Structural and phase transformations in high-nitrogen austenitic steel 05Kh20AG10N3MF during thermal action

The structural and phase transformations in high-nitrogen austenitic steel 05Kh20AG10N3MF under various thermal actions are studied. The transformations are shown to occur only in the Fe-Cr diffusion couple, and they resemble the phase transformations in an Fe-20% Cr binary alloy. At 1200°C, hightemperature phase separation is detected; at 550°C or below, low-temperature separation is detected; and at 700 and 800°C, ordering with the formation of a Laves phase is observed. The ordering-low-temperature separation phase transition in the steel differs substantially from that occurring in Fe-Cr binary alloys.     

Ordering and phase separation in the Fe-Cr-N system

The structures of Fe-Cr-N alloys are studied, and the types of chemical interactions in the Fe-Cr and Fe-N diffusion couples are determined at various temperatures. At 1200°C, austenite is shown to form in the alloys as a result of a tendency toward phase separation in the Fe-Cr and Cr-N diffusion couples. Below 1150°C, chromium nitrides form in the alloys due to ordering in the Cr-N diffusion couple.     

利用经验电子理论(EET)分析Al对Fe-Cr高温合金冲击韧性的影响

采用Thermal calc热力学软件,计算了新型含铝Fe-Cr合金(/%:1.60C、26.00Cr、5.50Al、0.50Ni、0.12Y)的相图,结合X射线衍射分析,确定了该合金的相组成。在此基础上,运用固体与分子经验电子理论(EET),分析了普通高铬铸铁和含铝Fe-Cr合金的价电子结构、共价键键能以及晶格电子数,从电子层面揭示含铝Fe-Cr合金冲击韧性下降的微观机理。结果表明,添加5.50%Al后,Fe-Cr合金中出现了含铝结构单元,这些单元中最强键共价电子对数从0.48640降低为0.450 31,次强键共价电子对数也从0.199 83降低到0.118 88,同时晶格电子数有所减少,从而导致其冲击韧性下降。含5.50%Al的Fe-Cr高温合金冲击韧性的EET预测结果与实测结果吻合良好。     

Incipient chemical instabilities of nanophase Fe-Cu alloys prepared by mechanical alloying

We used Mössbauer spectrometry, X-ray diffractometry, a novel imaging method of electron energy loss spectrometry, and small-angle neutron scattering (SANS) to study early stage thermal instabilities of nanophase Fe-Cu alloys prepared by mechanical attrition. Mössbauer spectrometry confirmed previous reports of an extended Cu solubility in the body-centered cubic (bcc) phase of the as-milled material. Mössbauer spectrometry also provided evidence that in the compositional range of bcc-face-centered-cubic (fcc) two-phase coexistence, the bcc phase had a Cu concentration nearly the same as the overall composition of the alloy. After the as-milled powders were annealed at temperatures as low as 200 °C, however, Mössbauer spectrometry showed significant chemical unmixing of the Cu and Fe atoms. In annealed bcc Fe-20 pet Cu alloys, SANS measurements indicated that Cu segregated to grain boundaries. This segregation of Cu atoms to bcc grain boundaries did not alter significantly the tendency for grain growth, however. X-ray diffractometry showed that grain growth during thermal annealing was similar for all alloys, although grain growth was small at temperatures below 300 °C. The two-phase (bcc plus fcc) alloy of Fe-30 pc Cu was more unstable against chemical segregation than were the single-phase (bcc or fcc) alloys. Energy-filtered imaging indicated that the Cu atoms segregated to regions around the bcc grains, perhaps to the adjacent fcc crystallites.     

一氧化钛基金属陶瓷粘结相的研究

本文对铜基与铁铬基两个系列的合金用作一氧化钛（TiO）基金属陶瓷仿金材料的粘结相进行了研究。结果表明：纯Cu及Fe－Cr合金在TiO上的润湿性较差，不宜作TiO的粘接相，Cu中加入少量Mo，C，可明显改善其润湿性，Cu－Mo-C中加入适量的Al、Ni、Si，可使合金的颜色接近黄金且有较好的耐蚀性，并能与TiO保持很高的界面结合强度，适用于TiO陶瓷的粘结相，Fe－Cr合金中加入表达活性物质A中显著改善在TiO上的润湿性，抑制Fe与TiO间的界面反应，与TiO间形成很高的界面强度，可用做TiO陶瓷的粘结相。     

Application of the diffusion couple to study phase equilibria in the Fe-Cr-S ternary system at 600 °C

The sulfidation of Fe-Cr alloys has a large financial significance for industries that use fossil fuels, such as the electric utility industry. Therefore, the sulfidation of a series of Fe-Cr alloys was studied at 600 °C using a solid-state diffusion couple technique. The diffusion couple technique combined Fe_0.95S powder and FeCr binary alloys together in a configuration that allowed for post-heat-treatment microanalysis using an electron probe microanalyzer (EPMA). The results showed that only two different diffusion couple microstructures formed in samples spanning the entire Fe-Cr binary range. The Fe-rich alloy diffusion couples contained a surface αFeCr layer and an internal sulfide precipitate layer that contained three different sulfide phases. The Cr-rich alloy diffusion couples also possessed an internal precipitate layer, as well as a thick, triplex interfacial scale. The ternary elemental diffusion was described using diffusion paths plotted on the 600 °C isothermal section of the Fe-Cr-S phase diagram. The results also showed that samples with less than 51 wt Pct Cr were more sulfidation resistant. The accuracy of the existing Fe-Cr-S 600 °C isothermal section was assessed, and it was determined that the τ phase field had a larger composition than previously published.     

The diffuse-scattering method for investigating locally ordered binary solid solutions

Diffuse-scattering investigations comprise a series of maturing methods for detailed characterization of the local-order structure and atomic displacements of binary alloy systems. The distribution of coherent diffuse scattering is determined by the local atomic ordering, and analytical techniques are available for extracting the relevant structural information. An extension of such structural investigations, for locally ordered alloys at equilibrium, allows one to obtain pairwise interaction energies. Having experimental pairwise interaction energies for the various coordination shells offers one the potential for more realistic kinetic Ising modeling of alloy systems as they relax toward equilibrium. Although the modeling of atomic displacements in conjunction with more conventional studies of chemical ordering is in its infancy, the method appears to offer considerable promise for revealing additional information about the strain fields in locally ordered and clustered alloys. The diffuse-scattering methods for structural characterization and for the recovery of interaction energies are reviewed, and some preliminary results are used to demonstrate the potential of the kinetic Ising modeling technique to follow the evolution of ordering or phase separation in an alloy system. With the Materials Science Division, Argonne     

Ordering transformation and spinodal decomposition in Au-Ni alloys

A transmission electron microscope (TEM) was employed to study the ordering and phase separation processes in Au-40 at. pct Ni and Au-50 at. pct Ni alloys in order to test for the possible existence of a transient long-range-order (LRO) phenomenon within a disordered miscibility gap. An L1₀ LRO phase was found in the Au-50 at. pct Ni alloy when spinodally decomposed specimens were reannealed at ∼490 °C on a TEM hot stage. This observation, together with the literature results, indicates that a transient LRO did exist in the Au-Ni system, although it appeared during a reversion process. Attempts to find the L1₀ and L1₂ LRO phases during decomposition processes of a single-phase homogeneous solid solution were unsuccessful, except in very thin sections of the TEM foils. Elastic strain energy relaxation was employed to explain the experimental observations. Time-temperature-transformation (TTT) diagrams for the two Au-Ni alloys were constructed based on the TEM characterization. In bulk-annealed samples, only spinodal decomposition and discontinuous precipitation structures were observed.     

Co-Al基三元系中B2相的溶解度间隙与稳定性

Information on phase equilibria in the Co-Al based systems which are related to some magnetic and heat resistance materials is important for their microstructural control. Recently, it was proposed with a theoretical calculation on electronic band structure that some Heusler-type alloys Co2 XAl (X: Cr and Mn) should be a new type of spinelectronic materials so-called half-metallic ferromagnet. In the case of the Co2CrAl, however, magnetic properties expected from the theoretical work can not been experimentally obtained and the reason has been still unknown. On the other hand, a tunne- ling magnetoresistance (TMR) effect due to the half-metallic properties was reported in Co2 (Cr0.6 Fe0.4 ) Al alloy, but not the Co2CrAl alloy. In the present paper, it is reported that this discrepancy with the theoretical work in the Co2CrAl alloy is bought about by phase separation between A2 and B2 phases, and that the substitution of Fe for Cr can suppress the precipitation of A2 phase in the B2 phase. Such a phase separation is originally due to the miscibility gap between CoAl and Cr formed in the Co-Al-Cr ternary system as well as that reported by Hao et al. in the Ni-Co-Al-Fe system.     

Synthesis and Structural Characterization of V–4Ti–4Cr Alloy

V–(4–10)Ti–(4–5)Cr alloys are the potential candidate materials for the high performance structural applications in fusion power systems due to their favorable mechanical and physical properties. In the present study, the alloy design has been attempted through model based approach for pre mentioned composition range. Thermodynamic calculations have been carried out through Miedema model for this alloy system. The enthalpy difference—composition plot for the ternary V–Ti–Cr system indicate the stability of the solid solution phase in this composition range, which is also in agreement with the atomic size variation which is well within the 15 % size variation limit. Based on these plots, an alloy with a composition of V–4Ti–4Cr is considered as the reference composition for our study. The alloys have been prepared by melting the desired compositions of highly pure metals in a water cooled vacuum arc melting unit. For structural information, X-ray diffraction experiment has been carried out. The XRD pattern does not contain any signature for the secondary phase formation. The structure is bcc structure with small variation in lattice parameter from that of pure vanadium. Transmission electron microscopy characterization has also been carried out, which confirms the absence of fine secondary phases and the crystal structure as b.c.c.     

Ordering transformation and spinodal decomposition in Au−Ni alloys

A transmission electron microscope (TEM) was employed to study the ordering and phase separation processes in Au-40 at. pct Ni and Au-50 at. pct Ni alloys in order to test for the possible existence of a transient long-range-order (LRO) phenomenon within a disordered miscibility gap. An L1₀ LRO phase was found in the Au-50 at. pct Ni alloy when spinodally decomposed specimens were reannealed at ∼490°C on a TEM hot stage. This observation, together with the literature results, indicates that a transient LRO did exist in the Au−Ni system, although it appeared during a reversion process. Attempts to find the L1₀ and L1₂ LRO phases during decomposition processes of a single-phase homogeneous solid solution were unsuccessful, except in very thin sections of the TEM foils. Elastic strain energy relaxation was employed to explain the experimental observations. Time-temperature—transformation (TTT) diagrams for the two Au−Ni alloys were constructed based on the TEM characterization. In bulk-annealed samples, only spinodal decomposition and discontinuous precipitation structures were observed. JI-CHENG ZHAO, formerly Graduate Student, Department of Materials Science and Engineering, Lehigh University     

Chromium Extraction <Emphasis Type="Italic">via</Emphasis> Chemical Processing of Fe-Cr Alloys Fine Powder with High Carbon Content

Ferrous alloys are important raw materials for special steel production. In this context, alloys from the Fe-Cr system, with typical Cr weight fraction ranging from 0.45 to 0.95, are prominent, particularly for the stainless steel industry. During the process in which these alloys are obtained, there is considerable production of fine powder, which could be reused after suitable chemical treatment, for example, through coupling pyrometallurgical and hydrometallurgical processes. In the present study, the extraction of chromium from fine powder generated during the production of a Fe-Cr alloy with high C content was investigated. Roasting reactions were performed at 1073 K, 1173 K, and 1273 K (800 °C, 900 °C, and 1000 °C) with 300 pct (w/w) excess NaOH in an oxidizing atmosphere (air), followed by solubilization in deionized water, selective precipitation, and subsequent calcination at 1173 K (900 °C) in order to convert the obtained chromium hydroxide to Cr₂O₃. The maximum achieved Cr recovery was around 86 pct, suggesting that the proposed chemical route was satisfactory regarding the extraction of the chromium initially present. Moreover, after X-ray diffraction analysis, the final produced oxide has proven to be pure Cr₂O₃ with a mean crystallite size of 200 nm.     

X-ray diffraction and resistivity studies of titanium-molybdenum alloys

The phase transformation behavior of the metastable beta phase in hydrogen charged Ti-Mo alloys was investigated using electrical resistivity and X-ray diffraction techniques. Hydrogen charging was found to have little effect on athermal omega phase formation in a highly susceptible alloy (16 wt pct Mo) but suppressed athermal omega in alloys with compositions near the critical composition for the transition to diffuse (incommensurate) type omega (∼20 wt pct Mo). No evidence was found for a hydrogen induced omega phase in a concentrated alloy (30 wt pct Mo). The incipient stages of the beta + beta’ phase separation in Ti-30 wt pct Mo were detected after aging slightly below the beta transus.     

X-ray diffraction and resistivity studies of titanium-molybdenum alloys

The phase transformation behavior of the metastable beta phase in hydrogen charged Ti-Mo alloys was investigated using electrical resistivity and X-ray diffraction techniques. Hydrogen charging was found to have little effect on athermal omega phase formation in a highly susceptible alloy (16 wt pct Mo) but suppressed athermal omega in alloys with compositions near the critical composition for the transition to diffuse (incommensurate) type omega (∼20 wt pct Mo). No evidence was found for a hydrogen induced omega phase in a concentrated alloy (30 wt pct Mo). The incipient stages of the beta + beta’ phase separation in Ti-30 wt pct Mo were detected after aging slightly below the beta transus.     

Thermodynamic properties of the BaO-MnO flux system

Although a great number of works on BaO-bearing fluxes for refining Fe-Cr and Fe-Mn alloys have been carried out, there still remain several unresolved problems on using them in the refining process. The principal aim of the present study is to understand the thermodynamic properties of the BaO-MnO system, which has been shown to be very effective for dephosphorization of Fe-Mn alloys. The activity of manganese oxide in the BaO-MnO flux was measured at 1573 and 1673 K by equilibrating the flux, a Ag-Mn alloy, and a gas mixture of CO and CO₂ as functions of the flux composition and temperature. The influence of BaF₂, which is an effective additive for lowering the melting temperature of the flux, on the thermodynamics of the BaO-MnO system, including the solubility of MnO in the BaO-BaF₂ system, was also investigated.     

A new characterization method of the microstructure using the macroscopic composition gradient in alloys

A new experimental method to determine the phase boundary and phase equilibria is accomplished by using the transmission electron microscopic observation of alloys having the macroscopic composition gradient. The various phase boundaries,i.e., the coherent binodal and spinodal lines and incoherent binodal line, are distinctly determined for the Cu-Ti alloy system. Furthermore, the equilibrium compositions at the interface of precipitate/matrix are experimentally obtained for various particle sizes, and therefore, the Gibbs-Thomson relation is verified. It is expected that the composition gradient method proposed in the present study will become an important experimental method for microstructural characterization.     

Microstructure-property relationships in low-density Al-Li-Mg alloys

The present article describes an investigation of the microstructure and tensile properties of cast Al-Li-Mg alloys with very low densities, in the range 2.3 to 2.4 Mg/m³. Low density is achieved by adding Li and Mg in excess of the solubility limit, which prevents subsequent dissolution of the Al₂LiMg particles that form during solidification. A simple model developed during the course of this research allows prediction of the volume fraction of Al₂LiMg and alloy density from alloy composition. The model was used to select two alloy compositions for detailed investigation: A112Li6Mg and A116Li8Mg. The microstructures of the cast alloys consist of coarse Al₂LiMg particles embedded in an Al matrix containing Al₃Li particles. Both alloys exhibit low tensile elongation in the as-cast condition. Additional processing steps were used to modify the microstructural characteristics thought to be responsible for the low tensile elongation of the ascast alloys. The A116Li8Mg alloy, with an Al₂LiMg volume fraction of 0.25, does not exhibit increased tensile elongation as a result of processing, and the brittle nature of this material is attributed to the high volume fraction of the Al₂LiMg phase. The A112Li6Mg alloy, with an Al₂LiMg volume fraction of 0.13, exhibits a remarkable increase in tensile elongation after extrusion, an effect attributed to fragmentation and dispersal of a three-dimensional (3-D) network of the intermetallic phase in the as-cast alloy.     

Heterogeneous Creep Deformations and Correlation to Microstructures in Fe-30Cr-3Al Alloys Strengthened by an Fe<Subscript>2</Subscript>Nb Laves Phase

A new Fe-Cr-Al (FCA) alloy system has been developed with good oxidation resistance and creep strength at high temperature. The alloy system is a candidate for use in future fossil-fueled power plants. The creep strength of these alloys at 973 K (700 °C) was found to be comparable with traditional 9 pct Cr ferritic–martensitic steels. A few FCA alloys with general composition of Fe-30Cr-3Al-.2Si-xNb (x = 0, 1, or 2) with a ferrite matrix and Fe₂Nb-type Laves precipitates were prepared. The detailed microstructural characterization of samples, before and after creep rupture testing, indicated precipitation of the Laves phase within the matrix, Laves phase at the grain boundaries, and a 0.5 to 1.5 μm wide precipitate-free zone (PFZ) parallel to all the grain boundaries. In these alloys, the areal fraction of grain boundary Laves phase and the width of the PFZ controlled the cavitation nucleation and eventual grain boundary ductile failure. A phenomenological model was used to compare the creep strain rates controlled by the effects of the particles on the dislocations within the grain and at grain boundaries. (The research sponsored by US-DOE, Office of Fossil Energy, the Crosscutting Research Program).     

Dissolution of iron intermetallics in Al-Si Alloys through nonequilibrium heat treatment

Conventional heat treatment techniques in Al-Si alloys to achieve optimum mechanical properties are limited to precipitation strengthening processes due to the presence of second-phase particles and spheroidization of silicon particles. The iron intermetallic compounds present in the microstructure of these alloys are reported to be stable, and they do not dissolve during conventional (equilibrium) heat treatments. The dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical property studies. The dissolution of iron intermetallics improves with increasing solution temperature. The addition of manganese to the alloy hinders the dissolution of iron intermetallics. Nonequilibrium heat treatment increases the strength properties of high iron alloys until a critical solution temperature is exceeded. Above this temperature, a large amount of liquid phase is formed as a result of interdendritic and grain boundary melting. The optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.     

Investigation of the Oxidation Kinetics of Fe-Cr and Fe-Cr-C Melts under Controlled Oxygen Partial Pressures

In the current work, oxidation kinetics of Fe-Cr and Fe-Cr-C melts by gas mixtures containing CO₂ was investigated by Thermogravimetric Analysis (TGA). The experiments were conducted keeping the melt in alumina crucibles, allowing the alloy melt to get oxidized by an oxidant gas. The oxidation rate was followed by the weight changes as a function of time. The oxidation experiments were conducted using various mixtures of O₂ and CO₂ with $ P_{{{\text{O}}_{2} }} $ ?=?10^?2 to 10^4?Pa. In order to understand the mechanism of oxidation, the wetting properties between the alumina container and the alloys used in the thermogravimetric analysis (TGA) experiments and the change of the alloy drop shape during the course of the oxidation were investigated by X-ray radiography.The experiments demonstrated that the oxidation rate of Fe-Cr melt increased slightly with temperature under the current experimental conditions, but it is strongly related to the Cr-content of the alloy as well as the oxygen partial pressure in the oxidant gas mixture, both of which caused an increase in the rate. For the Fe-Cr-C system, the oxidation rate has a negative relationship with carbon content, viz. with increasing carbon, the oxidation rate of the alloy melt slightly decreased. The chemical reaction was found to be the rate determining step during the initial stages, whereas as the reaction progressed, the diffusion of oxygen ions through slag phase to the slag?Cmelt interface was found to have a strong impact on the oxidation rate. The overall impact of different factors on the chemical reaction rate for the oxidation process derived from the current experimental results can be expressed by the relationship: $ k_{1} = \frac{{dm}}{{dt}} = \Uplambda {\text C}_{\text{Cr}}^{0. 2 3} {\text{C}}_{{\text{CO}}_{ 2} } ^{ 0. 4 1}{\text{exp}}(\frac{{{{ - E}}_{\text{a}} }}{{{\text{R}}T}} ). $ A model for describing the kinetics of oxidation of Fe-Cr and Fe-Cr-C alloys under pure CO₂ was developed. Simulation of the oxidation kinetics using this model showed good agreement with the experimental results.