Inverse temperature dependence of toughness in an ultrafine grain-structure steel

Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile- to- brittle transition behavior of body- centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh- strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer- sized carbides. In the temperature range from 60 degrees to -60 degrees C, the yield strength was greater, leading to the enhancement of the toughness.