Ultrahigh strength in lightweight steel via avalanche multiplication of intermetallic phases and dislocation

Lightweight high-strength steel has been an important topic for structural materials. Recent studies show that B2 intermetallic phase, nucleated around crystal defects such as grain boundaries and dislocations, can be used to further strengthen mechanical properties of metal alloys. Due to the limited density of nucleation sites , however, the current thermomechanical processing leads to formation of low density and coarse B2 phase. In this study, a lightweight steel Fe-Mn-Al-C (70 wt.% Fe) is processed by a thermal-controlled high strain rate plastic deformation process, warm laser shock peening (wLSP), in which high-density dislocations can be generated and high mobility of solute atoms at elevated temperature can facilitate the precipitation of B2 intermetallic phase. Simulations based on coupled phase-field and dislocation dynamics model are performed to investigate the intermetallic precipitation and dislocation formation during wLSP process. It is found that high density dislocations can be obtained in wLSP due to the pinning effect of B2 intermetallic phase on dislocations. The interaction between B2 intermetallic precipitates and shock wave facilitates the generation of high-density dislocations, which in turn serve as nucleation sites for B2 phase formation. This strong coupling effect results in avalanche multiplication of ultrahigh density and ultrafine B2 precipitates, and significant improvements in both strength and ductility. The yield strength of the lightweight steel reaches 2030Mpa with ultimate strength of 2850 MPa, among the highest for lightweight mid-carbon steel. This work provides a new insight to produce lightweight metals with ultrahigh density and ultrafine intermetallic phases for high strength and ductility.