Characterization of pack cemented Ni2Al3 coating exposed to KCl(s) induced corrosion at 600 °C

Abstract

Oxidation of Ni₂Al₃ produced by pack aluminizing of pure nickel was studied with and without 0.10 mg cm^?2 KCl(s) deposit in an environment containing 5% O₂, 40% H₂O and 55% N₂ at 600 °C for up to 168 h. Oxide microstructure and composition was investigated by SEM/EDX, BIB, TEM and GDOES. Oxidised Ni₂Al₃ shows minimal weight gain, while adding KCl results in a small weight loss consistent with evaporation of KCl. On the surface of samples exposed to the gas environment only, a 30 nm oxide of Al oxide was present, but where KCl was present as deposit, 50–250 nm thick nodules form that are enriched in K, O and Al.     

Effect of Re content on precipitation behaviour of secondary phases in a single-crystal Ni-based superalloy during high-temperature thermal exposure

The precipitation of secondary phases in single-crystal Ni-based superalloys with different contents of Re are investigated using TEM and SEM. The results show that after full heat treatment, the size of secondary γ′ phase decreases gradually with increasing Re content. During long-term thermal exposure at 1093 °C, some needle-like and rod-like secondary phases precipitate from the γ matrix. With increasing Re content and prolonging of the thermal exposure, the amounts of precipitated phases clearly increase. In the alloy with 2·5 wt-% Re, the needle-like phases are identified as M₆C carbides. In the alloy with 3 wt-% Re, both M₆C carbides and μ phase are found. In the alloy with 3·5 wt% Re, the needle-like and rod-like precipitated phases are identified as μ and P phases. Stacking fault was found in the P phase and that part was identified as μ phase by SAD; that is, there is intergrowth of P and μ phases.     

Effects of Si,Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract

Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO₂ and Ar–20CO₂–20H₂O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr₂O₃ layer and an inner SiO₂ layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn₃O₄ and MnCr₂O₄ above alloy grain boundaries and Mn₃O₄, Cr₂O₃ and MnCr₂O₄ on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.     

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.     

Ceramic–metal composites produced by laser surface treatment

Abstract

The injection of SiC particles (150 μm size) into laser surface melted commercial purity titanium, Ti–6Al–4V (wt-%) alloy, and Ti–2·5Cu (wt-%) alloy has been investigated using 1·75 kW laser power, 5 mm beam diameter, 0·15 g s^?l powder flowrate and traverse speeds ranging from 7 to 20 mm ^?l. Partial dissolution of SiC occurred and fine dendrites of TiC nucleated at the particle/matrix interfaces and also within the matrix. Silicon enrichment of the matrix and a eutectic constituent were observed. The microhardness of the melted zone was increased to 600–650 HV (500 g).

MST/964     

Characterisation and thermodynamic calculations of biodegradable Mg–2.2Zn–3.7Ce and Mg–Ca–2.2Zn–3.7Ce alloys

In the present study, the effect of Ca (0.5–6?wt-%) content on the microstructure, phase formation, and mechanical properties and in vitro degradation behaviour of Mg–2.2Zn–3.7Ce alloys were investigated. Microstructural analysis and thermodynamic calculations also showed that Mg–2.2Zn–3.7Ce alloy contain α-Mg, Mg₁₂Ce and CeMgZn₂, while after adding 0.5?wt-% Ca to Mg–2.2Zn–3.7Ce alloy, IM1 (Ca₃Mg_xZn_15?x) (4.6?≤?x?≤?12) phase was detected. Further addition of Ca to 6?wt-% resulted in forming Mg₂Ca besides α-Mg, Mg₁₂Ce and IM1 with the absence of CeMgZn₂. The tensile strength and elongation of the Mg–Ca–2.2Zn–3.7Ce alloys increase with increasing Ca content up to 1.5?wt-%, while further addition of Ca to 6?wt-% has a reversed effect. Similarly, the degradation rate of the alloys increased first with increasing Ca content and then decreased.     

Effects of hot pressing temperature on microstructure,hardness and corrosion resistance of Al2NbTi3V2Zr high-entropy alloy

A novel lightweight high-entropy alloy Al₂NbTi₃V₂Zr was fabricated by vacuum hot pressing. The effects of sintering temperature (1200–1550°C) on the microstructure, hardness and corrosion resistance of the alloy were investigated. Results showed that Al₂NbTi₃V₂Zr mainly consisted of simple cubic matrix and (Zr, Al)-based intermetallic phase (α-phase) at sintering temperatures of 1200–1350°C. Moreover, the matrix phase transformed from simple cubic to body-centred cubic phase, and (Ti, Zr, Al)-based intermetallic precipitated from the matrix at temperature of 1450°C. The fabricated Al₂NbTi₃V₂Zr alloy had low density of 5.05–5.23?g?cm^–3, high hardness of 510–728?HV and excellent corrosion resistance in 10?wt-% HNO₃ solution.     

Dissolution kinetics and solubility limit of CaF2 in molten KCl–NaCl salt

The interaction between NaCl–KCl salt of an equimolar composition and CaF₂ was investigated in the 1023–1123 K temperature range. The solubility limit was determined as 2.1, 2.5, and 3.4 mol.% for 1023, 1073, and 1123 K, respectively. The linear dependence of the specific weight loss with time and the activation energy of the dissolution (124.6 ± 8.7 kJ/mol) indicate that CaF₂ dissociation of the ions is the limiting stage of the process. Thermodynamic properties (partial enthalpy and entropy) of CaF₂ in a dilute ternary –NaCl–KCl–CaF₂ liquid solution were estimated using the values of the solubility limit and by applying a regular solution approach.     

Microstructure and mechanical properties of as-extruded AZ80–xSn magnesium alloys

The effect of 0–2?wt-% Sn addition on AZ80 magnesium alloys after 350°C extrusion has been studied by analysing microstructure and mechanical properties. The results indicated that dynamically recrystallised grains were fine and homogeneous with less than 1?wt-% Sn addition. In AZ80–0.5Sn alloy, a large number of Mg₁₇Al₁₂ precipitated phases formed in grains and at grain boundaries during extrusion process. With more than 1?wt-% Sn addition, the size of dynamically recrystallised grains increased and the number of Mg₁₇Al₁₂ phases decreased. The strength of as-extruded AZ80–0.5Sn alloy enhanced largely as compared with that of the as-extruded AZ80 alloy. AZ80–0.5Sn alloy had the outstanding tensile and compressive properties.     

Effect of microstructure on KCl corrosion attack of modified AISI 310 steel

Abstract

The effect of microstructure on KCl corrosion attack was studied using a specifically chosen modified AISI 310 austenitic steel in a 15% (v/v) H₂O (g) + 5%(v/v) O₂ (g) + N₂ (g) (balance) atmosphere at 600°C for 168 h. The material was a targeted choice as it allows investigation of different microstructures i.e. as-received (without sigma phase) and heat-treated (29% σ-phase per area) microstructures. The corrosion attack was studied with light optical, scanning electron and transmission electron microscopy as well as X-ray diffraction. The heat-treated sample showed a corrosion attack that was 5 times higher than the as-received sample. In the heat-treated sample, the σ-phase was selectively attacked. At the corrosion front, chlorine (but not potassium) was detected in the selectively attacked σ- phase but not in the unattacked adjacent matrix. Therefore, the corrosion attack was propagated by preferential σ-phase attack by chlorine species.     

Fe–AI–Zn ternary phase diagram at 450°C

Abstract

Several Fe–Al and Fe–Al–Zn alloys were vacuum melted, annealed at 450°C under atmospheric pressure, and then quenched in iced water. The structure and composition of phases were determined using microstructural, X-ray diffraction, and X-ray energy dispersive analyses. The maximum solubility of zinc was found to be 5·3 wt-% in FeAl₃, 20·9 wt-% in Fe₂Al₅, and 2·0 wt-% in FeAl at 450°C. Based on these results, an isothermal section for the aluminium rich corner of the Fe–Al–Zn ternary phase diagram is proposed.

MST/1196     

The role of superalloy precipitates on the early stages of oxidation and type II hot corrosion

Abstract

To meet materials challenges encountered in gas turbines, superalloys have been developed for high temperature, strength, oxidation and corrosion resistance. One strengthening method is using refractory metal additions to form carbide precipitates. However, such precipitates may be detrimental to the alloy’s environmental resistance. This paper reports how refractory metal carbide precipitates affect the early stages of oxidation and hot corrosion of two alloys: Rene 80 (nickel-based) and MarM 509 (cobalt-based). Samples were exposed at 700 °C in either dry synthetic air or 90 ppm SO_x, 10·5% CO₂, 8·5% O₂, 5% H₂O (balance N₂) with a 80/20 (Na/K)₂SO₄ deposit (1·5 μg/cm²/hour flux). The oxidation morphology and corrosion products were investigated by scanning electron microscope and energy dispersive X-ray analysis, to show that refractory metal carbide precipitates close to the metal surfaces disrupt protective oxide scale formation, thus providing inward transport routes for corrosive species.     

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract

The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO₂, intermediate Al₂O₃, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.     

Wettability of graphite by liquid aluminum under molten potassium halide fluxes

This study is a continuation of our previous work (Baumli et al. J Mater Sci 45:5177–5190, 2010) in which the wettability of graphite by liquid aluminum was studied under different molten chlorides containing K₂TiF₆ as function of the cation of the molten chloride. In the present paper, the same was studied as function of the anion in different potassium halides (fluxes). The fluxes consisted of different potassium halides (KX) + 10 wt% potassium hexafluoro-titanate (K₂TiF₆). The potassium halides studied were potassium iodide (KI), potassium chloride (KCl), and their equimolar mixture. For perfect wettability of graphite by liquid Al under different molten KX-10 wt% K₂TiF₆ systems at 800 °C, certain critical values of the salt:Al mass ratio should be achieved, the value of which increases from KI toward KCl. Comparing the present results with the results of our previous study, we found that the critical value of the salt:Al mass ratio increases with the cation size and decreases with the anion size of the salt.     

Microstructure of alumina ceramic/Ag–Cu–Ti brazing alloy/Kovar alloy joint

Abstract

The microstructure of the alumina ceramic/Kovar alloy joint brazed with Ag–35·2Cu–1·8Ti (wt-%) was studied. The effects of brazing temperature on the microstructure were also discussed. It was found that the microstructure of the joint brazed at 1173 K for 5 min was TiO + TiNi₃ + TiFe/eutectic Ag–Cu/TiFe₂ + TiNi₃/TiFe₂ + Cu (s.s) +Ag (s.s). When the brazing temperature was >1193 K, there was no TiO formed on the alumina ceramic/brazing alloy interface.     

Low density steel 1·2C–1·5Cr–5Al designed for bearings

Abstract

A novel alloy design, designated as 1·2C–1·5Cr–5Al, has been proposed with high aluminium(～5 wt-%) and more carbon(～1·2 wt-%) addition into the classical 1C–1·5Cr bearing steel for lowering density and improving performance simultaneously, which is approximate 8 wt-% lighter than convention. In order to understand preliminarily the suitability of the novel alloy for bearing application, the martensite starting temperature and hardness, related to microstructure evolution and mechanical properties, respectively, after partial austenitisation treatment with undissolved carbides have been investigated carefully. The martensite starting temperature is comparable with conventional 1C–1·5Cr alloy. The hardness of 860±3 HV20 achieved is much higher than convention.     

Microstructure and Wear Resistance of Semisolid TiB2/7050 Composites Produced by Serpentine Tube Pouring Technique

TiB₂/7050 (3, 6, and 9 wt%) composites slurries with globular were synthesized by in situ reaction and serpentine tube pouring techniques. The results showed that the semisolid 7050 alloy and 3, 6, and 9 wt%TiB₂/7050 composites with average grain diameter of 28, 25, 20, and 19 µm and shape factor of 0.77, 0.83, 0.90, and 0.93, respectively, can be obtained at 660 °C pouring temperature. With increasing TiB₂ content and curves number, the α-Al grain size was decreased. The composite melts have an effect of “self-stirring” when they flow through the serpentine tube, which is beneficial to make the primary nuclei with globular grains. Moreover, the wear resistance of TiB₂/7050 composites improved obviously with increasing in situ TiB₂ particles content and that the wear rate of 9 wt%TiB₂/7050 composite was 79% lower than that of 7050 matrix alloy under 100 N applied load, 30 min sliding time, and 0.15 m/s sliding velocity.     

Microstructure,phase evolution and corrosion behaviour of the Zn–Al–Mg–Sb alloy coating on steel

ABSTRACT

In the present work, the influence of antimony (Sb) addition in Zn–Al–Mg alloy on the microstructure, phase characteristic, solidification behaviour and corrosion resistance of hot dipped Zn–0.5Al–0.5Mg–xSb (x?=?0, 0.1, 0.3 and 0.5?wt-%) coated steel wires were evaluated. Thermal analysis revealed that cooling rate of the liquid metal using the steel mould (5.3°C?s^–1) was higher than using ceramic mould (0.3°C?s^–1). Based on the phase analysis and verified by thermodynamic calculations, it was revealed that Zn₁₁Mg₂ and Zn₂Mg phases appeared for Zn–Al–Mg alloy at slow and fast cooling rates while, the Mg₃Sb₂ phase was observed after addition of Sb at both cooling rates. Corrosion behaviour of the alloys determined through electrochemical measurements shows that Zn–Al–Mg alloy with 0.3?wt-%Sb has the lowest corrosion rate indicating an excellent corrosion resistance.     

Investigation of thermal properties of Al–Si matrix reinforced fine SiCp composites

Abstract

The characterisation of thermal expansion coefficient and thermal conductivity of Al–Si matrix alloy and Al–Si alloy reinforced with fine SiC_p (5 and 20 wt-%) composites fabricated by stir casting process are investigated. The results show that with increasing temperature up to 350°C, thermal expansion of composites increases and slowly reduces when the temperature reaches to 500°C. The values of both thermal expansion and conductivity of composites are less than those for Al–Si matrix. Microstructure and particles/matrix interface properties play an important role in the thermal properties of composites. Thermal properties of composites are strongly dependent on the weight percentage of SiC_p.     

Properties of SiCf/SiC composites fabricated by slurry infiltration and hot pressing

Abstract

Two-dimensional SiC fibre reinforced SiC ceramic matrix composites (SiC_f/SiC) were fabricated by vacuum infiltration and hot pressing using a 200 nm thick pyrolytic carbon coated Tyranno SA3 fabric and 50 nm sized β-SiC powder. Hot pressing was carried out at 1750°C for 3 h in an Ar atmosphere under a pressure of 20 MPa. Al₂O₃–Y₂O₃–MgO sintering additive (10 wt-%) and polyvinyl butyral resin (45 wt-%) with respect to the matrix SiC were found to be the optimum contents for the high density composite. Vacuum infiltration with a force gradient produced much higher amount of slurry infiltration than simple dipping. Much improved density of 3·02 g cm^?3, compared to the previous reports, was achieved for the SiC–SiC_f containing approximately 67 vol.-% of fibre. This composite showed a step increase with a stress–displacement behaviour during the three-point bending test due to the fibre reinforcement. The displacement for failure and flexural strength were 0·58 mm and 342 MPa respectively, which were much larger than those for monolithic SiC.