Ductile-to-brittle transition behavior of high-interstitial Fe-Cr-Mn alloys

The ductile-to-brittle transition behavior of high-interstitial Fe-Cr-Mn alloys with different N and C contents is discussed in terms of the deformation microstructure and the mode of brittle fracture. The combined addition of N+C improved the low-temperature toughness by decreasing the ductile-to-brittle transition temperature, compared to the addition of N alone, by effectively increasing the free-electron concentration and enhancing the metallic component of interatomic bonding. Transmission electron microscopy observations on deformed regions beneath the fracture surface of Charpy impact specimens tested at low temperatures indicated that α′-martensite was formed at the intersections of deformation twins in the N+C alloy, unlike in the N alloy. Thus, it is suggested that the formation of α′-martensite exerts a beneficial influence on low-temperature toughness because it suppresses the initiation of brittle crack by reducing the internal stresses of intersecting twins. On the other hand, the ductile-to-brittle transition temperature of the N+C alloys increases with increasing C content, which could be explained by the occurrence of intergranular fracture resulting from the excessive content of C above a certain level.