Mechanisms of Cleavage-Fracture Initiation in Notched and Smooth Specimens of 3% Silicon Iron

Abstract

The mechanisms of cleavage fracture in coarse-grained 3% silicon iron have been studied between −196 and +60° C using uniaxial tensile and notch-bend tests. Emphasis was placed on determining the critical event for fracture under the various experimental conditions. In the tensile tests at the lowest temperatures (−196 to −125° C) fracture occurred below the slip yield stress and was twin-nucleated. Between −125 and ∼ −50° C twinning coincided with macroscopic yielding by slip and fracture occurred after an approximately constant plastic strain of < 1%. It was deduced that the critical event for fracture throughout this range (−196 to ∼ −50° C) was the nucleation of a suitable microcrack. Between ∼ −50 and +55° C the critical event was the growth of grain-size microcracks, themselves nucleated at cracks in grain-boundary carbides or pearlite colonies. The deformation and fracture mechanisms in the notched specimens were in sharp contrast to those described above. Throughout the range +40 to −160° C twins were not formed at the fracture stress and fracture was determined by the growth of slipnucleated carbide cracks. Between −160 and −196° C the fracture mechanism was probably associated with mechanical twinning. The reasons for the different behaviour of the two types of specimen are discussed.