi MEA against Si3N4 balls over typical operating temperatures from 25 to 800 °C. The FeCrNi MEA exhibits excellent wear resistance at an intermediate temperature of 600 °C. The microstructure characterization of the wear tracks reveals a transition in the wear mechanism from abrasive wear at 25 °C to oxidative-delamination wear at 600 °C. The improved tribological performances are mainly attributed to the high Cr content, which promotes the formation of the compact oxide layer and the precipitation of the hard sigma (σ) phase.

Effects of Mo and Cu Contents on Sigma Phase Precipitation in 25Cr-5Ni-Mo-Cu-1Mn-0.18N Duplex Stainless Steel

Effects of Mo and Cu Contents on Sigma Phase Precipitation in 25Cr-5Ni-Mo-Cu-1Mn-0.18N Duplex Stainless Steel

Effect of Sigma Phases on Moderate-Temperature Tensile Properties of Z3CN20.09M CASS Used for Primary Coolant Pipe of Nuclear Power Plant

The effect of sigma phases on the moderate-temperature tensile properties of Z3CN20.09M casting austenite stainless steel was investigated by means of isothermal treatment, scanning electron microscopy, transmission electron microscopy, instrumented nanoindentation, tensile testing, and finite element simulation. The results show that the yield strength and ultimate tensile strength of aged specimens tensile tested at moderate temperature increase remarkably with an increasing sigma phase, while the elongation at break decreased. The strain-hardening rate of aged specimens with sigma phases is higher than that of unaged specimens without sigma phases at a certain low-strain range. However, the value of the strain-hardening rate of aged specimens is lower than that of unaged specimens when the strain exceeds a certain value. The effect of the sigma phase on the tensile properties at moderate temperature is also more significant. This can be attributed to the many high-energy σ/γ2 and α/σ/γ2 incoherent phase boundaries caused by the precipitation of sigma phases. On the one hand, these boundaries hinder the movement of dislocations and subsequently accumulate dislocations to some extent, so strength is enhanced and the strain-hardening rate is improved. On the other hand, microcracks at these interfaces initiate and propagate more easily when the strain exceeds a certain value. Thus, the elongation value and the strain-hardening rate decrease.

Effect of Precipitation of Alpha Line and Sigma Phases on the Microstructure and Corrosion Resistance of the Duplex Stainless Steel SAF 2205

Effect of Precipitation of Alpha Line and Sigma Phases on the Microstructure and Corrosion Resistance of the Duplex Stainless Steel SAF 2205

Precipitation of Sigma and Chi Phases in Cast Standard Duplex Stainless Steel

Precipitation of Sigma and Chi Phases in Cast Standard Duplex Stainless Steel

Sigma phase embrittlement-induced failures of heat-resistant stainless steel traveling grate links

Traveling grates (TG) are used as beds over which green pellets are subjected to a series of thermal cycles, namely preheating, induration, and firing to make the pellets suitable for charging in the blast furnaces of an integrated steel plant. In this work, chronic failures of TG links after a service life of 2.5–3 years are investigated. A comparative analysis of failed, used and new TG links was carried out. Fractography of the failed link revealed an intergranular brittle fracture near the surface followed by a transgranular fracture with a signature of decohesion in the bulk of the fracture surface. Microstructural analysis revealed the presence of pre-existing grain boundary chromium carbides (Cr23C6) in the new (unused) link, which can facilitate easy crack initiation and propagation. Furthermore, failed and used TG link revealed the presence of extensive precipitation of needle-like sigma phase confirmed by a combination of X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy techniques. Such precipitation of the sigma phase occurs during exposure to a susceptible high-temperature range. The presence of the sigma phase is known to embrittle the austenitic stainless steel and such embrittlement is confirmed by a significant increase in hardness and decrease in Charpy impact toughness of failed and used TG links compared to the new TG link. Thermodynamic and kinetic simulations confirmed a high susceptibility of the existing alloy composition to extensive sigma phase precipitation in a wide temperature range. A new alloy composition with higher nickel (∼30 wt%) and free from tungsten is proposed to reduce the susceptibility towards in-service embrittlement induced by sigma phase precipitation.

High-temperature tribological behavior of CoCrFeNiV high-entropy alloys: A parallel comparison with CoCrFeNiMn high-entropy alloys

The present work investigates the high-temperature tribological behavior of CoCrFeNiV fabricated by vacuum hot pressing sintering, and parallel experiments were conducted with CoCrFeNiMn as a comparison. It was found that the wear rate of CoCrFeNiV is almost an order of magnitude lower than for CoCrFeNiMn. Compared with CoCrFeNiMn of a single FCC, the precipitation of the hard sigma (σ) phase from the addition of V element made CoCrFeNiV possess excellent hardness (975 ± 11 HV) and high-temperature wear-resistance. The detailed characterization of the worn surface revealed that mild adhesion and oxidative wear are the dominant wear mechanisms of CoCrFeNiV. With the increase of temperature, the tribo-oxidation composed of V2O5, NiO and Cr2O3 play an excellent anti-oxidation lubrication role during the friction process. The higher volume fraction of the hard σ phase and the dense oxide glaze layer resulted in wear of approximately 0.289 × 10–4 mm3/Nm at 800 °C.

Elucidating microstructural evolution and its effect on the mechanical properties of FSWed alumina-forming austenitic steel for power plant applications

Alumina-forming austenitic (AFA) steel is considered as a promising candidate used for advanced energy systems. Fusion welding of austenitic steels usually requires the introduction of a certain delta (δ) ferrite to avoid thermal cracking, but for the AFA steel, the presence of δ ferrite will greatly accelerate the precipitation of hard and brittle sigma (σ) phase at its target service temperature.In this work, for the AFA steel with a typical composition, we demonstrated a superior capability of friction stir welding (FSW) in avoiding the formation of δ and σ phases, and developing the favorable weld microstructures under various welding conditions. The microstructure evolution of AFA steel during FSW with various rotational speeds (300–700 rpm) was studied in detail. The resulting FSW joints all exhibit higher yield strength (YS) and ultimate tensile strength (UTS) than the base metal, especially at low rotational speed, while retaining reasonable ductility, despite decreasing with reducing rotational speed. The associated relationship between the microstructure and mechanical properties of the resulting FSW joints was well correlated. This work provides an effective method to obtain the fully-austenitic welded joints of AFA steel with excellent strength-ductility synergy.