Open Collections

Precipitation and recrystallization in a binary magnesium-neodymium alloy

University of British Columbia

The influence of precipitate state and annealing temperature on the recrystallization of a Mg-2.8wt.%Nd alloy has been investigated. Precipitation kinetics at 190°C, 350°C and 400°C were studied in order to understand precipitate evolution during recrystallization. Precipitation was studied primarily through electrical resistivity measurements, and modelled using a mean radius model. It was found that predicting the kinetics required the spatial distribution of solute to be considered. Pre-aging conditions were selected in order to study the influence of either pre-existing or concurrently formed precipitates during recrystallization. After aging, the samples were cold rolled to a strain of 20%. The microstructures were characterized primarily through EBSD, and also through hardness measurements. Pre-aging the samples at 400°C for three hours resulted in a dispersion of stable β precipitates during annealing. This led to a recrystallized microstructure with recrystallization nucleation sites similar to those previously reported in the literature. Pre-aging the sample at 190°C for 24 hours lead to the formation of metastable β'' precipitates which strengthened the sample, but dissolved rapidly upon annealing at higher recrystallization temperatures. When samples previously solutionized at 545°C or aged at 190°C were subsequently annealed at 350°C, recrystallization stagnated. This was attributed to concurrent precipitation pinning grain boundaries. In all samples, irrespective of aging condition, recrystallization was observed primarily in twins and shear bands. The twins which recrystallized were found to be {10-11} contraction twins and {10-11}{10-12} contraction-extension twins. As the nuclei forming within these regions were not randomly oriented, recrystallization in these alloys did not randomize the texture. The work presented in this thesis increases understanding of recrystallization in Mg-Nd alloys. In particular, the means by which Nd interacts with and affects the recrystallizing microstructure have never been studied in detail. Furthermore, this work points to possible ways in which new magnesium alloys and thermomechanical processes could be designed to improve final material properties.

Text

2015-08-13

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/54396

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/