Peter N. Kalu

Microstructure and tensile properties of nanostructured Cu-25wt%Ag

The microstructure and textures of nanostructured Cu-25wt%Ag are shown to relate to its phase transformation, fabrication and mechanical properties. The analysis was undertaken by transmission electron microscopy, scanning electron microscopy, X-Ray texture, tensile tests and analytical modeling. The Cu-25wt%Ag is described by two components in the model: (1) eutectic component which is a composite of nanosized Cu and Ag (2) proeutectic Cu component embedded with Ag nano-fibers formed below the eutectic temperature. In both cast and forged samples, the Cu and Ag always have cube-on-cube orientation relationship in proeutectic component, but they have the orientation relationship only in selected eutectic areas. The orientation relationship maintains the same in cast, forged and drawn Cu-Ag materials only in proeutectic regions. The deformation enhances the tensile strength of the materials by decreasing the size of various Cu and Ag phases in the radial direction of the wire in both components.

Characterization of Al-8090 superplastic materials using orientation imaging microscopy

A fundamental understanding of the deformation mechanisms of superplasticity requires a detailed characterization of the microstructure. For certain studies, optical microscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques used in conventional mode may be inadequate for detailed analysis. This paper presents the use of a newly developed technique, ‘orientation imaging microscopy’ (OIM) in characterizing the microstructure of an Al-8090 alloy deformed in uniaxial tension to strains of 15, 70 and 660% at 520°C and 5 10 4 s 1 strain rate. In OIM the microstructure is constructed from the measured crystal orientations obtained from points on the specimen surface distributed in a hexagonal grid. Neighboring measurements with a misorientation greater than a specified value, v, misorientation angle criteria (designated by the researcher) are deemed to define the location of grain boundaries. These boundary lines can be interpreted as high angle grain boundaries or subgrain boundaries depending on the value of v. The implications of the analyses on superplastic deformation are discussed.

Examinations of Cu-Ag Composite Conductors in Sheet Forms

Cold rolled Cu-24 wt% Ag composite was characterized and the effects of crystallographic structure and defect anisotropy and microstructure refinement on properties of the composite were studied. Characterization was carried out with high resolution scanning electron microscopy (SEM), x-ray diffractometer and mechanical testing. All the deformed samples exhibited {110}-texture that had impact on the anisotropic properties of the materials. The yield and tensile strengths were higher in the long transverse (LT) than in the rolling direction (RD). The resistivity was higher in the RD than in the LT. Increasing the rolling strain increased both the mechanical strength and electric resistivity. The property changes with strain were related to the Cu and Ag lamellae thickness. The thickness of the lamella was inversely proportional to the deformation strain. It was observed that the smaller the thickness of the lamellae, the higher the strength and the electric resistivity. A closer examination of the Cu and Ag components revealed that while the lamellae were well aligned in the LT direction, they were curved in the RD. The curved lamellae observed in the RD were attributed to the development of shear bands during rolling. Both the texture and shear bands were related to the anisotropy of the properties.

The effect of off-axis thermomechanical processing on the mechanical behavior of textured 2095 Al-Li alloy

Abstract This investigation was conducted to study the influence of off-axis deformation in the thermomechanical processing on the properties of Al–Li 2095 HT72 alloy sheet. The thermomechanical processing involved several stages. First, the as-received hot rolled sheet was solution heat treated at 510°C for 30 min. Samples were then cut from the sheet and stretched off-axis at 0, 30, 60 and 90° angles (stretch axis) to the original rolling direction. All the specimens were then aged at 180°C for 24 h. Tensile coupons were then machined from each of these aged samples at 0, 30, 45, 60 and 90° (tensile axis) to the rolling direction, then tested to failure. The crystallographic texture and tensile properties of the processed samples were evaluated, and were compared to the solution heat-treated material. In the solution heat-treated condition, the material was highly textured with a very strong Brass component. Off-axis stretch followed by aging had little or no effect on the texture development of the solution heat-treated material. Upon testing, samples with stretch axis at 0° showed pronounced mechanical anisotropy; specimens tested at 45° showed about 40% drop in tensile and yield strength values when compared to those tested at 0°. However, stretching along 60° or 90° directions minimized mechanical anisotropy. The texture characteristics of all tested samples were determined and related to the changes in the mechanical properties. An attempt was made to correlate the data with some texture theories. Analysis of the results and normalization based on the Taylor’s factor showed that that crystallographic texture contributes only partly to the total mechanical anisotropy.

Correlation Between the Microstructure and Forces Generated During Friction Stir Processing of AA5052

The microstructure of friction stir processed AA5052 sheets is investigated using Scanning Electron Microscopy (SEM) and Orientation Imaging Microscopy. The correlation between the generated forces during processing and the microstructure is evaluated. Observations indicate that the finest microstructure are achieved when the plunging forces are at minimum.

Strain-induced selective growth in 1.5% temper-rolled Fe;1%Si.

Strain-induced selective growth was investigated in a 1.5% temper-rolled Fe∼1%Si alloy using the electron backscatter diffraction (EBSD) technique. The EBSD technique was used to quantify the presence of orientation spreads within grains and to show that this particular case of selective growth can be directly related to differences in stored energy as reflected in the geometrically necessary dislocation content. The differences in stored energy were sufficient to give rise to selective growth as evidenced by bi-modal grain sizes.

Texture evolution in Al-Li 2195 alloy during net shape roll forging

The quest to manufacture low-cost, large complex structures with minimum material wastage has instigated research into near-net shape (NNS) technologies. In this paper, the microstructural and texture evolution in Al-Li 2195 alloy resulting from a specific NNS processing called roll forging was evaluated. The processing consists of five stages and combines conventional ingot conversion (stages I and II), back extrusion (stage III) and ring rolling to produce hollow cylinder (stage IV). Considerable potential exists for the property enhancement of the components produced by this processing method if a better understanding of the evolution of microstructure and texture is developed.

Effect of state of stress on the cavitation behavior of Al 5083 superplastic material

In this paper we address the controversial issue of nucleation of cavities in Al 5083 alloys and their subsequent growth to coalescence and failure. We focus on the origin and growth of cavities not only during the primary processing of Al 5083 in sheet forms, but also during the manufacture of these sheets into SPF (superplastic forming) components. Experimental observations of pre-existing cavities in this alloy are made using optical and electron microscopy. The role of sheet rolling direction, and the state of stress during superplastic deformation on the cavity formation and coalescence are also discussed. The effect of the state of stress (uniaxial, plane strain, balanced biaxial, and tri-axial) on the growth characteristics of cavitation is also examined. It is found that the uniaxial model based cavitation cannot directly be extended to predict the behavior of more complex stress states, unless great care is taken to identify the right strain measure for the mapping process.

Microstructures of La 1− x A x (A = Ca or Sr)MnO 3−δ thin films by liquid-delivery metalorganic chemical vapor deposition

The microstructure of La 1– x A x (A = Ca or Sr)MnO 3–δ thin films grown by liquid-delivery metalorganic chemical vapor deposition on (001) MgO and (110) pseudo-cubic LaAlO 3 were studied by transmission electron microscopy. The La 1– x Ca x MnO 3–δ thin film on large lattice mismatched MgO exhibited very defective microstructures and consisted of two typical regions. The first region was close to the film–substrate interface and had an epitaxial relationship to the substrate with many differently oriented domains nucleated on the substrate surface. The second region consisted of columnar grains with some degree of texture. In contrast, the smaller lattice-mismatched La 1– x Sr x MnO 3–δ /(110) pseudo-cubic LaAlO 3 film had good crystalline quality with highly oriented columnar grains but exhibited complicated dislocation structures. Apart from the misfit dislocations formed at the film–substrate interface, two types of anomalous dislocations with limited contribution to relieving misfit stresses were also observed. One type of dislocation had extra planes in the film and some climbed into the substrate. These dislocations were considered to form from dislocation loops during nucleation of the film. The other type of dislocations had extra planes parallel to the film–substrate interface and glided into the substrate side resulting in a 2° tilt of the film with respect to the substrate. The complicated dislocation configurations present in the sample were related to the complex strain field in the film. The relative strains along the interface measured in the film were heterogeneous. The variations of the strains in the film were related to the local Curie temperature changes and second-order phase transitions of the film.

3D Stress-Strain Model of the Nb

The performance of Nb3Sn multifilament wires is highly strain-sensitive. Since the strain in each individual Nb3Sn filament rather than the overall strain on the Nb3Sn wire influences the superconducting properties, it is very important to build a relationship between the internal strain in each Nb3Sn filament and external loads. In this study, a 3D model is developed to address the relationship. It is found that the geometry of the cross-section of the filaments plays an important role in the relationship between the stress and strain.