Boron-induced unique precipitation behavior at the grain boundary and its effect on the brittle fracture in a Fe-Mn-Al-C-based austenitic lightweight alloy

Boron-induced unique precipitation behavior at the grain boundary (GB) and its effect on the brittle fracture were studied in a Fe–Mn–Al–C austenitic lightweight alloy. Adding 50 ppm wt.% of B promoted the GB segregation of boron at the early aging stage at 550 °C. The segregated boron forms the M 23 B 6 at the GBs during further aging treatment. The initial M 23 B 6 evolves the M 23 C 6 by the replacement of boron to carbon during prolonged aging. Meanwhile, the M 23 B 6 promotes the catalytic reaction that sequential formation of α-ferrite and κ-carbide, which results in the accelerated coverage of the GB cellular precipitates. The ductility of the alloy drops faster by cracking of GB α-ferrite and κ-carbide under the applied stress as the cellular precipitate covers the GB. This study provides new insights into the role of boron addition between the GB strengthener and the embrittler in a Fe–Mn–Al–C austenitic lightweight steel.