1990 Volume 30 Issue 8 Pages 632-638
The effect of Si and Al additions on the low temperature toughness and fracture mode of Fe-27Mn (in mass%) alloys was investigated in terms of the microstructure, heterogeneous deformation and stacking fault energy. The Fe-27Mn binary alloy has two constituent phases at 77 K; Epsilon martensite (ε) and metastable austenite (γ). It undergoes a ductile-to-brittle transition because of an intergranular fracture associated with heterogeneous deformation. The intergranular fracture is caused by stress concentration at grain boundaries on which large ε plates impinge. Silicon addition to the Fe-27Mn binary alloy is effective for refining ε plates and changes the fracture mode from intergranular to transgranular. The fracture is, however, a quasi-cleavage mode along ε plates so that Fe-27Mn-Si alloys also exhibit a ductile-to-brittle transition related to the formation of ε. The stress concentration at the intersection of ε plates results in the formation of microcracks along the ε plates leading to the quasi-cleavage fracture. Fracture modes of high manganese steels containing ε largely depend on the microstructure and the deformation behavior. Silicon addition probably affects not only the nucleation of ε, but also the cross slip behavior of partial dislocations at the intersection of ε plates through a decrease in the stacking fault energy of γ, thus leading to the change in fracture mode.
Aluminum addition to high manganese steels is so effective for suppressing the γ-ε transformation that the low temperature embrittlement associated with the formation of ε does not occur with even a small Al addition. In an Fe-27Mn-2.5Al alloy, for example, toughness is high even at 77 K because the γ phase is so stable that the transformation to ε does not occur during deformation at 77 K.
