Intermetallic Sigma Phase Research Articles

Empirical and Computational-Based Phase Predictions of Thermal Sprayed High-Entropy Alloys

AbstractDue to the wide range of compositional possibilities in the high-entropy alloy (HEA) field, empirical models and the CALPHAD method have been implemented to efficiently design HEAs. Although most design strategies have been tested on as-cast alloys, their validation for thermal sprayed HEA coatings is lacking. In this work, empirical models and the CALPHAD method under equilibrium and non-equilibrium conditions are assessed for phase prediction in five HEAs in the as-cast, laser clad and thermal sprayed conditions. High-velocity oxygen fuel coatings were prepared for these five HEAs, and their phases were identified by the x-ray diffraction analysis. These processes, even though their cooling rates vary significantly, show similar phase formation as indicated by a literature review and the current experimental study. The CALPHAD equilibrium calculation predicted most of the phases at specified temperatures. Furthermore, the CALPHAD-based non-equilibrium simulations correctly predicted the major phases present in the HEA coatings. The empirical models also show good prediction capability, but the intermetallic sigma phase is problematic for the parameter-based models. Therefore, the CALPHAD method can be used to efficiently design and develop HEAs prepared under conditions that encompass rapid cooling, such as occurring during thermal spray processes.

Interface analysis of additively manufactured pure molybdenum and AISI 304 stainless steel building-plate

In this study pure molybdenum was produced by laser powder bed fusion on an AISI 304 building-plate. Adhesion issues were found at the interface between the two materials since the very beginning of the production process. Interface analysis was performed in order to investigate possible origins of the failure. The investigation showed that large cracks could propagate and led to separation of the printed part from substrate when the dilution of Mo on AISI 304 platform was approximately in the range 40 ÷ 50. The brittle intermetallic sigma phase was extracted and analysed by XRD on Mo-AISI 304 interface specimens. It is assumed that the sigma phase impaired the interface strength, providing a preferential route for brittle crack propagation.

Ti content effect on microstructure and mechanical properties of plasma-cladded CoCrFeMnNiTix high-entropy alloy coatings

The CoCrFeMnNiTix (atomic ratios, x = 0, 0.3, 0.65 and 1) high-entropy alloy (HEA) coatings were fabricated via plasma cladding process. The effects of Ti content on the microstructure, mechanical properties and tribological performance of as-prepared coatings were investigated. The results show that with the atomic ratios of Ti increasing from 0 to 1, the microstructure of CoCrFeMnNiTix coatings evolves from single FCC structure with columnar dendrites to FCC structure with net-shaped interdendrites and columnar dendrites, hybrid phases of FCC structure, BCC structure and intermetallic sigma phase with equiaxed and/or snowflake-like dendrites and gradually increased grain size. The Ti addition can improve the mechanical properties of CoCrFeMnNiTix coatings and the CoCrFeMnNiTi1 coating has a microhardness of 903.23 ± 45.16 HV and a reduced nano-hardness of 10.02 GPa. The increase in Ti content leads to the more enhanced wear resistance of CoCrFeMnNiTix coatings. The dominant mechanism for the CoCrFeMnNiTix (x = 0 and 0.3) coatings is abrasive wear, the mechanisms for the CoCrFeMnNiTi0.65 coating are abrasive wear and oxidation wear, and main reasons for the wear loss of CoCrFeMnNiTi coating are abrasive wear, oxidation wear and spalling effects induced by brittle fracture.

Study of high temperature friction and wear performance of (CoCrFeMnNi)85Ti15 high-entropy alloy coating prepared by plasma cladding

(CoCrFeMnNi)85Ti15 high-entropy alloy (HEA) coating prepared on Q235 steel substrate via plasma cladding was investigated for its microstructure, hardness, and high temperature tribological performance. The microstructure of (CoCrFeMnNi)85Ti15 HEA consists of FCC and BCC solid solutions as well as an intermetallic sigma phase. The FCC solid solution is characterized by a lattice parameter of 3.669 Å and is enriched with Co-Cr, the BCC solid solution has a lattice parameter of 2.998 Å and is enriched with Fe-Mn, and the intermetallic sigma phase is enriched with Ni-Ti. The micro-hardness of the Q235 steel substrate, the CoCrFeMnNi HEA coating, and the (CoCrFeMnNi)85Ti15 HEA coating is 123.6 ± 6.2, 150.1 ± 7.4, and 910.5 ± 26.6 HV, respectively. As the hardness of the (CoCrFeMnNi)85Ti15 HEA coating is enhanced six times more than that of the CoCrFeMnNi HEA coating, it exhibits a lower wear rate and a better wear resistance at high temperatures. The best wear resistance, which corresponds to a wear rate of 4.08 × 10−6 mm3·N−1·m−1, is obtained at 400 °C. The wear mechanisms of the (CoCrFeMnNi)85Ti15 HEA coating are predominantly oxidation wear and contact fatigue, although there is evidence of oxidation wear and adhesive wear at 800 °C.

The influence of Ce micro-alloying on the precipitation of intermetallic sigma phase during solidification of super-austenitic stainless steels

The influence of Ce addition on the precipitation of ferrite (δ) and sigma (σ) phase in super-austenitic stainless steels during solidification was investigated via a combined experimental and theoretical approach. Thermodynamic calculations reveal that the secondary phase of δ and σ are precipitated in sequence near the end of the solidification process due to the segregation of Mo and Cr. The Ce addition improves the thermal stability of the δ, expands the range of Mo content for δ phase precipitation and increases the δ fraction in the solidified structure in the Gulliver–Scheil model. Experimental observation shows that a small addition of Ce accelerates δ nucleation in the interdendritic solute-rich liquid, refines the solidified structure and reduces Mo and Cr segregation, which suppresses precipitation of the σ phase. The σ phase precipitation is replaced by δ phase precipitation after a Ce addition of 0.016%. The change in the secondary phase from σ to δ shortens the time required for homogenization and reduces the nano-hardness of the interdendritic secondary phase, which is significant for hot working.

The effect of microstructure on the corrosion resistance of VDM® alloy C-4 in molten salts

High-temperature corrosion of VDM® Alloy C-4 was investigated in 3LiCl–2KCl and acidic KCl–AlCl3 melts in a wide temperature range (450–750 °C). The corrosion resistance of the alloy in molten salts depends on the oxidizing properties of the electrolyte and temperature. Holding VDM® Alloy C-4 above 650 °C may lead to the formation of secondary phases at the grain boundaries inducing undesirable intergranular corrosion. The secondary phase is represented by intermetallic sigma-phase; its chemical composition corresponds to the stoichiometric Ni(Mo,Cr)2. Time-temperature-precipitation and time-temperature-sensitization diagrams describing formation of sigma-phase in VDM ® Alloy C-4 were built to predict maximal working temperature and time of the material's application. The phenomenon of simultaneous decreasing ductility and increasing strength of the alloy caused by the long range order transformation was noticed after prolonged exposure of the material in the range from 550 to 650 °C. It was shown that unlike precipitation at the grain boundaries, formation of the long range ordered Ni2(Cr,Mo) phase did not increase the corrosion rate of VDM® Alloy C-4 in halide melts due to absence of micro galvanic pairs and growing of interatomic interaction force.

Additive manufacture of superduplex stainless steel using WAAM

Superduplex stainless steels have been used in the oil and gas industry for a couple of decades due to the combination of excellent mechanical properties and corrosion resistance. The present investigation addresses the applicability of wire and arc additive manufacturing for this steel grade. Due to the inherent rapid heating and cooling, the initial base metal microstructure will be substantially altered, and complex thermal cycles may cause the formation of brittle secondary phases, among which the frequently observed intermetallic sigma phase is most harmful. However, no intermetallic phases have been observed, which is consistent with the low heat input employed, and the high Ni content in the wire. The microstructure observations in terms of ferrite volume fraction, Cr nitrides precipitation and the formation of secondary austenite are discussed together with the hardness measurements, tensile testing and notch toughness testing. It is concluded that additive manufacturing of superduplex stainless steels by wire and arc process is feasible.

Influence of ageing time on hardness, microstructure and wear behaviour of AISI2507 super duplex stainless steel

The effect of ageing time on hardness, microstructure and wear behaviour of super duplex stainless AISI 2507 is examined. The material was solution treated at 1050 °C and water quenched, further the ageing has been carried out at 850 °C for 30 min, 60 min and 90 min. The chromium (Cr) and molybdenum (Mo) enriched intermetallic sigma phase (σ) were found to precipitate at the ferrite/austenite interface and within the ferrite region. The concentration of intermetallic sigma phase (σ), which was quantified by a combination of scanning electron microscopy and image analysis, increases with increasing ageing time, leading to significant increase in the hardness. The x-ray diffraction (XRD) and energy dispersive x-ray (EDX) was employed to investigate the element distribution and phase identification. Wear characterstics of the aged super duplex stainless steel were measured by varying normal loads, sliding speeds, sliding distance and compared with solution treated (as-cast) specimens. Scanning electron microscopy was used to assist in analysis of worn out surfaces. The outcomes suggested that the increase in percentage of sigma phase increases hardness and wear resistance in heat-treated specimens compared to solution treated specimens (as-cast).

Oxidation behaviour and phase transformations of an interconnect material in simulated anode environment of intermediate temperature solid oxide fuel cells

The oxidation behaviour and the phase transformations associated with high temperature exposure of a commercial ferritic interconnect steel, Crofer 22 H, was studied in a simulated solid oxide fuel cell (SOFC) anode atmosphere at 700 °C. Special emphasis was placed on the formation of the intermetallic sigma phase. No sigma phase was detected in the bulk alloy after 500 h of exposure of bare specimens. However, specimens which were pre-coated with a layer of nickel showed formation of an interdiffusion zone after as little as 2 h of exposure and sigma phase formation occurred after 10 h. The presence of the nickel layer, which simulates the contact between ferritic steel interconnects and a nickel mesh in a SOFC results in the formation of an austenitic zone and accelerated formation of a σ-phase rich layer at the ferrite/austenite interface. The ferritic steel is transformed into austenite due to the inward diffusion of nickel, σ-phase started to nucleate at the transformed austenite grain boundaries. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface due to more pronounced diffusion of Fe, compared to Cr, in the direction of the Ni-layer. Different possible mechanisms for the nucleation and growth of σ-phase were identified. The experimental results led to the conclusion that sigma nucleates in the austenite and grows following an isothermal eutectoid-like decomposition. The kinetics of σ-phase formation and the depth of the interdiffusion zone were found to follow a traditional diffusion relationship. It was observed that as the Ni-concentration increases the sigma-phase re-dissolves and thus the zone which, contains sigma phase moves deeper into the ferritic steel with exposure time. Interdiffusion processes between the nickel layer and the ferritic steel result not only in accelerated formation of σ-phase but also in the formation of Cr-rich oxides within the nickel layer.

Avaliação da Técnica de Voltametria Linear para Determinação Quantitativa de Fase Sigma no Aço Inoxidável Duplex UNS S31803

Resumo A precipitação de fases intermetálicas nos aços inoxidáveis duplex constitui um problema no que diz respeito à utilização destes materiais em determinadas faixas de temperatura, desta forma, fazem-se necessários métodos de quantificar, de forma não destrutiva, a presença de alterações microestruturais decorrentes de aporte térmico. Neste trabalho, ensaios eletroquímicos de voltametria linear, de tração e de microdureza Vickers foram realizados no aço inoxidável duplex UNS S31803 para avaliação da presença da fase intermetálica sigma na microestrutura do material. Amostras deste aço foram envelhecidas isotermicamente a 870 °C, em tempos de até 3600 segundos para induzir a precipitação de diferentes teores desta fase. Os ensaios de voltametria linear foram conduzidos em solução eletrolítica de 5% KOH, em uma célula eletroquímica de 3,3.10–6 m2 de área, com eletrodo de referência do tipo Calomelano Saturado e eletrodo auxiliar de fio de platina. Os resultados desta técnica apresentaram sensibilidade para detectar teores de fase sigma maior que 1% no material viabilizando sua aplicação como um Ensaio Não Destrutivo (END).

Tensile properties of the Cr–Fe–Ni–Mn non-equiatomic multicomponent alloys with different Cr contents

Abstract A series of Fe40Mn28Ni32 − xCrx (x = 4, 12, 18, 24 (at.%)) multicomponent alloys was prepared by vacuum arc melting. The Fe40Mn28Ni28Cr4, Fe40Mn28Ni20Cr12 and Fe40Mn28Ni14Cr18 alloys were ductile single phase fcc solid solutions. The Fe40Mn28Ni8Cr24 alloy had intermetallic sigma phase matrix and was extremely brittle after homogenization. The tensile properties of the Fe40Mn28Ni28Cr4, Fe40Mn28Ni20Cr12 and Fe40Mn28Ni14Cr18 solid solution alloys were examined in recrystallized condition with average grain size of ~ 10 μm. The yield strength increased from 210 MPa of the Fe40Mn28Ni28Cr4 alloy to 310 MPa of the Fe40Mn28Ni14Cr18 alloy. The elongation to fracture of the alloys decreased from 71% to 54%, respectively. Solid solution strengthening by the constitutive elements of the alloys was calculated using Labush approach. Strong solid solution strengthening by Cr was predicted. Gypen and Deruyttere approach was used to estimate solid solution strengthening of the Fe40Mn28Ni32 − xCrx alloys. Good correlation between predicted solid solution strengthening and the experimental yield strength values was found.

Evolution of sigma phase in 321 grade austenitic stainless steel parent and weld metal with duplex microstructure

Samples of 321 stainless steel from both the parent and welded section of a thin section tube were subjected to accelerated ageing to simulate long term service conditions in an advanced gas cooled reactor (AGR) power plant. The initial condition of the parent metal showed a duplex microstructure with approximately 50% ferrite and 50% austenite. The weld metal showed three distinct matrix phases, austenite, delta ferrite and ferrite. This result was surprising as the initial condition of the parent metal was expected to be fully austenitic and austenite+delta ferrite in the weldment. The intermetallic sigma phase formed during the accelerated ageing was imaged using ion beam induced secondary electrons then measured using computer software which gave the particle size as a function of aging time. The measurements were used to plot particle size, area coverage against aging time and minimum particle spacing for the parent metal. During aging the amount of ferrite in the parent metal actually increased from ∼50 to ∼80% after aging for 15 000 h at 750°C. Sigma has been observed to form on the austenite/ferrite boundaries as they may provide new nucleation sites for sigma phase precipitation. This has resulted in small sigma phase particles forming on the austenite/ferrite boundaries in the parent metal as the ferrite transforms from the austenite.

Precipitation of Second Phases in High-Interstitial-Alloyed Austenitic Steel

The precipitation reaction of an austenitic stainless steel containing N + C was investigated using transmission electron microscopy. The main precipitate formed during isothermal aging at 1123 K (850 °C) was M23C6 carbide, and its morphology gradually changed in a sequence of intergranular (along grain boundary) → cellular (or discontinuous) → intragranular (within grain interior) form with aging time. Irrespective of different morphologies, the M23C6 was consistently related to austenite matrix in accordance with the cube-on-cube orientation relationship. Based on the analysis of electron diffraction, two variants of intragranular M23C6 were identified, and they were related to each other by twin relation. Prolonged aging produced other types of precipitates—the rod-shaped Cr2N and the coarse irregular intermetallic sigma phase. The similarities and differences in precipitation behavior between N only and N + C alloyed austenitic stainless steels are briefly discussed.

Thermal, magnetic and composition analyses of the reverse transformation of intermetallic sigma phase to ferrite

The reverse transformation of the sigma phase to ferrite in a duplex stainless steel upon heating has been studied by using differential thermal analysis (DTA), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), quantitative metallography and magnetic susceptibility measurement. It has been demonstrated that the reverse transformation of the sigma phase to ferrite is sensitively affected by the morphology of the sigma phase. However, EDS has shown that the composition of the sigma phase does not significantly affect its reverse transformation behaviour. Magnetic susceptibility measurements and TEM have revealed that untransformed, residual ferrite is present even after prolonged annealing between 873 and 1173 K and hence it does play a role in the sigma-to-ferrite reverse transformation. Quantitative metallography was employed to follow the coarsening of the sigma phase upon annealing and DTA was used to estimate its enthalpies of dissolution. As a potential application, a method for temperature monitoring using duplex stainless steels and DTA is suggested.

Effect of Aging on the Toughness of Austenitic and Duplex Stainless Steel Weldments

In the present study, the effect of aging heat treatment at 650, 750, and 850°C on the impact toughness of 316L austenitic stainless steel, 2205 duplex stainless steel and their weldments has been investigated. Welding process was conducted using the TIG (tungsten inert gas) welding technique. Instrumented impact testing, at room temperature, was employed to determine the effect of aging treatment on the impact properties of investigated materials. Aging treatment resulted in degradation in the impact toughness as demonstrated by the reduction in the impact fracture energy and deformation parameters (strain hardening capacity, fracture deflection, and crack initiation and propagation energy). The degree of embrittlement was more noticeable in duplex stainless steel parent and weld-metal than in 316L stainless steel and became greater with the increase of aging temperature. The degradation in impact toughness was discussed in relation to the observed precipitation of the intermetallic sigma phase in the microstructure of the stainless steel weldments and the corresponding fracture surface morphology.

Austenite reformation in the heat-affected zone of duplex stainless steel 2205

The aim of the present work was to evaluate the amount of reformed austenite in the heat-affected zone (HAZ) of submerged arc welded (SAW) duplex stainless steel 2205. The model is based on calculations of the nucleation and growth of austenite. The present results are compared with other studies to evaluate the toughness properties and the probability of precipitation of brittle, intermetallic sigma phase. The results indicate that cooling the weld in air provides a satisfactory amount of reformed austenite and prevents formation of sigma phase.

Investigation Of Aging Heat Treatment OnMicrostructure And Mechanical Properties Of316L Austenitic Stainless Steel Weld Metal

Austenitic stainless steel alloys are used extensively in heat resistant structural components in power generating and chemical industries. Austenitic stainless steel weld metal exhibits a duplex microstructure consisting of a dispersion of high temperature delta-ferrite in austenite. It has been shown that aging between 500–700°C results in progressive dissolution of the delta-ferrite and precipitation of M23 C6 carbides and intermetallic phases. In this paper, the effects of aging at temperatures between 550–850°C for periods of up to 100 hours on the microstructure and mechanical properties of 316L austenitic stainless steel weld metal have been investigated. Austenitic stainless steel 316L plates of 20 mm thickness were welded using the gas tungsten arc welding technique. Tensile and impact test specimens were machined from the weld sections and subjected to various aging heat treatments. Based on the results of mechnical testing it was found that whereas the strength properties show comparatively small variations due to aging, the tensile ductility and impact toughness are strongly affected by increasing aging temperature and time, such that aging at 850°C for 5 hour results in a 90% reduction in the value of impact energy of the weld metal. The loss of impact toughness and tensile ductility has been discussed based on the dissolution of delta-ferrite and precipitation of the intermetallic sigma phase.

Energetics and phase diagrams of Fe-Cr and Co-Cr systems from first principles

The first principles computations of the total energies of complex phases have been addressed recently. The structural energy differences, calculated by FLAPW (Full potential augmented plane wave) method, enable us to utilize a more complete physical information about total energy of intermetallic phases and to propose a new model for their thermodynamic description. Our approach is based on the two-sublattice model, similarly as for solid solution phases, but the structural energy differences for end-members in the metastable or unstable structures are obtained by means of first-principles electronic structure calculations. Phase diagrams of Fe-Cr and Co-Cr systems containing the intermetallic sigma-phase (5 inequivalent lattice sites, 30 atoms in repeat cell) are described here as an example of application of our new model.

Second-phase particles in ferritic stainless steels containing 40% chromium

The nature and occurrence of secondary phases in experimental ferritic stainless steels containing 40% chromium is reviewed. Phases discussed include oxides, carbides, nitrides, sulphides, and the intermetallic sigma phase. The influence of these particles on the overall ductility and their contribution to brittle fracture and localized corrosion are examined by means of a variety of metallographic techniques. Control of the fabrication and processing parameters can minimize the deleterious effects of secondary phases, and can, in certain instances, even harness their properties advantageously.

New Crystal Data for Mo.63Ru.37(Mo5Ru3) A Superconducting Sigma Phase

AbstractNew crystal data for the intermetallic sigma phase in the Mo-Ru-system are reported and the indexing of the pattern given in the Powder Diffraction File has been extended. The material is tetragonal, P42/mnm (No. 136) with a = 9.5652(4) Å; c = 4.9362(3) Å; Vol = 451.63(7) Å3, Dc= 10.79 Mg m−3for the composition Mo.63Ru.37, and a = 9.5569(21) Å; c = 4.9418(20) Å; Vol = 451.35(38) Å3, Dc= 10.75 Mg m−3for Mo.70Ru.30.Powder data were obtained by the Guinier method and the indexing was corroborated by single crystal data. Transition to the superconducting state took place in the interval 6.8 K – 8.8 K.