Diffusion in the B2-b.c.c. phase of the Al–Fe–Ni system—application of a phenomenological model

A phenomenological model for multicomponent diffusion in B2-b.c.c. ordered phases is applied to the Al–Fe–Ni system. In the model, the activation energy for diffusion is expressed as a function of the degree of ordering, which is evaluated from thermodynamic data. These data are also used to evaluate the thermodynamic factors for diffusion. A CALPHAD-like assessment procedure is applied to the experimental data for diffusion in the B2-b.c.c. phase of the Al–Fe–Ni system. It is shown that the model is capable of representing the experimental data. The diffusivities from the model are used to simulate six different diffusion experiments. The results are compared with experimental data and it is concluded that the model yields satisfactory predictions.     

A phenomenological treatment of diffusion in Al–Fe and Al–Ni alloys having B2-b.c.c. ordered structure

A model for diffusion in ordered phases with B2-b.c.c. structure is presented and generalized to multicomponent alloys. The model makes use of thermodynamic data to evaluate the thermodynamic factor as well as the degree of ordering at a given temperature. The activation energy for diffusion is expressed as a function of the degree of order. The model is suitable for implementation in software for simulation of diffusional phase transformations. The available experimental data for the Al–Ni and Al–Fe systems is discussed. Some inconsistencies in the data for the Al–Fe system are pointed out. A CALPHAD-like assessment procedure is applied successfully to a selected set of experimental information showing that the model is capable of representing this information in the Al–Fe and Al–Ni systems. The practical use of the model is demonstrated by the computer simulation of a diffusion experiment. It is shown that the results of the simulation agree well with the experimental observations.     

Thermodynamic properties of coherent interfaces in f.c.c.-based Ag–Al alloys: a first-principles study

The thermodynamic properties of coherent interphase boundaries (IPBs) between the Al-rich-matrix and Guinier–Preston-zone (GP-zone) precipitate phases in Ag–Al are studied from first principles. The cluster-variation-method (CVM), with effective-cluster-interaction (ECI) parameters derived from the results of ab initio total energy calculations, is used to compute the interfacial free energies (γ) and composition profiles of flat {111} and {100} IPBs as a function of temperature (T). The calculated values of γ increase monotonically from zero to 35 (37) mJ/m² for {111} ({100}) IPBs as T is lowered from the critical temperature (calculated to be 760 K) to 450 K. Monte-Carlo simulations, based on the same set of ECIs used in the CVM work, have been performed to compute GP-zone morphologies at 450 K. Simulated precipitate shapes are found to be anisotropic, consistent with experimental observations. The CVM is used also to compute the gradient coefficient (κ) in the Cahn–Hilliard coarse-grained free energy. Calculated values of κ are found to display non-negligible concentration and temperature dependencies, in contrast to the predictions of regular-solution theory.     

Composition of Oxide formed on Copper and Copper–Manganese Alloys (5–40% Mn) in Oxygen at 150°–600°c

Abstract

The proportions of CuO and Cu₂O in films formed on copper and copper-manganese alloys in oxygen at temperatures of 150–400°c (600°c for higher Mn alloys) were estimated by coulometric reduction. Only Cu₂O was observed at 150°c. The proportion of CuO increased as the temperature was raised, reaching 82% for copper at 300°c and somewhat lower values for the alloys containing Mn. At temperatures of 350–400°c, the proportion of CuO declined. The oxide remained adherent above 400°c only for alloys with 20% and 40% Mn. From 400° to 600°c the proportion of CuO increased for the 20% alloy but neither CuO nor Cu₂O was found on the 40% alloy.     

Microstructure of Rapidly Solidified Alloys of the Sn–Zn–Bi–In System

The results of a study of the phase composition and microstructure of foils of Sn–8.0 Zn–3.0 Bi–X In (X = 1.5, 2.5, 4.5, 9.0) (wt %) alloys formed by rapidly quenching from the melt at a cooling rate of up to 5 × 10⁵ K/s have been presented. The dependence of the phase composition of the rapidly quenched foils on the concentration of In has been determined. It has been shown that, in rapidly quenched foils, crystallization occurs with the formation of supersaturated solid solutions based on β-Sn and γ phase (Sn₄In). The mechanisms and rates of decomposition of the supersaturated solid solutions at room temperature have been established. The specific features of the formation of the microstructure of the foils have been discussed. The grain structure has been studied by the electron back-scatter diffraction (EBSD) method; the formation of an elongated shape of grains and the high specific surface area of small-angle boundaries has been explained.     

Influence of Mo in the structure of rapidly solidified Fe–Mo–Cr–Mn–Si–C alloy

Abstract

An Fe–Mo–Cr–Mn–Si–C alloy was prepared in an induction furnace and was cast into cylindrical rod in a copper mould in castmatic equipment (low pressure casting). A single phase non-equilibrium featureless (no visible microstructures after deep etching) phase was observed over a certain range of thickness of the rod. In this present work, the extent of the featureless phase was studied with different concentrations of Mo (5–25 wt-%) for 5·5 mm diameter of cylindrical rod at a cooling rate of 1100 K s^–1. Light optical microscopy, scanning electron Microscopy and Vickers hardness tests were used to analyse the samples. The amount of the featureless area varies as the Mo content changes and the maximum featureless area was obtained for 7 wt-% of Mo. This single phase featureless structure exhibits very high hardness (>1350 HV) which can be used in many interesting applications with or without suitable heat treatments.     

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.     

The influence of Cr alloying on microstructures of Fe–Al–Mn–Cr alloys

The effects of increasing chromium content on the phase transformations in Fe–Al–Mn–Cr alloys have been investigated by means of transmission electron microscopy and energy-dispersive X-ray spectrometry. The experimental results revealed that increasing the chromium addition would expand both the A12α-Mn and DO₃ phase-field regions.     

Interrelation of wettability–microstructure–tensile strength of lead-free Sn–Ag and Sn–Bi solder alloys

A comparative investigation on the wettability and tensile strength of a Sn–2Ag, a Sn–40Bi and the traditional eutectic Sn–Pb solder alloys was carried out. The wettability is represented by thickness of covered layer (TCL) and spread area (SA) while the mechanical behaviour by the ultimate tensile strength (UTS). It is shown that the TCL of studied alloys decreased with the increase in the dipping temperature. It is also shown that TCL and SA have opposite behaviour with respect to the cooling rate. The Sn–Bi solder alloy has lower SA when compared with those of the Sn–Ag solder when similar cooling rates are considered. The Sn–Bi solder exhibits the best UTS/SA combination for dendritic spacings between 25 and 27?µm, associated with cooling rates ～2°C?s^?1, 2× lower than those of the Sn–Ag alloy. Besides, the Sn–Bi alloy has shown SA >70～80% associated with higher UTS (～80?MPa) as compared with the other alloys examined.     

Structure–Property–Functionality of Bimetal Interfaces

Interfaces, such as grain boundaries, phase boundaries, and surfaces, are important in materials of any microstructural size scale, whether the microstructure is coarse-grained, ultrafine-grained, or nano-grained. In nanostructured materials, however, they dominate material response and as we have seen many times over, can lead to extraordinary and unusual properties that far exceed those of their coarse-grained counterparts. In this article, we focus on bimetal interfaces. To best elucidate interface structure?Cproperty?Cfunctionality relationships, we focus our studies on simple layered composites composed of an alternating stack of two metals with bimetal interfaces spaced less than 100?nm. We fabricate these nanocomposites by either a bottom?Cup method (physical vapor deposition) or a top?Cdown method (accumulative roll bonding) to produce two distinct interface types. Atomic-scale differences in interface structure are shown to result in profound effects on bulk-scale properties.     

Modeling of Oxidation Kinetics of Y-Doped Fe–Cr–Al Alloys

Studies using advanced analytical techniques indicated that the reactiveelements (RE) segregate along the oxide grain boundaries and at theoxide–alloy interface during oxidation of -Al₂O₃forming alloys. The segregation results in inward oxygen diffusion along theoxide grain boundaries as the predominant transport process in the oxidegrowth. The present work establishes a mathematical model based on themechanisms of inward oxygen diffusion along the grain boundaries and oxidegrain coarsening. This model has been used to describe the oxidationkinetics of Y-doped Fe–Cr–Al alloys. The results showed a muchbetter agreement with the experimental data than the parabolic rate law. Byusing this model, the exponential number for the grain coarsening of aluminascales during oxidation was calculated to be 3. The activation energyfor oxygen diffusing along the grain boundaries was 450 kJ/mol. They arealso in good agreement with values reported in the literatures.     

Machining of a Zr–Ti–Al–Cu–Ni metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass machining chips were characterized using SEM, X-ray diffraction and nano-indentation. Above a threshold cutting speed, oxidation of the Zr produces high flash temperatures and causes crystallization. The chip morphology was unique and showed the presence of shear bands, void formation and viscous flow.     

Microstructural design of hardfacing Ni–Cr–B–Si–C alloys

This work reports the procedure for selection of alloying elements to refine the microstructure of hardfacing Ni–Cr–B–Si–C alloys by providing in situ formed nucleation agents. It is concluded that the refining element should be able to spontaneously produce precipitates at high temperatures with little solubility in their Cr-rich counterparts. After exploring the theoretical backgrounds on how to select the refining element, Nb and Zr were selected and the phase formation reactions of Zr- or Nb-modified Ni–Cr–B–Si–C alloys were calculated using Thermo-Calc® simulations. Detailed microstructural analyses of the rapidly solidified samples deposited from the modified alloys showed that addition of Nb in specific quantities induces a significant microstructural refinement in the original Ni–Cr–B–Si–C alloy without deteriorating its high hardness. The Nb-modified alloy could be used to further investigate the viability of microstructural refinement as an effective toughening mechanism for Ni–Cr–B–Si–C and similar alloy systems.     

Modelling of the age hardening behaviour of Al–Mg–Si alloys

In the present investigation a special control volume formulation of the classical precipitation model for coupled nucleation, growth and coarsening has been adopted to describe the evolution of the particle size distribution with time during thermal processing of Al–Mg–Si alloys. The analysis includes both isothermal and non-isothermal transformation behaviour. Well established dislocation theory is then used to evaluate the resulting change in hardness or yield strength at room temperature, based on a consideration of the intrinsic resistance to dislocation motion due to solute atoms and particles, respectively following heat treatment. The model is validated by comparison with experimental microstructure data obtained from transmission electron microscope examinations and hardness measurements, covering a broad range in the experimental conditions. It is concluded that the model is sufficiently relevant and comprehensive to be used as a tool for predicting the response of Al–Mg–Si alloys to thermal processing, and some examples are given towards the end.     

Friction stir welding of thick plates of Al–Mg–Sc alloy

A technology is developed for single-pass friction stir welding (FSW) of 11- and 35-mm-thick plates of Al–Mg–Sc alloys. The microstructural and mechanical heterogeneity of the welded joints is investigated. The welded joints obtained under the optimum welding conditions are free from macrodefects. The strength of the welded joint equals 98% of the strength of the parent metal, which is higher than the strength of fusion-welded joints. It is concluded that the FSW of thick plates of Al–Mg–Sc alloy can be used efficiently in practice.     

Comparative investigation of oxidation resistance and thermal stability of nanostructured Ti–B–N and Ti–Si–B–N coatings

Nanostructured Ti–B–N and Ti–Si–B–N coatings were deposited on silicon substrate by ion implantation assisted magnetron sputtering technique. To evaluate the oxidation resistance and thermal stability the coatings were annealed on air and in vacuum at 700–900°C. As-deposited and thermal-treated coatings were investigated by transmission electron microscope, selected area electron and x-ray diffraction, atomic force microscopy, Raman and glow discharge optical emission spectroscopy. Nanoindentaion tests were also performed. Obtained results show that Si alloying significantly improves the thermal stability of Ti–B–N coatings and increases their oxidation resistance up to 900°C. It was shown that formation of protective amorphous SiO₂ top-layer on the coating surface plays important role in the increasing of the oxidation resistance.     

Fatigue behavior of Zr–Ti–Ni–Cu–Be bulk-metallic glasses

The high-cycle fatigue (HCF) behavior of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (in at.%) bulk-metallic glass (BMG) was studied. Two batches of samples that are from different lots (Batches 59 and 94) are employed in present experiments. The HCF experiments were conducted, using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1 in air at room temperature and under tension-tension loading, where R=σ_min./σ_max.. (σ_min. and σ_max. are the applied minimum and maximum stresses, respectively). A high-speed and high-sensitivity thermographic-infrared (IR) imaging system was employed for the nondestructive evaluation of temperature evolutions during fatigue testing. No distinct sparking phenomenon was observed at the final fracture moment for this alloy. The fatigue lifetime of Batch 59 is longer than that of Batch 94 at high stress levels (maximum stresses >864 MPa). Moreover, the fatigue-endurance limit of Batch 59 (703 MPa) is somewhat greater than that of Batch 94 (615 MPa). The vein pattern and liquid droplets were observed in the apparent-melting region along the edge of the fractured surfaces. The fracture morphology suggests that fatigue cracks initiated from casting defects, such as porosities and inclusions, which have an important effect on the fatigue behavior of BMGs.