Atom Location By Channeling Enhanced Microanalysis Research Articles

Atom Location by Channeling-Enhanced Microanalysis (ALCHEMI) Study of Structure of Complex Concentrated Alloys Derived From FeAl Iron Aluminide

Atom Location by Channeling-Enhanced Microanalysis (ALCHEMI) Study of Structure of Complex Concentrated Alloys Derived From FeAl Iron Aluminide

Automating ALCHEMI at the nano-scale using software compatible with PC-controlled transmission electron microscopy.

Atom location by channeling-enhanced microanalysis (ALCHEMI) is a technique to obtain atom-site-specific information on constituent elements in a crystalline sample by acquiring a set of core electron transition spectra while tilting the incident beam. This methodology has been extended to a more quantitative technique called high-angular-resolution electron-channeled X-ray/electron spectroscopy (HARECXS/HARECES). There is a growing demand for analyzing smaller areas, such as small particles and multilayers. However, the minimum size of a region of interest probed by the present hardware-assisted automated HARECXS/HARECES scheme is limited to no smaller than 1 µm, not only by the size of the electron probe and its convergence angle but also by the movement of the probe position associated with the beam tilt due to aberrations of the hardware system. Herein, QED (quantitative electron diffraction), a commercial plug-in working on an integrated software platform, Gatan Microscopy Suite, was modified to enable the calibration and control of the probe to resolve the aforementioned limitation. In addition, a more sophisticated scheme for QED was developed to realize the ALCHEMI method for energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy or both concurrently. This allows access to ALCHEMI and its derivative methods, automatically executed with any type of current PC-controlled commercial microscope on an area as small as 30 nm, without modifying the hardware system.

Atomic site occupancy of alloying elements and Laves phase stability in γ-γ′ Co-base superalloys

The atomic site occupancy of alloying elements in the ZrCo2 and HfCo2 based Laves phases formed in the Co-9Al-9W-2Zr and Co-9Al-9W-2Hf alloys was experimentally determined by the combined techniques of energy-dispersive X-ray spectroscopy mapping with atomic resolution and the atom location by channeling enhanced microanalysis (ALCHEMI) method. The results show that both Al and W occupy the Zr site in the C15 and the Hf site in the C36 crystal structure. The atomic locations of Al and W were further confirmed by comparing calculated inelastic cross-sections with experimental ALCHEMI results. Using the determined atomic site occupancies of alloying elements, special quasirandom structure solid solution models with 192 atoms were constructed and employed in first-principles calculations. It is found theoretically that the energy of formation of the C15 structure is always lower than that of the C36 structure at 0 K in both ZrCo2 and HfCo2 phases, no matter whether Al and W are incorporated or not. However, further ab initio molecular dynamics simulations suggest that lattice vibration at finite temperature contributes significantly to the phase stability, stabilizing the C36 structure compared to the C15 type for the (Hf, Al, W)Co2 phase at finite temperature, which fits well with the experimental findings.

Site occupancy of alloying elements in the L12 structure determined by channeling enhanced microanalysis in γ/γ’ Co-9Al-9W-2X alloys

Knowledge about the sublattice site preference of alloying elements in the L12-γ′ phase of novel Co-base superalloys is a necessary pre-requisite to understand their influence on the properties of the alloys in general and the γ′ phase in particular. In the present study, the atomic site occupancy of the alloying elements in the L12-γ′ structure in Co-9Al-9W-2X quaternary alloys after long-term annealing at 900 °C for 5000 h was determined using the atom location by channeling enhanced microanalysis (ALCHEMI) technique in combination with energy-dispersive X-ray spectroscopy (EDX) composition analysis in a transmission electron microscope (TEM). The experimental ALCHEMI data were evaluated by comparing them with those calculated by the program ‘Inelastic Cross Section Calculator’ (ICSC). The results show that Co mainly occupies one sublattice site and Al/W are located at the other sublattice site in the L12 unit cell in the ternary alloy. The additional elements Ti, V, Mo and Ta which partition strongly to the γ′ phase tend to occupy the Al/W sublattice site, and Cr which partitions more to the γ phase also favors the Al/W sublattice site, while Ni weakly partitions into the γ′ phase and favors the Co sublattice site. The results of this study can provide evidence to the predictions on the site preference in literature based on the phase composition or on theoretical studies.

ALCHEMI studies of site occupancies in Cr-, Ni-, and Fe-substituted manganese cobaltite spinels

The effects of Cr, Ni, and Fe substitution into manganese cobaltite (MCO) spinels are of great interest due to the roles that the diffusion of these cations play in reaction layer development during high temperature exposure of MCO-coated alloys. Here we report a study on a series of model Cr-, Ni-, and Fe-substituted MCO spinel ceramics produced by consolidation of combustion-synthesized oxide powders. The cation site occupancies in these samples have been studied by X-ray spectrometry-based Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI) experiments in the transmission electron microscope, with the data being analyzed using the ordering tie-line approach. In Cr-substituted samples, the Cr ions lie on the octahedral B sites and the Co ions reside on the tetrahedral A sites with Mn occupying the remaining sites. In Ni-substituted samples, all of the Ni ions occupy the B sites and the Co and Mn ions tend to lie on A and B sites, respectively. In contrast to the Cr-substituted samples, there is some mixing of the Co and Mn ions on the two types of sites at lower Ni contents. In Fe-substituted samples with lower Fe contents, all of the Mn ions occupy the B sites, roughly equal proportions of Fe ions occupy the A and B sites, and Co ions fill the remaining sites. With increasing Fe content, the degree of order decreases which ultimately results in the High Fe sample exhibiting no channeling evidence for preferred site occupation. These ALCHEMI data could provide a useful insight into the role of cation sub-lattice site preference in the formation of reaction layers in MCO-coated stainless steels and superalloys.

(Invited) Capabilities of Analytical Transmission Electron Microscopy for the Analysis of Structural, Chemical and Electronic Properties Exemplified by the Study of Y-Doped (Ba,Sr)(Co,Fe)O3-δ

Transmission electron microscopy (TEM) is a standard technique for microstructure studies with a resolution that meanwhile has reached values better than 0.1 nm. Essential is the capability of obtaining images and diffraction patterns from the same small specimen region which facilitates crystal structure determination of small precipitates. Particularly unique is the possibility to perform atomic-scale chemical analyses by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). Information on the electronic material properties like, e.g., valence states or bond configurations between neighbor atoms is obtained by the electron near edge structure of ionization edges in EELS spectra. In addition to these well-known capabilities, many more imaging modes and evaluation techniques are available which yield additional material properties and render electron microscopy an extremely powerful technique. We will illustrate the capabilities of analytical TEM for the example of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) which is a promising and highly complex material for oxygen separation membranes. BSCF suffers from degradation of the oxygen diffusivity at temperatures below 840 oC which is attributed to the decomposition of the cubic phase into several secondary phases [1,2]. Composition and crystal structure analyses of the different phases combined with Co-valence state determination sheds light on the origin of the instability of the cubic phase. Indispensable is the determination Co-valence state in the cubic and hexagonal BSCF phases which required the introduction of a new evaluation technique due to the overlap of the Co-L2,3ionization edges with ionization edges of other elements in BSCF [3]. Doping with Yttrium has been used to stabilize the cubic Sr1−x Y x Co1−y Y y O3−δ phase [4] which also appears to be a promising strategy for BSCF. Substitution of B-site Co- and Fe-cations in BSCF by Y3+-cations with a fixed valence state should lead to the stabilization of the Co-valence and, hence, the stabilization of the cubic BSCF phase. The volume fraction of secondary phases is indeed substantially reduced in Y-doped BSCF. Only the hexagonal BSCF phase is observed which is confined to grain boundaries (Fig. 1). In contrast, a large volume fraction of different secondary phases is found in undoped BSCF (Fig. 2). A closer inspection suggests, however, that the suppression of secondary phases in Y-doped samples could be partially related to the lack of nucleation sites rather than the stabilizing effect of Y-doping. To achieve an improved understanding of the effect of Y-doping, the occupation sites of Y-cations were analyzed by atom location by channeling enhanced microanalysis (ALCHEMI) as proposed by Spence and Taftø [5]. ALCHEMI analyses demonstrate that Y-cations occupy A- and B-sites. Despite substitution of B-site cations, 25 % to 40 % of the Y-cations occupy A-sites depending on the doping level. This indicates the complex effect of Y-doping in BSCF which will be discussed.

(Invited) Capabilities of Analytical Transmission Electron Microscopy for the Analysis of Structural, Chemical and Electronic Properties Exemplified by the Study of Y-Doped (Ba,Sr)(Co,Fe)O3-δ

The effect of Y-doping on secondary phase formation was studied in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ to avoid degradation of the oxygen permeation in this material. B-site cations were substituted by up to 10 % Yttrium. The microstructure was studied by analytical transmission electron microscopy. This also includes the investigation of A- and B-site occupation by Y-cations using atom location by channeling enhanced microanalysis (ALCHEMI).

Ordering and site occupancy of D03 ordered Fe3Al–5 at%Cr evaluated by means of atom probe tomography

Addition of ternary elements to the D0(3) ordered Fe(3)Al intermetallic phase is a general approach to optimise its mechanical properties. To understand the physical influences of such additions the determination of the probability of site occupancies of these additions on the lattice site and ordering parameters is of high interest. Some common experimental techniques such as X-ray diffraction or Atom Location by Channelling Enhanced Microanalysis (ALCHEMI) are usually applied to explore this interplay. Unfortunately, certain published results are partly inconsistent, imprecise or even contradictory. In this study, these aspects are evaluated systematically by atom probe tomography (APT) and a special data analysis method. Additionally, to account for possible field evaporation effects that can falsify the estimation of site occupancy and induce misinterpretations, APT evaporation sequences were also simulated. As a result, chromium occupies most frequently the next nearest neighbour sites of Al atoms and local ordering parameters could be achieved.

Characterization of Al-Si ordering state in an alkali feldspar using atom location by channeling-enhanced microanalysis (ALCHEMI)

Atom location by channeling-enhanced microanalysis (ALCHEMI) was used to determine the occupancy of Al atoms in the T1o site (t1o) of alkali feldspar. Building on the method demonstrated by previous research, analytical electron microscopy proves to be a viable technique for fully characterizing the Al-Si ordering state in the feldspar framework. We applied this method to orthoclase from Itrongay, Madagascar, and to its heated counterpart. Our preliminary results give 0.74 and 0.37 as 2t1 (=t1o + t1m) and t1o, respectively, for the original orthoclase, vs. 0.54 and 0.28 for heated orthoclase. Single-crystal X-ray diffraction experiments have been performed, and the results agree with our ALCHEMI measurements. This new method promises to help resolve some complex issues relating ordering paths, for example, twinning in feldspars and domain intergrowth in plagioclase.

Neutron irradiation effect on site distribution of cations in non-stoichiometric magnesium aluminate spinel

Neutron irradiation effects on cation distribution in non-stoichiometric Mg–Al spinel were examined by ALCHEMI (Atom Location by Channeling Enhanced Microanalysis) method. Parameter n, or non-stoichiometry of MgO · nAl 2O 3 of the specimens, were n = 1.00, 1.01, 1.10, 1.48. These specimens were neutron-irradiated up to a fluence of 2.3 × 10 24 n/m 2 ( E > 0.1 MeV) at 500–530 °C in JMTR. Some specimens contracted by the irradiation and the arrangement of cations became more disorder. The other specimens showed very small swelling by the irradiation and the cation distribution became slightly ordered. The cation distribution of the contracted specimen returned stepwise to the pre-irradiated condition after the annealing at 700 °C. The cation distribution of the slightly swollen specimens did not change after the annealing up to 700 °C. Cation distribution in the T-site was more sensitively influenced by the irradiation.

Co location and valence state determination in ferromagnetic ZnO:Co thin films by atom-location-by-channeling-enhanced-microanalysis electron energy-loss spectroscopy

The Co valence state and the location of Co dopant atoms in ZnO:Co thin films revealing anomalous Hall effect below 50K have been determined by electron energy-loss spectroscopy (EELS) measurements and atom location by channeling enhanced microanalysis (ALCHEMI), respectively. The method of ALCHEMI-EELS to investigate the dopant in materials was brought. It was used to determine that the intrinsic ferromagnetism in ZnO:Co thin films derives from the +2 Co which substitute for Zn atoms at Zn sites in the ZnO matrix. The divalent Co state in ZnO:Co has been unambiguously proven by modeling the corresponding EELS data.

Incorporation of dopants into the lattice of ZnO nanoparticles to control photoactivity

Using a similar production process, a series of doped and undoped ZnO nanoparticles has been tested for photoactivity, using chemical methods, when exposed to UV radiation. Parameters such as crystallite size, dopant type and concentration were considered. It was confirmed that photoactivity increased significantly by a factor of 2 to 3 as the mean crystallite size was decreased from 100 to 20 nm. Analysis of a further series of materials doped with metal cations showed that, depending on the dopant type and concentration, photoactivity could be reduced by more than an order of magnitude. A variant of the method of atom location by channelling-enhanced microanalysis (ALCHEMI) was applied to these crystal systems to determine whether or not dopant atoms at a concentration of less than 1 at.% were incorporated on the cation sublattice site within the nanoparticles. This technique is described and discussed in some detail.

Site occupancies, point defect concentrations, and solid solution hardening in B2 (Ni,Fe)Al

A novel approach was used to characterize the site occupancies and point defect concentrations in the B2-ordered (Ni,Fe)Al single phase field over a wide range of composition and temperature. This approach combined atom location by channeling enhanced microanalysis (ALCHEMI) determinations of atomic site occupancies and vacancy concentration measurements. The triple defect structure was observed across the entire phase field. Qualitative thermodynamic predictions such as that Fe anti-site are more stable than Ni anti-sites were confirmed. However, in some portions of the phase field it was found that Ni anti-site concentrations were higher than expected from simplistic thermodynamic predictions of site preference. Such predictions are clearly inadequate if a quantitative determination of site occupancies is desired. The importance of this type of characterization in interpreting solid solution hardening (SSH) in ordered compounds was illustrated by considering three portions of the (Ni,Fe)Al phase field. In all three cases the solute atoms did not directly harden, but affected the hardness by altering the concentrations of other point defects.

Site occupancy preferences in the B2 ordered phase in Nb–rich Nb–Ti–Al alloys

The β-(Nb, Ti) solid solution phase in the Nb–Ti–Al system has recently been observed to extend to Al concentrations as high as 40 at.%. Five alloys with compositions (in at.%) of Nb– xTi–15Al, where x=17, 28.3 and 34, Nb–25Ti–25Al and Nb–24Ti–40Al were investigated. The β-transus temperatures, lattice parameters and β → B2 (β o) ordering temperatures are reported. Site occupancy preferences of the B2 lattice as a function of alloy composition were examined through the atom location by channeling enhanced microanalysis (ALCHEMI) technique with results presented as ordering tie lines. Ti substitution for Nb on Nb sublattice sites was observed in all of the alloys, with the strength of partitioning between Ti and Al atoms being dependent on alloy composition. A correlation between the partitioning behaviors of the atomic species and the Nb:Ti ratio of the alloy compositions was observed, along with changes in the lattice parameters and microhardness.

Hardening by Point Defects and Solutes in B2 Intermetallics

ABSTRACTThis paper provides a review of recent progress on point defect and solute hardening in binary and ternary B2 intermetallics. As is the case for disordered metallic solutions, the presence of point defects and solute atoms in ordered intermetallic compounds results in solid solution hardening (SSH). However, factors unique to ordered systems are often responsible for unusual hardening effects. Binary compounds with identical crystal structures can exhibit significantly different hardness behavior. Ternary solute additions to ordered compounds can give rise to apparent solid solution softening as well as unexpectedly rapid hardening. These effects arise from the interaction of multiple defect types as well as the presence of multiple sublattice sites available for solute occupation. Therefore, before the SSH behavior of ordered intermetallics can be properly studied, it is necessary to develop an understanding of the types and quantities of the point defects which are present. Three recent studies by the authors are reviewed. Much of the work was done on NiAl and FeAl in binary form as well as with ternary additions. Defect concentrations over wide ranges in alloy composition and quenching temperature were determined using the ALCHEMI (atom location by channeling enhanced microanalysis) technique combined with vacancy measurements. Hardness values were also measured. It was found that most of the observed SSH effects could be rationalized on the basis of the measured point defect concentrations.

How accurate is ALCHEMI for ordering measurements?

Atom location by channeling-enhanced microanalysis (ALCHEMI) promises a straightforward measurement of ordering in materials. On tilting the electron beam in a transmission electron microscope (TEM) about a superlattice Bragg position the electron channeling changes, weighting X-ray production from each of the sublattices differently. The overall X-ray signal, converted to an apparent composition via a kinematical non-channeling standardization, moves between the composition of the constitute sublattices. However, the accuracy of measurement always bothers many researchers. Results may vary from time to time, and in the worse cases, the composition outputs are nonsense. Table 1 gives an example of a series of ALCHEMIC measurements in a B2 Ti50Al42Mo8 intermetallic alloy.In general, two major sources cause errors in ALCHEMI: weak channeling and delocalization effects. The later only results in “systematic errors”, which can always be corrected or reduced by theoretical calculations. However, it is difficult to improve the error due to weak channeling.

Effect of alloying additions on β′ precipitation in NiAl–Ti base alloys

The precipitation of β′–Ni 2AlTi is investigated in NiAl–3Ti– X ( X=Cu, Hf) alloys to determine the effects of alloying additions. Hf enhanced the precipitation of β′ and both Ti and Hf were found to partition to the β′ phase. Atom Location by Channeling Enhanced Microanalysis (ALCHEMI) results indicate that Hf has a strong preference to reside on Al sites. Cu was found to enhance β′ precipitation, but only when added to replace Ni in the composition. In this case, the β′ phase was enriched in Ti but not Cu. By relating the stability of the phases with nearest neighbor pair interactions, it is shown that Hf increases the stability of β′ phase, while Cu decreases the stability of the matrix β phase relative to β′.

Phase equilibria and solid state transformations in Nb-rich Nb–Ti–Al intermetallic alloys

The solidification pathways and phase equilibria at 1100, 900 and 700°C in seven Nb-rich alloys of the Nb–Ti–Al system were examined. The as-cast microstructures revealed that all alloys had undergone β solidification, extending the range of the β phase field further into the ternary system than previously reported. Changes in the atomic site occupancy preferences with alloy composition were examined for the B2 ordered β0 phase through atom location by channeling-enhanced microanalysis (ALCHEMI). A β to δ massive-like transformation observed in cast material, was reproduced in continuously cooled homogenized samples. The formation of a new phase, θ, based on the I41/amd body centered tetragonal structure was observed below 1100°C. In addition, a metastable hP18 structure, produced through an ω-type transformation of the β0 phase, was also observed.

Atomic scattering factors for K-shell and L-shell ionization by fast electrons

Atomic scattering factors have been calculated for K-shell ionization for elements in the range Z = 6 (carbon) to Z = 50 (tin) and for L-shell ionization in the range Z = 20 (calcium) to Z = 60 (neodymium). The calculations are based on relativistic Hartree-Fock wave functions for the atomic bound states and Hartree-Slater wave functions for the continuum wave functions. The results are presented in tabular form such that accurate values of the scattering factors can be obtained by cubic spline interpolation for incident electron energies between 50 and 400 keV and for scattering vectors with magnitude s = sin straight theta / lambda up to 2.5 A(-1) (2straight theta is the scattering angle and lambda the wavelength of the incident electrons). A separate parameterization of the form factors is given for 2.5 </= s </= 20 A(-1), where they are small. In addition, a simpler but less accurate parameterization of the atomic scattering factors in an exponential form has been obtained by fitting the calculated form factors in the region s </= 2.5 A(-1). The scattering factors are suitable for the calculation of ionization cross sections for use in atom location by channelling-enhanced microanalysis (ALCHEMI).

An experimental and theoretical study of bonding mechanisms in ternary γ-TiAl + X (X = Cr, Mn, Nb) alloys

The local atomic order and the electronic structure of γ-(Ti 46 Al 54 ) 97 X 3 (X = Cr, Mn, Nb) alloys have been investigated by Electron Energy Loss (EELS) and X-ray absorption (XAS) spectroscopies, The substitution sites of solute atoms have been determined experimentally by Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI), Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) in the framework of a multiple scattering (MS) formalism. Furthermore, EXAFS provides a way to probe the local relaxation around the atom of interest. A strong elastic relaxation is found to occur around Mn solute atoms, while Nb solute atoms generate almost no elastic strain. From a calculation in the band structure (BS) formalism, the influence of the temary solute atom on charge density distribution has been studied and has shown that the addition elements mainly improve the intensity of the d-d bonds within the (001) planes. All these results are consistent with the previously observed mechanicai properties and dislocation microstructures.