Design of microstructure for achieving high strength in an Fe-10Mn-3Al-0.2C based alloy

Abstract

Yielding and work hardening behaviors were studied using α+γ and α' +γ fine lamellar structures in an Fe-10Mn-3Al-0.2C based alloy. An alloy of Fe-10.62Mn-2.84Al-0.17C-0.5Mo (wt. %) steel was prepared by vacuum induction melting. The ingot was homogenized at 1150 °C for 2 h, then hot-rolled to 4-mm-thick plates. The plates were solution-treated at 900 °C for 20 min then quenched with water. To obtain the α + γ lamellar structure, intercritical annealing was performed at 550 °C for 16 h (Fig. 1). A plate was then annealed at 800 °C for 1 min and cooled in air. Another plate was annealed at 850 °C for 3 min and water-quenched. Yielding in α'+γ lamellar structure occurred much faster than that in α+γ lamellar structure. After yielding, α'+γ lamellar structure showed abrupt increase of strength compared to gradual work hardening of α+γ lamellar structure. The origin of two different yielding and work hardening behavior was investigated using a transmission electron microscopy (TEM). TEM results demonstrate the importance of neighboring phase contacting with soft  and mechanical stability of  for yielding. Abrupt work hardening in α' +γ lamellar structure was related with deformation induced martensitic transformation of γ lamella. When γ lamella is mechanically stable, yielding occurs by the propagation of pile-up dislocations in α to γ or α' to γ. To understand this yielding mechanism, the stress for the propagation of pile-up dislocations in one phase into another phase is calculated using Hall-Petch behaviors of corresponding phases. Effective grain size of a lamella is determined to calculate the contribution of each boundary of a lamella to dislocation pile-up. Comparison of stresses required to drive α-to-γ and γ-to-α, and α'-to-γ and γ-to-α' propagations of pile-up dislocations suggests that α+γ and α'+γ lamellar structures yield by propagation of pile-up dislocations in α lamella to γ lamella or α' lamella to γ lamella. Based on the experimental results, the concept for achieving high strength was developed.