X-ray orientation microscopy using topo-tomography and multi-mode diffraction contrast tomography

Polycrystal orientation mapping techniques based on full-field acquisition schemes like X-ray Diffraction Contrast Tomography and certain other variants of 3D X-ray Diffraction or near-field High Energy Diffraction Microscopy enable time efficient mapping of 3D grain microstructures. The spatial resolution obtained with this class of monochromatic beam X-ray diffraction imaging approaches remains typically below the ultimate spatial resolution achievable with X-ray imaging detectors. Introducing a generalised reconstruction framework enabling the combination of acquisitions with different detector pixel size and sample tilt settings provide a pathway towards 3D orientation mapping with a spatial resolution approaching the one of state of the art X-ray imaging detector systems.     

Investigations of MX and γ′/γ″ precipitates in the nickel-based superalloy 718 produced by electron beam melting

Samples with a composition similar to the nickel-based superalloy Inconel alloy 718 were produced by electron beam melting of prealloyed powder and investigated with respect to type and composition of the strengthening precipitates. The matrix consists of γ grains orientated in nearly the same direction, almost like a single crystal. Coarse precipitates (<2 μm), mostly of the (Ti,Nb)(C,N,B) type with B1 structure, are aligned along the growth direction. TEM and APFIM investigations of the γ matrix revealed very fine γ″ precipitates of around 5–10 nm in size. Additionally, at small angle grain boundaries, coarser γ″ precipitates of 50–100 nm in size have been observed. The 0 01 γ//0 0 1 γ″ and {1 0 0} γ//{1 0 0} γ″ orientation relationship between γ and γ″, known from literature [M. Sundararaman, P. Mukhopadhyay, Mater. Charact. 31 (1993) 191–196], was confirmed. Some γ′ precipitates of 2–5 nm in size were observed by means of FIM.     

Transmission electron microscopy and X-ray diffraction studies of quantum wells

A series of In _x Ga_1?x As (x=0·47) quantum wells with InP barrier layers have been grown on InP substrates by metalorganic vapour phase epitaxy (MOVPE) at 625°C. The nominal well widths were defined during growth at (i) 25 Å, 39 Å, 78 Å and 150 Å for one sample and (ii) 78 Å for all 4 wells in another sample. The InP barrier widths have been kept constant at 150 Å. These layers have been characterized by X-ray diffraction (XRD) which from simulation gave the nominally 78 Å well width as 84 Å and the nominally 150 Å barrier width as 150·5 Å. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been carried out on etched and ion-milled samples for direct measurement of well and barrier widths. The well widths found from TEM are 25 Å, 40 Å, 75 Å and 150 Å. TEM micrographs revealed that, while the InP barrier layer is of good quality and the growth is confirmed to be epitaxial, dipoles are detected at the interface and the quantum well has some small disordered regions. These thickness measurements are in good agreement with earlier photoluminescence (PL) and secondary ion mass spectrometry (SIMS) studies.     

Tight-binding molecular dynamics simulations of the vibration properties of α-titanium

Phonon thermal anomalies in α-titanium are studied by means of tight-binding microcanonical molecular dynamics simulations. The frequencies of the zone centre [0001]LO and TO phonons, and the [0001]TA phonon at are determined at different temperatures via the power spectrum of the autocorrelation function associated with the corresponding projections of atomic velocities. It is shown that even at very low temperatures the effects of anharmonicity in vibrational properties are strong and dependent on both wave propagation direction and frequency. In particular, the frequencies of the TO and TA phonons are shown to decrease while the frequency of the [0001]LO phonon increases with crystal temperature, in agreement with experiment.     

Study of microstructure in vanadium-palladium alloys by X-ray diffraction technique

Present study considers microstructural characterization of vanadium-based palladium (V-Pd) alloys, which are widely used in marine environment due to their high corrosion resistance. The X-ray diffraction line profile analysis (XRDLPA) have been used to assess the microstructure in body centred cubic (bcc) V-Pd alloys having four different nominal compositions in wt.%. X-ray diffraction line broadening analysis on V-Pd alloys has been performed by using different methods like the Warren-Averbach, double-Voigt and Rietveld methods. Finally microstructural defect parameters such as domain size (D), r.m.s. microstrain 〈ɛ ²〉^1/2, twin fault (β′), spacing fault (αɛ) and deformation stacking fault (α) were evaluated in these alloys by Fourier line shape analysis using Rietveld method in which the X-ray diffraction profiles of these alloys were described by the pseudo-Voigt function to fit the experimental data. From analysis it has been observed that twin fault, β′, and the spacing fault, αɛ, are totally absent in these bcc alloy systems because the twin fault, β′, has been observed to be either negative or very small (within experimental error limit) for these alloy systems and the spacing fault, αɛ, appears to be negative. This analysis also revealed that the deformation stacking fault, α, is significantly present in this alloy system and increases with Pd content.     

Scanning electron microscopy imaging of hydraulic cement microstructure

Use of the scanning electron microscope (SEM) with X-ray microanalysis allows study of clinker and cements; permitting measuring bulk phase abundance and surface areas of the phases, as well as bulk chemistry of constituent phases can be carried out. Direct imaging of hydraulic cements by SEM yields a more complete picture of both bulk and surface phase compositions. Mass percentages obtained by SEM imaging are in good agreement with percentages based upon QXRD and may differ significantly from those estimated by the Bogue calculations. The finer-grained phases (gypsum, tricalcium aluminate, and ferrite) show much higher surface areas per unit mass than the coarser-grained phases such as alite and belite. Such data are being applied to develop better relationships between the cement material properties and performance properties and to provide starting images for a cement hydration simulation model being developed at NIST.     

Microstructures and properties of biomedical TiNbZrFe β-titanium alloy under aging conditions

A metastable β-titanium alloy Ti–28Nb–13Zr–0.5Fe (TNZF alloy for short) was designed for implant biomedical application. The forged specimens were solute-treated at 850 °C followed by water quenching and then aged at 350 °C, 450 °C, and 550 °C for 2–6 h in order to evaluate the effect of phase transformation during ageing on the biomechanical compatibility of the alloy. The quenched microstructure consists of lath α″ martensite and β phase. A large quantities of shuttle-like ω phase precipitate at 350 °C, leading to the drastic increase of strength and elastic modulus and the decrease of plasticity. Ageing at 450 °C for 4 h, small amount of elliptic ω phase and dot α phase precipitate from β matrix. With increasing ageing time α precipitations begin to coarsen and precipitation free zones (PFZs) form around prior β grain boundaries. Needle-like α phase precipitates on grain boundaries and intra-grains when aged at 550 °C. Both PFZs and grain boundary α precipitates are prone to bring about the intergranular fracture and thus have adverse effects on the tensile strength and fracture plasticity. The quenched microstructure has good combination properties of high strength, high plasticity and low elastic modulus.     

The role of microstructure in the mechanical behaviour of Ti-1.6 wt.%Fe alloys containing O and N

The effects of small changes to the heat treatment temperature within the (α + β) phase field on the room temperature properties of a Ti-1.6 wt.%Fe-0.56 wt.%O-0.04 wt.%N alloy are described. To identify contributions from the individual alloying elements the binary Ti-1.6 wt.%Fe and ternary Ti-1.6 wt.%Fe-0.6 wt.%O and Ti-1.6 wt.%Fe-0.04 wt.%N alloys were also investigated. It was found that the interstitial elements affected the degree of disorder in the ω_ath phase, and that the magnitude of this disordering was not merely consistent with changes in Fe concentration. The strength and ductility of the alloys free of additional nitrogen were independent of annealing temperature, whereas the alloys containing nitrogen showed a marked dependency on the temperature. Alloys containing nitrogen displayed a prismatic rather than basal texture after processing.     

Transformations and structures in the Al-Zn-Mg alloy system: A diffraction study using synchrotron radiation and electron precession

The structure of the hardening η′-phase precipitates in Al-Zn-Mg alloys has been investigated by using precession electron diffraction and X-ray synchrotron radiation diffraction. The latter was recorded as a three-dimensional, continuous intensity distribution from a single alloy grain, from which patterns of remarkable sharpness from the precipitate particles could be extracted. High resolution electron microscopy revealed extensive structural variations with prolific faults in the η′ precipitates. The η′-structures are described in terms of two structure models based on two icosahedral elements that are inherent in the equilibrium η-MgZn₂ structure. The role of these icosahedra in the transformations of the alloy system is discussed.     

Nucleation and growth of Fe-rich phases in Al-5Ti-1B modified Al-Fe alloys investigated using synchrotron X-ray imaging and electron microscopy

Plate-like Fe-rich intermetallic phases directly influence the mechanical properties of recycled Al alloys;thus,many attempts have been made to modify the morphology of these phases.Through synchrotron X-ray imaging and electron microscopy,the underlying nucleation and growth mechanisms of Fe-rich phases during the solidification of Al-5Ti-1B-modified Al-2Fe alloys were revealed in this study.The results showed that the Al-5Ti-1 B grain refiner as well as the applied pressure both resulted in reduction of the size and number of primary Al3Fe phases and promoted the formation of eutectic Al6Fe phases.The tomography results demonstrated that Al-5Ti-1B changed the three-dimensional (3D) morphology of primary Fe-rich phases from rod-like to branched plate-like,while a reduction in their thickness and size was also observed.This was attributed to the fact that Ti-containing solutes in the melts inhibit the diffusion of Fe atoms and the Al3Fe twins produce re-entrant corner on the twin boundaries along the growth direction.Moreover,the TiB2 provides possible nucleation sites for Al6Fe phases.The nucleation mechanism of Fe-rich phases is discussed in terms of experimental observations and crystallography calculations.The decrease in the lattice mismatch between TiB2 and Al6Fe phases was suggested,which promoted the transformation of Al3Fe to Al6Fe phases.     

Microwave hydrothermal synthesis and visible-light photocatalytic activity of γ-Bi2MoO6 nanoplates

γ-Bi₂MoO₆ nanoplates have been successfully synthesized by a microwave hydrothermal approach for the first time, by using Bi(NO₃)₃, NH₃ and MoO₃ as the starting materials. The influences of the various preparation parameters (including the pH value, reaction temperature, and holding time) on the phase formation and morphology development were investigated in detail. XRD and TEM results showed that γ-Bi₂MoO₆ nanocrystals were formed indirectly with an intermediate phase of Bi₂O₃ in the microwave hydrothermal and hydrothermal synthesis. However, the reaction condition in the microwave hydrothermal approach was milder compared with the hydrothermal method. The photocatalytic activity of the nanoplates was evaluated by the degradation of the methylene blue solution under visible-light irradiation.     

Structural study by X-ray diffraction and transmission electron microscopy of the misfit compound (SbS1−xSex)1.16(Nb1.036S2)2

In the Sb-Nb-S-Se system, a new misfit layer compound (MSL) has been synthesized and its structure was determined by combining single crystal X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It presents a composite crystal structure formed by (SbS_1−xSe_x) slabs stacking alternately with double NbS₂ layers and both can be treated as separate monoclinic subsystems. The (SbS_1−xSe_x) slabs comprise a distorted, two-atom-thick layer with NaCl-type structure formed by an array of {SbX₅} square pyramids joined by edges (X: S, Se); the NbS₂ layers consist of {NbS₆} trigonal prisms linked through edge-sharing to form sheets, just as in the 2H-NbS₂ structure type. Both sublattices have the same lattice parameters a = 5.7672(19) Å, c = 17.618(6) Å and β = 96.18(3)°, with incommensurability occurring along the b direction: b₁ = 3.3442(13) Å for the NbS₂ subsystem and b₂ = 2.8755(13) Å for the (SbS_1−xSe_x) subsystem. The occurrence of diffuse scattering intensity streaked along c^* indicates that the (SbS_1−xSe_x) subsystem is subjected to extended defects along the stacking direction.     

Structure determination of chitosan-stabilized Pt and Pd based bimetallic nanoparticles by X-ray photoelectron spectroscopy and transmission electron microscopy

Chitosan (CTS)-stabilized bimetallic nanoparticles were prepared at room temperature (rt.) in aqueous solution. Palladium (Pd) and platinum (Pt) were selected as the first metals while iron (Fe) and nickel (Ni) functioned as the second metals. In order to obtain the noble metal core-transition metal shell structures, bimetallic nanoparticles were prepared in a two-step process: the preparation of mono noble metallic (Pd or Pt) nanoparticles and the deposition of transition metals (Fe or Ni) on the surface of the monometallic nanoparticles. The structures of the nanoparticles were studied using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The XPS results show that Pd and Pt exist mainly in zero valences. The presence of Fe and Ni in the bimetallic nanoparticles affects the binding energy of Pd and Pt. Moreover, the studies of O 1s spectra indicate the presence of Fe or Ni shells. The analyses of TEM micrographs give the particle size and size distributions while the high-resolution TEM (HRTEM) micrographs show the existence of noble metal core lattices. The results confirm the formation of noble metal core-transition metal shell structures.     

Effect of β-spodumene on the phase development in an alumina/aluminium-titanate system

The effect of β-spodumene additions on the in situ phase formation and abundances in an Al₂O₃–Al₂TiO₅ system in the temperature range 1000–1400 °C has been studied by neutron diffraction and differential thermal analysis. Results show that β-spodumene began to decompose by phase separation and partial melting at 1290 °C, followed by complete melting at 1330 °C. Formation of Al₂TiO₅ was observed to occur at 1310 °C and its abundance increased with temperature. The addition of β-spodumene as a sintering aid did not cause its reaction with alumina or rutile to form additional phases. Addition of β-spodumene in excess of 5 wt% resulted in pronounced vitrification, which partly recrystallised when cooled to room temperature. The temperatures of Al₂TiO₅ formation and melting of β-spodumene are consistent with the results of differential thermal analysis.     

Probabilistic prediction of minimum fatigue life behaviour in α + β titanium alloys

The objective of this work was to develop and demonstrate a probabilistic life prediction method for the prediction of minimum fatigue lives that are typically used in the design of fracture critical rotating turbine engine components. A Monte Carlo analysis was used to predict the variability in fatigue lives based on the distribution of microstructural features that lead to early crack initiation as well as the variability in small fatigue crack growth rates. Two titanium alloys, both with bimodal microstructures, were tested and analysed in this study. The distribution of critical microstructural features was calibrated based on test results and understanding of microstructure neighbourhood effects. Testing was conducted on both alloys and included both smooth and notched specimens. The predictions are presented and compared with the data for smooth and notch geometries for the various loading conditions. A parametric study was performed to identify the importance of several model inputs and to identify areas for future improvement.     

Damage mode tensile testing of thin gold films on polyimide substrates by X-ray diffraction and atomic force microscopy

In situ tensile testing has been performed on thin gold film, 320 nm thick, deposited on polyimide substrates. During the tensile testing, strain/stress measurements have been carried out by X-ray diffraction using the d-sin²ψ method. The X-ray stress analysis suggests crack formation in the films for stresses greater than 670 MPa. The surface of the deformed specimen observed by atomic force microscopy (AFM) exhibits both cracks and two types of straight-sided buckling patterns lying perpendicular to the tensile axis. These buckling patterns can have a symmetrical or asymmetrical shape. The evolution of these two kinds of buckling structures under tensile stress has been observed in situ by AFM and compared to X-ray stress data. The results indicate that symmetrical straight-sided buckling patterns are induced by the compressive stress during unloading, whereas the asymmetrical result from the delamination of the film during the tensile deformation.     

A combined single crystal X-ray diffraction and electron diffraction study of the T2phase in Al-Li-Cu alloys

Both single crystal X-ray diffraction techniques and convergent beam electron diffraction have been employed to examine the structure of theT ₂ (Al₆CuLi₃) phase in three particular Al-Li-Cu alloys. It is shown thatT ₂ displays icosahedral symmetry both in a high purity laboratory melt and in two impure alloys which had been processed industrially. Possible reasons for the five-fold symmetry ofT ₂ are discussed.     

Study of the hot deformation behaviour in Ti-5Al-5Mo-5V-3Cr-1Zr

Structural applications of near beta titanium alloys are gradually increasing in the aerospace industry because of their high specific mechanical properties and good corrosion resistance. Furthermore, a wide range of microstructures can be obtained by thermomechanical processes. This work determines by the use of EBSD technique the mechanism of restoration active in the near beta titanium alloy Ti-5Al-5Mo-5V-3Cr-1Zr for deformations in both α + β and β field near to the β transus temperature (T_β = 803 °C). Hot compression tests are carried out up to 0.7 true strain by means of a Gleeble^® 1500 machine at strain rates of 0.01, 0.1 and 1 s⁻¹. Dynamic recovery of β phase and rotation of the α grains take place predominantly in the α + β field. Further deformation produces continuous dynamic recrystallization of the β phase influenced by the strain rate. Dynamic recovery is observed during deformation above the T_β, where the misorientation is increasing towards the grain boundaries forming new small grains with a substructure at high strain rates and larger deformation. The stress exponent and the apparent activation energy for the sinh constitutive equations are determined and the microstructural features are correlated with the Zener-Hollomon parameter.