Creep behavior of INCOLOY alloy 617

The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)₂₃C₆ carbides and the δ-Ni₃Mo phase. M₆C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni₃(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)₂₃C₆ carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good creep response observed at 700 °C, compared with that found at 800 °C.     

σ-Phase Embrittlement in Some ferritic-austenitic stainless steels

Four commercially established ferritic-austenitic stainless steel grades have been investigated with respect to σ-phase embrittlement by exposure in the temperature interval 700–900 °C. The results illustrate the fact that this type of steels is rather susceptible to such an embrittlement, and this has to be considered when selecting steel grades for different applications. It is also shown that an increased austenite content in fact promotes σ-phase formation. Molybdenum as alloying element is found to favour σ-phase much stronger than chromium (4–5 times counted per wt%), hence enlarging the critical temperature range considerably towards higher temperature. Further, a simple “σ-equivalent” has been developed by which semiquantitative comparisons of the susceptibility to σ-phase embrittlement might be made between different steel grades, based on their alloy composition.     

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.     

Effect of component thickness on lifetime and oxidation rate of chromia forming ferritic steels in low and high pO2 environments

Abstract

Long term oxidation tests were carried out with a high-Cr ferritic steel at 800°C and 900°C in simulated cathode and anode gas of a solid oxide fuel cell (air and an Ar/H₂/H₂O mixture respectively). It was found that with decreasing sample thickness the life time of the steel decreases due to breakaway phenomena. This effect is caused by faster exhaustion of the chromium reservoir from the bulk alloy in the case of thinner components. During air exposure the oxidation rates increase with decreasing specimen thickness and this has to be taken into account in the calculation of the Cr-reservoir exhaustion. This thickness dependence is not found during the exposures in simulated anode gas. Hence, especially for thin walled components, the oxidation rates in anode gas are substantially smaller and thus the life times are longer than during air exposure. The differences in oxidation behaviour in the two environments are discussed on the basis of scale formation mechanisms involving microcrack formation in the surface oxide scale and depletion of major and minor alloying additions in the bulk alloy.     

The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys

Abstract

The present study focused on the influence of Ni on the microstructure and oxidation behaviour of Co–Re–Cr-based alloys. Alloys with three different Ni contents were tested in laboratory air at 800–1100 °C. A refinement and a reduction of the σ phase volume fraction as well as a change in the matrix microstructure were observed. Thermogravimetric measurements showed that the alloys with higher Ni contents possess a better oxidation resistance when exposed to higher temperatures. All alloys suffered from continuous mass loss during oxidation at 800 °C due to the formation of porous oxides scales, consisting of Co₃O₄, Co(Ni)O and Ni-doped CoCr₂O₄, which allow the evaporation of Re-oxides. At 900–1100 °C, only the alloy with 25 at. % Ni showed parabolic oxidation kinetics after a short period of transient oxidation. This is a result of the fast formation of a protective Cr₂O₃ layer. It was also found that exposure to air at 1000 °C leads to a phase transformation of the bulk material; an oxidation-induced formation of fine hexagonal close-packed (hcp) grains was observed near the oxide scales. It is supposed that the improved oxidation resistance of Ni-containing Co–Re–Cr alloys is a result of enhanced Cr diffusion caused by the Ni addition. The extensive formation of the fcc phase in the alloy matrix had a detrimental effect on the oxidation behaviour of the Ni-containing Co–Re–Cr-based alloys.     

On the transformations of the ψ-AlCuFe icosahedral phase

In the present investigation, different alloy compositions close to the ternary composition values, where the icosahedral phase in AlFeCu is obtained, have been studied. The specimens were obtained using a rapid solidification technique with subsequent thermal treatments of 600°C, 700°C, 800°C and 900°C. The obtained specimens were characterized with X-ray diffraction patterns (XRD) and transmission electron microscopy (TEM). The experimental results show different transformations of the icosahedral phase to crystalline phases between 800°C and 700°C .The cubic β-phase is a solid solution which regulates the formation and decomposition of the ψ-Al₆Cu₂Fe phase to different crystalline phases such as the tetragonal (Al₇Cu₂Fe) and monoclinic (Al₁₃Fe₄) phases.     

An analysis of the creep-environment interaction of Inconel alloy X-750 exposed at 1150°C

The effect of exposure of Inconel alloy X-750 to air at 1150°C on creep at 700°C and 400 MPa has been investigated. Contrary to the findings for exposure at 1050°C, exposure to air at 1150°C does not impair creep ductility. This is attributable to oxygen diffusion into the alloy at 1150°C being impeded. Evidence is given here to show that during exposure to air at 1150°C diffusion of chromium is fast enough to reach the surface and form a chromium oxide scale, which acts as a barrier to further oxygen diffusion.     

Effect of Ti on Morphology and Growth of Zn–Fe Intermetallic Phases

The effect of Ti in Fe substrate on the morphology and growth of Zn–Fe alloy phases in (Zn–0.2 wt.%Al)/Fe diffusion couples was investigated using optical and scanning electron microscopy. It is shown that ζ phase layer does not exist in (Zn–0.2 wt.%Al)/Fe couple, but it can be clearly observed in (Zn–0.2 wt.%Al)/(Fe–Ti) diffusion couples. Increasing of dissolved Ti in Fe substrate delays the disappearance of the ζ phase and promotes the formation of δ_k phase. The δ_p phase grows toward the ζ phase layer in the form of arborescence. The growth of total intermetallic phase layers in all diffusion couples is controlled by diffusion of Zn and Fe atoms within the layers at 380 °C. The thickness of total intermetallic layer increases with increasing of dissolved Ti in Fe substrate and reaches the maximum when Ti content is 0.4 wt.%.     

Non-isothermal aging of a high-Zn-containing Al–Zn–Mg–Cu alloy: microstructure and mechanical properties

The non-isothermal aging behaviour of a newly developed Al–Zn–Mg–Cu alloy containing 17?wt-% Zn was investigated. Hardness and shear punch tests demonstrated that during non-isothermal aging, the mechanical properties of the alloy first increased and then decreased. The best properties were obtained in a sample which was non-isothermally aged upto 250°C with heating rate of 20°C?min^?1, due to the presence of η′/η (MgZn₂) phases. This was confirmed by differential scanning calorimetery. After homogenisation, residual eutectic phases remained at triple junctions or in a spherical form. During aging, these phases transformed into rodlike S (Al₂CuMg)-phase at 400°C, with sizes ranging from 50 to 250?nm. The precipitation sequence in this high-Zn alloy was similar to that for conventional Al–Zn–Mg–Cu alloys.     

High temperature isothermal oxidation behaviour of an oxide dispersion strengthened derivative of IN625

Abstract

Gas atomised IN625 powder was mechanically alloyed with <1·0 Wt.% nano-yttria and consolidated by spark plasma sintering (SPS) to produce an oxide dispersion strengthened (ODS) alloy. The isothermal oxidation rate constant of the ODS alloy, and wrought IN625, was determined by thermogravimetric analysis. This was performed at 900 °C in static laboratory air for exposure times of up to 1000 h. It was found that the ODS alloy oxidised ~40x slower than wrought IN625, which is attributed to the reactive element effect. It is further proposed that the improvement in oxidation resistance of the ODS alloy, and the superior morphology of the oxide scale formed on the ODS alloy, may be related to the presence of Nb carbide, rather than δ-phase, in the ODS alloy.     

Effect of aging treatments and microstructural evolution on corrosion resistance of tungsten substituted 2205 duplex stainless steel

Abstract

The effect of partial substitution of tungsten for molybdenum on the microstructure and corrosion resistance in 22Cr–5Ni–3Mo duplex stainless steel (DSS) has been investigated following aging heat treatments in a temperature range of 600-1000°C. Electrochemical tests were carried out for the evaluation of corrosion resistance. Aging treatment had hardly influenced the general corrosion resistance. With the increase of aging time, the pitting corrosion resistance of the DSSs had decreased. After aging for 2 min at 700–900°C, the pitting potential of the 3Mo steel decreased remarkably, while that of the W substituted steel hardly changed. During aging, the intermetallic σ and secondary austenite (γ₂) phases were precipitated, and the pitting corrosion and intergranular corrosion resistance were significantly decreased after aging at 700–750°C for 10 h, which could be caused by the γ₂ formation. The γ₂ phase could effect the depletion of molybdenum and chromium in the γ₂ /α and γ₂/σ boundaries.     

Ageing response of a Al–Cu–Li 2198 alloy

The effects of different ageing treatments on microstructure evolution, properties and fracture are investigated in the present study. 2198 alloy exhibits strong ageing response during ageing. It is found that tensile properties, hardness and conductivity of 2198 alloy are very sensitive to ageing temperatures, which corresponds to different microstructures. In the naturally-aged condition (T3), only δ′ (Al₃Li) was detected. After artificial ageing (T8), large amounts of precipitates emerged and major precipitates that were detected turned to be δ′, θ′ (Al₂Cu) and T₁ (Al₂CuLi) phase. Exposure to higher temperature caused greater amounts of the precipitation. The constitution and morphology of precipitates varies with different ageing temperature; the major precipitates are δ′, θ′ when ageing below 160 °C, while above 160 °C, T₁ phase comes out in large numbers, becoming dominate strengthening phases gradually. Fracture transforms from a typical dimple type to a dimple-intergranular mixed type with the rise of ageing temperature.     

Effect of sintering temperatures on properties of BaO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glass/silica composites for CBGA package

BaO–B₂O₃–SiO₂–Al₂O₃ (BBSA) glass/silica composites synthesized by solid-state reaction method were developed for CBGA packages, and the effects of sintering temperature (900–950 °C) on the phase transformation, microstructure, thermal, mechanical and electrical properties were investigated. XRD results show that the major phases quartz and cristobalite, and the minor phase BaSi₂O₅ are detected in BBSA composites. Furthermore, it was found that the quartz phase transforms to cristobalite phase at 930–940 °C. The formation of cristobalite phase with higher coefficient of thermal expansion (CTE) led to the increase of CTE value of BBSA composites. However, excessive cristobalite phase content would degrade the mechanical properties and the linearity of thermal expansion of the ceramics. BBSA composites sintered at 920 °C exhibited excellent properties: low dielectric constant and loss (ε_r = 6.2, tanδ = 10^?4 at 1 MHz), high bending strength (179 MPa), high CTE (12.19 ppm/°C) as well as superior linearity of the thermal expansion.     

Study of effect of chromium on titanium dioxide phase transformation

MTiX samples with different atomic chromium percentages were synthesized by sol–gel method and calcined at 400 °C under air. The effects of Cr and temperature on titanium dioxide phase transition were studied. In situ measurement showed the presence of anatase phase for all samples at temperature < 500 °C. Without Cr content, the anatase–rutile transition takes place at 600 °C and the rutile fraction increases with increase of temperature. In the presence of Cr content, rutile phase appeared at 700 °C. Cr₂O₃ phase was shown only in the case of CrTi20 content at 800 °C which indicates that the segregation remains modest. We have also studied the anatase–rutile transition kinetics by using in situ X-ray measurements. It was found that the anatase phase stability increases as the chromium content increases. Results confirm that the transformation of anatase–rutile is of first order.     

Microstructure evolution and mechanical properties of GH984G alloy with different Ti/Al ratios during long-term thermal exposure

GH984G alloy is a low cost Ni–Fe based wrought superalloy designed for 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the microstructure evolution and tensile properties of GH984G alloy with different Ti/Al ratios during thermal exposure at different high temperatures are investigated. Detailed microstructure analysis reveals that the Microstructure of alloys with different Ti/Al ratios are similar after standard heat treatment, and the primary precipitates are γ′, MC, M₂₃C₆ and M₂B. However, η phase precipitates at grain boundary in the alloy with high Ti/Al ratio after thermal exposure at 750 °C for 570 h. By contrast, the microstructure stability of the alloy with lower Ti/Al ratio is excellent. There is no detrimental phase even if after thermal exposure at 750 °C for 5000 h in the alloy with lower Ti/Al ratio. γ′ coarsening plays a great role on the tensile strength, and the critical size range of γ′ could be defined as approximately 27–40 nm. The influence of η phase on tensile strength has close relationship with its volume fraction, the high volume fraction results in the decrease of tensile strength. The tensile strength of the alloy with lower Ti/Al ratio is obviously higher than the alloy with higher Ti/Al ratio and the yield strength has no obvious decrease during long-term thermal exposure at 700 °C. It is demonstrated that the thermal stability of microstructure and mechanical properties of GH984G alloy can be improved by moderately decreasing Ti/Al ratio to satisfy the requirement of A-USC plants.     

Induced formation of polar phases in poly(vinylidene fluoride) by cetyl trimethyl ammonium bromide

Poly(vinylidene fluoride) (PVDF) has been widely used as transducers, actuators,and sensors due to its exceptional electroactive properties among polymer materials and the electroactive properties of PVDF heavily depend on its polymorphs. The effects of cetyl trimethyl ammonium bromide (CTAB) on the crystallization behavior of PVDF during isothermal crystallization and annealing process were studied, and the results showed that the polar phases (β, γ) were induced by CTAB when PVDF was isothermally crystallized at 145–160 °C. Increasing the isothermal crystallization temperature resulted in an increased γ-phase formation with a concomitant decrease of α and β phases. A 100 % γ-phase formation occurred when the sample isothermally crystallized at 155 and 160 °C. The crystalline phase transition of the sample annealed at 160 °C for various time revealed that CTAB induced the transformation from α to γ′ phase and a higher concentration of CTAB resulted in shorter transformation time, as well as higher transformation degree.     

Influence of size and distribution of W phase on strength and ductility of high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca alloy processed by indirect extrusion

A high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca(wt%) alloy containing W phase(Mg_3Y_2Zn_3) prepared by permanent mold direct-chill casting is indirectly extruded at 350?C and 400?C, respectively. The extruded alloys show bimodal grain structure consisting of fine dynamic recrystallized(DRXed) grains and unrecrystallized coarse regions containing fine W phase and β2' precipitates. The fragmented W phase particles induced by extrusion stimulate nucleation of DRXed grains, leading to the formation of fine DRXed grains, which are mainly distributed near the W particle bands along the extrusion direction. The alloy extruded at 350?C exhibits yield strength of 373 MPa, ultimate tensile strength of 403 MPa and elongation to failure of 5.1%. While the alloy extruded at 400?C shows lower yield strength of 332 MPa,ultimate tensile strength of 352 MPa and higher elongation to failure of 12%. The mechanical properties of the as-extruded alloys vary with the distribution and size of W phase. A higher fraction of DRXed grains is obtained due to the homogeneous distribution of micron-scale broken W phase particles in the alloy extruded at 400?C, which can lead to higher ductility. In addition, the nano-scale dynamic W phase precipitates distributed in the un DRXed regions are refined at lower extrusion temperature. The smaller size of nano-scale W phase precipitates leads to a higher fraction of un DRXed regions which contributes to higher strength of the alloy extruded at 350?C.     

Investigation and improvement on structural stability and stress rupture properties of a Ni–Fe based alloy

The structural stability and stress rupture properties of a Ni–Fe based alloy, considered as boiler materials in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, was studied. Investigation on the structural stability of the existing alloy GH984 shows that the most important changes in the alloys are γʹ coarsening, the γʹ to η transformation and the coarsening and agglomeration of grain boundary M₂₃C₆ during thermal exposure. The stress rupture strength was found to be slightly lower than the requirement of 700 °C A-USC. The fracture mode of creep tested specimens was intergranular fracture. Detailed analysis revealed that η phase precipitation is sensitive to Ti/Al ratio and can be suppressed by decreasing Ti/Al ratio. The coarsening behavior of γʹ phase is related to Fe content. Adding B and P was suggested to stabilize M₂₃C₆ and increase grain boundary strength. Based on the research presented and analysis of the data, a modified alloy was developed through changes in composition. For the modified alloy, η phase is not observed and M₂₃C₆ is still blocky and discretely distributes along grain boundary after thermal exposure at 700 °C for 20,000 h. Moreover, the creep strength is comparable to the levels of Ni-based candidate alloys for 700 °C A-USC.     

Laboratory testing to evaluate candidate alloys for superheaters in waste-fired boilers

Two types of laboratory tests were used to evaluate the behaviour of a wide range of steels in chlorinating–sulphidising conditions: high temperature exposure after applying salt by dipping in an aqueous KCl/ZnCl₂ solution and salt-bed test in a ZnCl₂/NaCl/KCl/CaSO₄ salt mixture. The exposures were performed at 500 °C in a gas comprising N₂/HCl/SO₂/O₂/N₂. For the alloy group with 20–30 wt-% chromium and 25–65 wt-% nickel, the extrapolated metal loss was below 0.2 mm/year in the salt dip, up to 6 mm/year in the salt-bed test with 10% Cl and up to 20 mm/year in the 20% Cl salt-bed test. The intermediate alloy group showed poorer performance in the salt-bed; Esshete1250 showed also large spallation in the salt dip test. Results were compared with plant-exposed samples. Further refinement of the salt dip test is suggested for material ranking in the superheater region of a waste or biomass-fired incinerator.     

Phase stability in a powder-processed Al–Mn–Ce alloy

There has been considerable interest in Al-rich Al–Mn–Ce alloys due to the variety of crystalline and quasi-crystalline metastable phases that can be formed. Here we report a study of the effects of heat treatment on an Al–5Mn–2Ce (at.%) alloy processed by gas atomization and consolidated by warm extrusion. Characterization using X-ray diffraction and electron microscopy showed that the powder microstructure consists mainly of an amorphous phase, FCC Al, and a previously unreported phase, Al₂₀Mn₂Ce. The extrudate is fully devitrified and contains a mixture of FCC Al, Al₂₀Mn₂Ce, and Al₆Mn, with a small amount of Al₁₂Mn and Al₁₁Ce₃. Upon heat-treatment at up to 450 °C, the Al₂₀Mn₂Ce and Al₆Mn phases decompose to give a hard stable phase mixture with 72–73 % Al₁₂Mn plus 13–14 % each of Al₁₁Ce₃ and FCC Al. Heat treatments at 500 °C give a much softer phase mixture consisting of 60 % FCC Al, 22 % of an unknown Al₃(Mn,Ce) phase, 9 % Al₁₂Mn, 8 % Al₆Mn, and 1 % Al₁₁Ce₃. The formation of large volume fractions of Al₁₂Mn for heat-treatments at up to 450 °C suggests that the presence of Ce may stabilize this phase, and that more dilute Al–Mn–Ce compositions could form the basis for new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.