PhD Thesis Defense: Chao Yang

Friday, December 6, 2019, 4:00–6:00pm

Rm 232 (Jackson Conf Rm), Cummings Hall

"Effect of solute and particles on microstructural evolution during directional recrystallization"

Abstract

The effect of how solute and particles affect microstructural evolution during directional recrystallization was studied in this research using the Ni-Al system. Changes of grain morphology, boundary character and texture induced by solute and particles were investigated by comparing high-purity Ni with Ni-3 at.% Al and Ni-at.Al, respectively. Effect of how increasing drawing velocity during directional recrystallization will affect grain morphology was also elucidated.

Compared to high-purity Ni, Ni-3Al has a reduced upper limit for the drawing velocity that can form columnar grains, which can be explained by solute drag. It was found out solute drag effect was not only misorientation-angle dependent but also misorientation-rotation axis dependent, which narrowed the mobility difference of different misorientation high-angle boundaries. As a result, the boundaries between columnar grains and the small equiaxed grains ahead of them have a near-random distribution instead of a defined rotation relationship, and, thus, columnar grains can arise from random textures instead of arising from a single texture.

Ni-12Al with small particles can form columnar grains, but at a higher annealing temperature compared to its particle-free Ni-3Al counterpart, whilst columnar grains were not formed in Ni-12Al with large γ’ particles for the same annealing parameters. Columnar grains only formed after the γ’ particles dissolved. γ’ particle dissolution is not a trigger for columnar grain formation but is simply a prerequisite. Whether columnar grains can form or not was determined by the texture at the time of particle dissolution. If texture pinning exists after particle dissolution, abnormal grain growth occurs, and columnar grains form; if texture pinning is absent, columnar grains cannot form and normal grain growth occurs.

It is shown that the upper limit of drawing velocity for columnar grain propagation is higher than that for columnar grain nucleation. When the drawing velocity was increased during columnar grain propagation, there are always some columnar grains or some part of a columnar grain that stopped growing, which leads to a breakdown of the growth front as a result of growth competition.

Thesis Committee

For more information, contact Daryl Laware at daryl.a.laware@dartmouth.edu.