Recrystallization and grain growth in strain gradient samples

Annealing alloys that contain the critical amount of retained strain usually results in a large final grain size, because the recrystallization reaction is nucleation limited. Subsequent growth of these nuclei through strained material rapidly produces large grains. The process slows dramatically when the strain-free grains impinge upon one another. In past studies, critical strain behavior was studied using multiple tension or compression samples, each deformed to different strain levels. In this study, the recrystallization behavior of a nickel base superalloy was studied using wedge samples. When compressed between flat dies, the wedge shaped specimen provides a strain-gradient sample because the nominal strain varies as a function of position, from zero strain in the short region, through the critical strain level, to a high strain in the tall region. Observations made in wedge samples were compared to conventional compression tests, and the relationship analyzed using results of finite element calculations. An extrusion of powder metallurgy processed, nickel-base superalloy Rene' 88 was used