Mass spectrometric determination of activities for alloys with complex vapor species: Bi-Pb and Bi-Tl

For solutions from which complex species vaporize (Bi₂, Si₂, Al₂O, Sb₄, and so forth) new methods of determining the thermodynamic properties from mass spectrometric data are demonstrated. In order to test the feasibility of these new techniques, experiments have been carried out on the liquid Bi-Pb and Bi-Tl systems for which adequate thermodynamic data are available. In evaluating the thermodynamic properties, the ion current ratiosI _Pb ⁺/IBi₂/+ andI _Tl ⁺/IBi₂/+ were employed,e.g. $$\log {\text{ }}\gamma _{{\text{Bi}}} {\text{ = - }}\mathop {\int {\frac{{N_{Pb} }}{{1{\text{ + }}N_{Pb} }}d} }\limits_{N_{Bi} = 1}^{N_{{\text{Bi}}} = N_{Bi} } {\text{ }}\left\{ {{\text{log}}\frac{{{\text{1}}_{{\text{Pb}}}^{\text{ + }} {\text{ }}N_{Bi}^2 }}{{I_{Bi2}^ + {\text{ }}N_{Pb} }}} \right\}$$ Measuring these particular ion current ratios eliminates errors resulting from the fragmentation of the complex vapor species in evaluating the thermodynamic properties. A dimer-monomer technique, which corrects for fragmentation, was also demonstrated. The results using these two independent approaches are in good agreement with each other as well as with previous investigations. The activity coefficients in both systems adhere to the quadratic formalism over large composition ranges,e.g. $$\begin{gathered} \log {\text{ }}\gamma _{{\text{Pb}}} {\text{ = - 0}}{\text{.255 }}N_{Bi}^2 {\text{ }}N_{{\text{Bi}}} {\text{< 0}}{\text{.8}} \hfill \\ \log {\text{ }}\gamma _{{\text{Tl}}} {\text{ = - 0}}{\text{.805 }}N_{Bi}^2 {\text{ }}N_{{\text{Bi}}} {\text{< 0}}{\text{.7}} \hfill \\ \end{gathered} $$      

Thermodynamics of dilute bcc Fe-Si alloys

The thermodynamic properties of silicon in the α-phase of the Fe-Si system in the region 0.028 <x _Si < 0.084 and 1100 < °C < 1370 has beem measured by the emf cell Mo, Si(s) | SiO₂-Li₂O | (Si-Fe)(s), Mo. The results are expressed in the form $$\log {\text{ }}\gamma _{Si}^\alpha {\text{ = 1}}{\text{.19 }} - {\text{7070/}}T + [ - 6.30 + 18,300/T]x_{Si} $$      

Surface Tension of Liquid Fe-O Alloys: Revisiting Belton’s Two-Step Adsorption Model

The effect of oxygen adsorption on the surface tension of liquid iron was investigated using the constrained drop method. Experiments were carried out at 1823 K and 1873 K (1550 °C and 1600 °C) under a CO₂-H₂ gas mixture. The experimental results were interpreted using the Langmuir ideal adsorption model and Belton’s two-step adsorption model; the latter model showed better agreement with the experimental results. According to the two-step model, the surface tension of liquid Fe-O alloys at 1823 K and 1873 K (1550 °C and 1600 °C) can be respectively expressed as follows: $$ \sigma = 1882 - 260[0.25\ln (1 + 2407a_{\text{O}} ) + 0.75\ln (1 + 72a_{\text{O}} )]\quad \left[ {T = 1823\,{\text{K}}\left( {1550\,^\circ {\text{C}}} \right)} \right], $$ $$ \sigma = 1834 - 267[0.25\ln (1 + 1445a_{\text{O}} ) + 0.75\ln (1 + 46a_{\text{O}} )]\quad \left[ {T = 1873\,{\text{K}}\left( {1600\,^\circ {\text{C}}} \right)} \right]. $$      

Free energies of formation of WC and W2C,and the thermodynamic properties of carbon in solid tungsten

The activity of carbon in the two-phase regions W + WC and W + W₂C has been obtained from the carbon content of iron rods equilibrated with mixtures of metal plus carbide powders. From this activity data the standard free energies of formation of WC and W₂C have been calculated to be ΔG _f ⁰(WC) = -10,100 + 1.19T ± 100 cal/mole (-42,300 + 4.98T ± 400 J/mole) (1150 to 1575 K) ΔG _f ⁰(W₂C) = - 7300 - 0.56T ± 100 cal/mole (- 30,500 - 2.34T ± 400 J/mole). (1575 to 1660 K) The temperature of the eutectoid reaction W₂C = W + WC was fixed at 1575 ± 5K. Using available data for the solubility of C in solid W, the relative partial molar free energy of C in the dilute solid solution was calculated to be $$\Delta \bar G_C^\alpha {\text{ = 23,000 }} - {\text{ }}[{\text{0}}{\text{.68 }} - R\ln X_C^\alpha ]{\text{ }}T \pm 3000 cal/mole (96,200 - [2.85 - R\ln X_C^\alpha ]{\text{ }}T \pm 12,600 J$$ The heat solution of C in W obtained was \(\Delta \bar H_C^\alpha {\text{ = 23,000 }} \pm {\text{ 5000 cal/mole (96,200 }} \pm {\text{ 20,000 J/mole)}}\) and the excess entropy for the interstitial solid solution, assuming that the carbon atoms are in the octahedral sites, \(\Delta \bar S_C^\alpha {\text{ = (}}xs,i{\text{) }} = - {\text{1}}{\text{.5 }} \pm {\text{ 2 cal/deg - mole (}} - {\text{6}}{\text{.3 }} \pm {\text{ 8 J/deg - mole)}}\) .     

Determination of Gibbs Energy of Formation of Molybdenum-Boron Binary System by Electromotive Force Measurement Using Solid Electrolyte

The standard Gibbs energies of formation of Mo₂B, ??MoB, Mo₂B₅, and MoB₄ in the molybdenum-boron binary system were determined by measuring electromotive forces of galvanic cells using an Y₂O₃-stabilized ZrO₂ solid oxide electrolyte. The results are as follows: $$ \begin{aligned} \Updelta_{\text{f}} {\text{G}}^\circ \left( {{\text{Mo}}_{2} {\text{B}}} \right)/{\text{J}}\,{\text{mol}}^{ - 1} & = - 193100 + 44.10T \pm 700\left( {1198{\text{ K to }}1323{\text{ K}}\left( {925^\circ {\text{C to }}1050^\circ {\text{C}}} \right)} \right) \\ \Updelta_{\text{f}} {\text{G}}^\circ (\alpha {\text{MoB}})/{\text{J}}\,{\text{mol}}^{ - 1} & = - 164000 + 26.45T \pm 700\left( {1213{\text{ K to }}1328{\text{ K}}\left( {940^\circ {\text{C to }}1055^\circ {\text{C}}} \right)} \right) \\ \Updelta_{\text{f}} {\text{G}}^\circ \left( {{\text{Mo}}_{2} {\text{B}}_{5} } \right)/{\text{J}}\,{\text{mol}}^{ - 1} & = - 622500 + 117.0T \pm 3000\left( {1205{\text{ K to }}1294{\text{ K}}\left( {932^\circ {\text{C to }}1021^\circ {\text{C}}} \right)} \right) \\ \Updelta_{\text{f}} {\text{G}}^\circ \left( {{\text{MoB}}_{4} } \right)/{\text{J}}\,{\text{mol}}^{ - 1} & = - 387300 + 93.53T \pm 3000\left( {959{\text{ K to }}1153{\text{ K}}\left( {686^\circ {\text{C to }}880^\circ {\text{C}}} \right)} \right) \\ \end{aligned} $$ where the standard pressure is 1 bar (100 kPa).     

Thermochemical decomposition of hydrogen sulfide with nickel sulfide

In this research, the two-step thermochemical cycle shown below is proposed and experimental studies were made on the cycle. $$\frac{\begin{gathered} {\text{Ni}}_{\text{3}} {\text{S}}_{\text{2}} + {\text{H}}_{\text{2}} {\text{S}} = {\text{3NiS + H}}_{\text{2}} \hfill \\ {\text{3NiS = Ni}}_{\text{3}} {\text{S}}_{\text{2}} {\text{ + 0}}{\text{.5S}}_{\text{2}} {\text{(g)}} \hfill \\ \end{gathered} }{{{\text{H}}_{\text{2}} {\text{S = H}}_{\text{2}} {\text{ + 0}}{\text{.5S}}_{\text{2}} {\text{(g)}}}}$$ In the case where Ni₃S₂ alone was used without inert additions, nickel sulfide sintered or partly fused due to the melting point depression resulting from the thermal decomposition of formed NiS. Such sintering could be prevented by mixing the nickel sulfide powders with Al₂O₃ or MoS₂. The cyclic reactions were thereby shown to provide a stationary high decomposition rate of H₂S. Polysulfides, such as MS₂, have previously been employed in this kind of cycle. This research showed that the use of lower sulfides such as Ni₃S₂ may be regarded as rather promising based on the thermodynamic investigation of the respective reactions composing the cycle. The comparison between the sulfurization reactions of NiS to NiS₂ and of Ni₃S₂ to NiS further showed that the latter was superior to the former with respect to the kinetics and thermodynamical properties of the reaction.     

A study of strengthening mechanisms in tempered martensite from a medium carbon steel

Previous studies of strengthening mechanisms in quenched and tempered steels have been based almost entirely on microscopy. The density and distribution of residual dislocations in these materials have generally been poorly characterized and there is doubt as to the contribution of substructure to strength. In the present work, a 0.42 pct C steel has been hardened and tempered for 1 hr at seven temperatures in the range 250° to 550°C. The structures have been studied by X-ray diffraction line profile analysis as well as by electron microscopy. The measured 0.2 pct offset yield strength, over the whole tempering temperature range, could be represented by $$\sigma _{0.2{\text{ = }}} \sigma _0 {\text{ + }}\frac{{k_{\text{1}} }}{D}{\text{ ln }}\frac{D}{{2b}}{\text{ + }}k_{\text{2}} {\text{ }}\rho ^{1/2} $$ whereD is the mean planar spacing of carbide particles,ρ is the density of residual dislocations in the as-tempered structure, and b is the Burger’s vector of a dislocation in iron. The stress ct0 was found to be about 35 kgm mm^-2 for the steel used and probably includes an appreciable contribution from incremental interstitial solid solution strengthening. The constantsk ₁ andk ₂ have values which are consistent with both theory and previous experimental work with the Orowan mechanism and strain-hardening, respectively. The dislocation substructure contribution accounted for more than 35 pct of the yield strength of the steel tempered at temperatures up to 400°C.     

Reaction Mechanism and Kinetics of Enargite Oxidation at Roasting Temperatures

Roasting of enargite (Cu₃AsS₄) in the temperature range of 648?K to 898?K (375?°C to 625?°C) in atmospheres containing variable amounts of oxygen has been studied by thermogravimetric methods. From the experimental results of weight loss/gain data and X-ray diffraction (XRD) analysis of partially reacted samples, the reaction mechanism of the enargite oxidation was determined, which occurred in three sequential stages:

1. $4{\text{Cu}}_{ 3} {\text{AsS}}_{ 4} \left( {\text{s}} \right){\text{ + 13O}}_{ 2} \left( {\text{g}} \right){\text{ = As}}_{ 4} {\text{O}}_{ 6} \left( {\text{g}} \right){\text{ + 6Cu}}_{ 2} {\text{S}}\left( {\text{s}} \right){\text{ + 10SO}}_{ 2} \left( {\text{g}} \right) $
2. $ 6{\text{Cu}}_{ 2} {\text{S}}\left( {\text{s}} \right){\text{ + 9O}}_{ 2} \left( {\text{g}} \right){\text{ = 6Cu}}_{ 2} {\text{O}}\left( {\text{s}} \right){\text{ + 6SO}}_{ 2} \left( {\text{g}} \right) $
3. $ 6{\text{Cu}}_{ 2} {\text{O}}\left( {\text{s}} \right){\text{ + 3O}}_{ 2} \left( {\text{g}} \right){\text{ = 12CuO}}\left( {\text{s}} \right) $

The three reactions occurred sequentially, each with constant rate, and they were affected significantly by temperature and partial pressure of oxygen. The kinetics of the first stage were analyzed by using the model X?=?k ₁ t. The first stage reaction was on the order of 0.9 with respect to oxygen partial pressure and the activation energy was 44?kJ/mol for the temperature range of 648?K to 898?K (375?°C to 625?°C).     

The kinetics of formation of h2o and co2 during iron oxide reduction

The kinetics of the chemical reaction-controlled reduction of iron oxides by H₂/H₂O and CO/CO₂ gas mixtures are discussed. From an analysis of the systems it is concluded that the decomposition of the oxides takes place by the two dimensional nucleation and lateral growth of oxygen vacancy clusters at the gas/oxide interface. The rates of decomposition of the oxides under conditions of chemical reaction control are dependent not only on the partial pressures of the reacting gases at the reaction temperature but also on the oxygen activity of the prevailing atmosphere. Application of this model to the kinetic data leads to the determination of the maximum chemical reaction rate constants for the decomposition of the iron oxide surfaces. Assuming the reactions H₂ (g) + O(ads) → H₂O(g) andCO(g) + O(ads) → CO₂ (g) to be rate controlling the maximum chemical reaction rate constants for the reduction of iron oxides are given by $$\Phi _{{\text{H}}_{\text{2}} } = 10^{.00} exp \left( {\frac{{ - 69,300}}{{RT}}} \right)mol m^{ - 2} s^{ - 1} atm^{ - 1} $$ and $$\Phi _{CO} = 10^{4.40} \exp \left( {\frac{{103,900}}{{RT}}} \right)mol m^{ - 2} s^{ - 1} atm^{ - 1} $$ The maximum chemical reaction rate constants do not necessarily indicate the maximum rates which can be achieved in practice since these will depend on the limitations imposed by mass transport in the systems. The rate constants are important however since they indicate for the first time the upper limit of any reduction rate in these systems. The fractions of reaction sites which appear to be active on wüstite surfaces in equilibrium with iron are calculated. A direct relationship between chemical reaction rates on liquid iron surfaces and rates on atomically rough iron oxide surfaces is postulated.     

Effect of Slag Composition on the Concentration of Al2O3 in the Inclusions in Si-Mn-killed Steel

The thermodynamic equilibria between CaO-Al₂O₃-SiO₂-CaF₂-MgO(-MnO) slag and Fe-1.5 mass pct Mn-0.5 mass pct Si-0.5 mass pct Cr melt was investigated at 1873 K (1600 °C) in order to understand the effect of slag composition on the concentration of Al₂O₃ in the inclusions in Si-Mn-killed steels. The composition of the inclusions were mainly equal to (mol pct MnO)/(mol pct SiO₂) = 0.8(±0.06) with Al₂O₃ content that was increased from about 10 to 40 mol pct by increasing the basicity of slag (CaO/SiO₂ ratio) from about 0.7 to 2.1. The concentration ratio of the inclusion components, \( {{X_{{{\text{Al}}_{2} {\text{O}}_{3} }} \cdot X_{\text{MnO}} } \mathord{\left/ {\vphantom {{X_{{{\text{Al}}_{2} {\text{O}}_{3} }} \cdot X_{\text{MnO}} } {X_{{{\text{SiO}}_{2} }} }}} \right. \kern-0pt} {X_{{{\text{SiO}}_{2} }} }} \) , and the activity ratio of the steel components, \( {{a_{\text{Al}}^{2} \cdot a_{\text{Mn}} \cdot a_{\text{O}}^{2} } \mathord{\left/ {\vphantom {{a_{\text{Al}}^{2} \cdot a_{\text{Mn}} \cdot a_{\text{O}}^{2} } {a_{\text{Si}} }}} \right. \kern-0pt} {a_{\text{Si}} }} \) , showed a good linear relationship on a logarithmic scale, indicating that the activity coefficient ratio of the inclusion components, \( {{\gamma_{{{\text{SiO}}_{2} }}^{i} } \mathord{\left/ {\vphantom {{\gamma_{{{\text{SiO}}_{2} }}^{i} } {\left( {\gamma_{{{\text{Al}}_{2} {\text{O}}_{3} }}^{i} \cdot \gamma_{\text{MnO}}^{i} } \right)}}} \right. \kern-0pt} {\left( {\gamma_{{{\text{Al}}_{2} {\text{O}}_{3} }}^{i} \cdot \gamma_{\text{MnO}}^{i} } \right)}} \) , was not significantly changed. From the slag-steel-inclusion multiphase equilibria, the concentration of Al₂O₃ in the inclusions was expressed as a linear function of the activity ratio of the slag components, \( {{a_{{{\text{Al}}_{2} {\text{O}}_{3} }}^{s} \cdot a_{\text{MnO}}^{s} } \mathord{\left/ {\vphantom {{a_{{{\text{Al}}_{2} {\text{O}}_{3} }}^{s} \cdot a_{\text{MnO}}^{s} } {a_{{{\text{SiO}}_{2} }}^{s} }}} \right. \kern-0pt} {a_{{{\text{SiO}}_{2} }}^{s} }} \) on a logarithmic scale. Consequently, a compositional window of the slag for obtaining inclusions with a low liquidus temperature in the Si-Mn-killed steel treated in an alumina ladle is recommended.     

Iron redox equilibria in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags

Measurements have been made of the ratio of ferric to ferrous iron in CaO-Al₂O₃-SiO₂ and MgO-CaO-Al₂O₃-SiO₂ slags at oxygen activities ranging from equilibrium with pCO₂/pCO≈0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given by $\begin{gathered} \log {\text{ }}\left( {\frac{{Fe^{3 + } }}{{Fe^{2 + } }}} \right) = 0.3( \pm {\text{ }}0.02){\text{ }}Y + {\text{ }}0.45( \pm {\text{ }}0.01){\text{ }}\log \hfill \\ \left( {\frac{{pCO_2 }}{{pCO}}} \right) - 1.24( \pm {\text{ }}0.01) \hfill \\ \end{gathered} $ where Y=(CaO+MgO)/SiO₂, for melts with the molar ratio of CaO/SiO₂=0.45 to 1.52, 10 to 15 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and with 3 to 10 wt pct total Fe. Available evidence suggests that, to a good approximation, these redox equilibria are independent of temperature when expressed with respect to pCO₂/pCO, probably from about 1573 to 1873 K. Limited studies have also been carried out on melts containing about 40 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with $\frac{{Fe^{3 + } }}{{Fe^{2 + } }} \propto \left( {\frac{{pCO_2 }}{{pCO}}} \right)^{0.37} $ The nonideal behavior and the effects of basicity and Al₂O₃ concentration on the redox equilibria are discussed in terms of the charge balance model of alumino-silicates and the published structural information from Mössbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.     

Thermodynamics of the system NaF-AlF3. Part IV: The cryolite liquidus curve

The theory of the variation of activity of a binary compoundAXB with composition in a $$a_{A_x B} = {\text{ }}K \cdot a_A^x \cdot a_B $$ the activity coefficient should be defined as $$\gamma _{A_x B} {\text{ = }}K \cdot \gamma _A^x {\text{ }} \cdot {\text{ }}\gamma _B {\text{ = }}a_{A_x B} {\text{/(}}n_A^x {\text{ }} \cdot {\text{ }}n_B {\text{)}}$$ K is chosen so that^aA_xb is unity in the stoichiometric liquid (or, if preferred, so that γA_XB is unity). Such an activity coefficient possesses the property of approaching a limiting value at the stoichiometric composition, and the analog of Raoult’s law is^aA_xB∝ n_A ^x nB From this it follows that a plot of log (n_A ^x nB) against 1/T should show the same limiting tangent for points on both sides of the liquidus curve. Data for Na₃AlF₆ do not pass this test. With the aid of previously measured activities the discrepancy can be resolved if it be assumed that the solid material separating has a composition (2.824 ±0.005)NaF A1F₃; the calculated enthalpy of fusion of this material is 24,270 cal/gfw, and the standard deviation of the liquidus points from the least-squares line is ±2.3 deg.     

The Solubility and Diffusivity of Fluorine in Solid Copper from Electrochemical Measurements

The solubility and diffusivity of fluorine in solid copper were determined electrochemically using the double solid-state cell $$Ni + NiF_2 \left| {CaF_2 } \right|Cu\left| {CaF_2 } \right|Ni + NiF_2 .$$ In the temperature range 757 to 920°C, the diffusivity of fluorine in solid copper was found to be $$D_F \left( {{{cm^2 } \mathord{\left/ {\vphantom {{cm^2 } s}} \right. \kern-\nulldelimiterspace} s}} \right) = 9.32 \times 10^{ - 2} \exp \left( {\frac{{ - 98,910 {J \mathord{\left/ {\vphantom {J {mole}}} \right. \kern-\nulldelimiterspace} {mole}}}} {{RT}}} \right).$$ . The results obtained for the dissolution of fluorine as atoms in solid copper showed large scatter. However, the equilibrium dissolution of fluorine follows Sieverts’ law. Above the melting point (770°C) of CuF₂, the mean solubility of fluorine in solid copper, for the equilibrium Cu(s)+ CuF ₂(l), follows the relationship $$N_F^s (atom fraction) = 0.98 \exp \left( {\frac{{ - 79,500 {J \mathord{\left/ {\vphantom {J {mole}}} \right. \kern-\nulldelimiterspace} {mole}}}} {{RT}}} \right).$$      

The Diffusion Coefficient of Scandium in Dilute Aluminum-Scandium Alloys

The diffusion coefficient of Sc in dilute Al-Sc alloys has been determined at 748 K, 823 K, and 898 K (475 °C, 550 °C, and 625 °C, respectively) using semi-infinite diffusion couples. Good agreement was found between the results of the present study and both the higher temperature, direct measurements and lower temperature, indirect measurements of these coefficients reported previously in the literature. The temperature-dependent diffusion coefficient equation derived from the data obtained in the present investigation was found to be \( D \left( {{\text{m}}^{2} /{\text{s}}} \right) = \left( {2.34 \pm 2.16} \right) \times 10^{ - 4} \left( {{\text{m}}^{2} /{\text{s}}} \right) { \exp }\left( {\frac{{ - \left( {167 \pm 6} \right) \left( {{\text{kJ}}/{\text{mol}}} \right)}}{RT}} \right). \) Combining these results with data from the literature and fitting all data simultaneously to an Arrhenius relationship yielded the expression \( D \left( {{\text{m}}^{2} /{\text{s}}} \right) = \left( {2.65 \pm 0.84} \right) \times 10^{ - 4} \left( {{\text{m}}^{2} /{\text{s}}} \right) { \exp }\left( {\frac{{ - \left( {168 \pm 2} \right) \left( {{\text{kJ}}/{\text{mol}}} \right)}}{RT}} \right). \) In each equation given above, R is 0.0083144 kJ/mol K, T is in Kelvin, and the uncertainties are ±1 standard error.     

Thermodynamic studies of liquid copper alloys by electromotive force method: Part II. The Cu−Ni−O and Cu−Ni systems

Oxygen activities in liquid Cu?Ni melts have been measured at 1100°, 1200°, and 1300°C by the solid electrolyte electromotive force method. Addition of nickel lowers the activity coefficient of oxygen in liquid copper. The temperature dependence of \( \in _{\rm O}^{(Ni)} \) can be represented as follows: 1 $$ \in _{\rm O}^{(Ni)} = 17.0 - (3.60 \times 10^4 / T)$$ The solubility and activity of oxygen in liquid Cu?Ni alloys in equilibrium with NiO were measured at 1400°C. Oxygen solubility decreases with increasing nickel in the alloys. Nickel activities in the liquid Cu?Ni system were also calculated from the electromotive force data. Copper activities were evaluated by the Gibbs-Duhem integration. Activities of copper and nickel deviate positively from ideality. The activity coefficient of nickel in the liquid Cu?Ni system at 1400°C can be represented by the Darken's formulation: In 2 $$\gamma _{Ni} = 1.323 (1 - N_{Ni} )^2 ; N_{Ni} = 0.55 to 0.77$$ In 3 $$\gamma _{Ni} = 1.000 (1 - N_{Ni} )^2 + 0.005; N_{Ni} = 0.0 to 0.3$$ Excess entropies in the liquid Cu?Ni system are less than 0.1 eu. A complete set of thermodynamic properties of the system can be calculated by combining the free energy data of this study with the estimated enthalpy data from literature sources.     

Fabrication and Characterization of Naturally Selected Epitaxial Fe-{111} Y2Ti2O7 Mesoscopic Interfaces: Some Potential Implications to Nano-Oxide Dispersion-Strengthened Steels

The smallest features of ≈2 to 3 nm in nanostructured ferritic alloys (NFA), a variant of oxide dispersion-strengthened steels, include the Y₂Ti₂O₇ complex oxide cubic pyrochlore phase. The interface between the bcc Fe-Cr ferrite matrix and the fcc nanometer-scale Y₂Ti₂O₇ plays a critical role in the stability, strength, and damage tolerance of NFA. To complement other characterization studies of the actual nanofeatures (NF) themselves, mesoscopic interfaces were created by electron beam deposition of a thin Fe layer on a 5 deg miscut {111} Y₂Ti₂O₇ bulk single crystal surface. While the mesoscopic interfaces may differ from those of the embedded NF, the former facilitate characterization of controlled interfaces, such as interactions with point defects and helium. The Fe-Y₂Ti₂O₇ interfaces were studied using scanning electron microscopy, including electron backscatter diffraction, atomic force microscopy, X-ray diffraction, and transmission electron microscopy (TEM). The polycrystalline Fe layer has two general orientation relationships (OR) that are close to (a) the Nishiyama–Wasserman (NW) OR $ \left\{ {110} \right\}_{\text{Fe}} ||\left\{ {111} \right\}_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} $ 110 Fe | | 111 Y 2 Ti 2 O 7 and $ \left\langle {100} \right\rangle_{\text{Fe}} ||\left\langle {110} \right\rangle_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} $ 100 Fe | | 110 Y 2 Ti 2 O 7 and (b) $ \left\{ {100} \right\}_{\text{Fe}} ||\left\{ {111} \right\}_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} $ 100 Fe | | 111 Y 2 Ti 2 O 7 and $ \left\langle {100} \right\rangle_{\text{Fe}} ||\left\langle {110} \right\rangle_{{{\text{Y}}_{2} {\text{Ti}}_{2} {\text{O}}_{7} }} $ 100 Fe | | 110 Y 2 Ti 2 O 7 . High-resolution TEM shows that the NW interface is near-atomically flat, while the {100}_Fe grains are an artifact associated with a thin oxide layer. However, the fact that there is still a Fe-Y₂Ti₂O₇ OR is significant. No OR is observed in the presence of a thicker oxide layer.