Critical Effects of Microstructural Heterogeneities in Aluminium Alloys on Nucleation of Recrystallisation

Thermomechanical processing is traditionally used to alter the mechanical properties of metals, such as aluminium and its alloys. Aluminium is found in many different applications in our everyday lives, from aluminium foil and beverage cans to automobile sheet parts. In these various applications, different mechanical properties are required. Specific properties are achieved through different deformation and annealing schedules. Ultimately, the properties of the metallic product are controlled by its microstructure.

As the metal goes through various processing steps, its microstructure is altered. The microstructure is almost always heterogeneous in terms of crystallographic orientations, grain boundaries, impurities, crystal lattice defects, and residual strains. Predicting the onset of recrystallisation, the nucleation of recrystallisation, has been attempted through decades but still remains at best an estimate. This is critical, as nucleation to a large extent defines the recrystallised grain size and the crystallographic orientations. Thereby, nucleation of recrystallisation is expected to play a crucial role in determining the recrystallised microstructure of a metal.

The objective of this PhD thesis is to non-destructively explore the critical effects of microstructural heterogeneities in aluminium alloys on nucleation of recrystallisation in 3D and 4D. In this thesis, two experimental X-ray studies and one simulation experiment will be presented. The focus is primarily on two aspects of microstructural heterogeneity: the effect of second phase particles and the effect of local plastic strains. This is studied in two different aluminium alloys by different combinations of multimodal 3D X-rays techniques.

Particle stimulated nucleation has been studied in the past and is expected to play an essential role in recrystallisation of many industrially relevant aluminium alloys. With the aim of quantifying particle stimulated nucleation, a bulk volume of AA5182 alloy deformed to 75 % thickness reduction is studied by 3D synchrotron X-ray Laue micro-diffraction and laboratory absorption X-ray tomography. The novelty of this work is that particle stimulated nucleation is quantified in a large gauge volume of an industrially relevant aluminium alloy treated in a similar manner as it is in industry. It is found that upon annealing, nuclei often develop near large second phase particles. Particle stimulated nucleation is found to be by far the most frequent nucleation mechanism in this alloy under these processing conditions. Dependencies on particle size and chemical composition are found. Residual elastic strains are determined from the micro-diffraction data, and contradicting classical theory strains are found in the recrystallised nuclei. The microstructural variations, as particle type, particle clustering, particle size, and aspect ratio, are discussed. The presence of elastic strains in the nuclei and how these arise are assessed.

In most studies, particle stimulated nucleation is defined by a nearness criterion, i.e. the nuclei are particle stimulated if they are within a certain distance of a second phase particle. If using a nearness criterion leads to miscategorisation of particle stimulated nucleation events is studied. Miscategorisation of particle stimulated nucleation may take place if a nucleus has developed elsewhere and grows to be close to a particle. The possible magnitude of miscategorisation is studied by simulating an alloy with randomly distributed particles and nuclei in a virtual volume. Different simulation scenarios are created with different densities, numbers and sizes of particles and nuclei. It is found that the potential miscategorisation in aluminium alloys, of relatively low particle volume fraction should not play a significant role as long as the recrystallised volume fraction is kept low. However, the results may be influenced if the sample consists of a few very large nuclei or particles. The same simulation tool is used to evaluate potential miscategorisation in the experimental study of particle stimulated nucleation.

Nucleation of recrystallisation has in many years mainly been studied in 2D by different means of microscopy. However, it is expected that studying particle stimulated nucleation in 2D may underestimate the efficiency of the second phase particles. If a nucleus is seen in a 2D plane to be away from particles, it cannot be concluded not to be particle stimulated, as a particle can be present below the characterised plane. A numerical simulation is used to theoretically study if an area can correctly quantify the effect of particle stimulated nucleation in a bulk sample. The particles and nuclei are distributed in a virtual volume. The fractions of nucleation events observed in 2D and in 3D are compared, as the sizes of areas and volumes are altered. In all comparisons of 2D and 3D particle stimulated results, the 2D studies greatly underestimate the effect of second phase particles. This is further substantiated by a 2D study of the 3D volume used to study particle stimulated nucleation experimentally.

Effects of local plastic strain on nucleation is studied in a commercially pure AA1050 aluminium alloy. Other nucleation sites, than second phase particles, are expected to dominate nucleation in this material. For the first time, local plastic strains generated during deformation and nucleation of recrystallisation upon several heat treatments are followed using three different 3D synchrotron X-ray techniques: phase contrast tomography, diffraction contrast tomography and scanning 3D X-ray diffraction. A dog-bone-shaped sample is tensile deformed in several steps and mapped by synchrotron phase contrast tomography. The deformed microstructure is mapped by scanning 3D X-ray diffraction. Nucleation is initiated by several short annealing steps. In between these, the microstructure is mapped by diffraction contrast tomography and scanning 3D X-ray diffraction. The small second phase particles mapped by phase contrast tomography are used to find the local plastic strain. The results from this study are preliminary. Nucleation is found to occur near multi junctions, especially quadruple junctions and junctions of higher order. The nuclei are suggested to form via strain induced boundary migration. No obvious relation between the local plastic strain and the position of nuclei is found.

The results from these works show that nucleation of recrystallisation highly depends on the heterogeneities of the deformed microstructures. Furthermore, it is demonstrated how 3D and 4D characterisations enable detailed investigations of recrystallisation nucleation, providing new insights and statistically relevant results.

This work has resulted in several papers. A review paper describing the general theory of recrystallisation nucleation and the history of characterisation techniques can be found in [A]. The methodology of data registration of two large data volumes, one acquired by a laboratory X-ray setup and one by synchrotron Laue X-ray micro-diffraction, is described in [B]. A paper containing the results of the particle stimulated nucleation analysis has been submitted and a pre-print can be found in [C]. A numerical simulation of the artifacts of investigating particle stimulated nucleation in 2D and 3D is presented in [D]. Non-destructive 3D experimental results are used in a recrystallisation model, the paper can be found in [E]. A synchrotron Laue micro-diffraction experiment of an additive manufactured aluminium alloy sample can be found in [F]. The latter two papers address topics that are slightly outside the scope of this thesis, and my contributions to them have been somewhat small; therefore, they are not included here.