Deformation and damage in lath martensitic Aermet 100 steel under plate impact

Plate impact experiments are performed on hierarchically structured, lathed, martensitic Aermet 100 (A-100) steel to investigate its shock properties and spall damage under varying peak stresses and strain rates. Hugoniot relations are established up to shock stress of 15 GPa. The Hugoniot elastic limit and spall strength are determined from free-surface velocity profiles. The initial and postmortem samples are characterized using transmission electron microscopy, scanning electron microscopy, and electron backscatter diffraction. Crack length, width, and inclination angle are quantitatively analyzed. The spall strength of the A-100 steel ( ∼ 6.5 GPa) exceeds that of nearly all other steels, and shows an unusual insensitivity to peak stress and strain rate. Nanoscale twins form within thin martensitic blocks. Spall-induced cracks preferentially nucleate at incoherent packet boundaries rather than at coherent block or twin boundaries. Most cracks propagate at a distance between the mean block width and packet size. Cleavage and shear cracking modes are observed. Cleavage cracks occupy large portion ( > 60%) and cut through numerous thin blocks by opening along {100} plane in a group of block variants, but deviate upon crossing into adjacent packets, resulting in curved and roughly horizontal morphologies. Shear cracks constitute a minor fraction ( < 20%), are vertically oriented, and traverse multiple packets along straight paths via sliding, and are accompanied by considerable plastic deformation. These shear cracks often bridge cleavage cracks at different depths, ultimately coalescing into a main crack.