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 adding a large amount of 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-size carbides. In the temperature range from 60 to -60 ºC, the yield strength was greater, leading to the enhancement of the toughness.

This research article is published by Science Magazine, Vol.320(#5879) on 23 May 2008.