Σ-phase Precipitation Research Articles

Effect of Mo on intergranular corrosion behavior in super-austenitic stainless steel

The effect of the Mo element on the intergranular corrosion behavior was investigated on super-austenitic stainless steels. The σ-phase structure and compositional changes of stainless steels with different Mo contents during aging at 800–900°C were investigated by TEM, combined with electrochemical tests, and two different intergranular corrosion mechanisms were revealed. The low-Mo 904 L stainless steel induces severe intergranular electrochemical corrosion due to selective anodic dissolution at grain boundaries caused by decreased passivation ability and σ-phase composition changes. The σ-phase precipitation of 254SMo and AL-6XN with high-Mo leads to Cr and Mo depletion reduces passivation film continuity, and causes intergranular corrosion.

Effect of Mo on the σ-phase precipitation behavior of super-austenitic stainless steel

The aging precipitation behavior of super-austenitic stainless steels 904L (4.32% molybdenum (Mo)), 254SMo (6.06% molybdenum) and AL-6XN (6.29% molybdenum) with different molybdenum contents at 800–900°C was investigated by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The effect of molybdenum content on the σ-phase precipitation behavior of commonly used super-austenitic stainless steel was investigated. The results show that the super-austenitic stainless steels with different molybdenum contents formed different numbers and shapes of σ phases in the isothermal aging process, and the number of σ phases of 254SMo and AL-6XN is much larger than that of 904L, and the shape of the σ phases mainly shows coarse acicularity and elongation, while the σ phases of 904L show fine granularity.

Effect of Re Addition on Microstructure and Oxidation Resistance of Co-Cr-Ta-Ti-C Alloys

This study investigated the effect of Re addition on the microstructural evolution and oxidation resistance of Co-Cr-Ta-Ti-C alloys. The alloys tested in this study consisted of a Co-rich matrix with mixed HCP and FCC phases, together with TaTiC2- and Ti-based carbides. The addition of Re promotes the precipitation and growth of a CoCr-rich σ-phase. Differential thermal analysis (DTA) showed that the melting temperatures did not drastically change up to 10 at% Re addition, whereas 15 at% Re addition significantly increased the melting temperature of the alloys. Oxidation tests in air at 1200 °C showed that an excessive Re content could lead to a deterioration in oxidation resistance, mainly due to the accelerated formation of volatile Re oxides.

Characterization of σ-phase precipitation and effect on performance in duplex stainless steel S31803

A flange made of duplex stainless steel S31803 prematurely failed due to corrosion fatigue cracking in an offshore oilfield. This paper was carried out to determine the correlation among manufacturing processes, microstructures and properties of the failed flange by using various analysis methods. The results indicate that the σ-phase was present in heat affected zone and base metal, and the content of σ-phase was 17.0 vol% and 4.7 vol%, respectively. The hardness of heat affected zone was extremely high, up to 394HV0.3. The tensile and impact fractures of base metal were brittle cleavage fractures. The impact toughness of duplex stainless steel was remarkablely deteriorated due to the precipitation of σ-phase. The precipitation of σ-phase in base metal was owing to improper aging process. A large amount of σ-phase precipitation was caused by excessive welding heat input in heat affected zone.

Effect of sigma (σ) phase precipitation on the recrystallization kinetics of AISI 904L superaustenitic stainless steel

Microstructural evolutions, σ-phase precipitation, and recrystallization kinetics of cold-rolled AISI 904L stainless steel during annealing were studied in the present work. The microstructure and hardness evolution were different in the temperature ranges of T < 950 °C (with σ-phase precipitation) and T ≥ 950 °C. The former range was associated with the fall of hardness due to recrystallization softening, followed by hardening due to the concurrent σ-phase precipitation; while the latter range was associated with the continuous fall of hardness. Using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis, the recrystallization activation energy in these ranges were respectively determined as 325 and 257 kJ/mol, where the higher activation energy in the former range was correlated to the retardation effect of the σ-phase. Via applying the Kissinger's equation to the differential scanning calorimetry (DSC) results, the activation energy of ∼332 kJ/mol was obtained, which was consistent with the precipitation of the σ-phase.

Dynamic mechanical properties and σ precipitates strengthening of a NiCrFeCoMo0.2 high-entropy alloy additively manufactured by selective laser melting

Dynamic mechanical properties of the selected laser melting (SLM)-NiCrFeCoMo0.2 high-entropy alloy (HEA) are further improved by controlling the molten pool structure and the generation of σ-phase precipitation particles. In this study, a split Hopkinson pressure bar (SHPB) was used for testing the dynamical mechanical properties of the quenched SLM-NiCrFeCoMo0.2 HEA specimens. Microstructure and volume fraction of the σ precipitates were characterized by X-ray diffractometry (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and the small angle neutron scattering (SANS) technique. After heating to 800 ℃, holding for 8 h and water quenching, the molten pool boundary of the SLM-NiCrFeCoMo0.2 HEA disappear, and the ultimate compressive strength and the impact energy of specimens reach 1970 MPa and of 217.45 MJ·m−3, respectively. Due to the segregation of Mo elements in the molten-pool boundary of the as-build SLM-NiCrFeCoMo0.2 HEA, more σ precipitates tend to form at the residual molten pool boundary of the quenched SLM-NiCrFeCoMo0.2 HEA. The σ precipitates play key role in strengthening of the SLM-NiCrFeCoMo0.2 HEA.

Mechanical Characterization at Nanoscale of Austenite, Ferrite, and Sigma Phases via Hardness Measurement and Fretting Wear Behavior of a Duplex Stainless Steel

This study aimed at the mechanical characterization, on a nanometric scale, of the constituents obtained for different fractions in duplex stainless-steel plates subjected to 850, 950, 1000, and 1150 °C heating treatments via hardness measurements and determining their influences on the fretting wear behavior of the studied steel. The obtained ferrite (α)-, austenite (γ)-, and sigma (σ)-phase fractions were determined using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. The mechanical characterization was carried out using hardness measurement and fretting wear techniques via nanoindentation. For comparison purposes, the Vickers microhardness was also characterized to determine the effect of the σ phase, which eventually formed, on the obtained microstructure properties as a whole. Two distinct behaviors were observed, depending on the eventual formation of σ phase as a function of the treatment temperature: (i) specimens treated at 850 and 950 °C showed a hardening effect (HV0.5 values of 333 ± 15 and 264 ± 13, respectively) due to σ-phase precipitation (hereafter termed ‘as-aged’), and (ii) specimens treated at 1000 and 1150 °C (with HV0.5 values of 240 ± 13 and 249 ± 4, respectively) showed no σ-phase precipitation (hereafter termed ‘as-solubilized’). The increases in the microhardness values for the as-aged specimens were attributed to the hardness of the σ-phase precipitates (which showed nanohardness values varying in the 8.0–8.5 GPa range), which was approximately twice that of the austenite and ferrite grains (both phases showed nanohardness values in the 3.6–4.1 GPa range, on average). When formed (for fractions on the order of 8% and 3% at 850 and 950 °C, respectively), σ phase was mainly observed at the α/γ grain interfaces or boundaries. Fretting wear tests, using a diamond sphere with a radius of 10 μm as the counter body and a load of 20 mN, revealed the same wear mechanisms in the α/γ matrix for all studied conditions. However, as-solubilized specimens (heat-treated at 1000 and 1150 °C) displayed higher resistance to fretting micro-wear in the austenitic grains compared to the ferritic grains, indicating lower plastic deformation in the respective wear scars on the obtained tracks. In particular, as-aged specimens (heat-treated at 850 and 950 °C) exhibited lower coefficients of friction due to their higher surface resistances. The localized wear at σ-phase grains was much less pronounced than at ferrite and austenite grains. Overall, this study provides valuable insights into the mechanical behavior of microstructural changes in duplex steel at the nanometric scale.

Effect of annealing treatment on microstructural and mechanical properties of a hot-forged FeCrCuMn2Ni2 high-entropy alloy

The present study prepared an as-cast ingot of a FeCrCuMn2Ni2 high-entropy alloy using vacuum induction melting process, followed by hot forging at 650–750 °C. The samples were then annealed at temperatures between 600 and 1100 °C for 2 h, and were characterized in terms of their microstructural features and mechanical properties. No phase transformation was observed for the hot-forged and annealed samples. Compared to the as-cast state, the hot-forged sample exhibited higher compressive yield strength and hardness, but lower compressive strain. This behavior can be due to the precipitation of hard σ-phase with an average size of 0.55 µm during hot forging. It was further observed that the precipitates dissolved, when the temperature exceeded 1000 °C, resulting in discontinuous static recrystallization with an average grain size of 15.9 µm. Annealing between 600 and 800 °C decreased the compressive yield strength and compressive strain, which could be attributed to σ-precipitates morphological change and their growth up to an average size of 1.34 µm. On the other hand, when the annealing temperature increased above 800 °C, the compressive yield strength decreased and compressive strain increased. This behavior was attributed to the dissolution of a large amount of σ-precipitates, growth of the residual σ-precipitates, and softening of the alloy. Findings further revealed a decrease in the hardening capacity value of annealed samples in the range of 600–800 °C, resulting from the formation of σ-precipitates. In contrast, the hardening capacity value of the alloy increased with the annealing temperature, due to the grains size increase at higher temperatures.

Microstructure and Properties of Super-Ferritic Stainless Steels Used for Marine Construction after Fire Exposure

In this paper, the super-ferritic stainless steels (SFSSs) were simulating treated in a fire accident, and their mechanical properties and corrosion performance before/after fire exposure were studied. The experimental results demonstrated that SFSSs have a good combination property with the tensile strength, total elongation, and corrosion rate in FeCl3 solution of ∼600 MPa, ∼22.5%, and 0.03 mm/a. After fire exposure, the mechanical properties and corrosion performance were affected based on the exposure temperature due to the precipitation of intermetallic compounds. When the fire exposure temperature was 700–800°C, the plasticity was exasperated and the value decreased to only 1.5% at the condition of 800°C × 4 h due to the bulk σ-phase precipitation at grain boundaries. Meanwhile, the brittle fracture patterns emerged, and the corrosion rate in FeCl3 solution increased to 0.4 mm/a. When fire exposure takes place at 600°C for less than 4 h, the mechanical properties and the corrosion performance were little affected. Thus, it was possible to use SFSSs in marine environments.

The dissolution behavior of σ-phase and the plasticity recovery of precipitation-embrittlement super-ferritic stainless steel

In this study, a dissolution heat treatment was applied planning to recover the plasticity of an embrittled super-ferritic stainless steel caused by the precipitation of σ-phase, χ-phase and Laves phase. The dissolution behaviors of the brittle precipitation phase, and the concerned grain coarsening phenomena, as well as their effects on the mechanical properties were investigated. It was observed that the χ-phase dissolved first then did the σ-phase and therefore the Laves phase. The σ-phase dissolution kinetics showed a strong dependence on the reheating temperature. A weak grain growth phenomenon was found during the reheating process at 1100 °C. The Laves phase re-formation was observed during reheating at 1000 °C–1050 °C, which was generated by the Nb and Mo atoms diffusion. The plasticity of the tested steels was recovered after dissolution treatment at the temperature range of 1050 °C–1100 °C, during which the fracture mode was remodeled from brittle fracture to toughness fracture. The dissolution treatment at the temperature of >1050 °C was effective to recover the plasticity of precipitation- embrittlement SFSSs.

Heat treatment of laser powder-bed-fused Co–28Cr–6Mo alloy to remove its microstructural instability by massive FCC→HCP transformation

Laser powder bed fusion ( L -PBF) of Co–28Cr–6Mo alloy is a powerful tool for many engineering applications but one has to be familiar with its microstructural instability in the as-produced state. In our previous study [Roudnicka et al., Additive Manuf. 44 (2021) 102025], we depicted the phenomenon of this instability when exposing the material to temperatures above 673 K. Microstructural changes bringing about hardening of the material were studied thoroughly. In the present study, we follow up by revealing the effect of this hardening on mechanical properties in tension and show the beneficial effect of heat treatment on removing the observed instability. Hardening associated with increasing the proportion of HCP ε-phase and with the precipitation of σ-phase has a detrimental effect on material plasticity and can be negatively affect the fatigue too. We thus applied a two-stage heat treatment comprising solution annealing at 1473 K/2 h and aging at 1173 K/5 h. Not only it induced material stability but also increased the material strength with not such a detrimental decrease in material plasticity as was brought by direct aging at 1173 K/5 h. • L -PBF Co–28Cr–6Mo shows microstructural instability above 673 K. • The stability can be induced by massive FCC → HCP transformation. • Appropriate solution annealing and aging (STA) treatment stabilized the material. • STA enhanced material strength and preserved 70% of its plasticity.

Study on the σ-phase precipitation of SAF2906 duplex stainless steel

Abstract Samples of SAF2906 duplex stainless steel were aged at 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C and 950 °C for 1 h, after high temperature solution heat treatment. The morphology of 𝜎-phase in aged SAF2906 duplex stainless steel was then studied by means of scanning electron microscopy, X-ray diffraction and transmission electron microscopy, and the chemical composition of the 𝜎-phase was determined using X-ray energy dispersive spectrometry. The results indicated that the 𝜎 precipitation in SAF2906 duplex stainless steel formed both inside δ-ferrite with high Cr and Mo concentrations, and along δ-ferrite/γ-austenite interfaces; however, relative to the SAF2205 and SAF2507 duplex stainless steels, the content of Cr in 𝜎-phase increased significantly up to 70 %, which was much higher than Fe in the same phase and became the major element of 𝜎-phase.

Modelling and simulation of densification and σ-phase precipitation in PM duplex steel AISI 318LN during hot isostatic pressing

Following the development of hot isostatic pressing (HIP) with integrated rapid cooling technology, it is now possible to combine consolidation of encapsulated powder and subsequent heat treatment in a single process step in a pressure vessel. Using this technology, a near-net-shape product with target microstructure can be achieved. However, qualified final products still require traditional expensive and time-consuming trial-and-error tests. To eliminate these costly trial-and-error tests and achieve the required final properties of the products, it is urged to develop a combined numerical model to precisely predict the geometrical changes of the component during the HIP process and the evolution of significant microstructural features. This is realised in this work exemplarily by using duplex stainless steel AISI 318LN. This steel was selected due to requirements coming from applications regarding precise complex shaped near-net-shape components combined with high strength and toughness by avoiding brittle σ-phase precipitation. In this study, an existing finite element model for the densification in the HIP process was supplemented by a model for σ-phase precipitation kinetics based on microscopical characterisation. Moreover, the temperature dependent heat transfer coefficient (HTC) was implemented in the developed model to simulate the evolution and the distribution of the σ-phase during cooling. The numerically simulated final component dimensions and σ-phase volume fractions were verified by comparing with experimental data obtained from specimens and components of various shapes produced by both conventional HIP process and HIP with integrated rapid cooling. The high consistency between experimental and numerical results validates the accuracy of the developed simulation approach.

High-temperature phase characterization of AlCrFeNiTi compositionally complex alloys

In this work, alloys composed of the 5 base-elements Al, Cr, Fe, Ni and Ti and elements in minor amounts were produced by arc melting and powder metallurgy. The alloys’ phase distribution was analyzed via high energy X-ray diffraction (HEXRD), neutron diffraction and electron backscatter diffraction (EBSD). The two studied compositions have low concentrations of the elements Al and Ti, medium concentrations of the elements Cr and Fe and high amounts of Ni (chemical composition in at.-%: Al10Cr20Fe20Ni40Ti5, Al10Cr20Fe20Ni35Ti10). The goal then was to analyze the microstructure of these compositions at elevated temperatures in dependence of their chemical composition and the production method. Analysis of both compositions show the presence of B2-phases, L12-phases, Full Heusler-phases (H_L21), C14_Laves-phase and σ-phase. The combination of dilatometric tests with microstructure analysis via HEXRD and neutron diffraction gives the possibility of observing changes in phase amounts in dependence of temperature. A quantification of phase amounts in the compositions is supported by thermodynamic calculations of stable phases, applying the calculation-of-phase-diagrams (CALPHAD) method within the software Thermocalc. Due to its detrimental influence on ductility the existence of σ-phase is undesired in nickel-chromium-iron alloys and hence σ-phase precipitation with an associated dissolution of the phases B2 and L12 at elevated temperatures was studied in detail.

Microstructural instability of L-PBF Co-28Cr-6Mo alloy at elevated temperatures

In relation to the production of complex aircraft components, the additive manufacturing (AM) of Co-28Cr-6Mo alloy is highly discussed as it could bring many benefits regarding new designs, components lightening and shortening the design-to-product period. However, such components must undergo strict requirements for mechanical properties and material stability, even at elevated temperatures which might occur during part operation. So far, little is known about the thermal stability of Co-28Cr-6Mo alloy when produced by laser powder bed fusion (L-PBF), one of the mostly used AM technologies. In the present study, we reveal the instability of this material already at temperatures above 673 K. Material hardening accompanied with a significant loss of plasticity is caused by the breakdown of fine cellular microstructure with intercellular Mo segregation, precipitation of σ-phase and phase transformation of the matrix. Detail characterization by transmission electron microscopy was carried out for the maximum hardness state obtained after annealing the alloy at 1173 K for 5 h. The important findings of this study, representing an operational risk for L-PBF Co-28Cr-6Mo alloy, call for subsequent research into suitable heat treatment that would induce the stability of the material.

Effect of Grain Size on the Precipitation Behaviour in Super-Ferritic Stainless Steels During a Long-Term Ageing

A 27.6Cr-3.6Mo-2Ni alloy was solution treated and then aged for a long time to study the effect of grain size on precipitation behaviour by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The experimental results demonstrated that the average grain size increased from 46.3 ± 6.2 to 101.8 ± 13.5 μm and the grain boundary length per unit area decreased from 3.3 × 104 to 1.7 × 104 m/m2 with an increasing annealing temperature from 1100 to 1200 °C. After ageing at 800 °C, the σ-phase, χ-phase and Laves phase were observed. As the ageing time increased, the σ-phase notably increased, while the χ-phase and Laves phase gradually decreased before finally vanishing after ageing for 400 h. The σ-phase precipitation kinetics curves consisted of two parts, and the grain size had a significant effect on the first stage of the precipitation curves due to the abundance of nucleation sites in the specimens with finer grains. The Laves phase was transformed from Nb(C,N) particles by Nb diffusion. As the ageing time increased, the ferrite phase decreased due to the transformation of the ferrite phase to the σ-phase, and then C was expelled into the untransformed ferrite grains. Moreover, new Nb(C,N) particles were formed by Nb diffusion from the Laves phase, resulting in the absence of the Laves phase.

The precipitation behavior and its effect on mechanical properties of cold-rolled super-ferritic stainless steels during high-temperature annealing

The precipitation behavior, microstructure evolution and mechanical properties of a cold-rolled 27.6Cr–3.7Mo–2Ni ferritic stainless steel during high temperature annealing treatment were investigated. Experimental results demonstrated that the cold rolled microstructure was characterized by orientation-dependent heterogeneity, and many shear bands were found in the <111>//ND (normal direction) orientation grains. The elongated deformation microstructure gradually changed into equiaxed grains and three kinds of interphases including σ-phase, χ-phase and Laves phase were observed as the annealing temperature increased from 850 to 1000 °C. After annealing below 1000 °C, the σ-phase and χ-phase were observed to be located at grain boundaries, near the TiN particles as well as along with shear bands in grains, whereas the Laves phase distributed dispersedly in grains. After annealing at 1000 °C, both the σ-phase and χ-phase were almost completely dissolved, while the nanoscale Laves phases at grain boundaries were dominant. The grain boundary Laves phase precipitation at high temperature was attributed to the Nb segregation induced by cold rolling deformation. The σ-phase precipitation kinetics was calculated based the Johnson–Mehl–Avrami equation, and the fastest precipitation kinetics appeared at 850 °C with the activation energy of ~176.7 kJ·mol−1. The Vickers hardness, ultimate tensile strength and yield strength values decreased while the elongation value increased with increasing annealing temperature. The breakage pattern transformed from brittle to ductile fracture as the annealing temperature increases from 850 to 1000 °C.

Dynamic precipitation of σ-phase and element partitioning in equiatomic CoCrFeMnNi high-entropy alloy

We studied the development and element partitioning for σ-phase precipitation during hot compression of the CoCrFeMnNi HEA. The σ-phase mainly observed in the necklaced area (NA), where dynamic recrystallization (DRX) occurred but did not occur in the deformed area (DA). The σ-phase increases the hardness and elastic modulus measured by nanoindentation in NA, compared to that of DA. Several defects are transferred by the interface reaction of the crystal grains accompanied by the partitioning of the Cr and Ni elements. This results in the accelerated dynamic precipitation of the Cr-rich σ-phase.

On the compositional and structural redistribution during partial recrystallisation: a case of σ-phase precipitation in a Mo-doped NiCoCr medium-entropy alloy

In this work, we discussed the atomic-scale mass transport occurring in a Mo-doped NiCoCr medium-entropy alloy to form σ-precipitates during partial recrystallization at 700°C. We reveal the segregation of Cr/Mo to the fault planes of nano-twins/stacking faults in non-recrystallized zones. Upon annealing, the migration of recrystallized grain boundaries (GBs) redistributes the structure and composition, leading to the annihilation of faults and leaving behind the Mo-rich σ-precursors for precipitation. The segregated Cr/Mo solutes can also diffuse to the moving GBs and create additional σ-nucleation sites. The present results highlight the role of Mo in overcoming strength-ductility trade-off of the alloy.

Comprehensive study of phase transitions in equiatomic AlCoCrFeNi high-entropy alloy

In recent years, high-entropy alloys (HEAs) have attracted research attention because of their promising properties of high strength at elevated temperatures and high resistance to oxidation and corrosion. One of the most investigated families is the AlCoCrFeNi-based HEA. This series tends to form simple structures such as face-centered cubic (FCC), ordered and disordered body-centered cubic (B2 and BCC, respectively) and sigma phase (σ). Many studies aimed at calculating and predicting the thermodynamic and phase transitions in this system have been based on existing binary or ternary phase diagrams, in the absence of relevant thermodynamic experimental data. This has led to inaccurate phase predictions. In the present work, the phase transitions, thermal expansion and activation energies in the AlCoCrFeNi system have been measured and analyzed. It was found that at about 590 ℃, a FCC phase starts to precipitate from the B2/BCC matrix: the high-energy interphase at the FCC/BCC boundary leads to σ-phase precipitation. The activation energy for the growth of these phases was found to be 155.14 ± 13.47 kJ/mol. At 960 ℃, the σ-phase decomposed to a B2 matrix with “dot-like” BCC particles. The last phase transition was found to be a complex event, which contained two transitions: BCC → B2 at 1154 ℃ and FCC → B2 between 1203 and 1248 ℃: this transition was associated with a large volumetric shrinkage. Finally, at temperatures near melting there is a single B2 phase region.