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.     

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.     

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.     

Microdefects and electron densities in NiTi shape memory alloys studied by positron annihilation

1Introduction NiTi alloys are the most successful shape memory alloys as a result of their combination of good functional properties and excellent mechanical strength[1,2].The thermal and mechanical shape memory behavior in these alloys is dependent upon …     

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.     

Nb-Poor Fe–Nb–B nanocrystalline soft magnetic alloys with small amount of P and Cu prepared by melt-spinning in air

The effect of the addition of P and Cu to Fe₈₅Nb₆B₉ alloy on an as-quenched structure and soft magnetic properties in a nanocrystallized state has been investigated. The Fe₈₅Nb₆B₉ alloy melt-spun in air has an as-quenched structure of an amorphous phase and α-Fe grains with 20–45 nm in size. The coarse grains should still remain in the nanocrystallized structure, which deteriorates the soft magnetic properties. The simultaneous addition of 1 at.% P and 0.1 at.% Cu to the Fe₈₅Nb₆B₉ alloy decreases the α-Fe grain size to nanoscale in an as-quenched state, and realizes a uniform crystallized structure with high saturation induction of 1.61 T as well as high permeability of 41,000.     

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.     

Secondary precipitation in Al–Zn–Mg–(Ag) alloys

Secondary ageing of age-hardenable aluminium alloys occurs at temperatures below the solvus of GP zones after a preliminary ageing at a higher temperature. The phenomenon has technological interest, as it may be included in heat treatments giving a substantial benefit on the mechanical properties. In the present work, positron annihilation lifetime spectroscopy (PALS) is applied in combination with Vickers hardness measurements for an investigation on secondary ageing of Al–4wt.%Zn–3wt.%Mg–xAg, where x=0, 0.1, 0.2, 0.3, 0.5 wt.%. Ageing regimes have been characterised by the substantially different evolutions that are observed. The results shed light on the interplay between the formation of coherent solute aggregates (clusters or GP zones) and the precipitation of semi-coherent or incoherent precipitates, which are in competition to control the hardening effects. PALS data show that secondary ageing in the ternary Al–Zn–Mg alloys produces coherent aggregates even in the presence of a well-developed stage of semi-coherent or incoherent precipitation that is obtained if the alloys are first aged to peak hardness. In the presence of Ag, on the contrary, the effects of coherent aggregation during secondary ageing are observed only if the preliminary ageing is interrupted well before reaching peak hardness.     

Texture changes in the hcp → bcc → hcp transformation of zirconium studied in situ by neutron diffraction

The crystallographic texture of hot-rolled polycrystalline zirconium has been studied below and above the hcp–bcc transition temperature with HIPPO, the new time-of-flight neutron diffractometer at Los Alamos Neutron Science Center, making use of the multidetector capabilities and a vacuum furnace. Incomplete pole figures were extracted from diffraction spectra to determine the orientation distribution function and recalculate complete pole figures in situ at various temperatures. The texture analysis reveals that the orientation of grains in the new high-temperature (bcc) phase is related to the texture of the low-temperature (hcp) phase by Burgers relation, but with both an orientation selection and a symmetry variant selection. The cubic transformation texture is best explained if we assume preferential nucleation of the bcc phase in the hcp grain orientations that are most subject to mechanical twinning. After cooling, the new hcp texture closely resembles the original texture. Thermal cycling repeats this process with slight strengthening of the texture. The hexagonal transformation texture (after cooling) may be caused by nucleation and growth of untransformed domains or through variant selection by stresses imposed by neighboring grains.     

Two-phase growth in peritectic Fe–Ni alloys

Bridgman crystal growth experiments were carried out to investigate the solidification behavior of Fe–Ni alloys containing nominally between 4 and 4.5 at.% Ni. Due to macrosegregation, a radial concentration gradient was established across the cylindrical specimens. Due to this gradient, a series of solid/liquid interface morphologies was observed. Oriented two-phase microstructures, which formed either lamellar or fibrous δ-ferrite in an austenite (γ) matrix, were found in the central region of specimens with a composition of some 4.2 at.% Ni and a G/V ratio close to the critical ratio for solid/liquid interface breakdown. At slightly smaller concentrations, oscillatory two-phase structures formed which were similar to the 2-λ instabilities of off-eutectic alloys. The observations confirm that at low solidification rates the stable growth morphology in peritectic alloys cannot be selected by the highest growth temperature criterion. A recently developed nucleation and constitutional undercooling criterion (NCU) was applied to establish a solidification microstructure selection map. Reasonable agreement was obtained between calculated and experimental results. Based on eutectic growth theory the possibility of simultaneous two-phase growth in peritectic alloys is discussed.     

Nanocrystalline Fe–C alloys produced by ball milling of iron and graphite

A series of nanocrystalline Fe–C alloys with different carbon concentrations (x_tot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing x_tot. Eventually the size reaches a lower limit of about 6 nm in alloys with x_tot > 6.19 at.% (1.40 wt.%); a further increase in x_tot leads to the precipitation of carbon as Fe₃C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe₃C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy.     

Elastoplastic deformation of Al/SiCp metal–matrix composite studied by self-consistent modelling and neutron diffraction

Neutron diffraction was used to measure the lattice strains in Al/SiC_p metal–matrix composite under an external load applied while the sample was in situ in the neutron beam. The evolution of the internal stresses and of the critical-resolved shear stress during bending were predicted by elastoplastic models. Calculations based on these models were verified by comparison with the results of the diffraction experiment. It was found that the self-consistent model correctly predicts the distribution of stresses between the two phases of the Al/SiC_p composite. Finally, the parameters characterising elastoplastic deformation of the Al–matrix were determined.     

Relationship between microstructure,properties and reaction conditions for Cu–TiB2 alloys prepared by in situ reaction

Three Cu–TiB₂ alloys (0.45, 1.6, 2.5 wt.% TiB₂) have been prepared by a combination of in situ reaction and rapid solidification under optimized conditions. The relationship between microstructure, properties and in situ reaction condition for these Cu–TiB₂ alloys was investigated and analyzed by modeling. It is shown that the distribution and size of TiB₂ particles are strongly dependent on the choice of in situ reaction conditions and solute concentration; specifically, the size and aggregation level of TiB₂ particles tend to increase as the volume per cent of TiB₂ in the three Cu–TiB₂ alloys increases when the same in situ reaction conditions are used. The forming, coarsening and aggregation models of TiB₂ particles are established. The mechanical properties of three Cu–TiB₂ alloys are directly related to the distribution and size of TiB₂ particles, the change in the grain size and the residual solute concentration. A composite model reveals the separate contributions of these parameters to the mechanical properties.