<Emphasis Type="Italic">L</Emphasis><Subscript>3</Subscript>-XANES spectroscopy of the valence-unstable cerium in intermetallic compounds with 3<Emphasis Type="Italic">d</Emphasis> metals

The effective valence of cerium is studied by L ₃-XANES spectroscopy using synchrotron radiation in the following valence-unstable systems with different types of magnetic ordering: a CeNi matrix doped with neodymium, praseodymium, and gadolinium ions in the temperature range 5–300 K and the Ce₂Fe1_{7 − x} Mn _x intermetallic compound. The obtained dependences are discussed in terms of generally accepted models of states with an intermediate valence of rare-earth ions. Possible correlations between the effective valence of cerium and the magnetic properties of the substances are taken into account.     

Interface stability during displacement reactions between Cu<Subscript>2</Subscript>O and Co<Subscript>1−<Emphasis Type="Italic">X</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript> alloys at 1223 K

The stability of the reactive interface during the solid-state displacement reaction, Cu₂O+Co_1−X Fe _X =2Cu+(Co_1−X Fe _X )O, is studied as a function of Co-Fe alloy composition at 1223 K. For X≤0.03, the reaction zone has a layered structure, and the cation diffusion in (Co, Fe)O is the rate-limiting step. The interface is unstable in the early stages of the reaction; the instability decreases with time as the oxide thickness increases, and the interface becomes planar at long times. The time required for the attainment of interface planarity increases with the value of X. The reaction kinetics are consistent with the available cation-diffusion data in (Co, Fe)O. For X≥0.045, the product zone is a composite of Cu+(Co, Fe)O, and the rate is limited by the oxygen transport in copper. The transition to interface instability occurs when the oxide can support a cation flux that exceeds the maximum possible oxygen flux in copper. During the reaction, composition gradients develop in (Co, Fe)O because of higher diffusion rates for iron than for cobalt.     

Effect of alloying and aging on morphological changes from lamellar to equiaxed microstructure of <Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript>+<Emphasis Type="Italic">γ</Emphasis> titanium aluminides

The stability of a lamellar structure consisting of α ₂ and γ phases in alloys Ti-48Al, Ti-48Al-2Mo, Ti-48Al-4Nb, and Ti-48Al-1Mo-4Nb has been studied as a function of aging time and temperature. The alloys were solution treated (1400 °C, 30 min, and air-cooled (AC)) and aged at 1000 °C and 1100 °C for 1, 4, and 16 hours, respectively. The results indicate that the kinetics of lamellae to equiaxed transformation depends on alloy chemistry, aging time, and temperature. The Nb decreases and Mo increases the kinetics of transformation. The combined effect of Nb and Mo results in the highest volume fraction of equiaxed microstructure at a given aging time and temperature. The results have been discussed in relation to microstructural features and have been compared with those reported in other α ₂+γ alloys.     

A Derivation of a Quadratic Activity Coefficient <Emphasis Type="Italic">vs</Emphasis> Composition Relationship in a Quaternary System, <Emphasis Type="Italic">A-B-C-D</Emphasis>

In the literature, no direct derivation exists of the quadratic activity coefficient vs composition relationships for a quaternary system with high solute concentrations. Such relations for a ternary system (1-2-3) were derived by Darken by extending the results of a binary system (1-2), introducing a new concept of “hypothetical system” (2-3). To present a better scheme to find the activity coefficient-composition relations for multicomponent systems, derivations are made for a quaternary system A-B-C-D in the current work. Using a MacLaurin series expansion, the (Raoultian) activity coefficient, ln γ _i , of each component is equated with a quadratic expression of mole fractions (x), involving the activity coefficient at zero concentration ( g_i⁰ ) \left( {\gamma_{i}^{0} } \right) and nine interaction coefficients (ε). Subsequently, with the help of a Gibbs–Duhem equation, followed by a comparison of coefficients, most preceding 9 × 4, i.e., 36 interaction coefficients are eliminated, leaving behind only three self- and three ternary interaction coefficients, which are enough to express the activity coefficient vs composition relationships for the solutes B, C, and D, as well as for the solvent A. Setting the mole fraction x _D  = 0, the preceding expressions establish the same relations as proposed by Darken for the ternary system A-B-C. The derivation also clarifies how the quadratic concentration terms accompany the first-order interaction coefficients, not the second-order ones. Applications of the derived relations to determine simultaneously the activity coefficients g_i⁰ \gamma_{i}^{0} and the interaction coefficients ε in a new way in some iron- and steelmaking systems are presented. A new data on interaction coefficients in liquid iron at 1873 K (1600 °C), e_\textV^\textV = - 6. 1, \varepsilon_{\text{V}}^{\text{V}} = - 6. 1, has been generated through such an application.     

Crystallization of Co<Subscript>100-<Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>10</Subscript>Si<Subscript>12</Subscript> amorphous metallic alloys

Alloys of Co_78-x Pt _x B₁₀Si₁₂ were produced using the melt-spin process in order to study the crystallization behavior and ensuing magnetic properties of the Co-Pt amorphous alloys as a function of the Pt content. We showed that when x>15, well below its stoichiometric composition, CoPt intermetallic compound crystallized in the amorphous alloy. Below this composition, the main crystallization product was Co with Pt dissolved in its lattice. The nucleation of CoPt greatly altered the crystallized microstructures and magnetic properties of the Co-Pt amorphous alloys during annealing. In spite of the nucleation of CoPt with its high magnetic anisotropy, the highest coercivity was obtained when x was 15, free of the CoPt grains. It was also concluded that the Pt addition, in general, triggered crystallization to occur at a progressively lower temperature.     

Synthesis of <Emphasis Type="Italic">α</Emphasis>- and <Emphasis Type="Italic">β</Emphasis>-Rhombohedral Boron Powders<Emphasis Type="Italic"> via</Emphasis> Gas Phase Thermal Dissociation of Boron Trichloride by Hydrogen

The α-rhombohedral and β-rhombohedral crystal structures of pure elemental boron powders have been synthesized via gas phase thermal dissociation of BCl₃ by H₂ on a quartz substrate. The parameters affecting the crystal structures of the final products and the process efficiency, such as BCl₃/H₂ molar ratio (1/2 and 1/4) and reaction temperature (1173 K to 1373 K [900 °C to 1100 °C]), have been examined. The experimental apparatus of original design has enabled boron powders to be obtained at temperatures lower than those in the literature. The surface/powder separation problem encountered previously with different substrate materials has been avoided. Boron powders have been synthesized with a minimum purity of 99.99 pct after repeated HF leaching. The qualitative analysis of exhaust gases has been conducted using a Fourier transform infrared spectroscope (FTIR). The synthesized powders have been characterized using an X-ray powder diffractometer (XRD) and scanning electron microscope (SEM) techniques. The results of the reactions have been compared with equilibrium predictions performed using the FactSage 6.2 (Center for Research in Computational Thermochemistry, Montreal, Canada) thermochemical software.     

Quality assessment of artificially aged A357 aluminum alloy cast ingots by introducing approximate expressions of the quality index <Emphasis Type="Italic">Q</Emphasis><Subscript><Emphasis Type="Italic">D</Emphasis></Subscript>

The mechanical performance of A357 cast ingot aluminum alloy specimens subjected to 25 different artificial aging heat treatments has been experimentally investigated. For the quality evaluation of the artificially aged alloys, the quality index Q _D , proposed by the authors, has been involved. The index Q _D interprets quality as the potential of an alloy to offer combinations of tensile strength, ductility, and toughness at certain levels of values. The evaluation of the alloy quality using Q _D relies on the availability of the materials tensile flow curve. To facilitate the exploitation of Q _D for quality assessment, approximations of Q _D are developed. In the proposed approximations, Q _D is formulated as a function of ultimate tensile strength, yield strength, and elongation to fracture, or alternatively, as a function of Rockwell hardness E and Charpy impact energy values. Accurate estimations of the quality index Q _D are compared against Q _D values obtained by using the proposed approximations.     

Some studies on the thermal-expansion behavior of C-fiber,SiC<Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>, and <Emphasis Type="Italic">In-situ</Emphasis> Mg<Subscript>2</Subscript>Si-reinforced AZ31 Mg alloy-based hybrid composites

Magnesium alloy-based hybrid composites with carbon-fiber, SiC _p , and in-situ Mg₂Si reinforcements have been prepared by the squeeze-infiltration technique. The results of the studies done on the measurement of the coefficient of thermal expansion after thermal cycling of these composites show that the thermal cycling initially leads to rapid linear expansion of the composite. However, the expansion becomes stabilized after a few cycles, pointing toward formation of the stable interfaces due to the formation of stable precipitates. The model for the growth kinetics of these precipitates at the interface shows a rapid initial growth of the precipitates with the number of thermal cycles, which becomes staturated after a few thermal cycles. The thermal treatment of the composite lowers the coefficient of linear thermal expansion, which can be explained on the basis of further stabilization of the interfaces after the thermal treatment.     

Interdiffusion behavior of Pt-modified <Emphasis Type="Italic">γ</Emphasis>-Ni + <Emphasis Type="Italic">γ</Emphasis>′-Ni<Subscript>3</Subscript>Al alloys coupled to Ni-Al-based alloys

The effect of platinum addition on the interdiffusion behavior of γ-Ni + γ′-Ni₃Al alloys was studied by using diffusion couples comprised of a Ni-Al-Pt alloy mated to a Ni-Al, Ni-Al-Cr, or Ni-based commercial alloy. The commercial alloys studied were CMSX-4 and CMSX-10. Diffusion annealing was at 1150 °C for up to 100 hours. An Al-enriched γ′-layer often formed in the interdiffusion zone of a given couple during diffusion annealing due to the uphill diffusion of Al. This uphill diffusion was ascribed to Pt addition decreasing the chemical activity of aluminum in the γ + γ′ alloys. For a given diffusion couple end member, the thickening kinetics of the γ′ layer that formed increased with increasing Pt content in the Ni-Al-Pt γ + γ′ alloy. The γ′-layer thickening kinetics in diffusion couples with Cr showed less of a dependence on Pt concentration. Inference of a negative effect of Pt and positive effect of Cr on the Al diffusion in this system enabled explanation of the observed interdiffusion behaviors. There was no or minimal formation of detrimental topologically close-packed (TCP) phases in the interdiffusion zone of the couples with CMSX-4 or CMSX-10. An overlay Pt-modified γ + γ′ coating on CMSX-4 showed excellent oxidation resistance when exposed to air for 1000 hours at 1150 °C. Moreover, the Al content in the coating was maintained at a relatively high level due to Al replenishment from the CMSX-4 substrate.     

Ultrasound velocity and adiabatic compressibility of GdCl<Subscript>3</Subscript>-<Emphasis Type="Italic">M</Emphasis>Cl (<Emphasis Type="Italic">M</Emphasis> = Na,Cs) melts

The ultrasound velocity in GdCl₃ + MCl (M = Na, Cs) chloride melts is measured over wide temperature and composition ranges, and their adiabatic compressibility is calculated. A correlation is established between the relative deviations of these properties from their values in hypothetic ideal salt mixtures and the reciprocal alkali-metal cation radii. The role of lanthanide compression in the propagation of sound vibrations in the chlorides of cerium group REMs is revealed.     

Anomalous Mixing State in Room-Temperature Ionic Liquid-Water Mixtures: <Emphasis Type="Italic">N,N</Emphasis>-diethyl-<Emphasis Type="Italic">N</Emphasis>-methyl-<Emphasis Type="Italic">N</Emphasis>-(2-methoxyethyl) Ammonium Tetrafluoroborate

The mixing states of room-temperature ionic liquid (RTIL) H₂O mixtures (x = 0.0 mol pct to 99.5 mol pct H₂O) were investigated using wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and optical absorption in an ultraviolet and visible (UV-vis) region. The RTIL is N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF₄]. In a “prepeak” region of the WAXS, the scattered intensities increased at 85 mol pct to 95 mol pct. A medium-range order (MRO) in the liquid structure as observed in network-forming materials developed markedly. In the SAXS experiments, we can detect nanoscale fluctuations relating to polar and nonpolar regions. At 65 mol pct to 85 mol pct, the SAXS intensity increased unexpectedly. Furthermore, entirely different optical absorption spectra in the UV-vis region were observed as a macroscopic property from 90 mol pct to 95 mol pct. We suppose that these anomalies relate to the MRO of the liquid structure. All anomalies probably are induced by an intrinsic property in [DEME][BF₄]-H₂O mixtures.     

Thermal conductivity of NdCl<Subscript>3</Subscript>-<Emphasis Type="Italic">M</Emphasis>Cl (<Emphasis Type="Italic">M</Emphasis> = Na,K, Cs) melts

The thermal conductivities of individual NdCl₃, the K₃NdCl₆ chemical compound, an equimolar 0.5NdCl₃-0.5NaCl mixture, and eutectic 0.5NdCl₃-0.5KCl and 0.45NdCl₃-0.55CsCl mixtures are measured. For all melts, the concentration and temperature dependences of the thermal conductivity are found.     

Ultrasound velocity and adiabatic compressibility of LaCl<Subscript>3</Subscript> + <Emphasis Type="Italic">M</Emphasis>Cl (<Emphasis Type="Italic">M</Emphasis> = Li,Na, K,Rb, and Cs) melts

The ultrasound velocity in binary LaCl₃ + MCl (M = Li, Na, K, Rb, and Cs) melts is measured, and their adiabatic compressibility is calculated as a function of temperature and composition. A relation is established between the relative deviations of these properties from their values for hypothetic ideal salt mixtures, on the one hand, and the ionic potentials of the alkali-metal cations, on the other.     

<Emphasis Type="Italic">In-Situ</Emphasis> Scanning Electron Microscopy/Electron Backscattered Diffraction Observation of Microstructural Evolution during <Emphasis Type="Italic">α</Emphasis> → <Emphasis Type="Italic">γ</Emphasis> Phase Transformation in Deformed Fe-Ni Alloy

This article presents in-situ observation of ferrite (α)/austenite (γ) phase transformation in an Fe-8.5 at. pct Ni alloy deformed by rolling using an automated scanning electron microscopy/energy backscattered diffraction (SEM/EBSD) system. During heating, recrystallization in α phase and α → γ phase transformation independently occurred. The γ grains nucleated in unrecrystallized α grains were most probably incorporated into the grain interior of recrystallized α grains. They did not have any specific orientation relation (OR) with recrystallized α grains and grew in an isotropic manner. On the other hand, the intragranular γ grains nucleated in recrystallized α grains had a Kurdjumov–Sachs (K-S) OR with the α grains and grew in a considerably anisotropic manner. They preferentially grew along the common direction of surface traces of {110} _α /{111} _γ . Approximately half of grain boundary (GB) allotriomorphs had either the K-S OR or the Nishiyama–Wasserman (N-W) OR with the parent α grains. The γ allotriomorphs predominantly grew into the α grain having the special OR with themselves. The GB character distribution of γ phase at high temperatures was measured. The fraction of CSL boundaries was as high as 63 pct, particularly that of Σ3 grain boundaries (GBs) was 54 pct.     

Thermodynamic effect of alloying addition on <Emphasis Type="Italic">in-situ</Emphasis> reinforced TiB<Subscript>2</Subscript>/Al composites

From the viewpoint of thermodynamics, using the Wilson equation and an extended Miedema model, the effect of the alloying element on the stability of the precipitated phases during the fabrication of in-situ reinforced TiB₂/Al composites was evaluated. The result shows that additions of alloying elements, such as Mg, Cu, Zr, Ni, Fe, V, and La, can promote the formation of Al₃Ti and TiB₂ phases. Particularly, Zr has the most pronounced effect among these alloying elements. In addition, alloying elements can hinder the formation of AlB₂ to a small extent. The calculation results also show that it is easier for magnesium to react with the salts to form TiB₂ than aluminum during the fabrication of in-situ reinforced TiB₂/Al using the flux-assisted synthesis (FAS) technology.     

Reactive synthesis of <Emphasis Type="Italic">in-situ</Emphasis> ZrAl<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al composites

In this work, a reactive synthesis process is proposed to obtain ZrAl₃-Al₂O₃ particulate-reinforced aluminum matrix composites. The process involves the in-situ formation of Al₂O₃ and ZrAl₃ from Al-ZrO₂ green compacts. Upon compact heating, it is found that reduction of ZrO₂ by molten aluminum occurs at temperatures above 750 °C, leading to the development of ZrAl₃ and Al₂O₃ phases. Thermodynamically, it is found that the reduction of zirconium oxide is driven mainly by the dissolution of Zr in molten aluminum. Because the solubility of Zr in liquid aluminum is extremely small, the formation of ZrAl₃ is favored after relatively small Zr dissolutions. The first Zr-Al intermetallics to form at the lowest temperatures seem to be metastable, as infered from the measured atom ratios for Al : Zr of 2.83 : 1. At increasing temperatures, the reaction comes into completion, resulting in the formation of equilibrium intermetallic ZrAl₃ phases. The results obtained from differential scanning calorimetry (DSC) indicate that by increasing the scanning rates, both the reaction temperature and the exothermic peak intensity also increase. Alternatively, it is found that by reducing the amount of ZrO₂ in the green compact, the in-situ reaction temperatures also shift toward higher values.     

Defect structures in cosputtered thin films of transition-metal disilicides with C11<Subscript><Emphasis Type="Italic">b</Emphasis></Subscript>, C40 and C54 structures

Defect structures in crystallites of the stable phases in thin films of transition-metal (TM) disilicides (C11 _b MoSi₂, C40 TaSi₂, and C54 TiSi₂) produced by cosputtering and subsequent annealing have been investigated by transmission electron microscopy (TEM). Crystallites in thin films of MoSi₂, TaSi₂, and TiSi₂ all contain planar faults parallel to hexagonally arranged TMSi₂ planes, which are a characteristic feature commonly observed in all three crystal structures. These planar faults are twin boundaries in all cases. Twins in thin films of these disilicides, thus, have a common characteristic that the twin habit plane is parallel to hexagonally arranged TMSi₂ stoichiometric planes. For twins in thin films of C11 _b MoSi₂, and C54 TiSi₂, the twining elements can be deduced and the twin habit plane is found not to be parallel to the twinning (K ₁) plane, but to be perpendicular to it. Twins formed in C40 TaSi₂ thin films are different from those formed in C11 _b MoSi₂ and C54 TiSi₂ thin films, in that the crystal orientation of the twin is exactly the same as that of the matrix, since they are racemic twins that are only enantiomorphically (space groups of P6₂22 or P6₄22) related to each other. This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003.     

Co-deformation processing and modeling of <Emphasis Type="Italic">In-Situ</Emphasis> multiphase composites

A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB₂ particulate within a titanium trialuminide or near-γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al₃Ti+30 vol pct TiB₂) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB₂), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB₂ within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of the TiB₂ percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited by the constituents’ inherent capacity to plastically deform.     

Effect of heat treatments on <Emphasis Type="Italic">In-Situ</Emphasis> Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiAl<Subscript>3</Subscript> composites produced from squeeze casting of TiO<Subscript>2</Subscript>/A356 composites

In-situ Al₂O₃/TiAl₃ intermetallic matrix composites were fabricated via squeeze casting of TiO₂/A356 composites heated in the temperature range from 700 °C to 780 °C for 2 hours. The phase transformation in TiO₂/A356 composites employing various heat-treatment temperatures (700 °C to 780 °C) was studied by means of differential thermal analysis (DTA), microhardness, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). From DTA, two exothermic peaks from 600 °C to 750 °C were found in the TiO₂/A356 composites. The XRD showed that Al₂O₃ and TiAl₃ were the primary products after heat treatment of the TiO₂/A356 composite. The fabrication of in-situ Al₂O₃/TiAl₃ composites required 33 vol pct TiO₂ in Al and heat treatment in the range from 750 °C to 780 °C. The hardness (HV) of the in-situ Al₂O₃/TiAl₃ composites (1000 HV) was superior to that of nonreacted TiO₂/A356 composites (200 HV). However, the bending strength decreased from 685 MPa for TiO₂/A356 composites to 250 MPa for Al₂O₃/TiAl₃ composites. It decreased rapidly because pores occurred during the formation of Al₂O₃ and TiAl₃. The activation energy of the formation of Al₂O₃ and TiAl₃ from TiO₂ and A356 was determined to be about 286 kJ/mole.     

Influence of Ni Content on the Microstructure and Dry Sliding Wear Properties of Cr<Subscript>13</Subscript>Ni<Subscript>5</Subscript>Si<Subscript>2</Subscript>/<Emphasis Type="Italic">γ</Emphasis> Intermetallic Alloys

The effect of Ni content on microstructure, hardness, and wear resistance was studied for the Cr₁₃Ni₅Si₂-base intermetallic alloys toughened by Ni-base solid solution (γ). Volume fraction and microhardness of the Cr₁₃Ni₅Si₂ primary dendrite as well as the average hardness of the Cr₁₃Ni₅Si₂/γ alloy decrease with the increasing Ni content. The Cr₁₃Ni₅Si₂/γ alloys have excellent wear resistance under dry sliding wear test conditions, which increases under high contact load wear conditions and decreases under low contact load wear test conditions with the increasing Ni content. The high wear resistance is due to the combination of high toughness of γ and high hardness of Cr₁₃Ni₅Si₂ and formation of a transferred cover layer on the worn surface during wear process. The wear rate of the Cr₁₃Ni₅Si₂/γ alloy is governed by the slow process of microspalling or pullout of the cracked Cr₁₃Ni₅Si₂ primary dendrites. The Cr₁₃Ni₅Si₂/γ alloys have extremely low load sensitivity of wear and the load-sensitivity coefficient of wear decreases drastically as the Ni content increases.