Materials Science Forum Vols. 558-559

Abstract: A comprehensive investigation of microstructure evolution upon annealing in the temperature range between 100°C and 450°C in SPD-processed nanocrystalline iron had been conducted using transmission and orientation imaging microscopy, and XRD analysis. The asprocessed microstructure was typical of severely deformed metals consisting of grains with a mean size of 110 nm, each subdivided by a network of subgrain boundaries. Additionally, measurements of microstrains using XRD found a very high microstrain level of 0.003 in this material. After annealing at 200°C a drastic decrease of microstrains was observed; whereas no notable changes in the size of grains and subgrains as well in misorientation spectra have been revealed. Thus we relate this decrease of microstrains with recovery of non-equilibrium grain boundaries. Annealing at higher temperatures hadn’t led to further decrease of microstrains, but resulted in slight subgrain growth from 65 to 78 nm accompanied by increase of mean misorientation angle from 5° to 6.3°at 450°C. This indicates the occurrence of coalescence of subgrains as additional recovery mechanism.

Abstract: Both the presence and absence of an amorphous intergranular film (IGF) between the SiC grains have previously been reported in liquid-phase-sintered SiC ceramics (LPS-SiC). The dominant factor(s) responsible for the grain boundary structure in LPS-SiC has not been clearly revealed. In the present study, LPS-SiC ceramics containing different compositions of sintering additives were fabricated and characterized with respect to their grain boundary structure, using both scanning and transmission electron microscopy. The results suggest that the sintering additive composition plays a dominant role in the evolution of grain boundary structure in LPS-SiC.

Abstract: The kinetic and structural behavior of symmetrical <100> tilt grain boundaries with rotation angles 8.4°, 12.0°, 14.3° and 16.0° were investigated in-situ in a hot stage SEM in the temperature range between 380°C and 640°C. The results revealed that depending on the rotation angle the boundary either remained straight, became faceted or curved under the driving force provided by the boundary surface tension during high temperature annealing. The transition “facetedcurved boundary” was also found to depend on temperature. The observed behavior is attributed to the anisotropy of grain boundary energy with respect to boundary inclination.

Abstract: A novel high temperature heating method in combination with automated EBSD-data acquisition is presented. A commercially available infrared laser is utilized to heat samples up to a temperature of about 1000°C in high vacuum in a SEM while acquiring EBSD-data of the microstructure. First results on the γ-α-γ phase transformation between 840°C and 865°C in a microalloyed ferritic low carbon steel is presented.

Abstract: A new method for reconstructing a three-dimensional microstructure using the focused ion beam-orientation imaging microscopy (FIB-OIM) is introduced. The technique is important for the study of three-dimensional microstructures of materials because it can automatically align (register) a series of parallel sections with both topological information and orientation information at the sub-micrometer scale. Using voxel-based tessellation, a three-dimensional microstructure is reconstructed by registering each section. The application of the method to a cubic material is described and, based on the reconstruction, the grain shape and grain size distribution are characterized.

Abstract: The microstructures in 3 mol% Y2O3-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1100°-1650°C were investigated to clarify cubic-formation and grain-growth mechanisms. The cubic phase in Y-TZP appeared at 1300°C and its mass fraction increased with increasing sintering temperature. High-resolution transmission electron microscopy (HRTEM) and nanoprobe X-ray energy dispersive spectroscopy (EDS) measurements revealed that no amorphous layer existed along the grain-boundary faces in Y-TZP, and Y3+ ions segregated not only along the tetragonal-tetragonal phase boundaries but also along tetragonal-cubic phase boundaries. Scanning transmission electron microscopy (STEM) and nanoprobe EDS measurements revealed that the Y3+ ion distribution was nearly homogeneous up to 1300°C, but cubic phase regions with high Y3+ ion concentration clearly formed inside grains at 1500°C. These results indicate that cubic phase regions are formed from the grain boundaries and/or the multiple junctions in which Y3+ ions segregated. We termed such a new diffusive transformation phenomenon “grain boundary segregation-induced phase transformation (GBSIPT)”. The grain-growth mechanism is controlled by the solute-drag effect of Y3+ ions segregating along the grain boundary.

Abstract: The stress induced migration of planar grain boundaries in aluminium bicrystals was measured. Symmetrical <100> tilt grain boundaries with misorientation angles in the range between 5.7° and 17.8° were examined. Boundary migration under a shear stress was observed to be ideally coupled to the lateral translation of grains. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with the respective boundary geometry. The temperature dependence of grain boundary mobility was measured, and the corresponding activation parameters were determined. The activation enthalpy of boundary migration was found to be independent of misorientation angle in the investigated misorientation range and amounts to H=1.44 eV.

Abstract: The microstructural development of cold-rolled lath martensite structure in the low carbon steels and ultra-low carbon steels are studied and compared. In low carbon steel of as-quenched specimens, very thin austenite films exist at boundaries of adjacent laths, but do not exist in ultra-low carbon steel. After cold rolling for the low carbon steel, the lamellar dislocation cells, irregularly bent laths and kinked laths regions are frequently observed and, in some instances, the disappearance of initial lath boundaries is observed. The existence of retained austenite films suggests that the lath boundaries rarely disappear during cold-rolling in the low carbon steel.

Abstract: The diffusion behavior of Ti3+ along basal dislocations in sapphire has been investigated by SIMS technique. High-density unidirectional dislocations were introduced by the high-temperature mechanical deformation, and Ti3+ ions were subsequently diffused along the dislocations. The SIMS diffusion profiles clearly showed diffusion tail due to the short circuit diffusion along the dislocations called pipe diffusion. Lattice diffusion coefficient and pipe diffusion coefficient of Ti3+ at 1300°C were measured to be 1.0±0.2×10-19 [m2/sec] and 2.0±0.6× 10-13 [m2/sec], respectively.

Abstract: Quasicrystals are aperiodic long-range ordered solids with a high potential for many modern applications. Interest is nowadays paid to the development of economically viable large-scale synthesis procedures of quasicrystalline materials involving solid-state transformations. The kinetics of the high-temperature phase transition from the complex ω-phase to the icosahedral quasicrystalline (iQC) ψ-phase in AlCuFe nanopowders was here examined by in-situ time-resolved X-ray diffraction experiments using synchrotron radiation. In-situ XRD experiments will allow insight on the influence of uniaxial applied pressure on the kinetics of phase transitions leading to the formation of single-phase QC nanopowders and further contribute to the optimization of sintering procedures for nano-quasicrystalline AlCuFe alloy powders.