Influence of Hydrogen and Water Vapour on the Kinetics of Chromium Oxide Growth at High Temperature

In support of the selection of structural materials for heat exchangers in helium-cooled high temperature reactors, the oxidation behaviour of the Ni-base chromia-former alloy 230 was investigated at 850 °C in diluted helium atmosphere with a low water vapour content. In such a media, the equivalent partial pressure of oxygen (imposed by the $ P_{{{\text{H}}_{2} {\text{O}}}} $ / $ P_{{{\text{H}}_{2}}} $ ratio) is very low ( $ P_{{{\text{O}}_{ 2} }}^{\text{eq}} $ around 10^?16 Pa). The equivalent partial pressure of oxygen has no straight influence on the parabolic rate constant (k _p); on the other hand, $ P_{{{\text{H}}_{2} }} $ and $ P_{{{\text{H}}_{2} {\text{O}}}} $ demonstrate a complex influence on k _p. Photoelectrochemistry analyses revealed that this oxide could simultaneously contain two types of cationic defects. Specific oxidation tests with D₂O showed that the oxide scale also contains hydrogen. A mechanist model is proposed in order to describe the scale growth using both cationic defects. Those theoretical results show, at least qualitatively, how $ P_{{{\text{H}}_{2} }} $ and $ P_{{{\text{H}}_{2} {\text{O}}}} $ may concurrently influence the oxidation rate.     

\left\{ {10\bar 12} \right\} twinning behavior in magnesium single crystal

In order to investigate $\left\{ {10\bar 12} \right\}$ tensile twinning behavior, the magnesium single crystal was deformed by compressing along the $\left[ {2\bar 1 \bar 10} \right]$ direction at room temperature, as $\left\{ {10\bar 12} \right\}$ tensile twinning easily takes place when the compression direction is perpendicular to the c-axis. Numerous $\left\{ {10\bar 12} \right\}$ primary tensile twins were activated during deformation, and the Schmid factor (SF) criterion was applied to the six $\left\{ {10\bar 12} \right\}$ twin variants. The analysis shows that the majority of the $\left\{ {10\bar 12} \right\}$ primary twins belong to high SF variants, and high SF twin boundaries provided nucleation sites for low SF variants. The $\left\{ {10\bar 12} \right\}$ secondary tensile twins were formed inside the high SF of wide $\left\{ {10\bar 12} \right\}$ primary twin bands, and the basal plane of the $\left\{ {10\bar 12} \right\}$ secondary twin was tilted about 60° with respect to the original parent matrix. In the case of the $\left\{ {10\bar 12} \right\}$ secondary tensile twin, relatively low SF variants were activated while counterparts with higher SF variants were absent.     

Quasi-Ternary System Ag2Se-CdSe-Ga2Se3

Phase equilibria in the quasi-ternary system Ag₂Se-CdSe-Ga₂Se₃ were investigated by differential thermal and x-ray phase analysis methods. Phase diagrams of nine vertical sections were constructed. The boundaries of seven single-phase fields were determined which are solid solution ranges of system components and intermediate phases. We constructed the isothermal section at 820 K and the liquidus surface projection, and have determined the position in the system of six invariant processes with the participation of liquid: $ {\text{L}}_{{{\text{U}}_{1} }} + {\upzeta} {\leftrightarrows} {\upbeta} + {\upeta} $ L U 1 + ζ ? β + η (1145 K), $ {\text{L}}_{{{\text{U}}_{ 2} }} + \upzeta \leftrightarrows \upgamma + \upeta $ L U 2 + ζ ? γ + η (1138 K), $ \text{L}_{{U_{3} }} + \upeta \leftrightarrows \updelta + \upgamma $ L U 3 + η ? δ + γ (1113 K), $ {\text{L}}_{{{\text{E}}_{ 1} }} \leftrightarrows \upbeta + \updelta + \upeta $ L E 1 ? β + δ + η (1083 K), $ {\text{L}}_{{{\text{E}}_{ 2} }} \leftrightarrows \upalpha + \upbeta + \upvarepsilon $ L E 2 ? α + β + ε (969 K), $ {\text{L}}_{{{\text{E}}_{ 3} }} \leftrightarrows \upbeta + {\updelta} + \upvarepsilon $ L E 3 ? β + δ + ε (963 K). Two invariant processes in the sub-solidus part, $ \upbeta + \updelta \leftrightarrows \upeta + \uplambda $ β + δ ? η + λ and $ \upbeta + \updelta \leftrightarrows \upvarepsilon + \uplambda $ β + δ ? ε + λ at 968 and 938 K, respectively, were investigated as well.     

Water Vapour Effects on FeO Scale Growth: Differences Between Iron and Steel

High purity iron and a low carbon, low silicon steel were oxidised at temperatures of 800–1,200 °C, in atmospheres of N₂–H₂–H₂O and N₂–O₂–H₂O. Scales of wüstite grew at low oxygen potentials, and of FeO/Fe₃O₄/Fe₂O₃ at high oxygen potentials, both according to parabolic kinetics after an initial transient period. The iron and steel behaved similarly in the O₂/H₂O gases, but not in H₂/H₂O, where the steel oxidised much more slowly than the iron. The rate for steel increased with $ p_{{H_{2} O}} $ at fixed $ p_{{O_{2} }} , $ but for iron was almost independent of $ p_{{H_{2} O}} , $ whilst rates for both metals increased with $ p_{{O_{2} }} $ at fixed $ p_{{H_{2} O}} $ . These results are discussed using point defect models involving hydroxyl anions and cation vacancies. Scaling rates in O₂/H₂O also increased with $ p_{{H_{2} O}} , $ a result attributed to gas phase transport within oxide pores which were present in the scales, but absent in wüstite grown in H₂/H₂O.     

Characterization of crystallographic properties of GaN thin film using automated crystal orientation mapping with TEM

The GaN thin film deposited on an amorphous glass substrate was analyzed by using transmission electron microscopy with a new automated crystal orientation mapping tool. Film deposition was made at 600°C for 4 h by the hyperthermal neutral beam (HNB) source. Columnar crystals oriented to the [0001] direction without significant disordering were clearly observed. Electron diffraction patterns indicated that the crystals have mainly two different zone axes, [2 $\bar 1$ $\bar 1$ 0] and [10 $\bar 1$ 0]. This crystallographic and microstructural information provides the guidance for future works for the HNB source to obtain GaN thin films of higher quality on amorphous substrates.     

Microstructures and morphologies of B2-Ordered NiAl(Co) and FeAl(Co)

Fine dispersion of disordered phases is obtained in a Ni-Al-Co and Fe-Al-Co ternary system. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Fe in B2-ordered FeAl(Co) with different stoichiometries. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and B2-FeAl(Co) increased appreciably by the fine precipitation of (Ni,Co)₂Al, α-Fe, and overage softening occurred after prolonged aging. In case of B2-ordered (Ni,Co)Al, the (Ni,Co)₂Al phase had a hexagonal structure and took a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appeared in the hexagonal (Ni,Co)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix was (0001)_p//(111)_B2 and $[\bar 12\bar 10]_p //[\bar 110]_{B2}$ , where the suffix p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardened appreciably by the fine precipitation of the (Ni,Co)₂Al phase. On the other hand, in case of B2-FeAl(Co), the disordered α-Fe phase was present as a precipitate in a B2-FeAl(Co) matrix and had a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix was typically hardened by the precipitation of α-Fe.     

Relationship of Ca, B, and AlP in Al–12.6Si alloy

P modification has been widely used in Al-Si piston industry, but trace of Ca element has great influence on the P modification efficiency. In this work, it is found that primary Si can be heterogeneously nucleated by AlP in near eutectic Al-12.6Si alloy, but Ca element may destroy the P modification efficiency, whereas the addition of B can recover the P modification efficiency in near eutectic Al-12.6Si alloy with high Ca containing. The microstructure transformation was related to the reaction of Ca, B, and AlP. According to the thermodynamic calculation, Ca may react with AlP and form Ca3P2 compound in Al-Si alloy, whereas, when B was added into the melt, AlP could be reformed. The reaction of Ca, B, and AlP can be shown as follows: 2AlP +3Ca→Ca3P2+2Al; Ca3P2+18B+2Al→3CaB6+2AlP. In addition, with B added into the Al-12.6Si alloy with Ca and P addition, the mechanical properties were improved compared with single Ca and/or P addition.     

The role of a preceding chemical reaction and reactive clusters in phase transitions of intermetallic compounds

By means of chemical and X-ray phase analysis with the use of a rotating disc electrode, it is found that the dissolution of Mg-Cu and In-Bi intermetallic compounds proceeds via magnesium ionization and subsequent $Mg_2 Cu\xrightarrow[{ - Mg^{2 + } }]{}MgCu_2 $ and $In_2 Bi\xrightarrow[{ - In^{3 + } }]{}InBi$ InBi phase regrouping. It is postulated that the process is accompanied by the formation of a defective crystal lattice and the appearance of reactive clusters, which interact to produce, at first, nuclei of a new phase and, then, the crystal lattice of a new intermetallic compound that is enriched in the electrochemically less active component.     

Modeling the Correlation of Composition-Processing-Property for TC11 Titanium Alloy Based on Principal Component Analysis and Artificial Neural Network

In the present investigation, the correlation of composition-processing-property for TC11 titanium alloy was established using principal component analysis (PCA) and artificial neural network (ANN) based on the experimental datasets obtained from the forging experiments. During the PCA step, the feature vector is extracted by calculating the eigenvalue of correlation coefficient matrix for training dataset, and the dimension of input variables is reduced from 11 to 6 features. Thus, PCA offers an efficient method to characterize the data with a high degree of dimensionality reduction. During the ANN step, the principal components were chosen as the input parameters and the mechanical properties as the output parameters, including the ultimate tensile strength ( $ \upsigma_{\text{b}} $ ), yield strength ( $ \upsigma_{0.2} $ ), elongation ( $ \updelta $ ), and reduction of area (??). The training of ANN model was conducted using back-propagation learning algorithm. The results clearly present ideal agreement between the predicted value of PCA-ANN model and experimental value, indicating that the established model is a powerful tool to construct the correlation of composition-processing-property for TC11 titanium alloy. More importantly, the integrated method of PCA and ANN is also able to be utilized as the mechanical property prediction for the other alloys.     

The Effect of Texture on the Corrosion Behavior of AZ31?Mg Alloy

Magnesium (Mg) grains show anisotropic corrosion behavior, which implies that the single-phase, hot-rolled Mg alloy AZ31 sheet, if highly textured, will have different corrosion performance depending on its crystallographic orientation of the grains. Its rolling surface, dominated by (0001) basal crystallographic planes, is more corrosion resistant than its cross-section surface, which is mainly composed of $ \{ 10\overline{1} 0\} $ and $ \{ 11\overline{2} 0\} $ prismatic crystallographic planes. Furthermore, grain refinement by hot rolling is beneficial to the overall corrosion resistance of AZ31 because of the dissolution of AlMn(Fe) intermetallic precipitates in the alloy. Surface compressive deformation machining can lead to refined grains and an expected preferred grain orientation, thus improving the corrosion resistance of AZ31 alloy.     

Texture Evolution in a Ti-Ta-Nb Alloy Processed by Severe Plastic Deformation

Titanium alloys are extensively used in a variety of applications because of their good mechanical properties, high biocompatibility, and corrosion resistance. Recently, ??-type Ti alloys containing Ta and Nb have received much attention because they feature not only high specific strength but also biocorrosion resistance, no allergic problems, and biocompatibility. A Ti-25Ta-25Nb ??-type titanium alloy was subjected to severe plastic deformation (SPD) processing by accumulative roll bonding and investigated with the aim to observe the texture developed during SPD processing. Texture data expressed by pole figures, inverse pole figures, and orientation distribution functions for the (110), (200), and (211) ??-Ti peaks were obtained by XRD investigations. The results showed that it is possible to obtain high-intensity share texture modes ({001}??110??) and well-developed ?? and ??-fibers; the most important fiber is the ??-fiber ({001} $ \left\langle {1\bar{1}0} \right\rangle $ to {114} $ \left\langle {1\bar{1}0} \right\rangle $ to {112} $ \left\langle {1\bar{1}0} \right\rangle $ ). High-intensity texture along certain crystallographic directions represents a way to obtain materials with high anisotropic properties.     

The MPAW of Ti-3Al-2.5V Thin Sheets and Its Effects on Mechanical and Microstructural Properties

This research work deals with joining of Ti-3Al-2.5V titanium alloy thin sheets by means of microplasma arc welding (MPAW). An experimental set-up was developed to produce specimens welded in butt joint under controlled welding parameters, such as voltage, current, travel speed, and shielding gas flow rate. The performance of MPAW process was examined by mechanical properties tests and microstructural characterization. Results show that tensile strength and elongation of the welded specimens for a range of specific input heat are comparable to those of the base material (BM). Scanning electron microscopy (SEM) images of the fracture surface presented characteristics of ductile rupture. Studies on microstructure morphology of the specimens at the fusion zone (FZ) and heat-affected zone (HAZ) reveal occurrence of phase transformation from high temperature β phase to acicular $ \alpha^{\prime} $ phase, while the BM is of equiaxed α with intergranular β. An increasing variation in hardness was measured at the HAZ and FZ, which can be attributed to the presence of acicular $ \alpha^{\prime} $ phase and decreasing the amount of β phase at these regions. Based on the experimental results, it can be stated that MPAW process is an effective method for joining Ti-3Al-2.5V thin sheets provided appropriate welding parameters are used.     

Phase Identification of Cu-In Alloys with 45 and 41.25 at.% In Compositions

In this work, the thermal stability of Cu-In alloys with 45.0 and 41.2 at.% In nominal compositions was investigated by differential scanning calorimetry (DSC), scanning electron microscopy, wavelength dispersive spectroscopy, and in-situ synchrotron x-ray powder diffraction (S-PXRD) over a temperature range from 25 up to 400 °C. The studied samples are mainly composed of a Cu₁₁In₉ phase together with minor amounts of the B phase (based on the NiAs-Ni₂In type structure) and, in one of the samples, with a minor amount of pure In. No evidence of the Cu₁₀In₇ (41.2 at.% In) phase was detected, not even in the sample with 41.2 at.% In nominal overall composition. The combined use of the S-PXRD and DSC techniques allowed us to identify two phase transitions involving the Cu₁₁In₉ phase, one of them corresponding to the $ \upeta^{\prime} \rightleftharpoons {\text{B}} + {\text{Cu}}_{11} {\text{In}}_{9} $ reaction at T = 290 °C and the other to the peritectic $ \upeta^{\prime} + {\text{L}} \rightleftharpoons {\text{Cu}}_{11} {\text{In}}_{9} $ reaction at T = 311 °C.     

Creep Properties and Deformation Mechanisms of a FGH95 Ni-based Superalloy

By means of full heat treatment, microstructure observation, lattice parameters determination, and the measurement of creep curves, an investigation has been conducted into the microstructure and creep mechanisms of FGH95 Ni-based superalloy. Results show that after the alloy is hot isostatically pressed, coarse γ′ phase discontinuously distributes along the previous particle boundaries. After solution treatment at high temperature and aging, the grain size has no obvious change, and the amount of coarse γ′ phase decreases, and a high volume fraction of fine γ′ phase dispersedly precipitates in the γ matrix. Moreover, the granular carbides are found to be precipitated along grain boundaries, which can hinder the grain boundaries’ sliding and enhance the creep resistance of the alloy. By x-ray diffraction analysis, it is indicated that the lattice misfit between the γ and γ′ phases decreases in the alloy after full heat treatment. In the ranges of experimental temperatures and applied stresses, the creep activation energy of the alloy is measured to be 630.4 kJ/mol. During creep, the deformation mechanisms of the alloy are that dislocations slip in the γ matrix or shear into the γ′ phase. Thereinto, the creep dislocations move over the γ′ phase by the Orowan mechanism, and the $ \left\langle { 1 10 } \right\rangle $ 〈 1 10 〉 super-dislocation shearing into the γ′ phase can be decomposed to form the configuration of (1/3) $ \left\langle { 1 12 } \right\rangle $ 〈 1 12 〉 super-Shockleys’ partials and the stacking fault.     

Secondary recrystallization in Fe-3% Si alloy with (110)[001] single-component texture

After the primary recrystallization of a preliminarily deformed (110)[001] single crystal, the texture also has the preferred (110)[001] orientation. Furthermore, the texture contains weak orientations, a major part of which is formed at the sample surface and can be described by a spectrum of scattered orientations {120}〈210〉…{351}〈103〉. A further heating leads to two concurrent processes taking place in the samples, i.e., the normal growth of Goss grains and secondary recrystallization. Abnormally grown crystals are represented by a quartet of orientations related with the initial Goss orientation by a rotation around [011], [01 $\bar 1$ ], [101], and [10 $\bar 1$ ] axes at an angle of ～30°. The crystallographic relationship between the initial and final grain orientations can be explained by their closeness to special misorientations as follows: Σ9, Σ19a, Σ27a, and Σ33a (rotation around 〈110〉 axes to close angles).     

Thermodynamic Roles of Metallic Elements in Carburization and Metal Dusting

This paper addresses the thermodynamic aspects of metallic elements (typically Fe, Ni, and Cr) in carburization and metal-dusting processes in CH₄–H₂, CO–CO₂, and CO–H₂O–H₂ carburizing gas mixtures, and their possible contributions to understanding of carburization and metal-dusting phenomena. Carburization requires $a_C \left( {gas} \right) > a_C \left( {M_Z C/M} \right) \cdot a_C \left( {M_Z C/M} \right)$ is solely temperature-dependent, while a_C (gas) is dependent not only on temperature but also on gas chemistry and total pressure. In general, metallic elements tend to be carburized at higher temperatures in CH₄–H₂, but at lower temperatures in CO–CO₂ and CO–H₂O–H₂ carburizing gas mixtures. For metal dusting to occur, $a_C \left( {M_Z C/M} \right)$ (gas)> 1 (first-type) and $a_C \left( {gas} \right) > a_C \left( {M_Z C/M} \right)$ ; 1 (second-type) should be satisfied. Possible regimes for first- and second-type metal dusting are discussed for pure Fe and Ni, and the range for first-type metal dusting of Ni is considerably broader than that for Fe.     

Comparative TEM investigation on the precipitation behaviors in Cu–15wt%Sn alloy

The decomposition and precipitation behaviors of a quenched Cu–15wt%Sn alloy as a function of aging temperature were investigated using transmission electron microscopy (TEM). Focused ion beam (FIB) was employed to assist TEM specimen preparation. At 300 °C, the decomposition of the supersaturated α′ phase occurred at grain boundaries, displaying a cellular morphology. The lamellae were found with ζ and α phases, rather than with the equilibrium ε and α phases. The ζ and α phases exhibit a well-defined orientation relationship (OR) as $ (1\bar{1}0)_{\alpha } //(0001)_{\zeta } ,\;[11\bar{2}]_{\alpha } //[\bar{1}2\bar{1}0]_{\zeta } $ . On the other hand, at 320 °C, only incipient lamellar structures of several micron meters were observed, which were composed of the δ and α phases. At the same time, abundant intragranular precipitation of the ε phase in the form of platelets was observed, and OR as $ (1\bar{1}1)_{\alpha } //(001)_{\varepsilon } , $ [110] _α //[100] _ε exists between ε phase and the α phase. These contrasting precipitation behaviors are discussed from the viewpoint of crystallographic coherency of these phases.     

Metal Dusting of Nickel–Aluminium Alloys

Alloys of γ-Ni(Al), γ–γ′ Ni(Al)–Ni₃Al, γ′–Ni₃Al and β-NiAl were exposed in 1 h cycles to a carbon-supersaturated CO–H₂–H₂O gas mixture (a _C = 36.7, ${{p_{{\rm O}_2}}} $  = 2.83 × 10^?26 atm) at 650 °C and an overall pressure of 1 atm. It was found that all alloys except β-NiAl had been attacked by metal dusting, leaving a layered structure of nickel particles, graphite and catalytically grown nano-sized carbon filaments as the corrosion product. Carbon uptake and metal wastage rates were slowed with increasing aluminium content for the single-phase alloys. However, the γ–γ′ two phase alloy had the overall highest metal loss rate. Surface morphologies reflected uniform attack for the γ and γ–γ′ alloys, whereas on γ′ a pitting type of attack was observed. Amorphous alumina formation was identified on the surface of the γ′ and β alloys, and is thought to be the major factor providing protection against dusting attack.     

Transient Oxidation of Cu-5at.%Ni(001): Temperature Dependent Sequential Oxide Formation

The transient oxidation stage of a model metal alloy thin film was characterized with in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and analytical high-resolution TEM. We observed the formations of nanosized NiO and Cu₂O islands when Cu-5at.%Ni(001) was exposed to oxygen partial pressure, $ {\text{pO}}_{ 2} = 1 \times 10^{ - 4} \,{\text{Torr}} $ and various temperatures in situ. At 350 °C epitaxial Cu₂O islands formed initially and then NiO islands appeared on the surface of the Cu₂O island, whereas at 550 °C NiO appeared first. XPS and TEM revealed a sequential formation of NiO and then Cu₂O islands at 550 °C. The temperature-dependent oxide selection may be due to an increase of the diffusivity of Ni in Cu with increasing temperature.     

Effects of Crystallographic Orientation on Corrosion Behavior of Magnesium Single Crystals

The corrosion behavior of magnesium single crystals with various crystallographic orientations was examined in this study. To identify the effects of surface orientation on the corrosion behavior in a systematic manner, single-crystal specimens with ten different rotation angles of the plane normal from the [0001] direction to the $ [ 10\overline{1} 0] $ direction at intervals of 10° were prepared and subjected to potentiodynamic polarization and potentiostatic tests as well as electrochemical impedance spectroscopy (EIS) measurements in 3.5?wt.% NaCl solution. Potentiodynamic polarization results showed that the pitting potential (E _pit) first decreased from ?1.57?V _SCE to ?1.64?V _SCE with an increase in the rotation angle from 0° to 40°, and then increased to ?1.60?V _SCE with a further increase in the rotation angle to 90°. The results obtained from potentiostatic tests are also in agreement with the trend in potentiodynamic polarization tests as a function of rotation angle. A similar trend was also observed for the depressed semicircle and the total resistances in the EIS measurements due to the facile formation of MgO and Mg(OH)₂ passive films on the magnesium surface. In addition, the amount of chloride in the passive film was found first to increase with an increase in rotation angle from 0° to 40°, then decrease with a further increase in rotation angle, indicating that the tendency to form a more protective passive film increased for rotation angle near 0° [the (0001) plane] or 90° [the $ ( 10\overline{1} 0) $ plane].