Fe–1C–3Mn low-density nano-bainitic steels: Influence of 6.1 and 8.0 wt.% Al on transformation kinetics, microstructure and mechanical properties

Two 1C–3Mn low-density steels with 6.14 and 8.0 wt.% Al (S1, S2) were designed and austempered above Ms to clarify how high C–Al–Mn contents affect bainite (α B ) kinetics, retained-austenite (γ R ) stability and mechanical response. Dilatometry shows that S1 transforms comparatively rapidly, reaching bainite plateaus after ∼5 h at 350 °C and ∼12 h at 270 °C, whereas S2 at 300 °C exhibits a long bainite transformation taking ∼60 h, evidencing a strong kinetic penalty associated with the higher Al level at 3 wt.% Mn. X-ray diffraction and alloy-specific T 0 calculations reveal large γ R fractions (≈ 41–46 %) with carbon contents Cγ ≈ 1.08–1.46 wt.% that place all conditions close to their T 0 lines, confirming incomplete-reaction bainite. SEM, TEM and ACOM-TEM show bainitic ferrite interleaved with film-like and blocky γ R ; the bainitic lamellae thickness ranges from ≈ 43 nm (S1–270 °C) to ≈ 102 nm (S1–350 °C), with S2–300 °C intermediate (≈ 63 nm). TEM/ACOM further reveals dense dislocation tangles and slip-band–like planar defects in γ R and at α B /γ R interfaces, together with weak L1 2 -type short-range order but no resolvable κ-carbide or cementite. Tensile tests show that S1–270 °C attains the highest strength (UTS ≈ 1.45 GPa, El ≈ 6.9 %), whereas S1–350 °C and S2–300 °C show a higher total elongation at comparable UTS levels (UTS ≈ 1.30–1.33 GPa, El ≈ 14–17 %). The results demonstrate that in these 1C–(6–8)Al–3Mn steels the governing parameters are the stability and morphology of γ R and the bainite lamellae thickness, which jointly control the degree of incomplete reaction and the relative contributions of TRIP and dislocation hardening.