Guilin Wu

Recrystallisation kinetics of aluminium AA1200 cold rolled to true strain of 2

Abstract Commercial purity Al AA1200 cold rolled to a true strain 2 was annealed at 270°C for a series of times. The microstructural parameters V V, the volume fraction recrystallized, S V, the interfacial area density separating recrystallised grains from deformed matrix and , the mean chord length of recrystallising grains, were measured stereologically using the EBSD technique. The recrystallisation kinetics were studied and the growth rates calculated by the extended Cahn–Hagel equation. It was found that the recrystallisation kinetics exhibited a two stage behaviour: an early stage characterised by decreasing growth rates, and a later stage with approximate constant growth rates. On average cube grains grow about two times faster than other grains and the growth rates of the three rolling texture components, brass, copper and S, are almost the same.

In-situ measurement of annealing kinetics of individual bulk grains in nanostructured aluminium

Non-destructive three-dimensional X-ray diffraction (3DXRD) was used to characterise the coarsening and growth of bulk crystallites in very heavily deformed aluminium in-situ during isothermal annealing. It was found that initially during the annealing, coarsening by recovery dominates. Later recrystallisation starts and, by fast growth of recrystallisation nuclei, some very big grains evolve. This occurs simultaneously with recovery coarsening of other parts of the microstructure. Consequently, very broad grain size distributions are observed. The 3DXRD results show that the nuclei (those crystallites that end up being very large by fast recrystallisation growth) do not have an initial size advantage compared to those coarsening much slower by recovery. Kinetics curves for these two categories of grains are determined. Data of this type are considered very important for understanding the thermal response of nanometals and thus also for instructing thermal treatment for optimal mechanical properties.

Development of a strong Goss texture during annealing of a heavily rolled Al—0.3% Cu alloy

The evolution of microstructure and texture during isochronal annealing of a heavily cold rolled Al-0.3% Cu alloy has been characterized using electron backscatter diffraction. It is found that the rolling texture of this alloy is dominated by the Brass component and that recrystallization during annealing leads to the formation of a pronounced Goss texture. It is suggested that the development of the strong Goss texture in Al-0.3% Cu is caused by preferred growth of Goss-oriented grains into the Brass-oriented matrix.

Direct observation of nucleation in the bulk of an opaque sample

Remarkably little is known about the physical phenomena leading to nucleation of new perfect crystals within deformed metals during annealing, in particular how and where volumes with nearly perfect lattices evolve from structures filled with dislocations, and how local variations at the micrometer length scale affect this nucleation process. We present here the first experimental measurements that relate directly nucleation of recrystallization to the local deformation microstructure in the bulk of a sample of cold rolled aluminum, further deformed locally by a hardness indentation. White beam differential aperture X-ray microscopy is used for the measurements, allowing us to map a selected gauge volume in the bulk of the sample in the deformed state, then anneal the sample and map the exact same gauge volume in the annealed state. It is found that nuclei develop at sites of high stored energy and they have crystallographic orientations from those present in the deformed state. Accordingly we suggest that for each nucleus the embryonic volume arises from a structural element contained within the voxels identified with the same orientation. Possible nucleation mechanisms are discussed and the growth potentials of the nuclei are also analyzed and discussed.

Microstructural path model and strain dependence of recrystallisation in commercial aluminium

Abstract The isothermal recrystallisation of commercial purity aluminium alloy AA1200 cold deformed to either a true strain of 2 (86·5% reduction in thickness) or 4 (98·2% reduction in thickness) was studied phenomenologically in each material by means of quantitative microscopy. The microstructural path descriptors, V V, the volume fraction recrystallised, and S V, the interfacial area density separating recrystallised grains from deformed grains were measured stereologically by electron backscatter diffraction and microstructural path model parameters were deduced for each strain. The effects of strain were delineated and compared with the results of recrystallisation in a slightly different commercial aluminium alloy AA1050 deformed to a true strain of 2·3.

Crystallographic Analysis of Nucleation at Hardness Indentations in High-Purity Aluminum

Nucleation at Vickers hardness indentations has been studied in high-purity aluminum cold-rolled 12 pct. Electron channeling contrast was used to measure the size of the indentations and to detect nuclei, while electron backscattering diffraction was used to determine crystallographic orientations. It is found that indentations are preferential nucleation sites. The crystallographic orientations of the deformed grains affect the hardness and the nucleation potentials at the indentations. Higher hardness gives increased nucleation probabilities. Orientation relationships between nuclei developed at different indentations within one original grain are analyzed and it is found that the orientation distribution of the nuclei is far from random. It is suggested that it relates to the orientations present near the indentation tips which in turn depend on the orientation of the selected grain in which they form. Finally, possible nucleation mechanisms are briefly discussed.

Simultaneous Enhancement of Mechanical and Magnetic Properties in Extremely-Fine Nanograined Ni-P Alloys

Exploring structural effects that influence both the mechanics and magnetism in nanocrystalline materials, particularly extremely-fine nanograined ones with grain sizes down to several nanometers, is of high interest for developing multifunctional materials combining superior mechanical and magnetic performances. We found in this work that electrodeposited extremely-fine nanograined Ni-P alloys exhibit a significant enhancement of magnetization, simultaneously along with an increase in hardness, after low-temperature annealing. The relaxation of non-equilibrium structures, precipitation of the second phase and the segregation of P atoms to grain boundaries (GBs) during annealing have then been sequentially evidenced. By systematically comparing the variations in macroscopic and microstructural investigation results among several Ni-P alloys with different P contents, we suggest that the second phase has little effect on magnetization enhancement, and essentially both the structural relaxation and GB segregation can play important roles in hardening by governing GB stability, and in the improvement of magnetization by enhancing Ni–Ni atom exchange interactions.

Heterogeneous multi-layered IF steel with simultaneous high strength and good ductility

Multi-layered IF steel samples were designed and fabricated by hot compression followed by cold forging of an alternating stack of cold-rolled and annealed IF steel sheets, with an aim to improve the strength of the material without losing much ductility. A very good combination of strength and ductility was achieved by proper annealing after deformation. Microstructural analysis by electron back-scatter diffraction revealed that the good combination of strength and ductility is related to a characteristic hierarchical structure that is characterized by layered and lamella structures with different length scales.

Structural refinement and property optimization in an Fe-23Cr-8.5Ni duplex stainless steel

An Fe-23Cr-8.5Ni duplex stainless steel was used to prepare samples with different volume-fraction-weighted grain sizes (d), ranging from the nano-scale to the micrometerscale by cold rolling and subsequent annealing. The cold rolled sample with d of 72 nm showed a high yield strength of about 1.3 GPa but only a small tensile elongation. An abrupt increase of ductility was observed as d increased to 375 nm, resulting in a good combination of yield strength of 738 MPa and tensile elongation of 29%. Further increase of d up to the micrometer-scale results in continued decreases in yield strength but with only a limited improvement in the ductility.

Nucleation of recrystallization at selected sites in deformed fcc metals

The objective of this thesis is to explore nucleation of recrystallization at selected sites in selected face-centered-cubic (FCC) metals, namely cold rolled columnar-grained nickel and high purity aluminum further deformed by indenting. Various techniques, including, optical microscopy, electron backscattered diffraction (EBSD), electron channeling contrast (ECC) and synchrotron X-ray technique, differential-aperture X-ray microscopy (DAXM), were used to characterize the microstructures, to explore nucleation sites, orientation relationships between nuclei and deformed microstructures, and nucleation mechanisms. In the cold rolled nickel samples, the preference of triple junctions (TJs) and grain boundaries (GBs) as nucleation sites is observed. The majorities of the nuclei have the same orientations as the surrounding matrix or are twin-related to a surrounding deformed grain. Only a few nuclei are observed with orientations different from the surrounding matrix. Hardness measurements at TJs in the deformed sample indicate a weak correlation between the difference in hardness among the three grains at the TJs and the potentials of the junctions to form nuclei: the higher the difference, the more likely is nucleation. In the weakly rolled and indented aluminum samples, it is found that hardness indentations lead to large orientation rotations near indentation tips. In initial grains of different crystallographic orientations, the grains with higher stored energy (SE) in the rolled microstructures have higher average hardness values and higher nucleation probabilities. In general, indentations with higher hardness values have higher nucleation potentials. The orientations of the nuclei from different indentations in a given grain are observed not to be randomly distributed, but clustered in limited orientation spaces. The orientation spread observed near the indentation tips in the deformed state covers the orientations of the nuclei observed in the annealed state. Whereas the above results are obtained by the EBSD technique and thus are 2D observation, the nucleation at hardness indentations is also investigated non-destructively by the DAXM technique. By first characterizing the deformation microstructure within a selected gauge volume near a hardness indentation, then annealing the sample and measuring the same volume again, nucleation is directly correlated to the deformation microstructures in the bulk of the sample. It is found that the nuclei evolve from embryonic volumes at areas of high SE below the surface and develop because of an advantage of fast migrating boundaries surrounding the initial embryonic volumes. All nuclei have crystallographic orientations as those present within the embryonic volumes in the deformed state. It is further suggested that boundaries between nuclei and the deformed matrix of less than 5° hinder subsequent growth of the nuclei. For all the observed cases, it is suggested that the nucleation mechanism may be strain induced boundary migration (SIBM), but the boundaries are not those conventionally considered, namely original grain boundaries, but are strain induced dislocation boundaries. Project period: