Amorphous Fe–B alloys in B–Fe–Ag multilayers studied by magnetization and Mössbauer measurements

Bulk and local magnetic properties were studied in [1 nm B + 1 nm ⁵⁷Fe + x nm Ag]₅, x = 1, 2, 4, 5 and 10, multilayer samples. Although Ag does not mix with either of the other two elements the magnetic properties of the multilayers are strongly influenced by the Ag thickness below x = 5, whereas no such effect is observed above this value. The Mössbauer measurements indicate a complete amorphization of the thin Fe layers in each sample, as a result of intermixing with the B layers. The variation of the magnetic properties is explained by the variation of the average B concentration of the amorphous Fe–B layers, which depends on the thickness of the Ag barrier layers. The magnetization measurements indicate ferromagnetic behaviour of the ultra-thin amorphous layers with the presence of less than 10% superparamagnetic moments for x = 5 and 10. The average B concentration of the amorphous Fe–B alloy, as estimated from the Fe hyperfine fields, is around 40 at%. It is significantly lower than the 60 at% nominal B concentration, suggesting the presence of an unalloyed B layer, as well. This picture is supported by transmission electron microscopy investigations which reveal two amorphous layers of different B concentration in between the crystalline Ag layers.     

Atom configurations in Pd–Au and Pd–Au–D alloys: A neutron total scattering and Reverse Monte Carlo study

The effect of the alloying elements on the distribution of deuterium in Pd–Au has been investigated by total neutron scattering. The data are analyzed by Reverse Monte Carlo modeling in order to assess the type of interstitial sites occupied with deuterium and how this is correlated with the distribution of the Pd and Au atoms on the host metal lattice. The results show that in Pd–Au alloys deuterium occupies both octahedral and tetrahedral interstitial sites: the overall occupancy of tetrahedral sites increases with increasing Au content and decreasing overall D content. Short-range ordering (SRO) is identified in the D occupancy of interstitial sites in the sample with higher Au content. Indications of SRO and D-induced reorganization in the metal lattice are discussed.     

Stress–strain relationship between ferrite and martensite in a dual-phase steel studied by in situ neutron diffraction and crystal plasticity theories

The stress–strain relationship between ferrite and martensite phases in the commercial dual-phase DP980 steel was studied using in situ neutron diffraction and the crystal plasticity finite element method (CPFEM). The phase identification method based on the image quality of electron backscatter diffraction and a filtering process was used to obtain information concerning individual crystallographic orientations for ferrite and martensite phases in DP980 steel. The (2 0 0) and (2 1 1) lattice strains of ferrite and martensite phases were measured along the loading and transverse directions as a function of macroscopic stress using in situ neutron diffraction. A CPFEM based on representative volume elements (RVE) was applied to determine the microscopic hardening parameters for each phase by fitting the measured macroscopic stress and measured (2 0 0) and (2 1 1) lattice strains. The microscopic hardening parameters for each phase successfully captured the influence of the crystallographic orientation of the ferrite phase on the localization of shear strain and the behavior of ductile failure in RVE of the unit cell during uniaxial tension.     

Small-angle neutron scattering of precipitates in Ni–Ti shape memory alloys

Small-angle neutron scattering was performed on polycrystals of Ni–(46–49) at.% Ti quenched in ice water from the solid solution. The presence of small precipitates of a radius of about 1 nm was found for Ni–(46, 47 and 48) at.% Ti. Assuming a composition of Ni₄Ti₃ of the precipitates, their volume fraction varies from 7% to 0.3%. No precipitates are found if the Ti content is closer to stoichiometric NiTi. The formation of these precipitates already during quenching seems to suppress the formation of martensite. Ni–(47.9 and 48.5) at.% Ti were further aged for 1 h at 553 K, and small-angle scattering shows a fully established precipitate microstructure. The particles have a radius of about 1.5 nm and a mean interparticle distance of 4.8–5.8 nm. From the integrated small-angle scattering curves, a volume fraction of Ni₄Ti₃ particles of about 20% is obtained.     

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract

Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.     

Radiation-induced atomic redistribution in Aging Fe–Ni alloys upon neutron irradiation

The structural and phase transformations and atomic redistribution induced by neutron irradiation have been investigated in aging fcc Fe–Ni alloys using special alloying with elements M (Si, Ti, Al, Zr) that form intermetallic compounds. It has been established that the mechanism and kinetics of disturbance of regions of Ni–M atomic order in atomic displacement cascades upon neutron irradiation are linked to the chemical activity and diffusion mobility of alloying elements. Comparison with the laws of the deformationinduced dissolution of intermetallic compounds has been conducted.     

Pre-precipitate clusters and precipitation processes in Al–Mg–Si alloys

The solute clusters and the metastable precipitates in aged Al–Mg–Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg–Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of β″ after 10 h artificial aging at 175°C varies depending on pre-aging conditions, i.e. pre-aging at 70°C increases the number density of the β″ precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70°C serve as nucleation sites for the β″ in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the β″ precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al–Mg₂Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al–Mg–Si alloys is discussed.     

Critical scattering of Ni–Nb–Y metallic glasses probed by quantitative anomalous small-angle X-ray scattering

Phase-separated Ni–Nb–Y metallic glasses were prepared by means of rapid quenching from the melt. Different stages of spinodal decomposition were obtained for Ni-contents between 66 and 71 at.%. From anomalous small-angle X-ray scattering experiments performed at the K-absorption edges of nickel, yttrium and niobium different correlation lengths, of between 15 and 5.5 nm, were found for the different concentrations. Moreover, from the quantitative analysis of the resonant invariant the chemical concentrations of yttrium and niobium in the random density fluctuations were deduced. The results are compared to a partially crystallized sample annealed at 773 K over 30 min.     

Formation of amorphous phase with crystalline globules in Fe–Cu–Si–B and Fe–Cu–Zr–B immiscible alloys

The microstructure of rapidly solidified melt-spun ribbons of (Fe_0.75M_0.10B_0.15)_100−xCu_x (M = Si, Zr) alloys was investigated focusing on amorphous-phase formation and the solidification structure. In this study, Fe–Cu–Si–B and Fe–Cu–Zr–B alloys were designed to show amorphous-phase formation and liquid-phase separation simultaneously. Amorphous-phase formation was confirmed in both Fe–Cu–Si–B and Fe–Cu–Zr–B alloys. Minor exceptions in a combination map of mixing enthalpy and quaternary predicted phase diagram are acceptable range for designing a quaternary Fe–Cu-based alloy system that shows liquid-phase separation in Fe-based and Cu-based liquids and the formation of an Fe-based amorphous phase.     

Mechanical alloying and amorphization in Cu–Nb–Ag in situ composite wires studied by transmission electron microscopy and atom probe tomography

We have studied deformation-driven alloying in a Cu–5 at.% Ag–3 at.% Nb in situ composite by transmission electron microscopy and atom probe tomography. In addition to alloying at interfaces, amorphization of nanosized Cu areas was observed after heavy wire drawing (true strain: η = 10.5) at some of the Cu–Nb interfaces. We discuss the alloying in terms of trans-phase dislocation-shuffling and shear banding mechanisms where lattice dislocations penetrate the interfaces between abutting phases. We interpret local amorphization in terms of the thermodynamic destabilization of a Cu–Nb crystalline phase between 35 and 80 at.% Cu due to enforced mixing. Deformation-driven mechanical alloying and amorphization are hence closely associated phenomena.     

Nano-scale phase separation in amorphous Fe–B alloys: Atomic and cluster ordering

Nano-scale phase separation encountered in metallic glasses is investigated for amorphous Fe₈₀B₂₀ and Fe₈₃B₁₇ alloys in an interaction field calculated via electronic theory of alloys in pseudopotential approximation combined with MC equilibration and reverse MC simulation. The phenomenon is identified regarding three topological aspects: (1) pure Fe-clusters as large as ～0.9 nm and pure Fe-contours ～0.72 nm thick are found to exist; (2) Fe-rich highly deformed-bcc regions are observed; (3) B-centered prismatic units are found to form distinct regions of high and low coordination. Polytetrahedral order prevails for B-prisms. All topological aspects are compiled into a two-dimensional projection model for predicting contributions to short- and medium-range order and corresponding spacing relations. The outcome geometrically involves proportions approximating the golden ratio.     

Magnetic domain structures in Co–Ni–Al shape memory alloys studied by Lorentz microscopy and electron holography

Magnetic domain structures in recently developed Co–Ni–Al ferromagnetic shape memory alloys were examined by Lorentz microscopy and electron holography, and relations of the martensite variants (crystallographic domains) and the magnetic domains were discussed. Direct observations of the magnetic domain walls by Lorentz microscopy and the magnetic lines of force by electron holography revealed that each martensite variant was divided into fine magnetic domains under a low magnetic field, e.g. about 0.2 mT. Although an applied magnetic field of about 0.4 T made each variant a large single magnetic domain, a similar configuration of multiple magnetic domains to the previous one appeared when the applied field was removed. In situ Lorentz microscopy studies have demonstrated that magnetic domain structures are sensitive to the crystal structure and/or microstructure in Co–Ni–Al alloys, i.e. a magnetic domain structure favorable to the parent phase is not inherited to the parent phase, but a distinct domain structure is observed in the martensitic phase.     

Defects caused by precipitation of in liquid copper–nickel–aluminium–bronze alloys dissolved carbon and removal by treatment of melt

Castings made of copper–nickel–aluminium bronze (CuAl₁₀Fe₅Ni₅) alloys regularly show defects in the thick, slowly solidifying parts of the castings, which give rise to rejections. Metallographic examination has been made on material of scrap castings showing porosity accompanied with film-like inclusions located beside the iron rich κ_II phases. Investigations of large failed cast structures of copper–nickel–aluminium bronze show the same characteristic defects on which fatigue cracks initiate and grow. Investigation has been made to the nature and the cause of appearance of the film-like inclusions. Microanalysis indicates a high intensity of carbon at the place of the film-like inclusions. Hereafter, an investigation has been made into the solubility of carbon in liquid copper–nickel–aluminium bronze, and it is found that besides hydrogen, also carbon is soluble in copper–nickel–aluminium bronze alloys. The appearance of the carbon as flakes in the fracture surface of materials with defects does suspect there is a nucleating effect on the formation of microporosity causing the defects. To prevent the formation of the casting defects by the interaction between solved hydrogen and carbon, it is necessary to remove the carbon as far as possible by treatment of the melt.     

Premartensitic phenomena and other phase transformations in Ni–Mn–Ga alloys studied by dynamical mechanical analysis and electron diffraction

Ni–Mn–Ga ferromagnetic ordered shape memory alloys are known to exhibit phonon softening and soft mode condensation into a premartensitic phase prior to the martensitic transformation itself. In the present work, this unique behaviour of Ni–Mn–Ga system has been studied as a function of electron concentration (i.e., alloy composition) and a region on the phase transformations diagram which corresponds to the stability of the intermediate phase has been determined, being completely in the ferromagnetic zone. The results were mainly obtained by means of dynamical mechanical analysis and transmission electron microscopy. The two types of lattice instability which might occur in the parent phase driving it to either the soft mode condensed intermediate phase or to the martensitic phase are discussed in this work, together with precursor phenomena and the intermartensitic transformation observed in alloys with the highest electron concentration.     

Temperature memory effect in Cu–Al–Ni shape memory alloys studied by adiabatic calorimetry

The temperature memory effect exhibited by Cu–Al–Ni shape memory alloys was studied by means of adiabatic calorimetry and microscopic observations. The harmonic, anharmonic and electronic contributions to the lattice specific heat were estimated by using the experimental data of the metallic components. The obtained results provide an accurate baseline for the quantitative study of the martensitic phase transformations as a function of the thermal history in these alloys. The specific heat of a Cu–Al–Ni sample was measured from 140 to 350 K throughout the phase transition region, and the temperature memory effect was carefully studied. These results are in good agreement with the optical observations as a function of temperature. The global behaviour of the martensitic transformation as regards the temperature memory effect is discussed and interpreted in terms of the microscopic mechanisms of nucleation and motion of the martensite plates.     

Phase transformations in Au(Fe) nano- and microparticles obtained by solid state dewetting of thin Au–Fe bilayer films

Sub-micrometer-sized particles of Au–Fe alloys were obtained by solid-state dewetting of single-crystalline Au–Fe bilayer films, deposited on c-plane sapphire (α-Al₂O₃) substrates. Depending on the annealing parameters, precipitation of an Fe-rich phase occurred on the side facets of the particles in an interface-limited reaction. Based on the literature values of surface and interface energies in the system, the precipitates were expected to grow inside the Au(Fe) particles, resulting in an (Fe) core–(Au) shell morphology. However, more complex, time-dependent precipitate morphologies were observed, with faceted Fe-rich precipitates attached to the parent faceted Au-rich particles of the same height being dominant at the last stages of the transformation. Our high-resolution transmission electron microscopy observations revealed a nanometric segregation layer of Au on the surface of Fe-rich particles and at their interface with sapphire. This segregation layer modified the surface and interface energies of the Fe-rich particles. A thermodynamic transformation model based on the concept of weighted mean curvature was developed, describing the kinetics of precipitations and morphology evolution of the particles during the dewetting process. Employing the values of surface and interface energies modified by segregation resulted in a good qualitative agreement between theory and experiment.