Aluminum-alloyed low-density steels have recently received significant attention because of the advantage in strength-todensity ratio [1] and superior corrosion resistance. [2][3][4] Aluminum is a strong ferrite-forming element and expands both α-ferrite and δ-ferrite phase region. Thus, the addition of aluminum in steel can result in δ-ferrite that always exists below solidification temperature, in spite of the addition of austenitic stabilizing elements. Under the action of deformation, the dualphase composed of δ-ferrite and austenite is expected to be elongated and forms micro-laminated microstructure. [5][6][7][8][9][10][11][12][13][14][15][16][17] Except prior δ-ferrite lamellae, through diversified alloy design, hot working process, and heat treatment process, the microstructure transformed from prior austenite can be designed as dual-phase or multiphase constituents, containing austenite, [5][6][7][8][9][10][12][13][14] α-ferrite, [5][6][7][8]12] martensite, [5][6][7][8][9][10][11][12][13] pearlite, [10] and bainite, [10] and the fraction range of each phase is also wide. Accordingly, the mechanical properties also vary among different phases. Hence, micro-laminated low-density steels have a broad application prospect, with extraordinary combinations of mechanical properties. [8,[10][11][12][13] Based on the microstructure, the deformation-induced evolution of the microstructure and the fracture mechanism are also complex. In general, δ-ferrite is a brittle phase, while the complex microstructure transformed from the original austenite has superior plasticity and resistance to crack growth. [17,18] Cracks propagate preferably into δ-ferrite or at the interface l ayer along the δ-ferrite {100} planes. [15,18] The embrittlement of δ-ferrite is because of higher aluminum content. [17][18][19] With the increase of aluminum content, the reduced mobility of dislocations [19,20] and the formation of ordered structure [17][18][19] result in lower ductility and toughness. Given that the retained austenite stability is critical for fracture resistance, [17] previous studies have focused on controlling the content and stability of retained austenite.However, studies on α-ferrite phase transformation on δ-ferrite and corresponding effect on mechanical properties are rare. Li et al. [14] reported that coarse δ-ferrite is harmful to mechanical properties. The grain refinement of ferrite is simultaneously beneficial to both strength and toughness.
