Low-Temperature Fracture of High Purity Iron and Its Relationship to the Grain Boundary Character

The mode of fracture and the dictile-to-brittle transition temperature (DBTT) of high purity iron (⪰99.999%) have been investigated with two sets of specimens of different grain boundary character. Wire specimens of 0.4 mm diameter were tested in tension at a strain rate of 8.33×10^-5/s. The grain structure of one set of specimens is of bamboo-type with high angle boundaries, which are nearly parallel to the {100} or {110} planes of the grains separated by the boundary and nearly normal to the specimen axis. As expected, these specimens fracture in the intergranular mode and DBTT is between 110 and 125 K. Specimens of the other set have coarse grain structure. Most of the boundaries are of small angle or near-twin type with random boundary plane orientation. These specimens fracture in the transgranular mode at and below 50 K; DBTT for the intergranular fracture, if any, is below 4.2 K. This is in contrast to the occurrence of intergranular fracture even at 77 K for less pure iron specimens (99.99% or below) with the grain boundary character of the second type. Thus, DBTT for the intergranular fracture, which is the common fracture mode in pure iron, depends strongly on the purity and on the grain boundary character of iron specimens. If the grain boundary character is unfavorable to the intergranular fracture, DBTT decreases from above 77 K to below 4.2 K by increasing the purity from 99.99% to 99.999%. In specimens favorable to the intergranular fracture, DBTT is about 120 K even in 99.999% pure iron.