Cyclic deformation behaviour and Bauschinger effect in ductile cast iron

The cyclic deformation behaviour of nodular cast iron in low cycle fatigue was investigated. To understand the mechanisms of hardening, mechanical and microstructural approaches were developed. A first attempt was conducted in terms of effective and internal components of strengthening. Results are discussed in the light of the dislocations arrangements. They were found to be in accordance with previous works on α-iron or Fe-Si polycrystals. In particular, the effective stress is strongly modified by strain rate, and the internal component of the hardening is especially influenced by applied plastic strain. However, for cumulated plastic strain lower than 1, nodular cast iron exhibits a specific Bauschinger effect characterized by a high internal stress and an unusual hysteresis loop shape. This effect is mainly due to inhomogeneities in deformation between inclusions and matrix and the development of enhanced dislocation density in the matrix from the interface. The Bauschinger effect is finally reduced when plastic flow spreads in the matrix with further cycling. A composite model based on microstructural measurements is then proposed to account for the evolution of the Bauschinger effect during deformation.