Amplitude-dependent internal friction of dislocation interactions in dual-phase steel

Dislocation interactions in the ferrite matrix of a dual-phase steel have been studied as a function of martensite volume fraction (11-25%) and tensile straining (0-5%) by strain-induced amplitude-dependent (strain amplitude varying from 10(-7) to 10(-4)) internal friction measurements at room temperature, using a high-frequency composite oscillator (40 kHz). Results showed that a higher volume fraction of martensite increases the internal stress in the softer ferrite matrix and restricted the dislocation motions, giving rise to a lower amplitude-dependent internal friction. Application of tensile straining further shortened the effective mobile dislocation length which was revealed by the internal friction measurements and this resulted in a high initial strain hardening rate of the high-martensite samples. Reduction in the martensite volume fraction and the mobile dislocations in the ferrite matrix through process control resulted in an increase in the amplitude-dependent internal friction, indicating a longer effective mobile length of dislocations which, in turn, delayed early necking of the dual-phase samples during tensile straining.