Structure of phases in the δ-AI2O3 fiber studied by convergent beam electron diffraction

The phases in the δ-Al₂O₃ fibers were investigated using the methods of transmission electron microscopy (TEM): convergent beam electron diffraction (CBED) and high-resolution electron microscopy (HREM). A phaseγ′-Al₂O₃ discovered previously by Vewerly in oxide layers with an fcc structure was found and new atomic positions are proposed. A new structure ofδ-Al₂O₃ was also observed. It has aPmma space group and lattice parameters ofa _δ = 2a _γ′,b _δ = l.5a _γ′, andc _δ ≈a _γ′ The correlation of the observed A1₂O₃ lattices to the spinel lattice is discussed and translation of atom positions during theγ′ →γ →δ transformation is studied. All anions must change their positions by a small amount; one-third of the cation positions inγ′ and more than 90 pct of cation positions inδ experience a large translation during that transformation. This implies that for theγ′ it→γ} →δ transformation, the positions of cations in both lattices are important. The results are discussed in relation to the fiber-matrix interaction under spinel formation during thermal loading ofδ-Al₂O₃-fiber-reinforced aluminum piston alloys.     

Automatic analysis of electron backscatter diffraction patterns

The ability to measure lattice orientation in individual crystallites enables a more complete characterization of microstructure by combining lattice orientation with morphological features. Lattice orientation can be obtained by analyzing electron backscatter diffraction patterns (EBSPs). However, current computer-aided EBSP analysis techniques make it impractical to obtain the number of measurements needed for statistically reliable characterizations of microstructure. An effective fully automated technique for determining crystallographic orientation from EBSPs is described. Bands are identified by linear regions of correlation in the image intensity gradient direction. The most probable orientation is then found using the angles between the detected bands. The reliability of the technique was tested using a set of 1000 patterns obtained from annealed oxygen-free electrical grade (OFEC) copper. The orientation of each test pattern found using automatic indexing was checked against the corresponding orientation as determined by manual indexing. Ninety-nine percent of auto-indexed orientations were found to lie within 5 deg of the misorientation angle of the manual-indexed orientations. By simulating noise in the test patterns, it was found that image quality has a strong effect on the reliability of the technique. An image quality parameter is described which allows the reliability of the technique to be predicted for a pattern of given quality.     

Convergent beam electron diffraction analysis of theT 1 (Al2CuLi) phase in Al-Li-Cu alloys

Point and space group analysis of largeT ₁ (Al₂CuLi) crystals was performed by convergent beam electron diffraction. The structure ofT ₁ was determined to be hexagonal, possessing a 6/mmm point group and P6/mmm (No. 191) space group. The lattice constants were found to bea ≈ 0.497 nm andc ≈ 0.93 nm. This structure is in agreement with an existing model of T₁, although discrepancies between the observed and calculated intensities of certain reflections were evident. Electron probe X-ray microanalysis of theseT ₁ crystals indicates the composition is on the copper-rich side of stoichiometric Al₂CuLi. The slight deviation in the composition ofT ₁ from stoichiometry and the presence of planar defects in the microstructure may account for the discrepancy in the intensity of certain reflections.     

Convergent beam electron diffraction study of Al3Zr in Al-Zr AND Al-Li-Zr alloys

Point and space group analysis of metastable and equilibrium A1₃Zr precipitates in Al-Zr and Al-Li-Zr alloys was performed by convergent beam electron diffraction (CBED). The results obtained for the metastable Al₃Zr showed it possesses a cubic Pm3m (No. 221) space group with a lattice parameter of 0.408 nm. The equilibrium Al₃Zr (β) phase was determined to be body-centered tetragonal having a space group I4/mmm (No. 139) and lattice constants of a = 0.401 nm and c = 1.73 nm. In the lithium containing alloys, it was shown that if lithium incorporation into the metastable Al₃Zr particles occurred, it did not alter its structure. X-ray microanalysis of extracted metastable Al₃Zr precipitates was conducted using energy dispersive X-ray spectrometry (EDS), and the composition of these particles was determined to be ≈ 60 wt%Al and ≈ 40 wt%Zr. Since this composition is significantly different from stoichiometric Al₃Zr (48 wt%Al, 52 wt%Zr), it suggests that a metastable phase field exists in the α + β phase region of the Al-Zr phase diagram.     

Advances in deformation twin characterization using electron backscattered diffraction data

This article reports on recent progress in the effort to develop an automated, crystallographically based twin identification and quantification routine using large sets of spatially correlated electron backscattered diffraction (EBSD) data. The proposed analysis scheme uses information about the most probably occurring twin types and the macroscopic stress state, taken together with the crystallographic theory of deformation twinning, to identify and classify twinned areas in a scanned cross section of a material. The key features of the analysis are identification of potential twin boundaries by their misorientation character, validation of these boundaries through comparison with the actual boundary position and twin-plane matching across the boundary, and calculation of the Schmid factors for the orientations on either side of the boundary. This scheme will allow researchers to quantify twin area fractions from statistically significant regions and, in turn, estimate twinned volume fractions with reasonable reliability. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Advances in deformation twin characterization using electron backscattered diffraction data

This article reports on recent progress in the effort to develop an automated, crystallographically based twin identification and quantification routine using large sets of spatially correlated electron backscattered diffraction (EBSD) data. The proposed analysis scheme uses information about the most probably occurring twin types and the macroscopic stress state, taken together with the crystallographic theory of deformation twinning, to identify and classify twinned areas in a scanned cross section of a material. The key features of the analysis are identification of potential twin boundaries by their misorientation character, validation of these boundaries through comparison with the actual boundary position and twin-plane matching across the boundary, and calculation of the Schmid factors for the orientations on either side of the boundary. This scheme will allow researchers to quantify twin area fractions from statistically significant regions and, in turn, estimate twinned volume fractions with reasonable reliability. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Quantitative deformation studies using electron back scatter patterns

The diffuseness of electron back scatter patterns (EBSPs) is observed to increase with plastic strain. The application of this technique to deformation studies has been limited by the lack of a general method of measuring the pattern quality. The degradation of EBSPs by cold work was thus thoroughly investigated using the A1 6061 alloy for the purpose. Methods of enhancing the Kikuchi band contrast by removal of the background intensity variation from digital images of 〈112〉 zone axes present in the EBSP have been developed. The contrast of the Kikuchi bands was quantified using the root mean square intensity of averaged band profiles, while the sharpness of the patterns was assessed by the attenuation of high frequency components of Fourier transforms of the enhanced images and of the averaged band profiles. Tilt was found to effect contrast but not sharpness, while cold work reduced both. However, surface contamination produced effects that were very similar to those of specimen deformation. A method is presented for quantitative determination of EBSP quality, which is independent of grain orientation and is based on the first moment of power spectra (the square of the Fourier transform) of features common to all patters to be compared.     

Radiation dosimetry for electron beam CT

PURPOSE: To measure the radiation dose profile, multiple-scan average dose (MSAD), and computed tomography dose index (CTDI) for electron beam CT and to determine the accuracy of ionization-chamber and manufacturer estimates of patient dose. MATERIALS AND METHODS: High-resolution dose profiles along the longitudinal axis were acquired at several positions within the scan plane with use of radiographic film. The full-width-at-half-maximum values, peak radiation dose, CTDI, and MSAD were calculated from the digitized film profiles. CTDI was also measured with an ionization chamber. RESULTS: The full-width-half-maximum value of the radiation profiles were significantly wider than the nominal scan width for the 6-mm single-section and 8-mm multisection modes. In the single-section mode, the CTDI underestimated the MSAD by 15%-30%. The multisection radiation profile was nonuniform and asymmetric. CONCLUSION: Patient doses in electron beam CT are approximately 125% of the ionization-chamber CTDI measurements in the single-section mode. For the multisection mode, the average patient dose over the scan volume is approximately 70%-85% of the ionization-chamber CTDI measurements.     

Measurement of residual strain in An AlSiCw composite using convergent-beam electron diffraction

Residual strains were introduced into an AlSiC_w composite by in situ cooling a thin foil from room temperature to -160°C. A detailed analysis was conducted using convergent-beam electron diffraction (CBED) to quantify the elastic residual stresses and strains in the matrix near the end and side of a SiC whisker. Large hydrostatic and effective stresses were measured in the matrix near the side of the whisker; the maximum stresses were located near the Al/SiC_w interface and decreased to zero approximately 1 μm (∼2 whisker diameters) from the interface. Residual strains were also observed in the matrix near the whisker end, but these strains could not be measured due to the complexity of the strain field. At the whisker end, the largest residual strains were located near the Al/SiC_w interface and decreased to zero approximately 0.5 μm (∼1 whisker diameter) from the interface. Finite element techniques were used to predict the residual strains in the composite material and these results were compared to experimental measurements.     

The FE-lspd model for electron beam dosimetry

The FE-lspd model is a two-component electron beam model that distinguishes between electrons that can be described by small-angle transport theory and electrons that are too widely scattered for small-angle transport theory to be applicable. The two components are called the primary beam and the laterally scattered primary distribution (lspd). The primary beam component incorporates a simple version of the Fermi-Eyges model and dominates dose calculations at therapeutic depths. The lspd component corrects erros in the lateral spreading of the primary beam component, thereby improving the accuracy by which the FE-lspd model calculates dose distribution in blocked fields. Comparisons were made between dose profiles and central-axis depth dose distributions in small fields calculated by the FE-lspd, Fermi-Eyges and EGS4 Monte Carlo models for a 10 MeV beam in a homogeneous water phantom. The maximum difference between the dose calculated using the FE-lspd model and EGS4 Monte Carlo is about 6% at a field diameter of about 1 cm, and less than 2% for field sizes greater than 3 cm diameter. The maximum difference between the Fermi-Eyges and Monte Carlo calculations is about 18% at a field diameter of about 2.5 cm. A comparison was made with the central-axis depth dose distribution measured in water for a 3 cm diameter field in a 10 MeV clinical electron beam. The errors in the dose distribution were found to be less than 2% using the FE-lspd model but almost 18% using the Fermi-Eyges model. A comparison was also made with pencil beam profiles calculated using the second-order Fermi-Eyges transport model.     

Simple models for the omega phase transformation

In the late sixties, it was recognized that the omega phase transformation occurring in bcc Ti, Zr, and Hf alloys was a displacive transition which could be described, at least qualitatively, by simple models. Since the displacive wave responsible for the transition to perfect omega was a Brillouin zone boundary wave, displacements and volume changes were rather small, in contrast to the classical martensite case. The nonideal, or modulated omega phase requires further consideration, and calls for soliton models, for example. Recent first principles electronic structure calculations, briefly reported upon here, have also shed light on the athermal omega transformation. This paper is based on a presentation made in the symposium “Pre-transformation Behavior Related to Displacive Transformations in Alloys≓ presented at the 1986 annual AIME meeting in New Orleans, March 2-6, 1986, under the auspices of the ASM-MSD Structures Committee.