Effect of 0.1 at.% zirconium on the cyclic oxidation resistance ofβ-NiAl

The effect of 0.1 at.% Zr (0.2 wt.% Zr) on the cyclic oxidation of hipped -NiAl was studied. Oxidation testing was performed in static air at 1100–1200 °C, using 1-hr exposure cycles for test times up to 3000 hr. The weight change versus time data were modeled with the COSP computer program to analyze and predict cyclic-oxidation behavior. Zr additions significantly change the nature of the scale-spalling process during cooling so that the oxide spalls near the oxide-air interface at a relatively low depth within the scale. Without Zr, the predominantly -Al₂O₃ scale tends to spall randomly to bare metal at relatively high effective-scale-loss rates, particularly at 1150°C and 1200°C. This leads to higher rates of Al consumption for the Zr-free aluminide and much earlier depletion of Al, leading to eventual breakaway (i.e., failure).     

Influence of Hot Isostatic Pressing on the Microstructure and Mechanical Properties of a Spray-Formed Al-4.5 wt.% Cu Alloy

Al-4.5 wt.% Cu alloy was spray atomized and deposited at varied spray heights ranging from 300 to 390 mm. The average grain sizes decreased from ~ 29 to ~ 18 μm and a concomitant increase in the hardness and the 0.2% yield strength (YS) with increase in the spray height. The respective hardness values of SF-300, SF-340, and SF-390 are 451 ± 59, 530 ± 39, and 726 ± 39 MPa and the YS are 108 ± 7, 115 ± 8, and 159 ± 10 MPa. The transmission electron micrographs revealed the morphological changes of the Al₂Cu phase from irregular shaped to small plate-shaped and then subsequently to spheroidal shape due to high undercooling encountered during spray atomization with increase in spray height from 300 to 390 mm. The porosity of the spray formed deposits varied between 5 to 12%. Hot isostatic pressing of spray deposits reduced the porosity to less than 0.5% without any appreciable increase in grain size. A dislocation creep mechanism seems to be operative during the secondary processing. A comparison between as-spray formed and hot isostatically pressed deposits exemplifies improvement in mechanical properties as a result of elimination of porosity without affecting the fine grain sizes achieved during the spray-forming process.     

Effects of Austempering after Hot Deformation on the Mechanical Properties of Hot Rolled Si-Mn TRIP Steel Sheets

Excellent mechanical properties are obtained by austempering after hot deformation without subsequent heat treatment in the present Si-Mn TRIP steel sheets. Isothermal holding time after finishing rolling has affected the mechanical properties of this steel. The results show that the sample exhibits a good combination of ultimate tensile strength and total elongation when it is held at the bainite transformation temperature after hot deformation. The stability of retained austenite increases with an increase of isothermal holding time, and a further increase in the holding duration results in a decrease of it. The tensile strength, total elongation and strength ductility reach the maximum values (774MPa, 33% and 25542MPa% respectively) for this sort of hot rolled Si-Mn TRIP steel using the optimal technology.     

Hot Workability and Flow Characteristics of Aluminum-5 wt.% B4C Composite

Flow behavior of aluminum-5 wt.% boron carbide (Al-B₄C) composite was investigated by carrying out compression tests over a range of strain rates (10^?4-10⁰ s^?1) and temperatures (200-500 °C). The flow stress data obtained from these tests at true strain 0.5 were used to develop processing map. The stable and instable flow regimes in the map were characterized by the microstructural examination using Scanning Electron Microscopy and Electron Backscattered Diffraction. The optimum condition for processing of Al-5%B₄C composite was found to lie between 425 and 475 °C at the strain rate of around 10^?4 s^?1. A strain-compensated Sellars-McG Tegart constitutive equation was established to model high-temperature deformation behavior of the material.     

Competitive Effect of Water Vapor and Oxygen on the Oxidation of Fe–5 wt.% Al Alloy at 1073 K

The effect of oxygen on the oxidation of Fe–5wt.% Al alloy was investigated at 1073 K in N2–12.2 vol.% H₂O, O₂–12.2 vol.% H₂O, and N₂–O₂–12.2 vol.% H₂O with various amounts of oxygen. The results showed S-shaped oxidation curves that consisted of three stages: slow-incubation, rapid transition, and relatively slow oxidation. The amount of oxidation increased with increasing oxygen contents up to 0.9 vol.% O₂ and then rapidly decreased. On the oxygen-rich side, a slow incubation oxidation stage was observed and its duration increased with increasing oxygen content. The extent of oxidation decreased gradually with decreasing oxygen content from the critical value and the incubation period disappeared. In the transient period, Fe₂O₃ was formed on the lean oxygen-content side and elongated voids were formed in the outer Fe₃O₄ and FeO layer. It was suggested that the differences in the morphology of Fe₂O₃ formed on the surface affected by the dissociation and gas-transport process due to differences in oxygen partial pressure at the gas–scale interface.     

Experimental and Thermodynamic Exploration of the High-Temperature Microstructures of Co-30 wt.% Cr- (2.5-5.0 wt.%) C Alloys

Cobalt alloyed with high contents in Cr and C potentially may lead to hard alloys candidate for high temperature applications involving reduced wear degradations. The Co-30wt.%Cr-xC system was explored from x = 2.5 to 5 wt.% by both experiments and thermodynamic calculations. The stable microstructures at 1000, 1100 and 1200 °C and the temperatures of solidus and liquidus were of interest. When the carbon content is between 2.5 and 3.5 the alloys display (hypo-)eutectic microstructures composed of FCC Co-based matrix and M₇C₃ carbides. Between 4 and 5 wt.%C, the microstructures contain coarse pro-eutectic M₇C₃ carbides, leading to a total volume fraction of M₇C₃ close to 50% or higher. The C-richest alloys also contain graphite, instead cementite as predicted by calculations. A better agreement between experiments and calculations about microstructures and solidus temperature can be obtained by forbidding cementite in the calculation conditions. The obtained hardness level is about 650 Hv but decreases when the heat-treatment temperature increases.     

Effect of Zn Additions on the Oxidation of Cu–2 at.% Al and Cu–4 at.% Al Alloys in 1 atm O2 at 800 °C

Four ternary Cu–Zn–Al alloys containing 5 or 10 at.% Zn and 2 or 4 at.% Al plus an alloy containing 2 at.% Al and 15 at.% Zn have been oxidized at 800 °C in 1 atm O₂, and their behavior has been compared with that of the corresponding binary Cu–Zn and Cu–Al alloys. For the alloy containing 4 at.% Al, which is already able to form external alumina scales, the addition of Zn is only effective in reducing the mass gain during the fast, initial-oxidation stage. Conversely, the addition of 15 at.% Zn to Cu–2Al is able to prevent the formation of external scales containing mixtures of the Cu and Al oxides, resulting in the formation of external alumina scales after an initial stage of faster rate, producing a limited third-element effect. Finally, the addition of Al to both Cu–5Zn and Cu–10Zn is able to prevent the internal oxidation of Zn, producing a kind of reversed third-element effect. Possible mechanisms for these effects are examined on the basis of general treatments concerning the scaling behavior of ternary alloys.     

Heat Treatment of Closed-Cell A356 + 4 wt.%Cu + 2 wt.%Ca Foam and Its Effect on the Foam Mechanical Behavior

In this investigation, aluminum-silicon alloy foam is developed by adding certain amounts of copper and calcium elements in A356 alloy. Addition of 4 wt.%Cu + 2 wt.%Ca to the melt changed bubbles morphology from ellipsoid to spherical by decreasing Reynolds number and increasing Bond number. Compression behavior and energy absorption of the foams are assessed before and after aging. Solid solution treatment and aging lead to the best mechanical properties with 170% enhancement in yield strength and 185% improvement in energy absorption capacity as compared to non-heat-treated foams. The metallographic observations showed that bubbles geometry and structure in the A356 + 4wt.% Cu + 2 wt.%Ca foam are more homogeneous than the A356 foam.     

Effect of Asymmetric Hot Rolling on the Texture Evolution of Fe–3%Si Steel

In Fe–3%Si steel, the hot rolling process affects not only the hot rolling texture but also the primary recrystallization texture. Here, the effect of asymmetric hot rolling was studied by comparing the difference in the texture evolved between asymmetric and symmetric hot rolling. The effect of asymmetric hot rolling on the texture of primary recrystallized Fe–3%Si steel was also studied. The symmetric hot rolling of Fe–3%Si steel produces a rotated cube texture at the center but Goss and copper textures near the surface. Asymmetric hot rolling tends to produce Goss and copper textures even at the center like the texture near the surface. After primary recrystallization, the dominant texture at the center changes from {001} <210> to {111} <112> and the new texture has a higher fraction of the grains which make the low energy boundary with Goss grains than that of symmetric hot rolling.     

Electrochemical Corrosion Behavior of Magnetron-Sputtered Al-Mo Gradient-Coated Steel in 3.5 wt.% NaCl Solution

A gradient three-layer Al-Mo coating was deposited on steel using magnetron sputtering method. The corrosion properties of the coating were studied in 3.5 wt.% NaCl solution using electrochemical techniques, whereas the hydrogen-induced cracking (HIC) resistance was examined by constant-load tests using notched tensile specimens. These results were compared with conventional electroplated cadmium-coated steel. The results show that the gradient Al-Mo coating exhibits better corrosion and HIC resistance when compared to electroplated cadmium. This was due to the excellent corrosion resistance of the bottom aluminum-rich layer, while the top Mo-rich layer provided good lubrication properties.     

Analysis of Plastic Flow Instability During Superplastic Deformation of the Zn-Al Eutectoid Alloy Modified with 2 wt.% Cu

The aim of this work is to analyze the plastic flow instability in Zn-21Al-2Cu alloy deformed under 10^?3 s^?1 and 513 K, which are optimum conditions for inducing superplastic behavior in this alloy. An evaluation using the Hart and Wilkinson–Caceres criteria showed that the limited stability of plastic flow observed in this alloy is related to low values of the strain-rate sensitivity index (m) and the strain-hardening coefficient (γ), combined with the tendency of these parameters to decrease depending on true strain (ε). The reduction in m and γ values could be associated with the early onset of plastic instability and with microstructural changes observed as function of the strain. Grain growth induced by deformation seems to be important during the first stage of deformation of this alloy. However, when ε > 0.4 this growth is accompanied by other microstructural rearrangements. These results suggest that in this alloy, a grain boundary sliding mechanism acts to allow a steady superplastic flow only for ε < 0.4. For ε values between 0.4 and 0.7, observed occurrences of microstructural changes and severe neck formation lead to the supposition that there is a transition in the deformation mechanism. These changes are more evident when ε > 0.7 as another mechanism is thought to take over.     

Effect of Double Austenitization on the Microstructure and Hardness of High-Carbon Steel 1.4% C – 11.7% Cr

Metal Science and Heat Treatment - The effects of single and double austenitization at 900, 590, 1000 and 1050°C on the microstructure and hardness of quenched and tempered steel 1.4% C...     

Water-vapor-effect on the oxidation of Fe-21.5 wt.%Cr-5.6 wt.%Al at 1000°C

Fe-21.5 wt. %Cr-5.6 wt. %Al oxidation, at 1000°C, in dry or wet oxygen shows that steam has an influence on the oxide-scale growth mechanism. Steam modifies the kinetics of early-stage oxidation. In dry oxygen, an initial fast linear regime is observed during one hour. Under wet conditions, weight-gain curves follow the same parabolic regime over the entire oxidation test. The scale structure strongly depends on the presence of steam in the gaseous environment. With dry oxygen, the scale is composed mainly of-Al₂O₃ after the initial formation of-Al₂O₃ identified by ESCA and RHEED. The kinetics transient stage corresponds to the necessary time for the internal part of the initial-Al₂O₃ scale to transform into a continuous-Al₂O₃ diffusion barrier. Under wet oxygen conditions, transient oxides are identified as (Mg, Fe) (Cr, Al)₂O₄, MgAl₂O₄ (orthorhombic), Al₂O₃ (hexagonal), these oxides transform into MgAl₂O₄ (cubic), Cr₃O₄, Fe₂O₃,-Al₂O₃, with time. When water vapor does not change drastically oxidation kinetics, the induced presence of iron and chromium in the oxide scale could be responsible for weakening the protectiveness of alumina scales.     

Hot Tearing of Sand Cast Mg-5 wt.% Y-4 wt.% RE （WE54） Alloy

Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented ＂CRC＂ equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE （WE54） alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction （fs） in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.     

Effect of Heat Treatment on Electrochemical Properties of Mg–9 wt.%Al–2.5 wt.%Pb Alloy in Sodium Chloride Solution

Mg–Al–Pb alloy can serve as a good candidate for the anode material in seawater-activated batteries. The effect of solution and aging treatment on electrochemical properties of Mg–9 wt.%Al–2.5 wt.%Pb alloy in 3.5 wt.% NaCl solution was investigated through scanning electron microscopy and electrochemical tests. The results indicate that the discharge activity of Mg–9 wt.%Al–2.5 wt.%Pb alloy decreases after solution treatment, although its anodic efficiency increases slightly. In contrast, its discharge performance and anodic efficiency, which are crucial for the application of batteries, are both enhanced after aging at 200°C for 12 h.     

Effect of Cold Deformation on the Friction–Wear Property of a Biomedical Nickel-Free High-Nitrogen Stainless Steel

The microstructural,mechanical and corrosion properties of different cold-rolled biomedical nickel-free highnitrogen stainless steels(NFHNSSs) were investigated to study the effect of cold deformation on its dry wear resistance as well as corrosion–wear behaviors in distilled water and Hank's solution. The results indicated that NFHNSS was characterized by stable austenite and possessed excellent work-hardening capacity; due to increasing cold deformation,the corrosion resistance just decreased very slightly and the dry wear rate decreased initially but subsequently increased,while the corrosion–wear resistance was improved monotonically in both distilled water and Hank's solution in spite of the presence of corrosive ions. The friction coefficients for different cold-rolled NFHNSSs were very close under the same lubricating condition,but they were the largest in distilled water compared to that in dry wear tests and Hank's solution.