Materials Science Forum Vols. 715-716

Abstract: Grain refinement taking place in a commercial 7055 aluminum alloy under equal channel angular pressing (ECAP) was examined in the temperature interval 250375°C. It was shown that the formation of recrystallized grains occurs through continuous dynamic recrystallization (CDRX). At 250°C, a low rate of dynamic recovery and high volume fraction of second phase particles provide the rapid formation of stable three-dimensional arrays of low-angle boundaries and their gradual transformation into high-angle boundaries. Increasing temperature leads an increase in the average crystallite size produced by ECAP from 0.7 μm at 250°C to 1.3 μm at 375°C. The effect of temperature on CDRX kinetic is discussed.

Abstract: This paper describes the stabilization of nanocrystalline grain sizes in Pd and Fe by the addition of Zr solute atoms. The grain size as a function of annealing temperature was measured by both x-ray diffraction (XRD) line broadening analysis and microscopy methods. The latter methods showed that the XRD grain size measurements for the samples annealed at the higher temperatures were not valid. It appears that thermodynamic stabilization may still be operative in the Fe-4at.% Zr alloy but not in the Pd-19at.% Zr alloy from the experimental results and calculations of the enthalpy of segregation.

Abstract: The detailed microstructure in front of recrystallization boundaries and their migration during annealing were traced using ex-situ electron backscatter pattern maps of one and the same surface area taken after annealing. It is observed that many protrusions/detrusions form on the recrystallizing boundaries. During annealing, the recrystallization boundary segments migrate in a stop-go type of fashion, while protrusions and detrusions alternately form and disappear. The correlation between the protrusions/detrusions and the stop-go type of migration are briefly discussed.

Abstract: Distinctive microstructure engineering of amorphous to nanocrystalline electroceramic thin films is of high relevance for integration in low to high temperature operating MEMS-devices. Up to now, kinetic rules of nucleation, crystallization and grain growth of precipitation-based ceramic thin films are unknown. In this study, general rules for the crystallization and grain growth kinetics of a pure single-phase metal oxide thin film with only one kind of cation, i.e. ceria, made by spray pyrolysis from a precursor with one single organic solvent is discussed [1,. The near-and long range disorder is studied via Raman, DSC investigation of crystallization enthalpy, XRD, SEM and TEM for amorphous to fully crystalline state. These 400 nm thick-thin films were dense, crack-free and amorphous directly after deposition on a sapphire substrate. Briefly, above deposition temperature crystallization sets in with respect to temperature and persists over a broad temperature range from 400 to 950°C. In this regime, biphasic amorphous-crystallien films exist and grain growth proceeds simultaneously to crystallization. Isothermal grain growth studies showed that after short dwell times of 10-20h stable microstructures established following self-limited grain growth law [. In this state, driving force for the crystallization is the reduction of free enthalpy for phase transformation and interface diffusion prevails. A transition to classical grain curvature-driven parabolic grain growth kinetics appeared once the material reached the fully crystalline state for average grain sizes larger than 140 nm and higher annealing temperatures. Volume diffusion was then activated in addition to the interface diffusion. It was found that once crystallized the material shows independent on processing route equal XRD density and microstrain, as well as Raman characteristics. However, dependent on processing conditions i.e. choice of organic and, according, deposition temperature of the film amorphous states vary and affect strongly crystallization and grain growth history for the biphasic films.

Abstract: AISI 304L austenitic stainless steel was cold rolled to 90% with and no inter-pass cooling to produced 89% and 43% of deformation induced martensite respectively. The cold rolled specimens were annealed by isothermal and cyclic thermal process. The microstructures of the cold rolled and annealed specimens were studied by the electron microscope. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and strain heterogeneity of the microstructure. The results indicated possibility of ultrafine austenite grain formation by cyclic thermal process for austenitic stainless steels those do not readily undergo deformation induced martensite. Keywords: Austenitic stainless steel, Grain refinement, Cyclic thermal process, Ultrafine grain

Abstract: This paper considers the use of Holographic Optical Elements (HOEs) to shape the weld beam and hence control the grain size of the weld bead and the grain growth and phase transformations in the HAZ. Welds have been produced on carbon steel with the introduction of a nickel based filler powder, using different energy densities produced by the HOEs. Cross sections of the welds have been analysed in terms of the weld profile, weld pool shape and grain size in the deposit and the HAZ. Electron BackScatter Diffraction (EBSD) coupled with Energy Dispersive X-ray Spectroscopy (EDS) has been used to study the microstructures developed. The results have shown that by utilising HOEs the grain size within the weld pool can be controlled such that a more equiaxed grain structure is developed when compared with the coarse columnar grains seen with a Gaussian beam.

Abstract: The objective of this study was to determine the effect of deformation mode on recrystallization behavior of severely deformed material. Commercial purity AA3104 aluminum alloy was deformed via high pressure torsion and equal channel angular pressing to different strains and then annealed to obtain the state of partial recrystallization. The microstructure and the crystallographic texture were analysed using scanning and transmission electron microscopes equipped with orientation measurement facilities. The nucleation of new grains was observed in bulk recrystallized samples and during in-situ recrystallization in the transmission microscope. Irrespective of the applied deformation mode, a large non-deformable second phase particles strongly influenced strengthening of the matrix through deformation zones around them. It is known that relatively high stored energy stimulates the nucleation of new grains during the recrystalization. In most of the observed cases, the growth of recrystallized grains occurred by the coalescence of neighboring subcells. This process usually led to nearly homogeneous equiaxed grains of similar size. The diameter of grains in the vicinity of large second phase particles was only occasionally significantly larger than the average grain size. Large grains were most often observed in places far from the particles. TEM orientation mapping from highly deformed zones around particles showed that orientations of new grains were not random and only strictly defined groups of orientations were observed.

Abstract: Two major types of Cube bands/segments have been observed in heavily (90%) cold rolled Al-0.1wt %Mn using the EBSD technique with a FEG SEM: i) intergranular transition bands as thin cube segments aligned along RD between S and Cu oriented grains and ii) as transgranular strain localized bands situated in some particular grains. Their evolution is studied by light annealing at 275°C and 300°C and EBSD observations of exactly the same areas to directly correlate local deformation substructure with recrystallization. Only the intergranular cube transition bands give rapid recrystallization nucleation to cube grains of dimension >10µm. In particular the fastest growing cube grains have a near 40°<111> relation with part of their surroundings.

Abstract: Ingot breakdown is the most significant process for achieving high-quality billets in the cogging process. Especially microstructure evolution and grain refinement during the plastic deformation are of technical interest for the metal forming industry. To obtain refined and uniform microstructures after processing, it is necessary to understand deformation mechanisms as well as recovery-and recrystallization phenomena. Polycrystalline pure nickel and cast structured austenitic stainless steels were deformed by a compression test at different warm forming temperatures (800-1200°C) to investigate the influence of initial grain size, strain and strain rate on the structural refinement process. To get rid off static-and postdynamic softening processes the samples were immediately water quenched after deformation. By using the electron back scatter diffraction technique, the microstructural evolution and the crystallographic orientations were captured after deformation.

Abstract: During large-strain plastic deformation, subgrain structures typically develop within the grains. At large enough equivalent strains above, say 0.5, recrystallization occurs via abnormal coarsening of the subgrain structure or abnormal (sub-) grain growth (AsGG). The fraction of subgrains that develop into new, recrystallized grains has been quantified as a function of texture spread (Grain Reference Orientation Deviation) using Monte Carlo simulation. When this fraction is combined with the known monotonic increase in mean misorientation with strain, the recrystallized grain size can be predicted as a function of von Mises strain. The prediction is in good agreement with experimental results drawn from the literature.