Cumulative contribution of grain structure and twin boundaries on cyclic deformation behavior of a 20Mn-0.6C- TWIP steel: Experimental and theoretical analysis

Abstract We elucidate that the cyclic deformation behavior of TWIP steels is governed by the cumulative effect of grain size and twin boundaries, by investigating fatigue properties of 20Mn-0.6C TWIP steels with two different grain size of 15 μm and 50 μm. At low strain amplitude (Δe/2 = 0.6%), the 15 μm 20Mn-0.6C TWIP steel exhibited a small initial cyclic hardening followed by a long stage of secondary hardening until saturation. In contrast, a short hardening stage followed by a long softening stage was observed in 50 μm 20Mn-0.6C TWIP steel. At high strain amplitude (Δe/2 = 1.6%), a rapid cyclic hardening followed by a long saturation stage up to fracture was observed in 15 μm 20Mn-0.6C TWIP steel. However, a rapid cyclic hardening followed by a long saturation stage up to fracture was noted in 50 μm 20Mn-0.6C TWIP steel. Fatigue life increased with decreasing grain size and strain amplitude and the ratio of plastic strain increased at the expense of elastic strain with increasing strain amplitude. The hardening ratio of the 15 μm 20Mn-0.6C TWIP steel was significantly higher as compared to 50 μm 20Mn-0.6C TWIP steel at the identical strain amplitude. This intriguing phenomenon is attributed to abundant boundaries leading to high work-hardening ability of 15 μm 20Mn-0.6C TWIP steel. Additionally, the spacing between striations at the crack initiation zone increased with reducing grain size and increasing strain amplitude. The increased striation spacing implied that the speed of fatigue crack propagation was enhanced with the number of fatigue cycles, which promoted maximum stress value involving cyclic hardening.