<Emphasis Type="Italic">In Situ</Emphasis> Studies of Intercritically Austempered Ductile Iron Using Neutron Diffraction

Intercritically austempered ductile irons hold promise for applications requiring fatigue durability, excellent castability, low production energy requirements, reduced greenhouse gas emissions, and excellent machinability. In the present study, four different ductile iron alloys, containing manganese and nickel as the primary austenite-stabilizing elements, were heat treated to obtain different quantities of austenite in the final microstructure. This article reports the microstructures and phases present in these alloys. Furthermore, lattice strains and diffraction elastic constants in various crystallographic directions and the transformation characteristics of the austenite were determined as a function of applied stress using in situ loading during neutron diffraction at the second generation Neutron Residual Stress Facility at the High Flux Isotope Reactor at Oak Ridge National Laboratory.     

<Emphasis Type="Italic">In Situ</Emphasis> Neutron-Diffraction Studies on the Creep Behavior of a Ferritic Superalloy

Precipitate strengthening effects toward the improved creep behavior have been investigated in a ferritic superalloy with B2-type (Ni,Fe)Al precipitates. In situ neutron diffraction has been employed to study the evolution of the average phase strains, (hkl) plane-specific lattice strains, interphase lattice misfit, and grain-orientation texture during creep deformation of the ferritic superalloy at 973 K (700 °C). The creep mechanisms and particle-dislocation interactions have been studied from the macroscopic creep behavior. At a low stress level of 107 MPa, the dislocation-climb-controlled power-law creep is dominant in the matrix phase, and the load partition between the matrix and the precipitate phases remains constant. However, intergranular stresses develop progressively during the primary creep regime with the load transferred to 200 and 310 oriented grains along the axial loading direction. At a high stress level of 150 MPa, deformation is governed by the thermally activated dislocation glide (power-law breakdown) accompanied by the accelerated texture evolution. Furthermore, an increase in stress level also leads to load transfer from the plastically deformed matrix to the elastically deformed precipitates in the axial direction, along with an increase in the lattice misfit between the matrix and the precipitate phases.     

First <Emphasis Type="Italic">In Situ</Emphasis> Lattice Strains Measurements Under Load at VULCAN

The engineering materials diffractometer, VULCAN, at the Spallation Neutron Source began commissioning on June 26, 2009. This instrument is designed for materials science and engineering studies. In situ lattice strain measurements of a model metallic material under monotonic tensile load have been performed on VULCAN. The tensile load was applied under two different strain rates, and neutron diffraction measurements were carried out in both high-intensity and high-resolution modes. These experiments demonstrated VULCAN’s in situ study capability of deformation behaviors even during the early phases of commissioning.     

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.     

Factors that influence <Emphasis Type="Italic">CSR</Emphasis> and <Emphasis Type="Italic">CRI</Emphasis>

The influence of various factors on CSR and CRI in the production of metallurgical coke at OAO MMK is evaluated.     

<Emphasis Type="Italic">In Situ</Emphasis> Observation of Strain Evolution in Cp-Ti Over Multiple Length Scales

The strain evolution in polycrystalline CP-Ti strip under tension was studied in situ and at two length scales using Synchrotron X-ray diffraction. To establish the bulk material behavior, experiments were performed at the Australian Synchrotron facility. Because of the relatively large grain size, discontinuous “spotty” Debye ring patterns were observed, and a peak fitting algorithm was developed to determine the individual spot positions with the necessary precision for strain determination. The crystallographic directional dependence of strain anisotropy during the loading cycle was determined. Strain anisotropy and yielding of individual crystallographic planes prior to the macroscopic yield point were further clarified by in situ loading experiments performed at the Advanced Light Source (ALS). The deviatoric strain accumulation and plastic response were mapped on a grain-by-grain basis. The onset of microscopic yielding in the grains was identified and correlated with the relative orientation of the grains with respect to the loading direction.     

Analysis of the <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis> transformation in a C-Mn steel by phase-field modeling

This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a shift toward diffusion control takes place to a degree that depends on cooling rate.     

Study of <Emphasis Type="Italic">γ</Emphasis>/<Emphasis Type="Italic">γ</Emphasis>′ Interfaces in Nickel-Based,Single-Crystal Superalloys by Scanning Transmission Electron Microscopy

The interfaces of γ/γ′ in crept Re-bearing, single-crystal, nickel-based superalloys have been studied by scanning transmission electron microscopy. Elements Ni and Ta are congregated at the γ′ side, ahead of the Re enrichment near interfaces, which shows the retarding effect on element diffusion during creep by Re enrichment. A growth step exists at the γ/γ′ interfaces with the apex of kinks congregated with heavy atoms, which may slow down interface displacement during creep.     

<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.     

Magnetic contribution to the interdiffusion coefficients in bcc (<Emphasis Type="Italic">α</Emphasis>) and fcc (<Emphasis Type="Italic">γ</Emphasis>) Fe-Ni alloys

The interdiffusion coefficients in bcc (α) and fcc (γ) Fe-Ni alloys below their Curie temperatures have been calculated based on the magnetic contribution to the free energy for interdiffusion. The free energy for interdiffusion due to magnetic ordering in bcc Fe-Ni alloys is positive. The calculated interdiffusion coefficients in bcc Fe-Ni alloys fit the experimental data quite well. In fcc Fe-Ni alloys, the magnetic contribution to interdiffusion depends on both temperature and composition and is abnormal for Ni compositions in the Invar region. The free energy of vacancy formation is positive and the free energy of vacancy migration is negative, due to the effect of magnetic ordering. The interdiffusion coefficient in the ferromagnetic phase is lower than that extrapolated from the paramagnetic phase for Ni compositions of 50 at. pct and greater and is higher than that extrapolated from the paramagnetic phase for Ni compositions of 40 at. pct and lower.     

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.     

<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.     

Effect of strain rate on the strain-induced <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis>′-martensite transformation and mechanical properties of austenitic stainless steels

The effect of strain rate on strain-induced γ → α′-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3×10⁻⁴ and 200 s⁻¹. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γ → α′ transformation. A correlation between the work-hardening rate and the rate of γ → α′ transformation was found. Therefore, the changes in the extent of the α′-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the α → α′ transformation.     

Creep behavior of copper-chromium <Emphasis Type="Italic">in-situ</Emphasis> composite

Creep deformation and fracture behaviors were investigated on a deformation-processed Cu-Cr in-situ composite over a temperature range of 200 °C to 650 °C. It was found that the creep resistance increases significantly with the introduction of Cr fibers into Cu. The stress exponent and the activation energy for creep of the composite at high temperatures (≥400 °C) were observed to be 5.5 and 180 to 216 kJ/mol, respectively. The observation that the stress exponent and the activation energy for creep of the composite at high temperatures (≥400 °C) are close to those of pure Cu suggests that the creep deformation of the composite is dominated by the deformation of the Cu matrix. The high stress exponent at low temperatures (200 °C and 300 °C) is thought be associated with the as-swaged microstructure, which contains elongated dislocation cells and subgrains that are stable and act as strong athermal obstacles at low temperatures. The mechanism of damage was found to be similar for all the creep tests performed, but the distribution and extent of damage were found to be very sensitive to the test temperature.     

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.     

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.     

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.     

<Emphasis Type="Italic">In Situ</Emphasis> Micromechanical Testing for Single Crystal Property Characterization

An in situ method to fully characterize the single crystal properties for polycrystalline alloys is developed using microscale experimental and analysis techniques. The developed method can be applied to metallic engineering alloys that do not exist in single crystal form. Thus using this technique, testing and analysis on polycrystalline samples can yield the single crystal elastic and plastic properties required as input to micro- and mesoscale computational models such as those which rely on crystal plasticity theory. Compression and shear experiments are conducted on single crystal specimens of various crystallographic orientations. Analytical and numerical analysis of the experimental results yields a set of equations that can be solved for the single crystal elastic parameters. This novel methodology is demonstrated to produce reasonable elastic property prediction results for an aerospace aluminum lithium alloy, AA2070. Details regarding the experiments and analysis are provided to facilitate application of the technique to a wide range of polycrystalline material systems and properties.     

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.     

Optimization of the johnson-mehl-avarami equation parameters for <Emphasis Type="Italic">α</Emphasis>-ferrite to <Emphasis Type="Italic">γ</Emphasis>-austenite transformation in steel welds using a genetic algorithm

A nonisothermal Johnson-Mehl-Avarami (JMA) equation with optimized JMA parameters is proposed to represent the kinetics of transformation of α-ferrite to γ-austenite during heating of 1005 steel. The procedure used to estimate the JMA parameters involved a combination of numerical heat-transfer and fluid-flow calculations, the JMA equation for nucleation and growth for nonisothermal systems, and a genetic algorithm (GA) based optimization tool that used a limited volume of experimental kinetic data. The experimental data used in the calculations consisted of phase fraction of γ-austenite measured at several different monitoring locations in the heat-affected zone (HAZ) of a gas tungsten arc (GTA) weld in 1005 steel. These data were obtained by an in-situ spatially resolved X-ray diffraction (SRXRD) technique using synchrotron radiation during welding. The thermal cycles necessary for the calculations were determined for each monitoring location from a well-tested three-dimensional heat-transfer and fluid-flow model. A parent centric recombination (PCX) based generalized generation gap (G3) GA was used to obtain the optimized values of the JMA parameters, i.e., the activation energy, pre-exponential factor, and exponent in the nonisothermal JMA equation. The GA based determination of all three JMA equation parameters resulted in better agreement between the calculated and the experimentally determined austenite phase fractions than was previously achieved.