Relationship between grain size and ductile-to-brittle transition at room temperature in Fe–18Mn–0.6C–1.5Si twinning-induced plasticity steel

• The elongation increased and then sharply decreased beyond a critical grain size.
• The fracture mode was varied from ductile to brittle with increasing grain size.
• While fine grains exhibited mechanical twins, coarse grains showed ε-martensite.
• ε plates provided nucleation sites and propagation paths for cracks.
• Si-added TWIP steel exhibited superior tensile properties to previous TWIP steels.

The effect of grain size on tensile properties, particularly elongation (El), was systematically investigated using cold-rolled and annealed Fe–18Mn–0.6C–1.5Si (wt.%) twinning-induced plasticity (TWIP) steels. Whereas both the yield (YS) and ultimate tensile strengths (UTS) decreased with grain coarsening, the El increased up to a grain size of approximately 18 μm, and then sharply decreased beyond the critical grain size. This unusual reduction in El in coarse-grained specimens was caused by the transition of the fracture mode from ductile to brittle. The transition in fracture stemmed from a change in the deformation mechanism from mechanical twinning to strain-induced ε-martensitic transformation. The ε plates provided nucleation sites and propagation paths for cracks, causing quasi-cleavage brittle fracture in coarse-grained specimens. The cracks growing along the {101¯0} or (0 0 0 1) plane were bent by ε plates with different plane orientations and were blocked by γ-austenite plates. When the local stress in the γ plate near the crack tip was high, new ε-martensitic plates formed within the γ plate, enabling the crack to pass through the γ plate, and changed the crack path. The quasi-cleavage fractured surface consisted of (101¯1), (101¯2), and (21¯1¯1) planes. Ridges of bamboo-like structures observed on the fractured surface formed by the variation in crack path occurring at the ε plates with different orientations. The Si-added TWIP steel exhibited a superior combination of UTS and El of more than 70,000 MPa %.