Karen L. Torres

Gallium-enhanced phase contrast in atom probe tomography of nanocrystalline and amorphous Al–Mn alloys

Over a narrow range of composition, electrodeposited Al-Mn alloys transition from a nanocrystalline structure to an amorphous one, passing through an intermediate dual-phase nanocrystal/amorphous structure. Although the structural change is significant, the chemical difference between the phases is subtle. In this study, the solute distribution in these alloys is revealed by developing a method to enhance phase contrast in atom probe tomography (APT). Standard APT data analysis techniques show that Mn distributes uniformly in single phase (nanocrystalline or amorphous) specimens, and despite some slight deviations from randomness, standard methods reveal no convincing evidence of Mn segregation in dual-phase samples either. However, implanted Ga ions deposited during sample preparation by focused ion-beam milling are found to act as chemical markers that preferentially occupy the amorphous phase. This additional information permits more robust identification of the phases and measurement of their compositions. As a result, a weak partitioning tendency of Mn into the amorphous phase (about 2 at%) is discerned in these alloys.

The influence of voxel size on atom probe tomography data

A methodology for determining the optimal voxel size for phase thresholding in nanostructured materials was developed using an atom simulator and a model system of a fixed two-phase composition and volume fraction. The voxel size range was banded by the atom count within each voxel. Some voxel edge lengths were found to be too large, resulting in an averaging of compositional fluctuations; others were too small with concomitant decreases in the signal-to-noise ratio for phase identification. The simulated methodology was then applied to the more complex experimentally determined data set collected from a (Co(0.95)Fe(0.05))(88)Zr(6)Hf(1)B(4)Cu(1) two-phase nanocomposite alloy to validate the approach. In this alloy, Zr and Hf segregated to an intergranular amorphous phase while Fe preferentially segregated to a crystalline phase during the isothermal annealing step that promoted primary crystallization. The atom probe data analysis of the volume fraction was compared to transmission electron microscopy (TEM) dark-field imaging analysis and a lever rule analysis of the volume fraction within the amorphous and crystalline phases of the ribbon.

Field evaporation behavior in [0 0 1] FePt thin films

Though the atom probe has provided unprecedented atomic identification and spatial imaging capability, the basic reconstruction assumption of a smooth hemispherical tip shape creates significant challenges in yielding high fidelity chemical information for atomic species with extreme differences in fields required for field evaporation. In the present study, the evaporation behavior and accompanying artifacts are examined for the super-cell lattice structure of L1(0) FePt, where alternating Fe and Pt planes exist in the [0 0 1] orientation. Elemental Fe and Pt have significant differences in field strengths providing a candidate system to quantify these issues. Though alloys can result in changes in the elemental field strength, the intrinsic nature of elemental planes in [0 0 1] L1(0) provides a system to determine to what extent basic assumptions of elemental field strengths can break down in understanding reconstruction artifacts in this intermetallic alloy. The reconstruction of field evaporation experiments has shown depletion of Fe at the (0 0 2) pole and zone axes. Compositional profiles revealed an increase in Fe and atom count moving outward from the pole. The depletion at the low indexed pole and zone axes was determined to be the result of local magnification and electrostatic effects. The experimental results are compared to an electrostatic simulation model.

Correlation between TEM Imaging and Microanalysis for Atom Probe Reconstruction Verification

The atom probe instrument field evaporates atoms from a specimen of interest and these atoms (now ions) are collected on a position-sensitive, mass-spectrum detector. By reconstructing the trajectory path and impact position of each ion from the field evaporation event, a volumetric reconstructed rendering of the material is generated with near atomic precision for each individual atom. The reconstruction method of an atom probe volume is dependent upon a constant evaporation field [1]. When the evaporation process proceeds through an interface of two different phases, the field can change resulting in aberrations in the atom probe reconstruction. These aberrations typically appear as density variations across interfaces and/or incorrect morphologies of precipitates within the matrix [2]. To help validate the atom probe reconstructions, TEM imaging and microanalysis can be employed. This proceeding addresses specific examples where the coupling of TEM can assist in the validation of the atom probe reconstruction. In addition, the proceeding addresses some experimental difficulties in bridging the two microscopy techniques.

Comparison of Simulated and Experimental Order Parameters in FePt—II

Eight FePt thin film specimens of various thicknesses, compositions, and order parameters have been analyzed to determine the robustness and fidelity of multislice simulations in determining the chemical order parameter via electron diffraction (ED). The shape of the simulated curves depends significantly on the orientation and thickness of the specimen. The ED results are compared to kinematical scattering order parameters, from the same films, acquired from synchrotron X-ray diffraction (XRD). For the specimens analyzed with convergent beam electron diffraction conditions, the order parameter closely matched the order parameter as determined by the XRD methodology. However, the specimens analyzed by selected area electron diffraction conditions did not show good agreement. This has been attributed to substrate effects that hindered the ability to accurately quantify the intensity values of the superlattice and fundamental reflections.

Penetration of Fine Foundry Sand by Rigid Projectiles

In another paper [1], the authors presented an approach to penetration of a particulate target. This theory is based on the friction that the particles of target material present to the entire penetrator surface, including its shank. The shank of the penetrator affords a very large surface area compared to that of the nose. Even modest friction acting on the shank can provide a fairly large retarding force. Normal pressure acting on the projectile is assumed to be velocity-squared dependant, as indicated by a number of methods, including cavity expansion modeling [2]. Penetration of sand and soil has been considered by numerous investigators, e.g. [3–5]. These investigations did not directly address the frictional component of the net resisting force acting on the penetrator. A series of laboratory scale penetrations tests were performed. Data from these tests was used to evaluate the parameters in the model. Fine foundry sand is a high-density medium (1960 kg/m3 ) with a small amount of friction. This contrasts the target used by the authors in [1], which had a sizable amount of friction. Results from the theory are in excellent agreement with the experiments with velocities as high as 630 m/sec.Copyright

Dynamical diffraction simulations in FePt--I.

A series of multislice simulations to quantify the effect of various degrees of order, composition, and thickness on the electron diffracted intensities were performed using the L1₀ FePt system as the case study. The dynamical diffraction studies were done in both a convergent electron beam diffraction and selected area electron diffraction condition. The L1₀ symmetry demonstrated some peculiar challenges in the simulation, in particular between the {111} plane normal and the <111> direction, which are not equivalent because of tetragonality. A hybrid weighting function atom of Fe-Pt was constructed to account for S < 1 or nonequiatomic compositions. This statistical approach reduced the complexity of constructing a crystal with the probability that a particular atom was at a particular lattice site for a given order parameter and composition. Considerations of accelerating voltage, convergent angle, and thermal effects are discussed. The simulations revealed significant differences in intensity ratios between films of various compositions but equivalent unit cell numbers and degree of order.

Taylor Cylinder Testing of High Strength Steels for Hard Target Warhead Applications

A one-dimensional analysis of the Taylor impact test [4] has been used to estimate the quasi-static stress for several different alloys. One criticism of this work was the use of Taylor cylinder test data to estimate the quasi-static true stress/true strain compression diagram. The one-dimensional theory does accommodate this estimate. The purpose of this paper is to demonstrate that this process leads to acceptable results by analyzing a series of high, medium, and low strength materials.Copyright

Taylor Cylinder Test Reduction Using a One-Dimensional Theory

A one-dimensional analysis of the Taylor impact test [4] has been used to estimate the quasi-static stress for several different alloys. One criticism of this work was the use of Taylor cylinder test data to estimate the quasi-static true stress/true strain compression diagram. The one-dimensional theory does accommodate this estimate. The purpose of this paper is to demonstrate that this process leads to acceptable results by analyzing a series of high, medium, and low strength materials.Copyright

Atom Probe Tomography Studies of FePt Thin Films

FePt films deposited directly onto Si <001> wafers adopt a strong {111} fiber texture as well as the A1 disordered phase However if FePt films are deposited onto a Introduction Th ti bit f l t d it h d d t i Focus Ion Beam ‘lift-out’ of an Atom Probe Tip: T i t i th t ti f th t b d t t T i i El t Mi (TEM) i i . , MgO <001> substrate heated at 350°C or higher, the FePt film adopts the L10 ordered phase and the [001] orientation. Several CrRu seed-layers were processed before attaining the correct composition and thickness to facilitate the epitaxial growt. Fig 3(a) is an atom probe reconstruction of both the CrRu seed-layer and the FePt film. An isosurface has been added to indicated the CrRu/FePt interface. Fig 3(b) is an Fe atom map oriented to depict the Fe (001) planes. Fig 3(c) and 3(d) are spatial distribution maps (SDM) generated from a 5 nm by 5 nm cube near the (001) crystallographic pole. From these SDMs the d001 was determined to be 3.64Å. e magne c or area s orage ens y as ecrease o a s ze that is rapidly approaching the superparamagnetic barrier; the thermal stability limit where magnetization randomly fluctuates because of the small magnetic volume. The thermal stability of very small magnetic crystals or grains can be improved if the material has a large uniaxial magnetocrystalline anisotropy, Ku. The L10 phase of FePt has been identified as a candidate material for ultra-high magnetic storage media because of its high Ku [1]. When this intermetallic is sputter-deposited as a polycrystalline thin film, a metastable solid solution face-centered-cubic phase (A1) is formed [2]. By annealing at temperatures in excess of 500°C, the crystalline lattice atomistically orders into the desired hard ti L1 h Th h b li it d i t l t di h o ass s n e recons ruc on o e a om pro e a a se s, ransm ss on ec ron croscopy mag ng was performed on a Focus Ion Beam (FIB) lift-out specimen of an atom probe tip, as shown in Fig.1. The TEM was performed using a 200keV FEI Tecnai F20. The FIB milling was performed using a FEI Quanta 3D dual beam FIB. The phase identification of the films was determined by XRD using a Bruker Discovery D8 diffractometer operating at 40kV and 35 mA with Co K radiation as the source.