Effect of water vapour partial pressure on the chromia (Cr2O3)-based scale stability

The kinetics of the Cr₂O₃-based scale oxidation and volatilisation were studied in the presence of water vapour (H₂O). A commercial Cr₂O₃-based scale forming Type 310S stainless steel was examined at the ambient pressure (0.1?MPa) and 550°C in relatively low and high H₂O-containing environments of air-10% H₂O and air-70% H₂O, respectively. The increase in the partial pressure of H₂O (p_H2O) from 10 to 70% resulted in the transition of the oxidation and volatilisation kinetics from the parabolic rate law in air-10% H₂O to the paralinear rate law in air-70% H₂O. The kinetics transition was attributed to the increase in the Cr loss rate from the base scale after coupons exposure in air-70% H₂O. The significant role of Mn alloying element in the base scale protectiveness was also discussed in the context of the Cr₂O₃-based scale stability.     

Effects of Nitrogen Deficiency on the Kinetics and Mechanism of Electrolytic ZrN x Oxidation

Use has been made of potentiodynamic polarization curves, XRD, and scanning electron microscopy (SEM) in the electrolytic oxidation in 3% NaCl solution for specimens of nitrogen-deficient zirconium nitride (ZrN_0.67, ZrN_0.77, ZrN_0.87, and ZrN_0.97), as well as pure zirconium. In all cases, the anodic polarization curves have several stages which characterize during oxidation both active dissolution of ZrN _x and Zr in the electrolyte as well as the formation of surface layers of ZrOCl₂, ZrN _x O _y , and α‐ZrO₂ of monoclinic form. The corrosion resistance of single-phase ZrN _x specimens in 3% NaCl solution decreases in the sequence ZrN_0.97 → ZrN_0.87 → ZrN_0.77, and the initial stages of interaction between the specimen surface and the electrolyte largely determine the subsequent behavior of specimens. It is found that ZrN _x containing a large number of nitrogen atom vacancies, in particular ZrN_0.77, is closer in corrosion behavior to metallic zirconium than it is to stoichiometric ZrN (the reduction in the corrosion resistance is undoubtedly due to the reduction in the ionic-covalent components of the bonds in ZrN _x ).     

In Situ ceramic particle-reinforced aluminum matrix composites fabricated by reaction pressing in the TiO2 (Ti)-Al-B (B2O3) systems

Particulate TiB₂ reinforced aluminum-based metal matrix composites (MMCs) were successfully fabricated by means of the reaction processing method. TiB₂ particulates were formed in situ through the reaction of Ti and B in Ti-Al-B, TiO₂ and B in TiO₂-Al-B, and TiO₂ and B₂O₃ in TiO₂-Al-B₂O₃ systems. The results showed that in situ TiB₂ particulates formed in the Ti-Al-B system had a size of 5 μm and they exhibited block and rodlike structures. Moreover, coarse Al₃Ti blocks several tens of micrometers in size were also formed simultaneously. On the other hand, equiaxed Al₂O₃ and TiB₂ particulates with a size of less than 2 μm were formed in situ in the TiO₂-Al-B and TiO₂-Al-B₂O₃ systems. The Al₃Ti phase was completely eliminated in the TiO₂-Al-B system with increasing B content. Tensile tests revealed that the Al₂O₃ · TiB₂/Al composite fabricated from the TiO₂-Al-B system exhibits excellent mechanical properties. The yield strength of the Al₂O₃ · TiB₂/Al composite appeared to increase with increasing TiB₂ content. The yield strength of the Al₂O₃ · TiB₂/Al composite could be further increased by introducing CuO into the TiO₂-Al-B system. Such an increment in mechanical strength arose from the strengthening effect caused by the Al₂Cu precipitates. The incorporation of CuO had no effect on the in situ reaction process of the TiO₂-Al-B system. Finally, the effect of SiC addition on the microstructure and mechanical properties of the composites fabricated from the TiO₂-Al-B and TiO₂-Al-B-CuO systems was also investigated.     

The solubility ofRH2−x in Gd,Er, Tm,Lu and Y from ambient to 850°C

The solubility of hydrogen in Gd, Er, Tm, Lu and Y was determined from 25 to 850°C when the metal was in equilibrium withRH_2?x (x varies between 0.1 and 0.2 depending on the rare earth metal). The room temperature solubilities determined by the lattice parametric method were found to be <0.1, 3.6, 7.7, 20.6 and 19.0 at, pct H in Gd, Er, Tm, Lu and Y, respectively. The change in unit cell volume for each atomic percent hydrogen added was nearly the same for all metals. The solubility of hydrogen increases more rapidly with temperature in those metals with low solubility at room temperature. Thus the solubility of hydrogen at 850°C is nearly the same in all five of the metals studied, that is, 35.0, 36.2, 36.0, 36.0 and 37.3 at. pct H in Gd, Er, Tm, Lu and Y, respectively. The equilibrium pressure of H₂ in these studies was the equilibrium pressure of hydrogen in contact withRH_2?x at the temperature concerned. A change in slope was observed in the solubility curves of the Gd-, Er-, Tm- and Lu-H systems. The logC) (at. pct H inR) was plottedvs 1/T for each system. Straight lines were obtained at temperatures above and below the changes in slope of the solubility curves. A calculation of the approximate ΔH of solution ofRH_2?x in the metal sfrom the slope of the lines gave 4.35, 1.88, 1.28, 0.61 and 0.55 kcal/mole for Gd, Er, Tm, Lu and Y, respecitively in the low temperature portion. The change in slope which occurs at some point between 350°C and 650°C, depending on the metal, indicates a lower heat of solution ofRH_2?x in these metals at the higher temperatures. In Lu there appears to be yet another change in slope in the neighborhood of 250°C.     

Structural properties and stability of metastable phases in the Zr-Nb system: Part II. Aging of bcc (β) alloys and assessment of β-Zr

The lattice parameters (LPs) of the bcc (β) phase occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (T _R ) using neutron diffraction techniques. LP values at T = T _R were determined in alloys with Nb contents in the range 8.4 ≤ at. pct Nb ≤ 23 using both standard and high-resolution neutron diffractometry. These results support the applicability of Vegard’s law at T _R . In addition, a neutron thermodiffractometric technique is used to establish the LP vs T relation of the β phase in two Zr-Nb alloys with 18 and 23 at. pct Nb up to 700 K. Such high-temperature data are used in a generalized Vegard’s law approach to gain insight into the LP vs T relation of the β phase in pure Zr, which is metastable in this temperature range. With these new LP values, an assessment is performed of the thermal expansion coefficient of β-Zr covering both stable and metastable states. As a direct application of the present findings, the results of an aging experiment are discussed, which provide information on the composition of the β phase taking part in the bcc-hcp equilibrium in the Zr-Nb phase diagram at 900 K. Such composition has been the subject of contradictory reports in recent literature. The composition value arrived at in the present analysis agrees well with the results of a thermodynamic calculation of the Zr-Nb phase diagram (A. Fernández Guillermet: Z. Metallkd., 1991, vol. 82, p. 478)     

Observations and analysis of the influence of phase transformations on the instability of the thermally grown oxide in a thermal barrier system

Observations and measurements have been performed on a thermal barrier system comprising a Ptaluminide bond coat and a thermal barrier coating deposited by electron beam evaporation. Past research has highlighted a displacement instability in the thermally grown oxide (TGO), as it affects the failure mechanism in the thermal barrier coating (TBC). Phase transformations in the bond coat have also been identified, with a proposed role in the TGO instability. The present study assesses this influence by characterizing the transformations as well as their spatial correlation with the instability sites. Both the isothermal transformation from β→γ′ and the martensite transformation in the β have been addressed. Toward the end of life, the instabilities are preferentially located in the β phase, between neighboring domains of γ′. After cycling, the composition of the β grains is spatially uniform. Within the γ′, there are Ni and Al composition gradients in narrow layers near the interfaces with the β phase and the TGO. An evaluation suggests that the primary influence of transformation on the cyclic displacement of the TGO is to create a local misfit between the growing γ′ domains and the volume strain accompanying the martensite transformation in the intervening β phase, upon cooling and reheating.     

Strain hardening and instability in biaxially streched sheets

This work is concerned with the origins of the two different patterns of failure limits in biaxially stretched sheets which were recently described in Ref. 1: the brass-type in which the limit strain is insensitive to strain state, and that of ferritic steel in which the limit strain increases as the imposed strain-ratio, ρ = ε₂/ε₁, changes from zero (plane-strain tension) toward unity (balanced biaxial tension). An earlier proposal that different slip modes,i.e. wavy in ferrite vs planar in brass, might have contributed to these failurelimit differences was found not to be valid. There were two parts to the main experimental program: the prestraining of small sheets by proportional loading on different paths betweenρ = 0 andρ = 1, followed by tension testing, and a more direct measurement of strain hardening and instability between ρ ≅ — 1/2 (uniaxial tension) andρ = 0. The principal finding was that the overall hardening rate, essentially as it appeared in the material’s effective stress-strain curve, changed with the loading path. Inα brass it decayed as p was increased from ∼—1/2 to 1; in ferritic steel it increased; and in aluminum it was affected very little. Such changes in hardening rate cause similar changes in the material’s capacity for stable flow. The stable flow, in turn, is the base to which a quasistable-flow increment (whenρ is >0) is added in reaching the observed failure limit. Thus a base ofρ-dependent height can account for the failure-limit patterns. There is still no explanation for the ρ dependence of the hardening rate. AMIT K. GHOSH, formerly Graduate Student, Massachusetts Institute of Technology, Cambridge, Mass. 02139, This paper is based upon a thesis submitted by AMIT K. GHOSH in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology.     

Factors controlling hydrogen-induced amorphization of C15 Laves compounds

The radii of tetrahedral holes occupied by hydrogen atoms in the amorphous hydrides a-RFe₂H_x (R = a rare earth metal) are larger than the empirical minimum hole size, 0.040 nm, of the general stable hydrides A_xB_1−xH_y. However, the radii of the holes in the corresponding crystalline hydrides c-RFe₂H_x are smaller than this value. Thus, the higher stabilities of the amorphous hydrides which are considered to be the origin for the driving force of hydrogen-induced amorphization (HIA) in the C15 Laves compounds are interpreted on the basis of the differences in the hole sizes. Furthermore, the occurrence of HIA in C15 AB₂ Laves compounds has been analyzed in terms of the tetrahedral hole size, the stability of the compounds, the atomic size ratio, the valence electron concentration and the contraction or the expansion of the constituent elements and the holes. The atomic size ratio is the single most important factor controlling the occurrence of HIA in these compounds, and the compounds with the ratio above 1.37 are amorphized by hydrogenation.     

Removal of antimony from copper by injection of soda ash

The removal of Sb from molten copper is of importance in the development of processes which can smelt copper concentrates directly into copper in a single furnace. A promising method is injection of oxygen and sodium carbonate in a modified anode furnace. This study encompassed a thermodynamic analysis of the impurity removal reactions and an experimental investigation of antimony removal from molten copper in a 15 kW induction furnace. The results showed that the reaction was controlled by diffusion of Sb in the metal phase. The reaction between metal and injected flux can be divided into two subprocesses-. (1) “transitory contact” reaction to the injected flux particles as they rise through the melt and (2) “permanent contact” reaction across the interface between the metal bath and the supernatant slag layer. On the basis of the experimental work, the overall volumetric mass transfer coefficient (cm³/s) at 1473 K was expressed in terms of the two subprocesses as follows:(k _d A) _ov = (k _d A) _pc +(k _d A) _tc = 1.25Q _g ^0.29 + 0.28(H Q _f ) whereQ _g is the injection gas flow rate in normal liters per minute,H is the depth of injection in centimeters, andQ _f the rate of flux injection in grams per second.     

Microstructural development of a gas-atomized and hot-pressed super-α2 alloy

A variety of heat treatments have been employed to explore the microstructure in Ti-25Al-10Nb-3V-lMo alloy prepared by gas atomization and hot pressing. These treatments include quenching by oil cooling and water cooling and aging at temperatures between 530 °C and 950 °C. Quenching transformations from the β-phase field include the formation ofO phase in oil quenching and β (disordered) +O phase in water quenching. The metastable β phase decomposes intoO + “Ω”,O, or α₂ + β_o/B2 phase when the as-quenched alloy is aged at various temperatures. By comparing the selection area diffraction patterns, it has been found that the ordered w phase in the alloy studied in this article is distinct in structure to the “Ω type” (P3m1) and B8₂ phase which are formed in the parent matrix of the ordered β(B2,D0₃) phases. It has also been shown by X-ray diffraction (XRD) analyses that the lattice parameters of the as-agedO phase do not remain constant in the alloy at various temperatures.     

Microstructure and properties of In situ Al/TiB2 composite fabricated by in-melt reaction method

A novel in situ reaction process-in-melt reaction method was developed. TiB₂ particles form in situ through the reaction of TiO₂, H₃BO₃, and Na₃AlF₆ in an aluminum alloy melt. The results showed that the in situ TiB₂ particles formed were spherical in shape and had an average diameter of about 0.93 μm. Moreover, the distribution of TiB₂ particles in the matrix was uniform. The interface between the TiB₂ particles and the matrix showed good cohesion. The tensile strength and the yield strength of the composite increase with increasing TiB₂ content. When TiB₂ particle content in the matrix was 10 vol pct, the tensile strength, yield strength, and elongation of Al-4.5Cu/TiB₂ composite were 417 MPa, 317 MPa, and 3.3 pct, respectively.     

Activity-composition relations of MnO in MnO-CrO x -CaO-SiO2-containing melts

The MnO activities in (MnO-CrO _x -CaO-SiO₂)-containing melts, which were saturated with the (Mn, Cr)₃O₄ spinel phase, were determined at 1500 °C under an oxygen partial pressure of 10^−8.99 atm. This was done by equilibrating the samples with platinum. The activity of MnO in the melt was then calculated from the activity coefficient of manganese in the resultant Pt-Cr-Mn alloy. Darken’s quadratic formalism for ternary metallic solutions was used to calculate the activity coefficient of manganese in the Pt-Cr-Mn system, in which platinum was considered to be the solvent. It was found that an increase in the concentration of MnO in the melt increases both the MnO activity and the activity coefficient of MnO. For a constant MnO concentration in the (MnO-CrO _x -CaO-SiO₂)-containing melts, the activity of MnO can be increased by increasing the basicity of the melt. In order to obtain high-manganese recoveries from (MnO-CrO _x -CaO-SiO₂)-containing melts into an alloy phase, basic slags in which the activity coefficient of MnO is high should therefore be used.     

A thermodynamic study of copper-iron and copper-cobalt liquid alloys by mass spectrometry

The thermodynamic activities in the liquid Cu-Fe and Cu-Co binary systems were measured by a high temperature mass spectrometric technique. From the obtained data, the heats of mixing in the Cu-Co liquid binary were calculated. Both the Cu-Fe and Cu-Co systems are thermodynamically similar, with large positive deviations from ideality in the activities. The activity coefficients in the liquid Cu-Co system at 1823 K were found to be symmetrical and can be expressed by lnγ_Cu= 1.76X _Co ² The heat of mixing in the liquid Cu-Co system was found to be described by a cubic function δH_m = −17.9 _Co ³ − 7.6X _Co ² + 25.5_Co (kJ/mol) The activity coefficients in the liquid Cu-Fe system at 1873 K were found to satisfy two quadratic formalisms in the binary In γ_Cu = 2.10X _Fe ² − 0.13 0.65 ≤X _Fe ≤ 1 In γ_Fe = 1.79X _Cu ² + 0.04 0 ≤X _Fe ≤ 0.65     

The Interface of TiB<Subscript>2</Subscript> and Al<Subscript>3</Subscript>Ti in Molten Aluminum

In the grain refinement of aluminum, Al₃Ti and TiB₂ particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB₂ particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al₃Ti particles in the master alloys. The titanium in solution either forms Al₃Ti layers on the surface of TiB₂ particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB₂/Al₃Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al₃Ti on the surface of TiB₂ particles, a sign of the formation of an Al₃Ti layer on the surface of TiB₂ particles in grain refinement practice. The Al₃Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al₃Ti and TiB₂ particles leading to the loss of efficiency in grain refinement.     

Phase Equilibria Studies in the System ZnO-“FeO”-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-SiO<Subscript>2</Subscript> Relevant to Imperial Smelting Furnace Slags: Part II

The phase equilibria and the liquidus temperatures in the system ZnO-“FeO”-Al₂O₃-CaO-SiO₂ have been determined experimentally in equilibrium with metallic iron. Specifically, the effects of Al₂O₃ concentrations in Imperial Smelting Furnace slags are identified, and the results are presented in the form of pseudo-ternary sections ZnO-“FeO”-(Al₂O₃ + CaO + SiO₂) in which CaO/SiO₂ = 0.93 and (CaO + SiO₂)/Al₂O₃ = 5.0 and 3.5, respectively. It was found that, in the presence of Al₂O₃, the spinel phase is formed, the spinel primary phase field expands, and the wustite and melilite primary phase fields are reduced in size with an increasing Al₂O₃ concentration. The implications of the findings to industrial practice are discussed.     

Manganese nitrides

Conclusions An investigation was carried out into the kinetics of the process of formation of manganese nitrides in nitrogen and ammonia streams. It was established that the rate of formation of manganese nitrides is higher in ammonia than in nitrogen. The kinetic parameters of manganese nitride formation were calculated.It is shown that in nitrogen the only manganese nitride that can be synthesized in the pure form is Mn_N. In ammonia, depending on the temperature and duration of the nitriding process, it is possible to obtain Mn_N, Mn₂N, and Mn₃N₂ in the pure form, free from any other phases.A study was made of the chemical stability of manganese nitrides of compositions Mn₄N and Mn₃N₂ in water and in concentrated and dilute mineral acids. It was established that these two nitrides are stable in water. In mineral acids they dissolve, the rate of dissolution being higher in concentrated than in dilute acids. In nitric acid Mn₄N and Mn₃N₂ decompose with the liberation of part of the nitrogen in molecular form.Translated from Poroshkovaya Metallurgiya, No. 3(171), pp. 65–70, March, 1977.     

Thermodynamic behavior of Na2O-B2O3 melt

The activity of Na₂O in a Na₂O-B₂O₃ melt was measured at 1373 K by a chemical equilibration technique to understand the thermodynamic behavior of this slag system. Also, the activity coefficient of Na in Ag was preliminarily measured as fundamental thermodynamic data, to estimate the activity of Na₂O in the slag. Sodium in the silver melt, within the present concentration range, exhibits the Henrian behavior, and the Henrian activity coefficient of Na in Ag (γ ⁰ _Na) is estimated to be 37.5 at 1373 K, indicating a positive deviation from ideality. The activity of Na₂O in the slag varies from 1.86×10⁻⁶ to 4.99×10⁻⁴ when its content is increased from 11.0 to 64.9 mol pct; this indicates a significant negative deviation from ideality. The excess stability does not exhibit any pronounced peak, despite being drastically changed at specific slag compositions. Comparing the molar Gibbs free energy of mixing in various binary slags, Na₂O was considered to be more basic than BaO (CaO) was, followed next by MgO; also, P₂O₅ would be significantly more acidic than SiO₂ would be, followed next by B₂O₃. Comparing the heats of formation of solid compounds in binary slags, the larger the differences in the electronegativity values between slag components, the more the relative ionic characters of the melts seemed to be.     

Study of Structural and Mechanical Properties of Al/Nano-Al2O3 Metal Matrix Nanocomposite Fabricated by Powder Metallurgy Method

In this work, the (1???x) Al–xAl₂O₃ (x?=?0, 1, 2, 3, and 4 wt%) of metal matrix nanocomposites (MMNCs) has been manufactured using the powder metallurgy technique. Aluminium metal powder (Al) was used as the matrix material, and alumina nanoparticles (Al₂O₃) synthesized by the sol–gel method were used as the reinforcing material to produce the MMNCs. Two phases of Al₂O₃ have been identified, i.e. the α-phase (rhombohedral structure) and the δ-phase (orthorhombic structure) by X-ray diffraction patterns (XRD) of synthesized Al₂O₃ nanoparticles with an average crystallite size of 31.33 nm. The average particle size of the Al₂O₃ nanoparticle is obtained as 39.6 nm. The XRD patterns of the Al–Al₂O₃ nanocomposites contain the Al and Al₂O₃ peaks that confirm the development of the MMNC without any solid-state reaction during the manufacturing process. FESEM micrographs show an almost uniform distribution of Al₂O₃ particles in the Al metal matrix. The reinforcement of the Al₂O₃ nanoparticles in the Al metal matrix has shown an improvement in hardness by increasing the wt% of Al₂O₃ in Al matrix, and a maximum 24.8% improvement in hardness is observed for 4 wt% Al₂O₃ sample. An increase in wear rate is observed with the increasing wt% of Al₂O₃ in the Al metal matrix in Al–Al₂O₃ nanocomposite. The addition of Al₂O₃ nanoparticles in the Al matrix has resulted in improved corrosion performance of the samples with a maximum corrosion resistance efficiency of 85.6% for 4 wt% Al₂O₃ in Al metal matrix.

     

Crystallite orientation distribution analysis of the cold rolled and recrystallization textures in low-carbon steels

The development of the cold rolled and recrystallization textures in low-carbon rimmed and killed steels was investigated using the crystallite orientation distribution analyses. With increasing cold reduction low-carbon steels exhibit the simultaneous development of a partial <110> fiber axis parallel to the rolling direction and a <111> fiber axis parallel to the normal direction. The strongest individual texture component rotates from a {111} <110> at 60 pct cold reduction towards a {112} <110> at 80 pct. During the early stages of recrystallization the (110) and <111> fiber textures decrease in both the rimmed and killed steels. However, the decrease in the <111> fiber texture is greater in the rimmed than in the killed steel. With further recrystallization and grain growth this <111> fiber texture increases in both steels but to a greater extent in the killed steel. The strongest individual texture component after complete recrystallization is the {111} <110>, being ∼5.5 and ∼3.0 times random in the killed and rimmed steel, respectively.     

Combined refinement of diffusion coefficients applied on the Nb-C and Nb-N systems

A novel technique was used for the calculation of diffusion coefficients in the niobium carbides and nitrides prepared by reaction diffusion. The temperature ranges investigated were 1500 °C to 2100 °C for the Nb-C system and 1400 °C to 1800 °C for the Nb-N system. Three independent theoretical approaches were applied and their results are compared. In the metalloid-rich phases, the concentration-dependent diffusion coefficients were calculated from the concentration profiles; two models of layer growth were used to obtain the concentration-independent diffusion coefficients in all phases. It was found that the diffusion coefficient of carbon in δ-NbC_1−x shows a decrease with increasing metalloid concentration, whereas the diffusivity of nitrogen in δ-NbN_1−x is nearly independent of the nonmetal concentration. The concentration dependence of the carbon diffusion coefficients in δ-NbC_1−x is a result of a lower activation energy of carbon diffusion in the substoichiometric δ-NbC_1−x than in the δ-NbC. On the contrary, the activation energy of nitrogen in δ-NbN_1−x does not change with the nitrogen concentration. This behavior could be explained by the different occupancies of metal sublattices, which remain constant in δ-NbC_1−x but decrease with increasing nonmetal concentration in δ-NbN_1−x .