Neutron diffraction monitoring of ductile cast iron under cyclic tension–compression

To understand work hardening behavior during low-cycle loading, ductile cast iron containing 10.1 vol% spheroidal graphite, 19.6 vol% pearlite, and ferrite matrix was investigated in an in situ neutron diffraction study of up to four cycles of tensile–compressive loading with applied strains of ±0.01. The amplitudes of applied stress, Bauschinger stress, and Bauschinger strain were found to increase with increasing cycle number, indicating work hardening as cyclic loading progressed. Absolute values of ferrite lattice strain at maximum and minimum applied strains increased with increasing cycle number, indicating an increase in ferrite strength. Consequently, the stress contribution to the strength from ferrite increased as cyclic loading progressed. Cementite embedded in pearlite behaved as the hardest phase and maintained elastic deformation, but its stress contribution to strength was limited because the volume fraction was only about 2.2%. Meanwhile, graphite accommodated little stress. The increase in ferrite strength, caused by dislocation accumulation in ferrite during cyclic loading, played an important role in the work hardening of the ductile cast iron.