Papers by Keyword: Misorientation

Abstract: Crystallographic quality of the epitaxial layers depends on the process temperature, partial pressures of active components and the surface polarity and also on the crystallographic quality of the subsurface layer resulting from the preparation of the substrate. The polishing etching in hydrogen-propane atmosphere of 4H-SiC substrate of different orientations and polarity was studied. The optimization of the polishing etching has been achieved with respect to the flow of C3H8, the duration and the temperature of the process. The investigation of the surface of SiC substrate before and after in situ polishing-etching in H2+C3H8 atmosphere was carried out by Nomarski interference contrast microscopy (DIC) and atomic force microscope (AFM).

Abstract: A new recrystallization phase-field method is proposed, in which the three stages of recrystallization phenomena, i.e., recovery, nucleation and nucleus growth are sequentially taken into account in a computation. From the information of subgrain patterns and crystal orientations in a polycrystal that are obtained by a dislocation-crystal plasticity FE analysis based on a reaction-diffusion model, subgrain groups surrounded by high angle boundary are found out. Next, subgrains in the group are coalesced into a nucleus by rotation of crystal orientation and migration of subgrain boundaries through a phase-field simulation. Then a computation of nucleus growth is performed also using the phase-field method on account of an autonomic incubation period of nucleation, in which stored dislocation energy assumes a role of driving force. It is shown that the present method can numerically reproduce the three stages of recrystallization as a sequence of computational procedure.

Abstract: Microstructure evolution and mechanical behavior of alpha/beta Ti-6Al-4V (VT6) and near-beta Ti-5Al-5Mo-5V-1Cr-1Fe (VT22) titanium alloys during uniaxial compression at 600°C to a high strain of 70% was studied. The plastic-flow response for both alloys is characterized by successive stages of strain hardening, flow softening, and steady-state flow. During compression the lamellae spheroidized to produce a partially globular microstructure. Globularization in VT6 is associated with the loss of the initial Burgers-type coherency between the alpha and beta phases and the subsequent individual deformation of each phase. The misorientations of boundaries increase to the high-angle range by means of the accumulation of lattice dislocations. In VT22 alloy the alpha phase evolves similar to that in VT6 alloy, while in the beta phase mainly low-angle boundaries are observed even after 70 pct. reduction.

Abstract: The role of the α/γ orientation relationships during ferrite nucleation is investigated. EBSD measurements were performed on an especially developed high purity ternary iron-based alloy with 20 wt.% Cr and 12 wt.% Ni with both austenite and ferrite present at room temperature to measure the orientation relationship between the austenite and ferrite crystallites. The experimental results are compared to the nucleation models of Clemm and Fisher and Aaronson and co-workers.

Abstract: The development of deformation texture and microstructure was examined for four different initial textures. IF steel sheets with a majority of α-, ε-, and γ-fiber and near random texture were prepared and cold rolled. The specimens exhibited characteristic behaviors in rolling texture evolution and deformation-induced misorientation development, according to their initial textures, especially at small strain levels. Due to the orientation dependence of intra-granular misorientation accumulation, the different texture evolutions affected the induced misorientation distribution. A larger fraction of γ-fiber orientations was related to more prominent misorientation development, while the initial texture stability simultaneously affected the misorientation development. The unstable, initial ε-fiber texture showed a stronger tendency of misorientation accumulation than the stable α-fiber during the subsequent cold rolling.

Abstract: The relationship between the misorientation of the austenite crystallites and the favoured sites for ferrite nucleation has been investigated. Ex-situ EBSD measurements were performed on an especially developed high purity ternary iron alloy with 20 wt.% Cr and 12 wt.% Ni with both austenite and ferrite present at room temperature to measure the misorientation between the austenite crystallites. The experimental results are compared to the nucleation models of Clemm and Fisher and Aaronson and co-workers.

Abstract: The effect of low temperature recovery treatments on the recrystallization kinetics during subsequent high temperature annealing was investigated in three Al-2.5%Mg alloys with various Fe additions. Recovery treatments were carried out at 190oC for times ranging from 0.25 to 65 hrs. Recrystallization treatments were carried out at 280oC. The kinetics of recrystallization was followed using the techniques of hardness measurement, optical metallography and calorimetry.

Abstract: The dislocation substructure that appears in deformed metals and alloys have been extensively investigated in the past by transmission electron microscopy (TEM). They are known to form a broad variety of microstructures. These substructures are characterized by three main parameters, namely the density of the dislocations that are trapped in the tangles, their degree of patterning and the misorientation between the cells. The aim of the present work is to investigate the relationship between these features and the mechanical properties of the material.

Abstract: SiC substrates produced at II-VI, Inc. have been characterized using x-ray rocking curve mapping (topography). The rocking curves have been measured in the
-scan mode for the (0006) Bragg reflection of 6H and the (0004) reflection of 4H SiC substrates. The maps contain information extracted from the rocking curves, such as the peak angle (
) and the rocking curve broadening (FWHM). In the case when lattice distortion is present due to the elastic or plastic deformation, the peak angle (
) changes gradually upon scanning, with the d
/dx gradient proportional to the lattice curvature in the plane of diffraction. Multi-peak reflections and/or sharp change in the value of
indicate the presence of misoriented grains. X-ray rocking curve mapping of SiC substrates yields excellent measures of crystalline quality that contain important information on the lattice strain and sub-grain misorientation.

Abstract: The Cyclic Extrusion Compression (CEC) is one of the methods of severe plastic deformation (SPD), used for producing nanomaterials. The CEC method allows materials to be deformed to arbitrarily large strains without changing the initial shape of sample. Large hydrostatic compressive stresses are exerted during deformation avoiding sample cracking. Using the CEC method Cu and aluminum alloys nanomaterials were produced. It has been found that, after exerting true strain of about ϕ = 14, only some part of the sample changes into a nanomaterial, while the remainder volume still shows the ultrafine microstructure. The nanometric microstructure is created generally inside the areas of intersecting microbands. Large misorientation has been found between the microbands and the surrounding material, facilitating the formation of nanograin boundaries. The hardness of samples increases with the increase of deformation, however only to a boundary level of about 100 MPa. The stabilization of hardening, above a deformation of about ϕ = 4, suggests the activation of softening processes. Independently to the stabilization of properties, the refinement of nanograins is continued, indicating the development of anomalies in the hardening – grain size relationship.