Protrusions formed during primary recrystallization of cross-rolled coarse-grained niobium

The growth of recrystallization nuclei towards the deformed matrix generally does not proceed isotropically. Local disturbances in the migration along grain boundaries lead to the formation of protrusions. This important aspect in recrystallization is rarely explored in the literature, especially in the sense of understanding which microstructural mechanisms explain its formation. The aim of this work is to study the formation of protrusions during the recrystallization of a niobium oligocrystal. A coarse-grained niobium plate was cut out from the longitudinal section of an electron-beam cast ingot. Five coarse grains (A, B, C, D and E) were randomly selected for this study to evaluate likely orientation effects on protrusion formation. The plate was cold rolled with several passes up to 50% thickness reduction. The sheet was then cut into two parts. The former was straight rolled until 70% thickness reduction while the latter was rotated 90 degrees around the normal direction with respect to the initial rolling direction. Important orientation and rolling mode effects were noticed regarding their recrystallization behavior. Protrusions were more abundant in the cross-rolled specimens. It was not possible to identify and associate significant differences regarding either CSL-type special boundaries or highly-misoriented boundaries ahead of the boundaries showing protrusions.