Cleavage oriented iron single crystal fracture toughness

Fundamental understanding of atomic level mechanisms controlling cleavage fracture in bcc metals, and the corresponding brittle to ductile transition (BDT) has been a long sought, 'grand challenge' of science. This is particularly true for the BDT in Fe, which is among vital elements that underpin our technological civilization. A key obstacle to developing an understanding of the BDT in Fe is the absence of a reliable database on the temperature dependence of toughness in Fe. In ferritic alloys, the micro-arrest toughness of ferrite, Kmu(T), is hypothesized to control macroscopic cleavage. As a surrogate for Kmu(T), special techniques were developed to measure the arrest toughness, Ka(T), for cleavage oriented, Fe single crystals. Further, the mechanisms controlling cleavage and the BDT should be reflected in the loading rate dependence of static-dynamic initiation toughness, K Ic and KId. Thus KIc/d(T) were also measured for K-rate from 10-1 to 104 MPa√m/s. These studies led to the following conclusions: (1) Ka is semi-brittle, increasing from an average of ≈ 3.5 MPa√m at -196°C to ≈ 9 MPa√m at 0°C. (2) The (100) Ka are similar in the [010] and [011] and orientations, but cleavage does not occur on (110) planes. (3) The Ka for unalloyed Fe is about 150°C lower than that for Fe-3wt%Si, suggesting that equivalent Ka may occur at equivalent lattice sigmay. (4) Higher K-rate shift K Ic/d(T) curves to higher T. (5) The shifts of the KIc/d(T) and Ka(T) curves can be understood and modeled based on dislocation dynamics concepts for the glide of screw dislocations with a stress (and T) controlled activation energy, Ea, with a maximum value of about ≈ 0.5 eV. (6) This Ea is consistent with a double kink nucleation mechanism. Etch pit, slip trace and ledge patterns on side, fracture and sectioned surfaces of the crystals were characterized to study dislocation activity associated with cleavage and the BDT. The results showed extensive dislocation activity on {lcub}110{rcub}<111> and {lcub}211{rcub}<111> slip systems. Consistent with its semi-brittle nature, there is evidence of significant dislocation glide and multiplication during cleavage, perhaps associated with pre-existing dislocations.