1994 Volume 34 Issue 3 Pages 225-233
Experimental resultt by present author's research group at Tohoku University are reviewed. High purity iron which is is considered to show the inherent mechanical properties of iron was prepared and used to investigate the low temperature brittleness, temperature dependence of the yield stress, solution softening and hardening, and effects of small addition of solutes on these properties. Specimens were wires and tensile tested at various temperatures above 4.2 K.
Iron of 99.999% purity and higher shows transgranular fracture (TGF) below 50 K if the microstructure is unfavorable to the intergranular fracture (IGF), and shows IGF below 120 K if the microstructure is favorable. The ductile-to-brittle transition temperature of IGF strongly depends on the microstructure. Less pure iron, 99.99%, shows IGF at 77 K even though the microstructure is unfavorable to IGF. Oxygen segregated at grain boundaries promotes IGF, but the effect is small. Carbon segregated at grain boundaries prevents IGF by increasing the cohesion at the boundaries. The strong temperature dependence of the yield stress is the inherent nature of iron, although impurities promote the dependence. High purity iron shows a hump on the temperature-yield stress curve at the temperature range between 170 K and room temperature. The hump is suppressed by dissolved carbon, and also absent in impure iron. Hydrogen causes softening at the temperature range of the hump and hardening outside the temperature range. Hardening due to hydrogen often reported previously is an effect in impure iron. The softening and hardening due to carbon is additive to the softening due to hydrogen.