Inverse Temperature Dependence of Toughness in an Ultrafine Grain-Structure Steel

• DOI: 10.1126/science.1156084
• Publication Date: 2008-05-22T21:38:55Z
• Authors: Yuuji Kimura; Tadanobu Inoue; Fuxing Yin; Kaneaki Tsuzaki
• Keywords: 0203 mechanical engineering; 02 engineering and technology

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° to –60°C, the yield strength was greater, leading to the enhancement of the toughness.