Tensile properties of hot rolled Mg_(97)Zn_1Y_2 alloy sheets at elevated temperatures

The elevated temperature tensile properties of Mg97Zn1Y2 magnesium alloy sheets, hot rolled at 390, 420 and 450 ℃ respectively, were tested in a temperature range from room temperature to 250 ℃ with a strain rate of 1.0×10-3 s-1. The results show that the variations in yield strength for Mg97Zn1Y2 magnesium alloy sheets hot rolled at 390 ℃ and 420 ℃ with temperature resemble each other due to their similar morphology of the chain-shaped strengthening phase. The yield strength maintains at a high level of 283 MPa before 200 ℃ and decreases significantly at 250 ℃. Despite of the fine lamellar structure of Mg97Zn1Y2 magnesium alloy sheet hot rolled at 450 ℃, its yield strength decreases linearly owing to occurrence of the coarse grain, and drops to 239 MPa at 250 ℃. The elongation for all hot rolled Mg97Zn1Y2 magnesium alloy sheets increases slightly with increasing testing temperature.     

Phase Formation of the Yttrium Aluminates in the Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiC System

Phase formation sequence of the yttrium aluminates in the Y₂O₃-Al₂O₃-SiC ternary system as temperature increases were investigated via x-ray diffraction (XRD). Results showed that YAM (monoclinic), YAP (perovskite) and YAG (garnet) were the yttrium aluminates presented in the solid-state reacted samples at a fixed Al₂O₃:SiC ratio of 1:1. Formation of the yttrium aluminates depended on the temperature. The YAM, YAP and YAG started to form below 1150 °C, at 1300 °C, and at 1450 °C, respectively. Accordingly, two behavior phase diagrams of the Y₂O₃-Al₂O₃-SiC ternary system were recognized, one is in the temperature range of 1150-1300 °C and the other is in 1300-1450 °C, respectively. Thereafter, the phase equilibrium was reached in the temperature range of 1450-1700 °C. Effects of SiC on the phase formation processes in the ternary system were discussed.     

Microstructure and High Temperature Oxidation Property of Fe–Cr–B Based Metal/Ceramic Composite Manufactured by Powder Injection Molding Process

This study investigated the microstructure and high temperature oxidation property of Fe–Cr–B metal/ceramic composite manufactured using powder injection molding process. Observations of initial microstructure showed a unique structure where α-Fe and (Cr, Fe)₂B form a continuous three-dimensional network. High temperature oxidation tests were performed at 900, 1000 and 1100 °C, for 24 h, and the oxidation weight gain according to each temperature condition was 0.13, 0.84 and 6.4 mg/cm², respectively. The oxidation results according to time at 900 and 1000 °C conditions represented parabolic curves, and at 1100 °C condition formed a rectilinear curve. Observation and phase analysis results of the oxides identified Cr₂O₃ and SiO₂ at 900 and 1000 °C. In addition to Cr₂O₃ and SiO₂, CrBO₃ and FeCr₂O₄ formed due to phase decomposition of boride were identified at 1100 °C. Based on the findings above, this study suggested the high temperature oxidation mechanism of Fe–Cr–B metal/ceramic composite manufactured using powder injection molding, and the possibility of its application as a high temperature component material was also discussed.     

Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C.

Part III: Oxidation behaviour of optimised MoSi₂ composites The topic “Oxidation behaviour of particle reinforced MoSi₂ composites at temperatures up to 1700°C” is discussed in a three part publication. In the first part of this paper a literature survey on the oxidation behaviour of MoSi₂ and MoSi₂ composites has been given. In the second part an initial screening at 1600°C revealed those composites which may be suitable for high temperature applications. The low temperature oxidation behaviour of selected composites in the “pest” region was examined as well. Additionally, the effect of iron impurities on the high temperature behaviour of the composites was explained. The present part deals with a detailed investigation of an optimised MoSi₂/HfO₂ composite. These investigations include high temperature oxidation at 1500 to 1700°C and low temperature oxidation at 400 and 500°C.     

Temperature dependent surface morphology and lithium diffusion kinetics of LiCoO<Subscript>2</Subscript> cathode

In an attempt to understand the effect of synthesis temperature upon surface morphology and lithium diffusion kinetics of LiCoO₂, the compound was synthesized at four different temperatures, viz., 600, 700, 800 and 900 °C using a novel gelatin-assisted combustion method. LiCoO₂ synthesized at 800 °C is found to be a mixture of rhombohedral and cubic LiCoO₂ and a temperature of 900 °C leads to the formation of cubic LiCo₂O₄ compound, thus favoring lower temperatures such as 600 and 700 °C to prepare phase pure rhombohedral LiCoO₂. Cyclic voltametry and impedance spectral studies evidence that LiCoO₂ synthesized at 600 °C exhibits better electrochemical cycling behavior and considerably reduced internal resistance upon cycling, which are substantiated further from the higher lithium diffusion coefficient value. The study demonstrates the possibility and superiority of synthesizing electrochemically active LiCoO₂ with preferred surface morphology and better lithium diffusion kinetics at a relatively lower temperature of 600 °C, using a gelatin-assisted combustion method.     

Dissolving behavior of ammonium paratungstate in (NH4)2CO3–NH3·H2O–H2O system

The effects of temperature, ammonia concentration and ammonium carbonate concentration on the dissolving behavior of ammonium paratungstate were studied in (NH₄)₂CO₃–NH₃·H₂O–H₂O system. The results show that rising temperature, prolonging duration, increasing ammonia concentration and decreasing ammonium carbonate concentration favor dissolving of ammonium paratungstate at temperature below 90 °C, while the WO₃ concentration decreases after a certain time at temperature above 100 °C. Furthermore, the undissolved tungsten exists in the form of either APT·4H₂O below 90 °C or pyrochlore-type tungsten trioxide above 100 °C. In dissolving process, the ammonium paratungstate dissolves into paratungstate ions followed by partially converting to tungstate ion, resulting in the coexistence of the both ions. This study may provide a new idea to exploit a novel technique for manufacturing ammonium paratungstate and pyrochlore-type tungsten trioxide.     

Influence of Plasma Nitriding on the Microstructure, Wear, and Corrosion Properties of Quenched 30CrMnSiA Steel

The oil-quenched 30CrMnSiA steel specimens have been pulse plasma-nitrided for 4 h using a constant 25% N₂-75% H₂ gaseous mixture. Different nitriding temperatures varying from 400 to 560 °C have been used to investigate the effects of treatment temperature on the microstructure, microhardness, wear, and corrosion resistances of the surface layers of the nitrided specimens. The results show that significant surface-hardened layer consisting of compound and diffusion layers can be obtained when the oil-quenched steel (α′-Fe) are plasma-nitrided at these experimental conditions, and the compound layer mainly consists of ε-Fe_2-3N and γ′-Fe₄N phases. Lower temperature (400-500 °C) nitriding favors the formation of ε-Fe_2-3N phase in surface layer, while a monophase γ′-Fe₄N layer can be obtained when the nitriding is carried out at a higher temperature (560 °C). With increasing nitriding temperature, the compound layer thickness increases firstly from 2-3 μm (400 °C) to 8 μm (500 °C) and then decreases to 4.5 μm (560 °C). The surface roughness increases remarkably, and both the surface and inner microhardness of the nitrided samples decrease as increasing the temperature. The compact compound layers with more ε-Fe_2-3N phase can be obtained at lower temperature and have much higher wear and corrosion resistances than those compound layers formed employing 500-560 °C plasma nitriding.     

High-temperature mechanical properties and thermal shock behavior of ternary-layered MAB phases Fe2AlB2

The high-temperature flexural and compressive strengths, and thermal shock behavior in water of Fe₂AlB₂ were investigated from room-temperature (RT) to 1000 °C. The flexural strength varies in a narrow range of 200–250 MPa from room temperature to 1000 °C, without evident plastic deformation. The compressive strength of Fe₂AlB₂ decreases gradually from 1992 ± 176 MPa at RT to 1482 ± 127 MPa at 600 °C. However, the further increasing temperature results in quicker decrease of the compressive strength to 245 ± 7 MPa at 1000 °C although no plastic deformation is present in the temperature range of 600–800 °C. The brittle-ductile transition temperature (BDTT) is higher under flexure (>1000 °C) than compression (800–900 °C), which is attributed to the higher shear stress under compression. The water-quenched flexural strength exhibits features consistent with the quasi-static propagation of “long initial cracks”, with a critical temperature difference of 200–300 °C. The deduced cracks contribute to the decreasing retained strength. The uniaxial compress during hot pressing results in a weak anisotropy of mechanical properties.     

The Pr-rich portion of the Ni-Pr system

The Ni-Pr phase diagram on the Pr-rich side was revised using differential thermal analysis, differential scanning calorimetry, and scanning electron microscopy. The existence of four stoichiometric compounds, Ni₂Pr, NiPr, Ni₃Pr₇, and NiPr₃, was confirmed; however, the melting temperatures of 766 °C (NiPr), 566 °C (Ni₃Pr₇), and 562 °C (NiPr₃) are considerably different from the previously reported values. The same was true for the four eutectic reactions, which were determined to be as follows: liquid (L) (45±1 at.% Pr) ↔ Ni₂Pr+NiPr at a mean temperature of 732±5 °C; L (65±1 at.% Pr) ↔ NiPr+Ni₃Pr₇ at a mean temperature of 550±2 °C; L (72±2 at.% Pr) ↔ Ni₃Pr₇+NiPr₃ at a mean temperature of 557±2 °C; and L (77±1 at.% Pr) ↔ NiPr₃+Pr.     

The Al(L) + AlB12 ? AlB2 peritectic transformation and its role in the formation of high aspect ratio AlB2 flakes

A new family of metal matrix composites (MMCs) based on high aspect ratio AlB₂ is being developed. Preparation of these MMCs involves the conversion of low aspect ratio AlB₂ to high aspect ratio AlB₂. This conversion is associated with the Al_(L) + AlB₁₂ ↔ AlB₂ peritectic transformation. There is confusion surrounding the temperature at which this transformation occurs. Its value has been reported as high as 1,500 °C and as low as 980 °C. In this paper, the peritectic temperature has been investigated using calorimetric methods and determined to be 956 ± 5 °C. In addition, an unexpected conversion of low aspect ratio AlB₂ to high aspect ratio AlB₂ has been identified near the peritectic temperature.     

Effect of Mg content on the dimensional stability and tensile properties of heat treated Al-Si-Cu (319) type alloys

The present study was performed on A319.2 aluminum alloy containing 6.2% Si, 3.46% Cu, 0.35% Fe, 0.1% Mg. Small amounts of additives were added to the molten metal as follows: 0.5% Mg, (0.5%Mg + 0.03% Sr), (0.5%Mg + 0.03% Sr + 0.02% TiB₂). The fluidity of the molten metal as a function of temperature in the range 620–740 °C was measured using the Ragone testing technique. Two solution treatments were applied to test bars prepared from these alloy compositions: 6–8 h at 470 °C, and 6–8 h at 505 °C. In both cases, the test bars were quenched in hot water (60 °C), followed by immediate ageing for 5 h at 180 °C. The results reveal that Sr slightly improves the fluidity of the molten metal (～12%) at 720 °C. The addition of Mg leads to a noticeable increase in the alloy length (～2%) when the solution temperature is above 500 °C. At this temperature, incipient melting of Al₅Mg₈Si₆Cu₂ and Al₂Cu phase particles located at the grain boundaries was significant. Ageing at 180 °C contributes significantly to the alloy strength without much change in the dimension. The magnitude of the increase in alloy strength is strongly related to the solution temperature. Solution treatment for 8–10 h at 500 °C may be recommended.     

The Al(L) + AlB12 ↔ AlB2 peritectic transformation and its role in the formation of high aspect ratio AlB2 flakes

A new family of metal matrix composites (MMCs) based on high aspect ratio AlB₂ is being developed. Preparation of these MMCs involves the conversion of low aspect ratio AlB₂ to high aspect ratio AlB₂. This conversion is associated with the Al_(L) + AlB₁₂ ↔ AlB₂ peritectic transformation. There is confusion surrounding the temperature at which this transformation occurs. Its value has been reported as high as 1,500 °C and as low as 980 °C. In this paper, the peritectic temperature has been investigated using calorimetric methods and determined to be 956 ± 5 °C. In addition, an unexpected conversion of low aspect ratio AlB₂ to high aspect ratio AlB₂ has been identified near the peritectic temperature.     

Electrochemical study on corrosion behaviour of zircaloy-2 in aqueous medium containing permanganate ion

Abstract

The corrosion compatibility of zircaloy-2 was studied in strong oxidising permanganate based acidic (HMnO₄) medium using impedance spectroscopy and potentiodynamic anodic polarisation methods at room temperature (28°C) and elevated temperatures (65 and 90°C). The average corrosion rate of zircaloy-2 in HMnO₄ medium (0·417–8·33 mM) was found to be 1·32×10^?4 μm h^?1 at 28°C, 2·59×10^?4 μm h^?1 at 65°C and 3·53×10^?4 μm h^?1 at 90°C. At 28°C, the polarisation resistance R_p values obtained from impedance spectrum were higher (in kΩ) when compared to R_p values at 65 and 90°C, indicating a lower corrosion rate at lower temperature. Comparative studies with 2·5 mM H₂SO₄ and platinum (as a working electrode) showed the effects of oxidising nature and participation of additional redox reactions in HMnO₄ medium. Cyclic polarisation studies showed the absence of pitting attack on zircaloy-2.     

Central region of the Mg-Zn phase diagram

The phase equilibria in the Mg-Zn system from 0 to 85 wt pct Zn and from 335° to 93°C have been redetermined. In this region four intermetallic phases, Mg₇Zn₃, MgZn, Mg₂Zn₃, and MgZn₂, exist. Below 325°C, the Mg₇Zn₃ phase decomposes eutectoidally to magnesium solid solution + MgZn. The MgZn phase is stable over the temperature range from 335°C to 93°C; below 325°C, it is in equilibrium with the magnesium solid solution.     

High temperature oxidation behavior of Ni-Cr-Al based powder porous metal

This study investigated the high temperature oxidation behavior of newly developed Ni-Cr-Al powder porous metal. High temperature isothermal oxidation tests were conducted at 900, 1000 and 1100 °C temperatures for 24 h under an atmosphere of 79% N₂ + 21% O₂ gas. Oxidation weight gain vs. time curves represented typical oxidation behavior of parabolic shape. Weight gain increased with increasing oxidation temperature. Ni-Cr-Al porous metal mainly created oxides such as α-Al₂0₃, Cr₂O₃, NiCr₂O₄. The α-Al₂0₃ oxide could be still maintained up to 1100 °C oxidation temperature as a thick and stable protective layer. It was noted that Ni-Cr-Al porous metal had better high temperature oxidation resistance than those of other Ni-based and Fe-based porous metals. The catastrophic degradation of oxidation resistance especially at very high temperature was not observed up to 1100 °C in this porous metal. The micro-mechanism of high temperature oxidation of Ni-Cr-Al porous metal was also discussed.     

Corrosion behaviour of X52 pipeline steel in high H2S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract

High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H₂S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H₂S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H₂S concentration at a temperature of 90°C. When the temperature and H₂S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe²⁺ ions and the inward diffusion of H₂S and HS^? species.     

Unlubricated friction and wear behaviors of Al2O3/TiC ceramic cutting tool materials from high temperature tribological tests

The unlubricated friction and wear behaviors of Al₂O₃/TiC ceramic tool materials were evaluated in ambient air at temperature up to 800 °C by high temperature tribological tests. The friction coefficient and wear rates were measured. The microstructural changes and the wear surface features of the ceramics were examined by scanning electron microscopy. Results showed that the temperature had an important effect on the friction and wear behaviors of this Al₂O₃ based ceramic. The friction coefficient decreased with the increase of temperature, and the Al₂O₃/TiC ceramics exhibited the lowest friction coefficient in the case of 800 °C sliding operation. The wear rates increased with the increase of temperature. During sliding at temperature above 600 °C, oxidation of the TiC is to be expected, and the formation of lubricious oxide film on the wear track is beneficial to the reduction of friction coefficient. The wear mechanism of the composites at temperature less than 400 °C was primary abrasive wear, and the mechanisms of oxidative wear dominated in the case of 800 °C sliding operation.     

KCl-Induced Corrosion of a 304-type Austenitic Stainless Steel in O2 and in O2 + H2O Environment: The Influence of Temperature

The oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated in the temperature range 400–600 °C in 5% O₂ and 5% O₂ + 40% H₂O. Exposure time was up to 1 week. Prior to exposure, the polished samples were coated with 0.1 mg/cm² KCl. Uncoated samples were also exposed and used as references. The oxidized samples were analyzed by gravimetry and by ESEM/EDX, XRD, IC and AES. The results show that KCl is strongly corrosive. Corrosion is initiated by the reaction of KCl with the chromia-containing oxide that normally forms a protective layer on the alloy. This reaction produces potassium chromate particles, leaving a chromium-depleted oxide on the alloy surface. At 500 and 600 °C this results in rapid oxidation, resulting in the formation of a thick scale consisting of a mixture of hematite, spinel oxide ((Fe,Cr,Ni)₃O₄) and K₂CrO₄. The thick scale is poorly protective and permeable to e.g. chloride ions. The KCl-induced corrosion of alloy 304L in dry O₂ and in an O₂ + H₂O mixture increases strongly with temperature in the range 400–600 °C. The strong temperature dependence is explained partly by the temperature dependence of the chromate-formation reaction and partly by the ability of the chromium-depleted oxide to protect the alloy at low temperature. At 400 °C, the oxide was still protective after 168 h.     

Thermoelectric properties of n-type Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> compounds prepared by spark plasma sintering

In this study, the thermoelectric properties of 0.1 wt.% Cdl₂-doped n-type Bi₂Te_2.7Sb_0.3 compounds, fabrieated by SPS in a temperature range of 250°C to 350°C, were characterized. The density of the compounds was increased to approximately 100% of the theoretical density by carrying out consolidation at 350°C. The Seebeck coefficient, thermal conductivity, and electrical resistivity were dependent on a hydrogen reduction process and the sintering temperature. The Seebeck coefficient and the electrical resistivity increased with the reduction process. Also, electrical resistivity decreased and thermal conductivity increased with sintering temperature. The results suggest that carrier density and mobility vary according to the reduction process and sintering temperature. The highest figure of merit, 1.93×10⁻³ K⁻¹, was obtained for the compound consolidated at 350°C for 2 min.     

Preparation of Self-Healing α-Al2O3 Films by Low Temperature Thermal Oxidation

This paper reports a new approach to lowering the temperature necessary for the preparation of α-Al₂O₃. Oxidation of Al–Cr alloys, with Cr contents of 18, 23 and 27 %, was performed at temperatures ranging from 620 to 720 °C in air for 100 h. The resulting oxide films were analyzed by SEM, EDS, XRD and XPS. The results showed that α-Al₂O₃ films were obtained following oxidation of the 18 and 23 wt% Cr alloy samples at 720 °C and that rough surfaces were conducive to the formation of α-Al₂O₃ such that peened surface samples showed significant α-Al₂O₃ growth while polished samples showed no oxide by XRD. A 23 wt% Cr sample with a roughened surface exhibited the formation of α-Al₂O₃ at a temperature of 670 °C. Conversely, only a very thin oxide film was observed on a 27 wt% Cr sample after oxidation at 720 °C.