Effect of solute copper on hardness and ductile-to-brittle transition in α-iron

Effects of solute Cu on strengthening and ductile-to-brittle transition (DBT) of α-iron have studied for Fe-(0 to approximately 2)mass%Cu alloys. The Fe-Cu alloys exhibit massive ferritic transformation on water-quenching from solution treatment temperature. With increasing Cu content, the massive ferrite grains were refined and the hardness was raised. Effect of the solid solution hardening is predominant to the strengthening, but the refinement of massive ferrite grains does not affect so much. Charpy impact tests showed that Cu addition substantially lowers DBT temperature of α-iron. The embrittleness at low temperature was caused by the onset of cleavage cracks at ferrite grain boundaries where deformation twins impinged. Copper in solution suppresses the formation of deformation twins, and this results in the shift of DBT temperature to lower side.