Particle stimulation nucleation of recrystallization in two-phase Al-Mg₂Si alloys

John Liu

doi:10.17918/00010348

A series of Al-Mg₂Si alloys was produced with the original intention of studying particle stimulated recrystallization for grain size control. Dispersions with independent variation of particle size and spacing were required for the study. A finite difference diffusion model was developed to generate the appropriate cooling curves for their production. From metallographic sections, 2-D particle size and distribution were measured by an image analyzer. To characterize the dispersions in more detail, all particles were approximated as spheroids. From the measured 2-D particle parameters, 3-D particle size and distribution were calculated by the Saltykov/DeHoff analysis. Log-normal distribution was found. The 90 percent cold rolled and annealed (350°C) conditions were studied using measurements of hardness, electrical conductivity, optical metallography, SEM and TEM. The technique of convergent beam electron diffraction was used to obtain Kikuchi patterns, which were digitized and analyzed to obtain crystallographic information. TEM results of rolling plane samples showed gross heterogeneity in the misorientation development around particles. These were attributed to the interaction of shear bands and particles. The shear bands were clearly revealed by transverse section TEM foils prepared with a newly developed technique. They appear to form in single-phase alloy as well, but seem to be intensified by the presence of particles. A hypothesis was put forward to explain the interaction of shear bands with particle deformation zones which overlap. Particle stimulated nucleation of recrystallization (PSN) also appears to be operative to some extent. Extra solute inhibits dynamic recovery. Particle pinning slows down the recrystallization kinetics. The findings of this study raise some questions on the currently accepted models of recrystallization in two-phase alloys. With the assumption of macroscopically homogeneous strain, these models predict the conditions for PSN or recrystallized grain size based on the imposed strain and particle size distribution. The assumption of macroscopically homogeneous strain is no longer valid for alloys which show strain localization in the form of shear bands. It appears that shear banding is a more common phenomenon than generally thought. The role of shear banding on the microstructural development and the resultant mechanical properties is worth further studying.

Particle stimulation nucleation of recrystallization in two-phase Al-Mg₂Si alloys

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Particle stimulation nucleation of recrystallization in two-phase Al-Mg₂Si alloys

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