Severe deformation of nanostructured bainitic steel

Abstract

Abstract Nanostructured bainite has been developed to meet the stringent demands of ballistic impacts. In a previous report [Y.T. Tsai et al., Scripta Mater. 2016;115:46], the deformation behavior of nanostructured bainite under high strain rate (2×103 s-1) was studied using a split Hopkinson pressure bar. Deformation twinning was found in both blocky austenite and film austenite, and it was concluded that twinning occurred first in the blocky austenite, enhancing the initial work-hardening, followed by twinning in film austenite, promoting further straining. In the present work, the deformation structure has been further studied using a severely deformed sample subjected to a high strain rate (3×103 s-1). It is found that blocky austenite, due to the inherent size inhomogeneity, can produce various deformation twinning structures; i.e., one variant or two variants of dominating mechanical twins, including twin kinking. The dominating twins are so-called primary twins, the apparent twinning planes of which deflect from the ideal {111}γ, resulting from the fact that the apparent twinning planes are stepped, presumably due to dislocation locking during deformation. Small twins are also found to be dispersed in parent austenite matrix, and they are categorized as secondary twins. Strong evidence suggests that the formation of α’-martensite usually occurs in the intersection of two primary twins. For film austenite, the deformation structure is one-variant twinning, and the twinning planes are all ideal.